Figure 1. The fundamental factors of heat, water, pressure and oxygen availability that create physiological problems for homeothermic vertebrates in extreme environments in the desert, at high altitude and while diving to great depths.

Figure 2. Schematic showing the mechanism of a countercurrent heat exchanger, e.g. in the flipper of a diving mammal or the nasal passages of a desert kangaroo rat. In the flipper, warm blood flowing through arteries out to the cold tissues (top left, red) transfers heat to the cooler blood flowing back to the body from the flipper in the adjacent vein. As heat is lost from the arterial blood it cools while the venous blood is warmed. However, the arterial blood continuously flows to contact even cooler venous blood; hence heat is recovered along the entire length of the vessel that would otherwise be lost to the surrounding water. In the kangaroo rat's nose, dry inspired air (lower right) is heated by the nasal passages as it flows inwards, but at the same time causes evaporation of surface liquid, cooling the nasal tissues. During expiration, warm air heated to body temperature in the lungs (upper left) loses heat to the cooler nasal tissues as it is expired causing expired gas to be cooler than body temperature. Because cooler gas can hold less water vapour than warmer gas, there is a net recovery of water for the kangaroo rat that would otherwise have been lost.

Figure 3. The relationship between elevation above sea level (SL) and total atmospheric (barometric) pressure is shown by the red curve on the left. The relationship between depth below sea level and total ambient (hydrostatic) pressure is shown by the blue curve on the right. Note that the altitude scale (top) is 10-fold larger than the depth scale (bottom), and that the total pressure scale for elevation changes is less than 1% of that shown for depth changes. Ruppell's griffon point indicates the highest known altitude attained by a wild vertebrate; sperm whale and elephant seal points are the deepest documented dives by these species.