Hormones have been called the ‘chemical messengers’ of the body. They are substances that certain glands secrete into the bloodstream in very small amounts to regulate the function of other parts of the body. Glands that secrete hormones are called endocrine glands, and the organs they control are known as their target organs. Because hormones are carried in the blood they reach all parts of the body, but only the target organs respond to their messages. Some hormones control only one target organ whereas others have an effect on nearly all the cells in the body. The main endocrine glands are the pituitary, thyroid, parathyroids, pancreas, and adrenals. The testes in men and the ovaries in women are also concerned with the production of hormones, the male and the female sex hormones respectively. Hormones control the amounts of various chemical constituents in the body; they regulate the rate of chemical reactions inside body cells, during both normal activities and stressful and unusual situations; and they control the chemical processes involved in growth, sexual maturation and in a woman, menstruation, pregnancy and childbirth.

One of their most important functions is to maintain the body’s internal environment in a steady state, a process known as homeostasis. Essentially, the ‘internal environment’ refers to the fluids surrounding body cells. These fluids, blood and extracellular fluids, should contain certain chemicals at the correct concentration for cells to survive and function normally. In fact, some cells, and even whole organs, can survive and function outside the body if kept in a fluid whose chemical composition is similar to that of extracellular fluids. Specific hormones control the balance of water, vital minerals such as sodium, potassium and calcium, and glucose – the simple sugar that is the body’s main source of energy. They manage to do this despite the differing amounts of various chemicals that each person takes in, in food, and uses according to various needs.