Genetics determines the proteins and enzymes that our cells will have. It also determines the limits of our function. One may wonder why the human kidney is not as efficient as that of a mouse, or why our lungs are nowhere as efficient as those of a bird. We like to think of ourselves as the best of the animal kingdom, yet we can find examples of nearly any function that can be carried out better by some other animal. Also, it is easy to see how the body could be made to function better and be more efficient.
All animals are limited, as well as defined, by their genetics.
The tendency for homeostasis can be detected in animals everywhere you look. This is just the control system within the body to keep conditions as constant as possible. The best example is the thermostat that will turn on a furnace when a room gets too cold and turn it off when it gets too warm. The thermostat has a negative feedback relationship to the furnace.