The Mammalian Circulatory System by dfsdf224s


									 S E C T I O N
                                         The Mammalian Circulatory System


 Identify the major circulatory
 pathways in the body.
 Explain how different types of
 blood vessels contribute to
 the circulatory system.
 Describe the structure and
 function of blood cells and
 the role of hemoglobin.
 Identify how antibodies
 contribute to the body’s
 immune response.

 Figure 9.9 This illustration of
 the circulatory system dates
 from the thirteenth century.

 Throughout history, different theories have been                    Although Harvey established that blood travels
 proposed to explain the purpose and structure of                 in only one direction through the circulatory
 the heart and blood vessels. The ancient Greeks                  system (flowing away from the heart in arteries,
 believed that the heart was the seat of intelligence.            and back toward the heart in veins), he was never
 In the second century, the Greek physician, Galen,               able to find the place where the blood stops
 proposed that blood ebbed and flowed within the                   flowing away from the heart and begins its return
 body like the tides of the ocean, causing the heart              journey. This discovery was made in 1657 (not long
 to enlarge suddenly and rhythmically. He also                    after Harvey’s death) by Marcello Malpighi, an
 believed that the arterial and venous systems were               Italian physiologist. Malpighi used the recently
 largely separate, and that blood flowed out of both               invented microscope to identify the capillaries,
 vessels to be absorbed by the flesh. This theory                  which completed the picture of the overall
 held sway for over a thousand years. It was not                  pathway of blood through the body.
 until the seventeenth century that an Englishman                    Figure 9.10 shows the main pathway and the
 named William Harvey established that mammals                    largest vessels of the human circulatory system. As
 have a cyclic circulatory system.                                you saw in the previous section, the mammalian
                                                                  circulatory system is a closed, complete, double
      We b                                                        system. The blood vessels are organized into the
                                                                  three primary cycles. The route taken by the blood
 Put yourself in the shoes of William Harvey, a physician and
                                                                  within the heart is called cardiac circulation. The
 surgeon in the 1600s (before the invention of the microscope).
                                                                  pathway of the blood from the heart to the lungs
 What kinds of experiments could you conduct to demonstrate
 that the blood is pumped by the heart and moves in a cycle
                                                                  and back is called pulmonary circulation. The
 around the body? Make a note of your ideas, then go to the       route from the heart to the rest of the body is called
 web site shown below to find out how Harvey solved this          systemic circulation. The systemic circulatory
 puzzle. Go to Science Resources, then to BIOLOGY 11 to find      cycle includes all the blood vessels other than
 out where to go next. How do your ideas compare to Harvey’s      those associated with the lungs. Almost all of these
 methods? In his discussion, Harvey compares the cycle of         blood vessels are named.
 blood to the water cycle on Earth. Why does he make this            The average adult man has about five to six litres
 comparison, and in what way is it inaccurate?                    of blood, while the average adult woman has about                              four to five litres. Of the volume of blood in your
                                                                  body right now, 80 to 90% is currently in your

288      MHR • Internal Systems and Regulation
systemic circulation, while most of the remainder                     which is a small vein. Remember that all arteries
is in your pulmonary circulation.                                     carry blood away from the heart, while all veins
                                                                      carry blood toward the heart. This means that most
                                                                      arteries carry oxygenated blood, and most veins
                                                                      carry deoxygenated blood. The one exception is in
                                                                      the pulmonary circulatory cycle, where the
 jugular                                               carotid
                       head and arms                                  pulmonary artery carries deoxygenated blood from
 vein (also          CO2             O2                artery (also
 subclavian                                            subclavian
                                                                      the heart to the lungs, while the pulmonary vein
 vein from                                             artery to      returns oxygenated blood from the lungs back to the
 arms)                                                 arms)          heart to be pumped into systemic circulation.
                               lungs                                  Table 9.1
 artery                                                pulmonary
                                                                      Examples of blood vessels and their associated organs
 superior                                              vein
 vena cava                                                               Vessel     Artery or vein            Associated organs
                                                                       aorta       artery            principal arteries of the body,
                                                                                                     distributing to all organs
                                                                       carotid     artery            head region
 vena cava
                                                                       jugular     vein              head region
 hepatic                                               mesenteric      hepatic     both              liver
 vein                                                  arteries
                                                                       renal       both              kidney
 hepatic             liver   digestive
                                                                       vena cava   vein              principal veins of the body, collecting
 portal vein
                                                                                                     from all organs
                                                       renal artery
 renal vein                                                            celiac      artery            serves coelom (major body cavity)
                                                                                                     and contained organs (e.g., stomach)
 iliac vein                  kidneys
                                                       iliac artery

                                                                        Wo rd
                                                  O2                                LINK
                         trunk and legs                               Each of the many blood vessels in the body takes its name
                                                                      from a Latin, Greek, or English term for its associated organ or
                                                                      organs. Portal, for instance, derives from the Latin porta,
Figure 9.10 The human circulatory system. Only the main
                                                                      meaning gate. It is the name given to vessels that enter
vessels are shown here.
                                                                      particular organs. For example, the hepatic portal vein carries
                                                                      the end products of digestion into the liver, while the renal
   A circulatory system has three main elements:                      portal vein carries waste products into the kidneys for
the transport vessels, which conduct fluid from one                    processing.
area to another; the transport medium, which is the
specialized fluid tissue that carries substances
around the body; and the pumping mechanism.                              Blood travels from an artery to an arteriole and
The rest of this section describes the vessels (the                   then into a capillary network where gases, foods,
arteries, veins, and capillaries) and transport                       wastes, and hormones are exchanged across the
medium (blood) of the mammalian cardiovascular                        capillary wall between the blood and the
system. The mammalian pumping mechanism, the                          interstitial fluid bathing each cell of the body.
heart, is examined in Section 9.3.                                    The capillaries then empty into venules and
                                                                      progressively larger veins that carry blood back to
The Transport Vessels                                                 the heart.
                                                                         A key factor that has influenced the evolution of
As you have seen, the mammalian circulatory                           the circulatory system is the need to keep the blood
system includes three main types of blood vessels                     flowing as quickly as possible through this cycle. As
— the artery, the vein, and the capillary. The                        vessels get smaller and smaller, friction increases
classification of blood vessels also includes the                      and blood pressure drops (see Figure 9.11 on the
arteriole, which is a small artery, and the venule,                   following page). This means that blood moves very

                                                                                                Transport and Circulation • MHR                289
 slowly through the capillaries. The heart alone is                                                  Arteries
 incapable of exerting enough pressure to drive the                                                  A cross section of an artery or arteriole reveals
 blood around the entire circulatory system. So how                                                  three different structural layers (see Figure 9.12).
 can blood be forced all the way through the                                                         The outer layer is a covering of connective tissue
 capillary beds, into the veins, and back to the                                                     mixed with a few elastic fibres. The middle layer
 heart? Adaptations in each of the different kinds of                                                — the thickest of the three layers — is made up of
 blood vessels help keep the blood moving.                                                           alternating, circular bands of elastic fibres and
                                                                                                     smooth muscle. The inner layer is only a single cell
  132                        18                                                                      thick, and consists of smooth epithelial cells that
  110                        15
                                                                                                     serve to reduce friction as blood courses through
            Blood pressure

   99                        12                                                                         A distinguishing feature of the artery is its
                                                                                                     elastic walls. This elasticity allows the artery to
   66                         9
                                                                                                     first expand as a wave of blood passes through it,
   44                         6                                                                      and then snap back again. This movement keeps
                                                                                                     blood flowing in the right direction, and provides
   22                         3
                                                                                                     an additional pumping motion to help force the
                                                                                                     blood through the blood vessels. When you






                                                                                                     measure your pulse, what you actually feel is the

                                                                                                     rhythmic expansion and contraction of an artery as
                                                                                                     blood courses through it.
 Figure 9.11 Blood pressure in different parts of the body.
 Note that the pressure approaches zero in the capillaries.                                                                BIO            FACT
 When the blood must travel a long distance against the pull
 of gravity, as it must in large animals such as the giraffe, the                                     If the arteries, veins, and capillaries in your body were
 veins may show a negative blood pressure.                                                            arranged end to end, they would stretch a distance of
                                                                                                      approximately 160 000 km.



                                                                        B Capillary

                                                                     inner layer

                                                                     middle layer

                                                                     outer layer
                              A Artery                                                                C Vein

 Figure 9.12 Sections through an artery, capillary, and vein. At any given moment,
 about 30% of the blood in your systemic circulation will be found in the arteries,
 5% in the capillaries, and 65% in the veins.

290                          MHR • Internal Systems and Regulation
Veins                                                                 Capillaries
Compared to an artery, a vein has a thinner wall                      The smallest of the blood vessels, capillaries reach
and a larger inner circumference (see Figure 9.12).                   every corner of the body. The capillary wall is a
As a result, the vein lacks the elasticity of the                     single layer of endothelial cells. The size of
artery, but has a greater capacity. At any given time                 capillaries varies, but their average diameter is
in a living mammal, the venous system contains                        about 8 µm — just large enough for the largest
approximately twice as much blood as the arterial                     blood cells to pass through in single file.
system.                                                                  The capillary wall regulates the movement of
   Once blood reaches the veins after passing                         fluids and other materials into and out of the blood
through the fine network of capillaries, it is                         stream. Remember that in a closed circulatory
travelling very slowly. Since veins lack the ability                  system the blood itself is always contained in
to contract, some other mechanism must be used to                     capillaries, and never flows out to bathe the body’s
keep the blood moving toward the heart. For the                       cells directly.
regions above the heart, the pull of gravity can help
draw blood back down through the veins. Below                           Math
the heart, however, the blood must be pushed
                                                                      The flow of blood through a blood vessel is affected by the
against the force of gravity. This movement comes
                                                                      pressure gradient along the vessel, the radius of the vessel, the
largely from the contractions of the muscles, which
                                                                      length of the vessel, and the viscosity of the blood itself. The
exert pressure on the vessels lying between them.                     relationship among these factors can be expressed according
Although these muscle movements are not always                        to the following equation:
                                                                                                F = ∆P πr µ
rhythmic, they do assist the flow of blood. This is
especially true when the body is moving, which is                     where F = flow, ∆P = pressure gradient, = length, and
when a faster blood flow is most needed. Veins                         µ = viscosity. Assuming a constant pressure and viscosity,
cannot contract behind the blood to keep it moving                    calculate the change in blood flow that would result if the radius
forward, but they are equipped with one-way                           of a blood vessel were doubled. On a graph, plot the
valves that help to keep the blood moving toward                      relationship between vessel radius and blood flow through the
the heart. These valves prevent the blood from                        same vessel at a constant pressure and viscosity. Now calculate
flowing backward.                                                      the change in blood pressure that would result if blood flow
                                                                      remained constant but the original vessel became partially
                                                                      blocked so that its radius was 80% of its original value. On the
                          BIO       FACT                              same graph, plot the relationship of blood pressure to vessel
  Do you ever have trouble staying alert during a long exam?          radius at a constant blood flow and viscosity. For all of these
  Some studies have found that people who jiggle their feet           calculations, assume that the length of the vessel remains
  and legs perform better on long tests than those who sit            constant.
  still. This may be because the movement assists the flow of
  blood and speeds the delivery of oxygen around the body.

 arterial end
 of capillary                                                                                                              venous end
                                                                                                                           of capillary
     interstitial fluid                            glucose                                              water wastes
                                                       amino acids                                     carbon dioxide


Figure 9.13 The exchange of material between the                      a product of its concentration gradient. Nutrients diffuse out
circulatory system and the fluid surrounding the individual            of the blood, while carbon dioxide and other waste
cells of the body takes place across the wall of the                  materials diffuse in.
capillaries. The direction of diffusion of each material is

                                                                                                Transport and Circulation • MHR            291
                                    PLAY                                           function of these cells, which all belong to the
      To view and explore the paths that the circulatory system                    same tissue.
      takes, refer to your Electronic Learning Partner. To explore                 Red Blood Cells
      the circulatory system of the fetal pig, turn to Appendix 10.
                                                                                   Red blood cells (also called erythrocytes) make up
                                                                                   44% of the total volume of your blood. The average
                                                                                   adult male has about 5.5 million red blood cells
 Blood: The Transport Medium                                                       per millilitre of blood, while the average adult
 Blood is a collection of cells that have been                                     female has about 4.5 million red blood cells per
 specialized to perform a set of particular tasks within                           millilitre of blood.
 an organism. For this reason, blood is considered a                                  The red blood cell is specialized for oxygen
 tissue even though (unlike most of our body                                       transport (see Figure 9.15). Only about two percent
 tissues) it appears liquid. In fact, blood consists of                            of the dissolved oxygen that enters the blood
 two distinct elements. The fluid portion is called                                 stream is transported by the fluid portion of the
 plasma, and the solid or “formed” portion is made                                 blood (the plasma). Since the remainder is
 up of different kinds of cells. About 55% of blood                                transported by red blood cells, these cells vastly
 is plasma, which is made up of water plus                                         increase the oxygen carrying capacity of the blood.
 dissolved gases, proteins, sugars, vitamins, minerals,                            A mature red blood cell has no nucleus. Instead,
 and waste products. The remaining 45% of the                                      each disk-shaped red blood cell is packed with
 blood is composed of cells (see Figure 9.14).                                     about 280 million molecules of the respiratory
    Table 9.2 compares the features of the main                                    pigment hemoglobin, an iron-containing molecule
 components of the formed portion of human blood.
 Note the marked differences in the form and
  Table 9.2
  Cellular components of blood

                                                White blood cells
        Point of        Red blood
      comparison          cells            Leucocytes        Lymphocytes           Platelets

   Origin             red bone          red bone            spleen,             red bone
                      marrow            marrow              lymph glands        marrow, lungs

   Cells present      5 500 000         6000                2000                250 000
   per mm3 of         (male)
   blood (approx.)    4 500 000
                                                                                                      plasma 55%
   Relative size      small             largest             large               smallest
                      (8 µm diameter)   (up to 25 µm)       (10 µm)             (2 µm)
   Function           to carry oxygen   to engulf           to play a role in   to play a role in
                      and carbon        foreign particles   the formation of    the clotting of
                      dioxide to and                        antibodies          blood
                                                                                                       white blood
                      from cells
                                                                                                         cells 1%
   Life span          120 days          a few hours to a    unknown             7–8 days
                                        few days

                                                                                                        red blood
                                                                                                        cells 44%

                                                                                                    Figure 9.14 A medical device can be used to
                                                                                                    separate the three main components of the
                                                                                                    blood. When the blood is separated it settles
                                                                                                    into layers as shown here.

292         MHR • Internal Systems and Regulation
that binds with oxygen. Hemoglobin allows the               Table 9.3
oxygen to be transported in the blood.                      Blood colour is determined by
    As you saw in Chapter 8, the respiratory system         different respiratory pigments
relies on the blood being able to pick up oxygen                Pigment       Colour         Metal element        Animal
from the lungs and deliver it to the cells of the
                                                             hemoglobin         red               iron         mammals
body. For this to work, there must be some way to
ensure the red blood cells will bind with oxygen as                                                            reptiles
it is absorbed into the body, but will release this                                                            amphibians
bond in the presence of the cells that require                                                                 fishes
oxygen. The special properties of hemoglobin allow
                                                             hemocyanin        blue              copper        molluscs
this molecule to alternately pick up and release
oxygen as the body requires.                                 chlorocruorin     green              iron         some annelids

                                                             hemoerythrin       red               iron         some annelids
                      Red blood cell

                                                            Oxygen Pick-up and Release Factors
                                 8 µm
 2 µm                                                       In mammals, two factors play a major role in
                                                            determining when oxygen is picked up and when it
  A                                                         is released by the respiratory pigment. These
                                                            factors are the concentration of oxygen and the
                                                            acidity of the surrounding fluid.
                                                               The concentration of oxygen is usually measured
                                                            in terms of its partial pressure. In any mixture of
                                                            gases, the total pressure of the mixture is made up
                                                            of the individual pressures exerted by each of the
                                                            component gases. At sea level, the total atmospheric
                                                            pressure is about 101 kPa. Since oxygen makes up
                                                            about 21% of the atmosphere, the partial pressure
                                                            of oxygen = 0.21 × 101 kPa = 21.21 kPa . Figure 9.16
                                                            illustrates the different partial pressures of
                                                            respiratory gases measured in different parts
  B                                                         of the body.

Magnification: 6900x
                                                                                                     tissue at rest: 8 kPa
Figure 9.15 A red blood cell is a biconcave disk as shown
in both A and B. It takes its colour from the respiratory
pigment hemoglobin. Oxygenated blood is bright red; as         lung: 20 kPa
oxygen is released, the colour becomes a darker blue-red.
                                                                                                          tissue during
   A hemoglobin molecule contains four iron                                                               activity: 1.3 kPa
atoms, each of which forms part of a separate
binding site or heme group. When the
concentration of oxygen is high (as it is in the
capillaries that line the alveoli of the lungs), the
heme group may form a loose bond with a
molecule of oxygen. Theoretically, four molecules
of oxygen can become attached to one molecule of
hemoglobin. In practice, not all of the possible
                                                            Figure 9.16 The partial pressure of oxygen is highest in the
heme positions in any one molecule of hemoglobin            lungs, and lowest in the tissues where oxygen is consumed
will be occupied by oxygen.                                 by the cells to meet metabolic needs. The partial pressure of
                                                            carbon dioxide, in contrast, is highest in the tissues and
                                                            lowest in the lungs.

                                                                                       Transport and Circulation • MHR         293
    When the partial pressure of oxygen is low, the      bonded to carbon monoxide are no longer available
 bond that links oxygen to the heme group weakens.       for oxygen, which prevents oxygen from reaching
 The hemoglobin molecule will then tend to release       the body’s cells. Hemoglobin is very slow to release
 the oxygen molecules it carries. Similarly, an          carbon monoxide, so this toxic chemical can
 increase in acidity will loosen the bond and lead to    continue to harm the body even after the victim’s
 the release of oxygen. This acidity, in turn, is        exposure has ended.
 influenced by the presence of dissolved carbon
 dioxide. An increase in the concentration of          100                             PO2 in Gas Exchange Organ
 carbon dioxide results in an increase in
 acidity in the blood, which in turn weakens                                                                 Change

                                                          Percentage (%) O2 Saturation
 the bond between oxygen and hemoglobin.                                                                     in O2
    In the capillaries of the alveoli of the lungs
                                                                                                             in Tissues
 (where the partial pressure of oxygen is                    PO2 in Tissues                                  at Rest
 relatively high), the blood picks up oxygen.                at rest
 In the fluid surrounding the cells in other body                                                              Change
 tissues, the partial pressure of oxygen will be                                                              in O2
 relatively low as the tissue cells take up oxygen           PO2 in                                           Saturation
 to fuel their metabolic processes. At the same              Tissues                                          in Tissues
                                                             During                                           During
 time, the partial pressure of carbon dioxide                Activity                                         Activity
 will be relatively high as the cells release
 carbon dioxide (a waste product of respiration).
 Under these conditions, hemoglobin releases
 the oxygen it carries. The result of this process        0
                                                                         Partial Pressure of Oxygen PO2
 is that the oxygen carried by the blood is
 released at the same rate as the cells’ rate            Figure 9.17 The different partial pressures of oxygen in
 of respiration.                                         different parts of the body can be plotted against the
    A dissociation curve is a graphic representation     relative saturation of hemoglobin with oxygen. Since the
                                                         resulting curves depict the dissociation, or breaking apart,
 of how much oxygen is saturated in the hemoglobin       of the oxyhemoglobin, the graph is referred to as a
 at different partial pressures of oxygen in the blood   dissociation curve. During activity, a large increase in the
 (see Figure 9.17). The characteristic shape of the      amount of oxygen released to the tissues can be produced
 dissociation curve is the result of co-operation        by a relatively small decrease in the partial pressure of
 between the four heme groups on a hemoglobin            oxygen in those tissues.
 molecule. When no binding sites are occupied by
 oxygen, it is relatively difficult for the first oxygen
 molecule to bind. After the first heme group is
 occupied, however, subsequent oxygen binding
 becomes easier. As a result, hemoglobin picks up
 oxygen slowly at first and then more quickly.
    Another factor that influences the rate at which
 oxygen dissociates from hemoglobin is
 temperature. In cooler temperatures, hemoglobin
 releases oxygen more slowly. This is not a
                                                         Figure 9.18 Cold-blooded animals like this one must use
 significant issue in warm-blooded animals, whose         external heat sources to warm their blood before the blood
 body temperature remains fairly constant, but it is     can deliver oxygen to their cells effectively. Such animals
 an important factor in determining the activity         tend to be very sluggish at night and in cooler temperatures.
 level of cold-blooded animals such as amphibians
 and reptiles (see Figure 9.18).                             Red blood cells also contribute to the transport
    The loose bond that hemoglobin forms with            of carbon dioxide in the blood. Carbon dioxide is
 oxygen is the key to hemoglobin’s ability to pick up    released by the cells of the body as a waste product
 and deliver oxygen in response to the different         of cellular respiration. It diffuses into the
 requirements in different parts of the body. In         bloodstream from the interstitial fluid. About
 contrast, carbon monoxide forms a very tight bond       nine percent of the carbon dioxide released by cells
 with hemoglobin. The heme groups that are

294    MHR • Internal Systems and Regulation
is carried in the plasma. About 40 to 45% of the               the loss of blood from a bad injury. The body will
carbon dioxide that enters the blood stream binds              also adjust its red blood cell count to respond to
to hemoglobin, forming a compound called                       environmental changes. For example, people used
carbaminohemoglobin. The remainder of the                      to living at high altitudes may have almost double
carbon dioxide combines with water in the blood                the number of red blood cells as those who are
plasma to form carbonic acid (H2CO3 ). This                    used to lower altitudes.
conversion is facilitated by an enzyme in the blood.
                                                               White Blood Cells
As carbon dioxide converts to carbonic acid, the
partial pressure of carbon dioxide in the blood                White blood cells, which are also called leucocytes,
remains low, so carbon dioxide continues to diffuse            make up about one percent of your total blood
into the blood from the cells of the body.                     volume. The number of white blood cells present
   Carbonic acid tends to dissociate into                      in your blood may increase to more than double
bicarbonate ions (H2CO3 − ) and hydrogen ions (H+ ).           normal levels, however, when your body is fighting
An accumulation of hydrogen ions would increase                an infection. White blood cells play a number of
the acidity of the blood, and could be harmful.                different roles that help protect the body from
Hemoglobin plays a role in picking up the loose                disease-causing agents, or pathogens. In contrast to
hydrogen ions to help maintain the optimal pH in               red blood cells, all white blood cells have nuclei
the blood.                                                     and appear colourless.
   A red blood cell rarely lives more than three or               There are several different types of white blood
four months. When it dies, the remains are carried             cells. Two of the most important disease-fighting
into the liver where much of the iron from the                 cell types are macrophages and lymphocytes.
hemoglobin molecules is salvaged and recycled. A               Macrophages are phagocytic cells that can pass
healthy body replaces its red blood cells at the rate          through the walls of the capillaries to engulf and
of approximately one to two million every second.              digest pathogens. These cells behave much like
Any condition that reduces the level of oxygen in              amoebae and use pseudopodial action to move.
the blood will cause a reaction that stimulates the            Macrophages are part of the body’s innate immune
bone marrow to produce more red blood cells.                   response, which is the body’s generalized response
Such conditions might include certain illnesses or             to infection.

     THINKING             LAB

Comparing Dissociation Rates                                   Hemoglobin oxygen saturation in
                                                               different representative organisms
The dissociation rate of oxygen from hemoglobin varies
from one animal to the next. These different dissociation                        Percent saturation of hemoglobin with oxygen
curves will affect the rate at which oxygen is delivered to     Oxygen partial
                                                                pressure (kPa)   Worm      Fish     Reptile   Bird   Mammal
the tissues of the animal. In this lab, you will compare the
dissociation curves of five different animals.                        0            0         0         0        0          0

You Try It                                                            1           85        19         2        3          9
 1. The table shows oxygen saturation levels measured in              2           95        42        10        9         24
    five different animal species. Prepare a dissociation
                                                                      3            -        61        22      16          42
    graph and plot each animal’s dissociation curve on the
    same graph.                                                       4            -        71        37      22          59
 2. Examine the different dissociation curves. What factors           5            -        80        44      30          74
    would need to be controlled for a comparison among
                                                                      6            -        83        60      41          85
    these curves to be possible?
                                                                      7            -        85        70      52          89
 3. Which animal is most efficient at releasing oxygen in
    response to tissue demand? What adaptive advantages               8            -        86        78      60          92
    or disadvantages do you think having each of the
                                                                      9                     87        84      72          95
    dissociation curves gives the organism?
                                                                      10                    -          -      81          -

                                                                                        Transport and Circulation • MHR         295
    In contrast, lymphocytes are non-phagocytic cells
 that play a role in the body’s acquired immune
 response. This is the response that enables the
 body to recognize and fend off specific pathogens.
 There are two main types of lymphocytes: T cells,
 which mature in the thymus gland, and B cells,
 which arise from the bone marrow. There are
 several kinds of T cells and B cells, each
 contributing a specific part of the response that                                 heavy chain
 allows the body to become immune to certain
 toxins. Figure 9.19 illustrates some of the roles                                                               variable regions
 played by both macrophages and lymphocytes in
 the body’s reaction to pathogens.                                         light
    As shown in Figure 9.20, an antibody is a                              chain                   constant regions
 Y-shaped protein molecule that is made up of a
 heavy polypeptide chain bound to a light
 polypeptide chain. Each chain includes one region                     Figure 9.20 The structure of an antibody. Variations in the
 that remains constant and another region (at the tip                  amino acid sequence produce the distinctive shape of the
 of each branch of the Y) that is variable.                            variable regions on different antibodies.

                                            B Phagocytic macrophages               C After a pathogen has been destroyed, the
  A The first line of defence is the           engulf and destroy invading            antigens from the pathogen protrude from
    arrival of non-phagocytic                 bacteria. The accumulation             the cell wall of the macrophage.
    leucocytes at the infection site.         of dead macrophages and                                 D Receptor sites on the surface
    These cells release histamine,            bacteria is visible as pus at                             of helper T cells bind to the
    which causes blood vessels at             the site of the infection.                                antigens on the surface of the
    the site to dilate and become                                         pathogen                      macrophage. This union
    more permeable to fluid and                                                                          triggers the release of
    leucocytes. The increased blood                                                                     chemical messengers from
    flow and accumulation of fluid                                                                        both cells. These messengers
    makes the area swollen and hot.                                                                     cause T cells to multiply.
    The increase in temperature             A                                             T cell        Some of these T cells destroy
    alone may be enough to destroy                                                                      infected tissue cells, breaking
    or neutralize some pathogens.                                                                       the reproductive cycle of the
                                        macrophage                                    D

                                                                                                                 E The antibodies on
                                                                              C                         B cell     B cells bind to the
                                                                                                                   contributing to the
                                                                 antibody                                          destruction of the
                                                                        surface of                                 pathogens.
                                                                        plasma cell
                                                            antigen           cell
                                                                                                       F T cells bind to the B cell
                                                                                                         antibody-antigen complex.
                                                                              memory B cell
                                                                                                         This union of T and B cells
                                                G The plasma cells produce antibodies at                 activates the B cell, causing
                                                  a rate of 2000 per second, and release                 it to enlarge and divide,
                                                  them into the blood stream. Antibodies                 which produces plasma
                                                  and memory B cells remain in the blood,                cells and memory cells.
                                                  ready to fight a new infection by the
                                                  same pathogen.

 Figure 9.19 A simplified illustration of the body’s immune response, triggered by
 the entry of pathogens at the site of an infection. Pathogens may include bacteria,
 viruses, fungi, protists, etc.

296     MHR • Internal Systems and Regulation
The variable regions recognize antigens, or toxins,                           therefore help to protect the body from excessive
carried by invading pathogens. Most antibodies are                            blood loss after an injury.
so specific that they can bind with only one                                      While the process of clotting is not fully
antigen. Different types of B cells, however, carry                           understood, it is clear that a number of steps are
different kinds of antibodies, and this variation                             involved. Your blood does not clot until a blood
increases the possibility that the body will have an                          vessel is broken, which indicates that the first step
antibody that can recognize and bind to an                                    is triggered by the injury. Substances released by
invading pathogen. Once a B cell is activated, it                             the broken blood vessels attract platelets to the site
enlarges and divides to produce memory B cells                                of the damaged vessels. As the platelets collect,
and plasma cells. The plasma cells produce                                    they rupture and release certain chemicals. These
enormous quantities of the same antibody carried                              substances combine with other clotting agents in
by the B cell, and release these antibodies into the                          the plasma of the blood to produce the enzyme
bloodstream to fight the invading pathogens. After                             thromboplastin. As long as there are calcium ions
the infection has been fought off, memory B cells                             in the blood, the thromboplastin will react with
remain in the blood, ready to trigger another                                 prothrombin (a serum protein produced by the
immune response when necessary (see Figure 9.21).                             liver) to produce thrombin. Thrombin is an enzyme
                                                                              that reacts with fibrinogen (another plasma protein)
                                                                              to produce fibrin. Fibrin is an insoluble material
                                                                              that forms a mesh of strands around the area of the
 antibodies produced
  Relative amount of

                                                                              injury. This matted material serves to trap escaping
                                            Second exposure                   blood cells and form the clot.
                                            to same antigens
                           First exposure
                           to antigens                                                                       A Chemicals released by
                                                                                                               ruptured platelets react
                                                                                                               with plasma substances to
                                                                                                               produce thromboplastin.

                       0      2    4    6      8   10   12     14   16   18
                                              Time (weeks)

                                                                                                            B Thromboplastin reacts with
Figure 9.21 On the first exposure to an antigen, the
                                                                                  Ca2+                        prothrombin to produce
immune response takes time to produce the antibodies
                                                                                                              thrombin. This reaction is
necessary to fight the infection. If the same antigen is
                                                                                                              catalysed by calcium ions.
introduced again, the response is much faster and
generates much higher levels of antibodies.

    In addition to their role in fighting disease,
lymphocytes can (under particular conditions)                                                               C Thrombin reacts with
undergo changes to become a variety of cell types.                                                            fibrinogen to produce
If lymphocytes enter the bone marrow, they can be                                                             fibrin. Fibrin produces a
broken down into various cell fragments. If they                                                              fibrous mesh that traps
enter other tissues of the body, they can play a role                                                         escaping blood vessels.
in the construction of different kinds of connective
tissue fibres.

Platelets make up the third major component of the
formed portion of the blood. Platelets are not cells
— they are fragments of cells that were created
when larger cells in the bone marrow broke apart.
These fragments contain no nucleus and break
down quickly in the blood (each platelet lasts only
about a week to 10 days). Nevertheless, platelets                             Figure 9.22 Clotting helps to prevent the loss of blood
play an important role in clotting blood and                                  when an injury breaks blood vessels.

                                                                                                     Transport and Circulation • MHR     297
 Blood Plasma                                                      Table 9.4
 Plasma, the fluid portion of the blood, is the medium              The composition of plasma
 in which the blood cells are suspended. But this                              Constituent                   Percentage
 fluid does more than simply carry all the blood
 cells. Plasma itself contains substances that play an              Water                                       92%
 important role in maintaining the body’s well-being                Blood proteins                               7%
 (see Table 9.4). The dissolved protein components                    Fibrinogen
 of plasma include fibrinogen, serum albumin, and                      Serum albumin
                                                                      Serum globulin
 serum globulin. Serum albumin plays an important
 role in maintaining the blood volume and blood                     Other organic substances                     0.14%
 pressure. Serum globulin is the name given to a                      Nonprotein nitrogen (urea)
 number of protein components, some of which act                      Organic nutrients
 as antibodies to defend the body against disease.                  Inorganic ions:                              0.93%
    Plasma also plays a role in the transport of carbon                calcium, chlorine,
 dioxide in the blood. Carbon dioxide dissolves in                     magnesium, potassium,
                                                                       sodium, bicarbonates,
 the water portion of the plasma to form carbonic
                                                                       carbonates, phosphates
 acid. This carbonic acid is carried in the plasma
 from tissues to the lungs for gas exchange.

  Biology Magazine                           TECHNOLOGY • SOCIETY • ENVIRONMENT

      Fake Blood: A Real Alternative?                              every day. These shortages are not the only problem
                                                                   associated with the use of donated blood. Blood typing
                                                                   and careful screening are required to make sure the
                                                                   donated blood is compatible with the recipient’s blood
                                                                   type, and that it is not contaminated by bacteria or
                                                                   viruses. Supplies of donated blood must be refrigerated,
                                                                   and they have a shelf life of only 42 days. Even where a
                                                                   safe supply of compatible donor blood is available, the
                                                                   time it takes for donated blood to circulate around the
                                                                   patient’s body means that there is a delay in oxygen
                                                                   delivery to the patient. These problems have led
                                                                   researchers to look for artificial blood substitutes.

                                                                   A Challenge for Scientists
                                                                   Red blood cells are extremely efficient vehicles for
                                                                   transporting oxygen. Each red blood cell contains about
                                                                   280 million molecules of hemoglobin, the protein
                                                                   responsible for absorbing oxygen when the red blood cell
                                                                   passes through the lungs. In the human body, about five
                                                                   litres of red blood cells travel through approximately
                                                                   160 000 km of blood vessels, releasing oxygen to the
                                                                   body along the way. Creating a blood substitute that can
                                                                   match this efficiency is not just a matter of duplicating the
                                                                   molecular structure of hemoglobin, however. Every
                                                                   hemoglobin molecule is enclosed by a protective
      Donated natural blood has a limited shelf life.              membrane. Once this covering is removed, the
                                                                   hemoglobin molecule disintegrates into particles called
      Each year, approximately 600 000 Canadians undergo           dimers. These particles can cause severe damage to the
      surgery or medical treatments that require donated blood     lungs and kidneys. Therefore, the search for an artificial
      — but donations to blood banks have dropped almost           blood substitute starts off with a challenge — how can
      25% in the last ten years. As a result, hospitals in major   we mimic the oxygen-carrying capacity of hemoglobin
      Canadian cities face a critical shortage of fresh blood      without the risk of producing toxic dimers?

298       MHR • Internal Systems and Regulation
   When the fibrinogen and other clotting agents                  serves as a channel for many of the body’s systems.
are removed from the blood, the straw-coloured                   This means that blood is closely connected to other
liquid that remains is called serum. This liquid                 body systems that are responsible for digestion and
contains cellular nutrients, hormones, electrolytes,             for the action of hormones.
enzymes, hormones, antibodies, and waste materials.                 Capillaries in the walls of the small intestine
Serum from an animal or a person immune to a                     absorb many of the nutrients that are end products
particular disease can be injected into a patient to             of digestion. The blood also absorbs nutrients
provide temporary immunity from that disease.                    synthesized by cells in parts of the body other than
                                                                 the digestive tract. These materials, which include
Other Functions of the Blood                                     glucose and amino acids, are carried to the liver
                                                                 where they are converted into storage products or
Blood is sometimes called a connective tissue,
                                                                 prepared for transport to other parts of the body.
because it plays a role in linking all the body’s cells
                                                                 The blood also serves a critical body function by
and organs. As the blood circulates around the
                                                                 removing the waste products of cellular processes.
body, it provides an ideal pathway for the
                                                                 Substances such as uric acid, end products of
distribution of materials. Within a very short time,
                                                                 protein metabolism, and excess amounts of various
a substance that enters the bloodstream in one part
                                                                 mineral ions are carried by the blood to the
of the body will come into contact with almost
                                                                 kidneys for processing and excretion.
every other body part. In mammals, the blood

  Several pharmaceutical research companies are working          and chemotherapy, as well as in emergency
  on developing blood substitutes. Some of these                 situations such as accident scenes.
  substitutes rely on natural blood products. For example,
                                                                 Although blood substitutes can offer some medical
  one company collects blood from slaughtered livestock,
                                                                 advantages, real blood does much more than just deliver
  while another obtains human blood from blood banks. In
                                                                 oxygen. The artificial products being developed today still
  these cases, scientists extract and purify the hemoglobin
                                                                 lack two key ingredients of real blood: white blood cells
  molecules, which then become building blocks for the
                                                                 to fight infection, and platelets to help blood clot. The
  synthetic product. Other researchers are exploring ways
                                                                 search is on for a substitute that performs all of the
  to create artificial blood without using hemoglobin at all.
                                                                 blood’s vital functions, and the stakes are high. A
  One possibility is to use a perfluorocarbon (PFC)
                                                                 successful blood substitute could help save thousands
  emulsion that can act like a liquid sponge to absorb
                                                                 of lives every year.
  and deliver oxygen.

  Artificial blood molecules developed through these             Follow-up
  techniques are up to 30 or even 40 times smaller than           1. In the 1990s, concerns about the safety of donated
  red blood cells. Because they are so small, these                  blood led to a nationwide inquiry known as the
  particles can travel around blockages in vessels and into          Krever inquiry. What were the main recommendations
  very small capillaries. This lets them deliver oxygen              of the inquiry, and what changes were made in the
  around the body more quickly than red blood cells                  blood bank system as a result? Following the Krever
  can — something that could be especially important in              inquiry, has the search for alternatives to donated
  an emergency, where every second counts.                           blood become more important, or less important?
                                                                     Explain briefly.
  Blood substitutes can address many of the problems
  involved in dealing with real blood. The artificial products    2. Other than in surgeries, emergency transfusions, and
  being developed are compatible with all blood types and            medical treatments such as chemotherapy, what
  are free of contamination by bacteria or viruses. Some of          uses might a blood substitute have? Conduct
  these products can be stored without refrigeration for up          research to determine what risks may be associated
  to two years. In addition, once blood substitutes are in           with the use of a blood substitute. Given the potential
  the human body, they offer immediate oxygen delivery.              benefits and risks, prepare a brief report arguing
  They can keep working for about 36 h — long enough for             whether or not the use of artificial blood should
  the body to begin reproducing its own blood. These                 eventually replace natural blood in all cases.
  characteristics could make blood substitutes helpful in
  planned medical procedures such as scheduled surgery

                                                                                         Transport and Circulation • MHR       299
                          FAST FORWARD                            of the red blood cells. The four blood groups result
      For more information about digestion, turn to Chapter 10.   from different combinations of two protein
                                                                  markers, A and B (see Table 9.5).
                                                                  Table 9.5
    The blood also serves as a medium for                         The human blood groups
 conveying chemical messengers, or hormones, from
 their origins in various glands to the organs on                      Blood type        Protein markers     Serum antibodies
 which the hormones act. Hormones play a central                              A                 A                  anti-B
 role in regulating and co-ordinating the internal
                                                                              B                 B                  anti-A
 systems of the body. Without the bloodstream to
 serve as a pathway for these substances, the body                            O              neither          anti-A and anti-B
 would be unable to respond effectively to                                    AB           both A and B            neither
 fluctuations in its external or internal environment,
 and the finely-balanced mechanisms that keep the                  The blood serum carries antibodies against the protein
 many different components of the organism                        markers that are not present on the red blood cells. This
 functioning together would quickly break down.                   means that a person with type A blood will carry anti-B
                                                                  antibodies, a person of type O blood will carry both anti-A
                                                                  and anti-B antibodies, and a person with type AB blood will
 Human Blood Groups                                               carry neither antibody.

 William Harvey’s seventeenth-century discovery
 that the blood circulates through the body made it                  Figure 9.23 illustrates how these antibodies are
 possible for physicians to consider blood                        responsible for the clumping, or agglutination, of
 transfusions as a treatment for patients. The first               blood cells when incompatible blood groups are
 transfusions were conducted within 50 years of                   mixed together. For a blood transfusion to be
 Harvey’s work. A number of transfusions were                     successful, physicians must ensure that the donor
 successful, but there were also a number of                      and recipient blood are compatible. If a patient
 dramatic failures in which the patient died almost               with type O blood receives a transfusion of type B
 immediately after receiving the transfused blood. In             blood, the patient’s anti-B antibodies will react
 England, concerns about the risks meant that blood               with the type B red blood cells, causing them to
 transfusions were made illegal in 1678.                          agglutinate. Agglutinated red blood cells can clog
    In the early 1900s, scientists identified the four             blood vessels, blocking circulation and causing
 major human blood groups: A, B, AB, and O. Each                  severe damage to the body.
 blood group is characterized by the presence or
 absence of particular protein markers on the walls

          THINKING             LAB

 Operating Room Puzzle                                             1. Prepare a table illustrating the results you would expect
                                                                      to see if you combined type B blood with each of the
 Imagine you are part of a medical team preparing to
                                                                      other blood types.
 conduct a blood transfusion. You see on the patient’s chart
 that she has type B blood, and the label on the donor             2. Explain how you would set up an experiment to
 blood container also says B. As an extra precaution before           determine which label was incorrect. Is there a control
 beginning the transfusion, you mix samples of the patient            in this experiment?
 and donor blood together on a glass slide. You find that
                                                                   3. If it turns out that the label on the stored blood
 agglutination occurs.
                                                                      container is incorrect and you have no type B blood
 You Try It                                                           for the transfusion, what other blood type could be
                                                                      used instead?
 You need to discover whether it is the information on the
 patient’s chart or the information on the blood container
 that is incorrect. You have access to samples of blood
 types A, AB, and O, but there is no more type B blood in
 the blood bank.

300        MHR • Internal Systems and Regulation
                      Type O blood                                 A second factor that affects the compatibility of
                                                                human blood is the rhesus factor, another protein
                                            type O red cell
                                                                marker. People who carry the Rh protein are called
                                                                Rh-positive, while people without the protein are
                                                                called Rh-negative. Unlike the A and B antibodies,
 anti-A                                                         the anti-Rh antibody is not always present in the
                                                                blood. It is manufactured by the body only after an
                                                                exposure to the Rh protein marker. Therefore, the
                         anti-B                                 first transfusion of Rh-positive donor blood into an
                            +                                   Rh-negative recipient is usually safe, since the
                                                                anti-Rh antibodies develop over a two to four month
                 Type A blood transfusion                       period after the transfusion. A second transfusion
                                                                of Rh-positive blood will cause agglutination of the
                                                                blood entering the recipient’s bloodstream.
                                          type A red cell
                                                                   A similar problem may occur when a Rh-
                                                                positive father and a Rh-negative mother conceive
                                                                a child who is Rh positive, as red blood cells from
                                                                the fetus may leak across the placenta into the
                                                                mother’s circulatory system. As a result, the
                                                                mother’s immune system may be stimulated to
                                                                produce anti-Rh antibodies that can in turn cross
                                                                the placenta. These will destroy the child’s red
               Agglutination of type A cells
                                                                blood cells and put all subsequent pregnancies at
                                                                risk. The solution has been to give the mother an
                                                                injection containing anti-Rh antibodies midway
                                                                through her first pregnancy or no later than 72 h
                                                                after giving birth to a Rh-positive baby. The anti-Rh
                                                                antibodies destroy any of the child’s red blood cells
                                                                that have crossed the placenta before the mother’s
                                                                immune system begins producing antibodies.
                                                                   This section has examined the transport vessels
                                                                and the transport medium of the mammalian
                                                                circulatory system. The next section takes a closer
                                                                look at the third major element of this circulatory
                                                                system: the heart, the pump that powers the
                                                                circulatory system.

                                                                  Wo rd
                                                                Is it clotting or is it clumping? These two words apply to two
                                                                different processes. Blood clots when platelets are ruptured,
                                                                but blood cells clump when blood of an incompatible type is
                                                                injected into a patient.

Figure 9.23 The agglutination or clumping of red blood
cells that occurs when type A blood is injected into a type O
patient. The recipient’s anti-A antibodies recognize and bind
to the A markers on the donor blood, causing them to clump
together. The agglutination of blood cells can be fatal.

                                                                                          Transport and Circulation • MHR        301
              SECTION                   REVIEW

        1.     K/U List three functions performed by the blood and               7.    I Develop a simple test, based on using just a few
              identify the cells involved in each.                                    drops of a person’s blood, that could determine
                                                                                      whether or not that person was anemic. Provide
        2.     K/U   Complete the following table in your notebook.
                                                                                      detailed reasoning to support your design.
       Vessel          Artery or vein             Associated organs
                                                                                 8.    MC Iron deficiency, or anemia, often results in fatigue
      aorta                              principal arteries of the body,              and pallor. Explain how these symptoms could result
                                         distributing to all organs                   from inadequate iron in the diet. What reasons can
                                                                                      you suggest for why women are statistically more
      carotid         artery
                                                                                      likely to be anemic than men?
      jugular                            head region
                                                                                 9.    MC A pharmaceutical company develops an artificial
      hepatic         both                                                            red blood cell that is very effective at transporting
                                                                                      oxygen but is unable to transport carbon dioxide.
      renal                              kidney
                                                                                      Explain what would take place in the blood vessels if
      vena cava       vein                                                            this substance were used in a blood transfusion. How
                                                                                      would this affect the test patient? Assume the
                                                                                      transfusion has no effect on the level of plasma in
      celiac                             serves coelom (major body cavity)
                                         and contained organs (e.g., stomach)         the blood.

                                                                                10.     I In a typical individual, 100 mL of venous blood
        3.     K/U  Explain the difference between the body’s
                                                                                      holds 14 mL of oxygen, and the ventricles each have
              innate and acquired immune responses. Which cells
                                                                                      a volume of 70 mL. Given these data and additional
              are involved in both processes? Which cells are
                                                                                      information that you may have to research, how
              involved only in one or the other?
                                                                                      much oxygen does the blood absorb from the lungs
        4.     K/U The blood of a mother and her fetus sometimes                      in one minute?
              mix during birth, and the Rh factor in maternal blood
                                                                                11.    C Describe the events that occur in each step of
              can cross the placenta to enter the fetus. Knowing
                                                                                      the blood-clotting process illustrated below.
              this, explain how an Rh− mother carrying an Rh+ baby
              could encounter no complications during her first                                                        A
              pregnancy, but face blood compatibility problems
              during a second pregnancy with an Rh+ baby.

        5.     K/U Unlike most viruses, the human
              immunodeficiency virus (HIV) first binds to the
              receptor sites on a T cell and then becomes engulfed
              by that T cell. Explain why this characteristic helps to
              make HIV a difficult antigen for the body to fight.

        6.     C A volunteer firefighting team wants to increase the                          Ca2+
              water pressure delivered by its fire truck. They have
              only enough money in their budget to do either
              (a) replace their 75 mm diameter hose with a 100 mm
              diameter hose, or (b) buy a new pump that will
              increase the pressure by 25%. Which of these
              options will provide the greatest gain in pressure?
              How does this relate to the organization of the                                                          C
              circulatory system in mammals?

302            MHR • Internal Systems and Regulation

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