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Gas Exchange
Campbell Chapter 42
Pages 886 - 897
Gas Exchange in Animals
• Gas exchange = taking in
molecular oxygen (O2) from
the environment and disposing
of carbon dioxide (CO2) to the
Gas Exchange
• Cellular respiration is the breakdown of
organic molecules to make ATP. A supply of
oxygen is needed to convert stored organic
energy into energy trapped in ATP.
• Carbon dioxide is a by-product of these
processes and must be removed from the
• There must be an exchange of gases:
carbon dioxide leaving the cell, oxygen
Air Supply
The respiratory medium or source of
oxygen is:
1. Terrestrial Animals = air
2. Aquatic Animals = water.
Atmosphere is ~21% oxygen (O2)
Bodies of water – variable oxygen
content, but much less than in an equal
amount of air.
Respiratory Surface
• Gases are exchanged with the
environment at the respiratory surface.
• Gas movement is by diffusion.
• Respiratory surfaces are usually thin and
have large areas as well as adaptations to
facilitate the exchange.
• Gases are dissolved in water, so
respiratory surfaces must be moist.
Diffusion Rate
• The net diffusion rate of a gas across a
fluid membrane is
– proportional to the difference in partial
– proportional to the area of the membrane and
– inversely proportional to the thickness of the
Fick’s Law
• The rate at which a substance can diffuse is
given by Fick's law
Surface to Volume Ratio
• Rate of exchange of substances depends
on the organism's surface area that is in
contact with the surroundings.
• The ability to exchange substances
depends on the surface area : volume
• As organisms get bigger, their volume and
surface area both get bigger, but volume
increases much more than surface area.
Unicellular Organisms
• Single-celled organisms
exchange gases directly
across their cell
• The slow diffusion rate of
oxygen relative to carbon
dioxide limits the size of
single-celled organisms.
Simple Animals
• The cells of sponges, cnidarians, and
flatworms are in direct contact with
• Simple animals that lack specialized
exchange surfaces have flattened, tubular,
or thin shaped body plans, which are the
most efficient for gas exchange. However,
these simple animals are rather small in
Simple Animals
Respiratory Surfaces
• Some animals use their outer surfaces
(skin) as gas exchange surfaces.
(earthworms and some annelids)
• Arthropods, annelids, and fish use gills.
• Terrestrial vertebrates utilize internal
Outer Surface (Skin)
• Flatworms and annelids use their outer surfaces as gas
exchange surfaces. Earthworms have a series of
capillaries. Gas exchange occurs at capillaries located
throughout the body as well as those in the respiratory
• Amphibians use their skin as a respiratory surface. Frogs
eliminate carbon dioxide 2.5 times as fast through their
skin as they do through their lungs.
• Eels (a fish) obtain 60% of their oxygen through their
• Humans exchange only 1% of their carbon dioxide
through their skin.
• Gills have evolved many times in different
animal groups, and the specific anatomy
varies widely. But as a general rule, gills
consist of fine sheets or filaments of tissue
that extend outward from the body into the
Fish Respiration
• Some fish ventilate their gills by swimming with
mouth and gill slits open [e.g. sharks, which die
of asphyxiation if immobilized].
• But many fish can respire while stationary, and
do so by swallowing water through their mouths
and forcibly expelling it through the gills.
• Gills are out-foldings of the body
surface that are suspended in water.
• They increase the surface area for gas
• They are organized into a series of
plates and may be internal (as in
crabs and fish) or external to the body
(as in some amphibians).
Who Has Gills?
Gills are found in a variety of animal
arthropods (including some terrestrial
Variety of Gills
Efficiency of Gills
• Gills are very efficient at removing oxygen
from water: there is only about 1/20 the
amount of oxygen present in water as in
the same volume of air.
• Water flows over gills in one direction
while blood flows in the opposite direction
through gill capillaries. This countercurrent
flow maximizes oxygen transfer.
How Gills Work
• Fish maximize gas exchange in their gills
by a 'design principle' called
countercurrent exchange.
• Countercurrent exchange requires that
two fluids (in this case, the external water
and the blood in the gills) flow past each
other in opposite directions.
Counter-Current Exchange
When a fish swims, water moves over its gills from
anterior to posterior.
Blood flow in the gill capillary bed is oriented from
posterior to anterior. The blood picks up O2 from the
external water (and loses CO2) as it flows through the
gill capillaries.
This countercurrent arrangement insures that the most
O2 -depleted blood (entering the gill) is confronted with
the most O2 -depleted water (leaving the gill), and that
the most O2 -rich blood (leaving the gill) contacts the
most O2 -rich water (entering the gill). This
arrangement maximizes O2 absorption.
Counter-Current Exchange
Terrestrial Respiratory Systems
• Many terrestrial animals have their
respiratory surfaces inside the body and
connected to the outside by a series of
• Insects, centipedes, and some mites and
spiders have a tracheal respiratory
• Vertebrates have lungs.
Insect Tracheal Systems
• All insects are aerobic organisms - they must
obtain oxygen (O2) from their environment in
order to survive.
• The insect respiratory system is a complex
network of tubes (tracheal system) that delivers
O2-containing air to every cell of the body.
• Tracheae are the tubes that carry air directly to
cells for gas exchange.
Tracheal System
• Air enters the insect's body through valve-like
openings (spiracles) in the exoskeleton. These
are located laterally along the thorax and
abdomen of most insects.
• Air flow is regulated by small muscles that
operate one or two flap-like valves within each
spiracle -- contracting to close the spiracle, or
relaxing to open it.
• After passing through a spiracle, air enters a
longitudinal tracheal trunk, eventually diffusing
throughout a complex, branching network of
tracheal tube that subdivides into smaller and
smaller tubes that reach every part of the body.
• At the end of each tracheal branch, a special cell
(the tracheole) provides a thin, moist interface for
the exchange of gases between atmospheric air
and a living cell.
• Oxygen in the tracheal tube first dissolves in the
liquid of the tracheole and then diffuses into the
cytoplasm of an adjacent cell. At the same time,
carbon dioxide, produced as a waste product of
cellular respiration, diffuses out of the cell and,
eventually, out of the body through the tracheal
Insect Tracheal System
Human Respiratory System
Human Respiratory System
• This system includes the lungs, pathways
connecting them to the outside
environment, and structures in the chest
involved with moving air in and out of the
• The main task of any respiratory system is
to take in oxygen and remove carbon
• Lungs are ingrowths of the body wall and
connect to the outside by as series of
tubes and small openings.
• Lung breathing probably evolved about
400 million years ago.
• Lungs are not entirely the sole property of
vertebrates, some terrestrial snails have a
gas exchange structures similar to those in
• The lungs are large, lobed, paired
organs in the chest (also known as
the thoracic cavity).
• Thin sheets of epithelium (pleura)
separate the inside of the chest cavity
from the outer surface of the lungs.
• The bottom of the thoracic cavity is
formed by the diaphragm.
Pathway of Air
• Air enters the body through the nose, is warmed,
filtered, and passed through the nasal cavity.
• Air passes the pharynx (which has the epiglottis
that prevents food from entering the trachea).
• The upper part of the trachea contains the larynx.
The vocal cords are two bands of tissue that
extend across the opening of the larynx.
• After passing the larynx, the air moves into the
bronchi that carry air in and out of the lungs.
• Bronchi are reinforced to prevent their
collapse and are lined with ciliated
epithelium and mucus-producing cells.
• Bronchi branch into smaller and smaller
tubes known as bronchioles.
• Bronchioles terminate in grape-like sac
clusters known as alveoli.
• Alveoli are surrounded by a network of
thin-walled capillaries. Only about 0.2 µm
separate the alveoli from the capillaries
due to the extremely thin walls of both
• Only in the alveoli does actual gas
exchange takes place.
• There are some 300 million alveoli in two
adult lungs.
• These provide a surface area of some 160
m2 (almost equal to the singles area of a
tennis court and 80 times the area of our
Diffusion of O2 and CO2
Alveoli: Designed for Rapid Gas
• After branching repeatedly the bronchioles
enlarge into millions of alveolar sacs
• This arrangement produces an enormous
surface are for gas exchange
• Each alveolus is surrounded by a net of
• The diffusion distance from gas in the alveoli to
blood cells in the capillaries is very short
• Blood takes about 1 second to pass through the
lung capillaries
• In this time the blood becomes nearly 100%
saturated with oxygen and loses its excess CO2
Surfactants Prevent the Alveoli
From Collapsing
• At air/water interfaces there is a high surface
• The high surface tension would cause the alveoli
to collapse, but this is prevented by surfactants
• Surfactants are detergent-like phospholipids
which accumulate at the air/water interface and
lower the surface tension
• Reduced surfactant causes respiratory distress
syndrome (seen in premature infants and some
older persons)
• Respiratory pigments increase the oxygencarrying capacity of the blood. Humans have the
red-colored pigment hemoglobin as their
respiratory pigment.
• Hemoglobin increases the oxygen-carrying
capacity of the blood between 65 and 70 times.
• Each red blood cell has about 250 million
hemoglobin molecules, and each milliliter of
blood contains 1.25 X 1015 hemoglobin
• Oxygen concentration in cells is low (when
leaving the lungs blood is 97% saturated with
oxygen), so oxygen diffuses from the blood to
the cells when it reaches the capillaries.
• Ventilation is the mechanics of breathing in
and out.
• When you inhale, muscles in the chest
wall contract, lifting the ribs and pulling
them, outward. The diaphragm at this time
moves downward enlarging the chest
cavity. Reduced air pressure in the lungs
causes air to enter the lungs.
• Exhaling reverses theses steps.
Negative Pressure Breathing
Diaphragm Action
Negative Pressure Animation
Lung Volume
• Tidal volume is the amount of air that is
inhaled and exhaled in a normal breath.
• Vital capacity is the maximum amount of
air that can be inhaled and exhaled in a
single breath.
• Since the lungs hold more air than the vital
capacity, the air that remains in the lungs
is the residual volume.
Dead Space
• Only the air in the alveoli can exchange O2 and
CO2 with the blood
• When you breath in the first 150 mL fills tubes
which are outside of the alveoli (trachea,
bronchi, bronchioles, etc.)
• This part of the tidal volume is called the
anatomical dead space- it does not participate in
gas exchange
• There is also a functional dead space- not all of
the alveoli are perfused with blood; air in these
alveoli doesn't exchange with the blood and is
part of the dead space
Avian Respiration
• The avian respiratory system delivers
oxygen from the air to the tissues and also
removes carbon dioxide.
• In addition, the respiratory system plays
an important role in thermoregulation
(maintaining normal body temperature).
• The avian respiratory system is different
from that of other vertebrates, with birds
having relatively small lungs plus nine air
sacs that play an important role in
respiration (but are not directly involved in
the exchange of gases).
• The air sacs permit a unidirectional flow of
air through the lungs.
• Unidirectional flow means that air moving
through bird lungs is largely 'fresh' air &
has a higher oxygen content.
• In contrast, air flow is 'bidirectional' in
mammals, moving back & forth into & out
of the lungs. As a result, air coming into a
mammal's lungs is mixed with 'old' air (air
that has been in the lungs for a while) &
this 'mixed air' has less oxygen. So, in bird
lungs, more oxygen is available to diffuse
into the blood
• All material for this PPT was found on
various websites or is from Campbell
Biology 6e.