Physiology is the study of the mechanisms that allow an organism to maintain homeostasis. An organism has to interact with its environment, it can't build a shell around itself and ignore what is outside. Physiology is the study of how an organism interacts with its physical environment.
The aquatic environment differs greatly from the terrestrial environment. As terrestrial organisms, we understand the constraints of the terrestrial environment, but have little appreciation of the aquatic environment. In the terrestrial environment, ambient temperature rapidly changes. In temperate deserts, for example, the difference between daytime and nighttime temperatures can be extreme, resulting in 24 hour fluctuations of 20o C or more. Water has a high specific heat, which slows the rate it changes in temperature. Even a small pond in a harsh environment will only have a diurnal fluctuation of about 5o C and most bodies of water will change only a degree or two from day to night. The oceans might take weeks to change a degree. Similarly, the aquatic environment is buffered from seasonal fluctuations in temperature. Moreover, the coldest that the aquatic environment can physically become is 0 o C (freezing) and because it is most dense at 4o C, it freezes from the top down, so aquatic habitat is preserved under a layer of ice in extreme cold.
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While fish have less temperature change to cope with than terrestrial animals, they often suffer from a lack of oxygen availability. The concentration of oxygen (O2) in air is always 260 mg/L, while the concentration of available (dissolved) oxygen in water is, at most, about 14 mg/L and may be much less, even 0, depending on a variety of circumstances including temperature and bacterial activity. As a consequence, fish must be able to function with less oxygen than terrestrial animals. While the concentration of oxygen in the water is a fraction of that in the air, the pressure of the gas may be equal (or less); this has important physiological consequences and will be discussed in the next chapter. Gravity is the same in terrestrial and aquatic environments, but the effect of gravity on fish is much less because of buoyancy. There are large differences between air and water in light transmission and density (which affects viscosity and pressure waves) that has a profound effect on sensory function and locomotion. Finally, the air is chemically uniform, while water varies in pH, salinity, and other dissolved substances that make the maintenance of homeostasis more difficult.
Mammals and fish have similar needs [ha!] and maintain fairly similar internal environments, but their external environments (terrestrial vs. aquatic) differ greatly and this demands differences in physiological mechanisms. Throughout this course we will compare the better known mammalian systems with those in fish.
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