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					                 Gas Exchange

Getting oxygen to each cell of the body
Campbell et al. “Biology: concepts and connections”
Chapter 22
Overview: getting O2 from the atmosphere to
the mitochondria
1. Breathing: mechanically draws
   air into lungs
   – O2 diffuses into blood vessels
   – CO2 diffuses from blood vessels
     into lungs and is exhaled
2. The blood transports gases to
   the body
   – O2 attaches to hemoglobin
3. Gas exchange between blood
   and body cells
   – O2 serves as final electron
     acceptor in in respiration
   – ATP is produced in the
   – CO2 + H2O produced as a by-
Gas exchange in animals occurs across moist

• O2 must be in
  solution to be
  used in biological
• Rate of diffusion
  depends on:
   – Surface area
   – Thickness of
     surface (1/x2)
   – Relative
Gas exchange structures in animals

• Some smaller
  animals use their
  entire body
  surface for gas
  – Small size and/or
    shape (flatten) 
    large surface area
    to volume ratio
  – Must remain
Gas exchange structures in animals

Water contains ~ 20
times less available O2
than air.
Many animals living in aquatic              Exchange
environments use gills for gas exchange
Countercurrent Exchange in Gills
Gas exchange structures in animals

• Insects use a
  “tracheal system” of
  air sacs and
  tracheoles allowing
  direct exchange of
  gas between the air
  and cells
  – Circulatory system
    not involved
                               The tips of the tracheoles
  – All cells are near an      contain fluid / gas exchange
    air tube (tracheole)       occurs across the epithelial
The development of lungs enabled vertebrates
to colonize land
• Some ancient forms likely had gills and lungs and
  lived in shallow water
• Longer snout and muscular neck allowed them to
  lift their heads above water surface to gulp air
• Terrestrial tetrapods diverged into three major
  – Amphibians (small lungs / diffusion across skin)
  – Reptiles / Birds
                     Various types of lungs
  – Mammals
• Size and complexity of lungs relate to metabolic
Gas exchange structures in animals

Most terrestrial
have lungs
 Each human lung
contains about 150
  million alveoli
Breathing forces air into and out of our
lungs (negative pressure breathing)
Birds have more efficient lungs

Inhalation – all the air sacs fill
    - read air sacs fill with fresh air from atmosphere
    - front air sacs fill with depleted air which has pass through the lungs
Exhalation expels air from air sacs
    - air from rear air sacs passes through the lungs into front air sacs
    - air from front air sacs is expelled to the atmosphere
-- Two cycles required for air to pass through whole system
-- Countercurrent exchange system increases efficiency
Flow through lung & countercurrent exchange
 greater efficiency
Don’t forget to
• Breathing centers in
the brain send signals to
muscles in the rib cage
and diaphragm
• CO2 levels in the blood
are monitored
• Sensors in the aorta &
carotid arteries monitor
O2 levels in blood
• Breathing and heart
beat are coordinated
Gases (O2 and CO2) are
transported by the blood
• Gases will diffuse down
their concentration
gradients according to their
partial pressure
• Direction of flow is from
areas with high partial
pressures to those with
lower partial pressures
Carbon Dioxide is constantly being
produced and oxygen is being
consumed in the body’s cells
Hemoglobin carries oxygen in the blood

 • Oxygen is not highly soluble in water
 • Hemoglobin can reversibly carry oxygen in the
 • Hemoglobin also acts as a buffer and assists in
   transporting carbon dioxide

 • CO2 + H2O   H2CO3   H+ + HCO3-
                      Carbonic Acid   Hydrogen ion +
                                      Bicarbonate ion
Hemoglobin loading and unloading…

 Hemoglobin carries out multiple functions:
 -- Oxygen carrier
 -- Carbon Dioxide carrier
 -- Controls blood pH
Bohr shift  a change in the conformation of hemoglobin
which lowers its affinity for oxygen.
Developing fetus exchanges gases with
the mother’s bloodstream
• Baby’s lungs are filled with
amniotic fluid and are non-
• Network of capillaries in the
placenta serve as gas exchange
• Fetal hemoglobin has higher
affinity for oxygen than adult
• (note: smoking reduces oxygen
supply to placenta by ~25%)
• At birth gas exchange via the
placenta stops and CO2 levels in
blood increase (pH drops)
• Lower pH signals the brain to
start using the lungs
• WOW!!
                                  - Lung Cancer
  Smoking kills!!                 - Emphysema
                                  - Heart Disease
                                  - Many other cancers

Cigarette smoke contains toxin-laden particles
Rubber cast of human lungs… (courtesy of the
Anatomical Institute, Bern)
• During the first inspiration, the air travels through the
  nostrils, also called nares, of a bird, which are located at
  the junction between the top of the upper beak and the
  head. The fleshy tissue that surrounds them, in some           Bird Breathing
  birds, is called the cere. As in mammals, air moves
  through the nostrils into the nasal cavity. From there it
  passes through the larynx and into the trachea. Air
  moves through the trachea to the syrinx, which is
  located at the point just before the trachea divides in
  two. It passes through the syrinx and then the air
  stream is divided in two as the trachea divides. The air
  does not go directly to the lung, but instead travels to
  the caudal (posterior) air sacs. A small amount of air will
  pass through the caudal air sacs to the lung.
• During the first expiration, the air is moved from the
  posterior air sacs through the ventrobronchi and
  dorsobronchi into the lungs. The bronchi continue to
  divide into smaller diameter air capillaries. Blood
  capillaries flow through the air capillaries and this is
  where the oxygen and carbon dioxide are exchanged.
• When the bird inspires the second time, the air moves
  to the cranial air sacs.
• On the second expiration, the air moves out of the
  cranial air sacs, through the syrinx into the trachea,
  through the larynx, and finally through the nasal cavity
  and out of the nostrils.