CHAPTER 11 SUMMARY
Evolutionary Solutions to Gas Demands
•Internal respiration refers to the intracellular metabolic reactions that utilize O 2 and
produce CO2 during energy-yielding oxidation of nutrient molecules. External respiration
encompasses the various steps involved in the transfer of O2 and CO2 between the external
environment and tissue cells. The respiratory and circulatory systems function together to
accomplish external respiration.
•External respiratory processes must meet the demands of body size, metabolic rate, and
habitat gases through diffusion and bulk transport. Unlike diffusion, which is the random
movement of individual molecules, bulk transport involves the movement of the entire
medium carrying the molecule of interest. Gas exchange has been augmented by
enhancements to diffusion itself, and by efficient bulk transport.
•Diffusion follows Fick’s Law:
Rate of diffusion (Q) = P . A . D
∆X
• All but the smallest, thinnest animals have special respiratory surfaces—tissues that are
exposed to the environment and across which O 2 and CO2 diffuse, such as gills, tracheae,
and lungs. Circulatory fluids (capillaries in closed systems) enhance diffusion in all tissues.
Respiratory tissues and circulatory systems enhance diffusion with high surface areas (A),
short diffusion distances (∆X), and high permeabilities (D).
•Bulk transport includes ventilation and circulation. If ventilation is actively created (e.g.,
by muscles), it is called breathing. Ventilation can be tidal (external medium moving in and
out the same opening) or a more efficient flow-through mechanism (separate inflow and
outflow openings).
•Up to four steps are needed for gas movement: external ventilation (bulk), diffusion across
respiratory tissues, internal circulation (bulk), and diffusion between circulatory fluid and
cells.
•Water respirers must cope with less oxygen availability than do air respirers. They may use
simple diffusion across the integument, or may use gills. Gills are evaginations of tissue
usually protruding into the external medium. They have a thin epidermis with a high surface
area and are highly perfused by a circulatory system. In fishes, gills are ventilated by
muscle-driven flow-through breathing with inflow via the mouth and outflow via the
operculum.
•Air respirers have internalized exchanges surfaces that reduce dehydration. These surfaces
include lungs (invaginations forming sacs that exchange gas with circulatory fluid) and
tracheae (invaginated tubes leading directly to internal organs; found in insects and some
arachnids).
•In vertebrates, the lungs are housed within the closed compartment of the thorax, the
volume of which can be changed by contractile activity of respiratory muscles.
•Exchange of O2 and CO2 between the air in lungs and the blood in the pulmonary
capillaries takes place across the extremely thin walls of numerous tiny air sacs, or alveoli,
in mammals. In birds, exchange occurs in tiny air capillaries.
Respiratory Mechanics
•Active ventilation, or breathing, in vertebrates is the process of cyclically moving water
across the gills or air in and out of the lungs so that old medium that has already
participated in exchange of O2 and CO2 with the capillary blood can be exchanged for fresh
medium. In mammals, ventilation is mechanically accomplished by alternately shifting the
direction of the pressure gradient for airflow between the atmosphere and alveoli, using
negative pressure for inhalation.
• Pulmonary compliance refers to the distensibility of the lungs—how much they stretch in
response to a given change in the transmural pressure gradient, the stretching force exerted
across the lung wall. Elastic recoil refers to the phenomenon of the lungs snapping to their
resting position during expiration. These are important factors in mammals; however, the
lungs of birds are comparatively small and inelastic and do not change in volume during the
respiratory cycle.
•Mammalian alveolar surface tension, which is due to the attractive forces between the
surface water molecules in the liquid film lining each alveolus, tends to resist the alveolus
being stretched upon inflation (decreases compliance) and tends to return it back to a smaller
surface area during deflation (increases lung rebound). If the alveoli were lined by water
alone, the surface tension would be so great that the lungs would be poorly compliant and
would tend to collapse. Type II alveolar cells secrete pulmonary surfactant, a
phospholipoprotein that intersperses between the water molecules and lowers the alveolar
surface tension, thereby increasing the compliance of the lungs and counteracting the
tendency for alveoli to collapse.
•The amount of air moved in and out of the lungs in one minute, the pulmonary ventilation,
is equal to tidal volume times respiratory rate. However, not all of the air moved in and out
is available for O2 and CO2 exchange with the blood because part of it occupies the
conducting airways, known as the anatomic dead space. Alveolar ventilation, the volume of
air exchanged between the atmosphere and alveoli in one minute, is a measure of the air
actually available for gas exchange with the blood. Alveolar ventilation equals (tidal volume
minus the dead space volume) times respiratory rate.
•In all species of animals, energy is responsible for all or part of the contraction of the
inspiratory and expiratory muscles. Respiration in water assumes a greater metabolic cost
because of the fluid properties of water, which resist movement as compared to air.
Estimates of the metabolic cost of respiration in a resting fish vary from 5 to 50% (normally
approximately 20%) of the resting metabolic rate compared to only 2% in some mammals.
Gas Exchange
•Oxygen and CO2 move across body membranes by passive diffusion down partial pressure
gradients. In water, gas exchange is achieved by a one-way flow of water between the gill
lamellae. Net diffusion of O2 occurs first between the alveoli/gills and the blood and then
between the blood and the tissues as a result of the O 2 partial pressure gradients created by
continuous utilization of O2 in the cells and continuous replenishment of fresh O 2 provided
by ventilation.
•Net diffusion of CO2 occurs in the reverse direction, first between the tissues and the blood
and then between the blood and the alveoli/gills, as a result of the CO 2 partial pressure
gradients created by continuous production of CO 2 in the cells and continuous removal of
CO2 through the process of ventilation.
Gas Transport
•Because O2 and CO2 are not very soluble in the blood, they must be transported primarily
by mechanisms other than simply being physically dissolved (in most vertebrates). For
oxygen, binding to respiratory pigments such as hemoglobin (Hb) in the circulation is the
primary adaptation. The primary factor that determines the extent to which vertebrate Hb
and O2 are combined (the % Hb saturation) is the P O2 of the blood. In mammals, the
relationship between blood PO2 and % Hb saturation is such that in the PO2 range found in
the pulmonary capillaries, Hb is still almost fully saturated even if the blood P O2 falls as
much as 40%; this provides a margin of safety by assuring near-normal O2 delivery to the
tissues despite a substantial reduction in arterial PO2. On the other hand, in the PO2 range
found in the systemic capillaries, large increases in Hb unloading occur in response to a
small local decline in blood PO2 associated with increased cellular metabolism; thus, more
O2 is provided to match the increased tissue needs.
•The Bohr Effect refers to the effect of increased CO2, acidity, temperature, and organic
phosphates shifting the O 2-Hb dissociation curve to the right, favoring unloading in
situations where this is useful.
•Acidosis lowers Hb oxygen capacity (not just affinity) of fish Hb—the so-called Root Effect.
The Root effect is important when establishing the elevated PO2 values in fish swimbladders
and may also play a role during periods of lactic acid accumulation associated with strenuous
activity.
•Carbon dioxide picked up at the systemic capillaries is transported in the blood by three
methods: (1) physically dissolved; (2) bound to Hb; and (3) in the form of bicarbonate (HCO 3-
). The erythrocyte enzyme carbonic anhydrase indirectly catalyzes the conversion of CO 2 to
HCO3- according to the reaction: CO2 + H2O H2CO3 H+ + HCO3-. The generated H+
binds to Hb. This and the Hb-CO2 reaction are reversed in the lungs/gills as CO 2 is
eliminated to the alveoli or surrounding water.
Control of Respiration
•Ventilation involves two distinct aspects, both of which are subject to neural control: (1)
rhythmic cycling between inspiration and expiration and (2) regulation of the magnitude of
ventilation, which in turn depends on control of respiratory rate and depth of tidal volume.
•Respiratory rhythm of vertebrates is primarily established by pacemaker activity displayed
by inspiratory neurons located in the respiratory control center in the medulla of the brain
stem. When these inspiratory neurons autonomously fire, impulses ultimately reach the
inspiratory muscles to bring about inspiration. When the inspiratory neurons cease firing,
the inspiratory muscles relax and expiration takes place. If active expiration is to occur, the
expiratory muscles are activated by output from the medullary expiratory neurons at this
time.
•This basic rhythm is smoothed out by a balance of activity in the apneustic and
pneumotaxic centers located higher in the brain stem in the pons. The apneustic center
prolongs inspiration, whereas the more powerful pneumotaxic center limits inspiration.
•Three chemical factors play a role in determining the magnitude of ventilation in air
breathers: the PCO2, PO2, and H+ ion concentration of the arterial blood. The dominant
factor in the minute-to-minute regulation of ventilation in air breathers is the arterial P CO2.
An increase in arterial PCO2 is the most potent chemical stimulus for increasing ventilation.
Changes in arterial PCO2 alter ventilation primarily by bringing about corresponding
changes in the brain ECF H+ concentration, to which the central chemoreceptors are
exquisitely sensitive.
•The peripheral chemoreceptors are responsive to an increase in arterial H + concentration,
which likewise reflexly brings about increased ventilation. The resultant adjustment in
arterial H+-generating-CO2 is important in maintaining the acid-base balance of the body.
The peripheral chemoreceptors also reflexly stimulate the respiratory center in response to a
marked reduction in arterial PO2 (<60 mm Hg). This response serves as an emergency
mechanism to increase respiration when the arterial PO2 levels fall below the safety range
provided by the plateau portion of the O2-Hb curve.
•In fishes, chemoreceptors resembling those of mammals are located diffusely throughout
the gills. Branchial mechanoreceptors are important in regulating the position of the gill
during the respiratory cycle and interact with chemoreceptors to control the transition from
active breathing to ram ventilation.