CHAPTER 48 – GAS EXCHANGE IN ANIMALS
48.1 What Physical Factors Govern Respiratory Gas Exchange?
Respiratory Gases – the gases that animals must exchange are oxygen and carbon dioxide.
Cells need to obtain O2 from the environment to produce an adequate supply of ATP by
CO2 is an end product of cellular respiration, and it must be removed from the body to
prevent toxic effect.
Diffusion is the only means by which respiratory gases are exchanged between the
internal body fluids of an animal and the outside medium (air or water).
There are no active transport mechanisms to move such gases across biological
Diffusion is driven by concentration differences
-The concentrations of different gases in a mixture are expressed by the partial pressures of
To do this, the total pressure must be known first (this is measured with a barometer).
Barometric Pressure – atmospheric pressure.
Partial Pressure of Oxygen – at sea level is 20.9% of 760 mm Hg.
-The concentration of respiratory gases in a liquid such as water is a little more complicated
because the solubility of the gas in the liquid is involved.
Fick’s law applies to all systems of gas exchange
Fick’s Law of Diffusion – an equation that describes the factors that determine the rate of
diffusion of a molecule from an area of higher concentration to an area of lower concentration.
Q = DA x P1 - P 2
Q = rate a substance diffuses between 2 locations
D = diffusion coefficient
A = area over which substance diffuses
P1 and P2 = concentration of gas at 2 locations
L = distance between these locations
-Animals can maximize D by using air instead of water
-Other adaptations for maximizing respiratory gas exchange
maximize surface area for exchange (A)
maximize concentration gradient across exchange surface (C1 >> C2)
Air is a better respiratory medium than water
-Oxygen can be obtained more easily from air that from water for several reasons:
The O2 content of water is much higher that the O2 content of an equal volume of water.
Oxygen diffuses about 8000 times more rapidly in air than in water.
When an animal breathes, it does work to move water or air over its specialized gas
exchange surfaces. More energy is required to move water than to move air because water
800 times denser than air and about 50 times more viscous.
-The slow diffusion of O2 molecules in water affects air-breathing animals as well as water-
It limits the efficiency of O2 distribution from gas exchange surfaces to the sites of
cellular respiration in air-breathing animals.
Animal cells with low rates of metabolism are forced to be no more than a couple of
millimeters away from a good source of environmental O2.
-Therefore, there are severe size and shape limits on many species of invertebrates that
lack internal systems for distributing O2.
High temperatures create respiratory problems for aquatic animals
Most water breathers are ectotherms.
-As the temperature of the water gets warmer, their body temperature and metabolic
They need more O2 as the water gets warmer.
-Warm water holds less dissolved gas than cold water does.
As water temperature goes up, they must extract more and more O2 from an environment
that is increasingly O2 deficient, and a lower percentage of that O2 is available to support
activities other than breathing.
O2 availability decreases with altitude
-An increase in altitude reduces the O2 supply for air breathers.
-At all altitudes, O2 makes up 20.9% of the dry air.
-As you go up in altitude, the total amount of gas per unit volume decreases.
-Since the movement of O2 across respiratory gas exchange surfaces and into the body
depends on diffusion, its rate of movement depends on the PO2 difference between air and the
-The drastically reduced PO2 in the air at high altitudes constrains O2 uptake.
CO2 is lost by diffusion
-Respiratory gas exchange is a two-way process:
CO2 diffuses out of the body as O2 diffuse in.
-The direction and rate of diffusion of the respiratory gases across the respiratory exchange
surfaces depend on the partial pressure of the gradients of the gases.
-The partial pressure gradients of O2 and CO2 across these gas exchange surfaces are quite
The amount of CO2 in the atmosphere is extremely low (0.03%).
-This means that for air-breathing animals, there is always a large concentration
gradient for the diffusion of CO2 from the body to the environment.
-The partial pressure gradient for CO2 does not decrease with altitude
The partial pressure gradient for O2 decreases with altitude.
-Getting rid of CO2 is not a problem for water-breathing animals because CO2 is much more
soluble in water than is O2.
48.2 What Adaptations Maximize Respiratory Gas Exchange?
-Common ways the respiratory systems of different organisms maximize the exchange of O2
and CO2 with the environment include adaptations for:
Increasing the surface area over which diffusion of gases can occur;
Maximizing partial pressure gradients;
Minimizing the diffusion path length through an aqueous medium.
Respiratory organs have large surface areas
-Many anatomical adaptations maximize the specialized body surface area (A) over which
respiratory gases can diffuse.
External Gills - highly branched and folded extensions of the body surface that provide a large
surface area for gas exchange with water.
They are found in larval amphibians and in the larvae of many insect species.
They consist of thin, delicate tissues.
They minimize the length of the path (L) traversed by diffusing molecules of O2 and CO2.
They are vulnerable to damage.
Internal Gills – protective body cavities for gills have evolved in many animals.
e.g., these can be found in many mollusks and arthropods, and in all fishes.
Lungs – internal cavities for respiratory gas exchange with air.
Used by air-breathing vertebrates.
Respiratory gases the gases that animals must exchange are oxygen and carbon dioxide. cells need to obtain o2 from the environment to produce an adequate supply of atp by cellular respiration. co2 is an end product of cellular respiration, and it must be removed from the body to prevent toxic effect. diffusion is the only means by which respiratory gases are exchanged between the internal body fluids of an animal and the outside medium (air or water). there are no active transport mechanisms to move such gases across biological membranes. The concentrations of different gases in a mixture are expressed by the partial pressures of those gases. to do this, the total pressure must be known first (this is measured with a barometer). partial pressure of oxygen at sea level is 20. 9% of 760 mm hg.