The cells of an animal can only remain healthy if the conditions are just right. The processes that take place in them are upset if the temperature is too high or too low, or if the fluid around or inside them is too acid or alkaline. Homeostasis is the name given to the processes that help keep the internal conditions constant even when external conditions change. The word means, “keeping things stable”.
There are a number of organs in the body that play a part in maintaining homeostasis. For example, the skin helps keep the internal temperature of bird and mammals bodies within a narrow range even when the outside temperatures change (see Chapters 5 and 16); the lungs control the amount of carbon dioxide in the blood (see Chapters 8 and 16); the liver and pancreas work together to keep the amount of glucose in the blood within narrow limits (see Chapter 5) and the kidneys regulate the acidity and the concentration of water and salt in the blood (also see Chapter 16). How the kidneys do this will be described later in this chapter.
Hormones are chemicals that carry messages around the body in the blood and are central to many of the homeostatic processes mentioned above. Their role will be described in more detail in Chapter 16.