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.