49
Gas Exchange in
Animals
Gas exchange systems are made up of surfaces
and the mechanisms that ventilate and
perfuse those surfaces
 Ventilation is ambient fluid (water or air)
flowing past exterior of exchange surface
 Perfusion is circulatory fluid (blood,
hemolymph) flowing inside exchange
surface
O2 and CO2 are respiratory gases
exchanged by diffusion along their
concentration gradients
 Partial pressure is concentration of a gas in
a mixture
○ Barometric pressure – atmospheric pressure
at sea level is 760 mm Hg
○ Partial pressure of O2 (PO2) is 159 mm Hg or
20.9% of atmosphere
www.allstar.fiu.edu/aero/images
Oxygen is easier to obtain from air than
from water:
 O2 content of air is higher than water
 O2 diffuses much faster through air
 Air and water must be moved by animal
over its gas exchange surfaces to obtain
O2 – it requires more energy to move
denser/more viscous water across
membranes than air
O2 diffusion is slow
 even cells with low
metabolism must be only
1–2 mm from O2 source
 limits size and shape of
species without internal
systems for gas exchange
 these species have evolved
○ larger surface areas
○ central cavities, or
○ specialized respiratory
systems
An aquatic animal’s body
temperature and metabolic rate
rise with an increase in water
temperature
 Dilemma: need for O2 increases
while available O2 decreases in
warmer water
○ Cold water holds more
oxygen
Increase in altitude reduces
available O2 for air breathers
due to lower partial pressure
of O2 at high altitudes
www.as.ua.edu/ant/bindon/ant475/altitude
Respiratory systems have adaptations to maximize
exchange of O2 and CO2
 Increase surface area
 Example: Gill filaments, aveoli in lungs
 Maximize partial pressure gradients
 Example: Blood flow – always low oxygen levels in blood returning
to lungs
 Minimize diffusion path length
 Example: Capillaries surrounding aveoli in lungs create short
diffusion length for the oxygen
 Minimize diffusion taking place in aqueous medium
http://www.biologyreference.com/
images/biol_02_img0187.jpg
http://www.sbs.auckland.ac.nz/services/micrographics/images/Gills.jpg
Adaptations - Insects
Insects have a tracheal
system:
 Spiracles in abdomen
open to allow gas
exchange and close to
limit water loss
 Spiracles open into
tracheae, that branch
to tracheoles, that end
in air capillaries
 Limits insect size
Adaptations - Fish
Fish gills use countercurrent flow to maximize gas
exchange
 Water flows unidirectionally into mouth, over
gills, and out from under opercular flaps
 Constant water flow maximizes PO2 on external gill
surfaces and blood circulation minimizes PO2 on
internal surfaces  maximizing O2 diffusion
Basking shark
www.flmnh.ufl.edu/fish/education/questions
Goliath grouper
www.florida-scuba.com/images/special/
Adaptations - fish
Countercurrent flow optimizes
PO2 gradient
 Afferent blood vessels bring
blood to gills and efferent
vessels take blood away
 Blood flows through lamellae
in direction opposite to water
flow
Adaptations - Birds

Birds – can sustain high levels of activity
(migration) at very high altitudes
Differences from mammals:
Unidirectional
Bird lungs are compressed during
inhalation and expand during exhalation
Bird lungs have unidirectional air flow (as
opposed to bidirectional in mammals) to
maintain high PO2 gradient  eliminates dead
space
 air sacs receive inhaled air – not sites of gas exchange
 Air enters through trachea, divides into bronchi, then into
parabronchi, and then into air capillaries
Adaptations - Birds
Adaptations - Mammals
Gas exchange in mammals
 Ventilation in lungs is
tidal – air flows in and
out along same path
Tidal volume (TV)
 amount of air that
moves in and out per
breath at rest
 measured by a
spirometer
Adaptations - Mammals
 Inspiratory (IRV) – breathe in as much as
possible
 expiratory reserve (ERV) – forcefully
exhale
 Vital capacity (VC) = tidal volume +
inspiratory reserve volume + expiratory
reserve volume
 Residual volume (RV) - is air that cannot
be expelled from lungs
 Total lung capacity - is sum of vital
capacity and residual volume
Adaptations – Mammals
 Two features offset
inefficiency of tidal
breathing in mammals:
○ Enormous surface area
○ Very short path length for
diffusion
Adaptations - Mammals
Air pathway in humans
 Air enters human lung through
oral cavity or nasal passage 
pharynx (area for both food and
air)
 Below pharynx, the trachea leads
to the lungs
○ At beginning of larynx is
larynx, or voice box
 Trachea branches into two bronchi
(both have cartilagenous rings for
support)
○ bronchi branch repeatedly into
bronchioles
○ bronchioles terminate in
alveoli
○ Capillaries surround and
lie between alveoli
 Diffusion path
between blood and
air is less than 2
mm
 Less than diameter
of a red blood cell
Adaptations - Mammals
Mammalian lungs produce two
secretions that affect ventilation:
mucus and surfactant
 Mucus lines airways and captures
dirt and microorganisms
○ Mucus escalator is group of cells
with cilia that sweep mucus and
particles out of airways
 Surfactant reduces surface tension of
liquid
○ fluid covering alveoli has surface
tension that makes lungs elastic
○ Lung surfactant is released by
alveolar cells when stretched – less
force is needed to inflate lungs
http://themodulator.org/archives/Trachea-thumb.jpg
Adaptations - Mammals
 Premature babies may not have developed
the ability to make lung surfactant.
○ Without it, they have great difficulty breathing,
known as respiratory distress syndrome
○ Treatments include respirators, hormones to speed
lung development, and aerosol surfactants.
Adaptations - Mammals
Human lungs are suspended inside right and left thoracic cavity
○ Diaphragm is a sheet of muscle at bottom of these
cavities
○ Pleural membrane lines each cavity and covers each
lung
 No real space but there is thin film of fluid allowing
movement during breathing
 The pleural space contains fluid to help membranes
slide past each other during breathing
- Negative pressure is created in pleural space
- Slight negative pressure present in between breaths
keeps alveoli inflated
- Injury that effects this pressure will result in
difficulty inflating lung – “collapsed lung”
www.tcnj.edu/~mckinney
 Inhalation begins when diaphragm contracts
○ Diaphragm pulls down on thoracic cavity and on
pleural membranes
○ Pleural membranes pull on lungs, air enters
through trachea, and lungs expand
 Exhalation begins when diaphragm relaxes
 Intercostal muscles, located between ribs, can also
change volume of thoracic cavity
○ External intercostal muscles lift ribs up and
outward, expanding cavity
○ Internal intercostal muscles decrease volume by
pulling ribs down and inward
Transport of gases in blood
Hemoglobin is a protein with
four polypeptide subunits
 Each polypeptide
surrounds heme group
that contains iron (Fe)
that can bind one O2
molecule
 One molecule of
hemoglobin can bind up
to four molecules of O2
http://fig.cox.miami.edu/~cmallery/150/chemistry
Transport of gases in blood
Hemoglobin will pick up or
release O2 depending on
PO2 of environment
 If PO2 of plasma is high
(lungs), hemoglobin will
pick up its maximum of
four O2 molecules
 As blood circulates
through tissues with lower
PO2, hemoglobin will
release some O2
Transport of gases in blood
 Carbon monoxide (CO)
○ Binds to Hb with much higher
affinity than does O2
○ Deadly poison  reduces ability of
Hb to transport O2
http://yalenewhavenhealth.org/library/health
guide/en-us/images/media/medical/hw
http://www.thrombosis-consult.com/articles/images/
Transport of gases in blood
Myoglobin is single polypeptide molecule in muscles and can bind
one molecule of O2
 has higher affinity for O2, binds it at lower PO2 values when
hemoglobin molecules would release their O2
 provides reserve for high metabolic demand for O2

Very high in diving mammals
www.brooklyn.cuny.edu/bc/ahp/SDPS/graphics/
Transport of gases in blood – affinity
of Hemoglobin for oxygen
Bohr effect – pH effect on hemoglobin affinity
 When blood pH falls, H+ ions bind to Hb  decreases its
affinity for O2  releases O2
○ Blood pH decreases as it picks up CO2, lactic acid, fatty
acids at tissues
 O2-binding curve shifts to right
  Hb releases more O2 to tissues where pH is low
http://members.aol.com/Bio50/LecNotes/LNPics
Transport of gases in blood
CO2 in the blood
 Very little CO2 is transported by blood, most of it is
transported to lungs in form of bicarbonate ions
 CO2 is highly soluble, moves easily thru cell membranes
into blood, where PCO2 is lower
○ CO2 reacts with H2O to form carbonic acid
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3–
○ Most (~68%) CO2 is transported as bicarbonate ion
 ~22% transported bound to Hb
 ~10% transported dissolved in blood
Transport of gases in blood
Brain stem generates
breathing rhythm via
autonomic nervous
system
○ Neurons within medulla
increase firing rate just
prior to inhalation
○ If spinal cord is cut below
pons, breathing continues
but is irratic
 Medulla is still able to get
signals to lungs
○ If spinal cord is cut below
medulla, breathing stops

In humans and mammals,
breathing rate is more
sensitive to changes in PCO2
than to PO2
 PCO2 of blood is primary
metabolic feedback for
breathing
 The major site of sensitivity to
PCO2 is on medulla’s ventral
surface


CO2 in blood lowers pH, actually more sensitive
to pH
Sensitivity to PO2 is monitored by
carotid and aortic bodies in blood
vessels leaving heart


If PO2 falls, chemo-receptors in these bodies send
nerve impulses to brain stem to stimulate
breathing
This works at high altitudes or when blood
pressure is low
CO2
O2