Most of us are not even aware that another system works constantly, along with the nervous system and the muscles, to direct our bodily functions. The system depends on chemical substances called hormones that are released by special glands called endocrine glands. These glands have no opening or ducts but release their secretions directly into the blood stream as it passes through their.tissues. You can see theirlocation in the body in Fig.
Each hormone acts on a certain organ in a specific way. Many of the effects of hormones arlong term changes, for example, the changes that take place in the body during growth and sexual maturity. However, some of the changes may be quick like the ones that occur whenthe heart rate and breathing rate is increased or decreased. Alias of some of the major endocrine glands a!ong with the hormones they secrete is given in Table. You are not expected to remember each of them. The information is providedin case, you are curious to know more about these glands. You can see that these hormones are vital for the proper functioning of the body. They regulate the chemical reactibns in the body and consequently play a critical role in maintaining nohal physical conditions or homeostasis. If hormones are to serve a regulatory function. they must be produced at the proper time. in proper amounts. This is controlled by the brain, receiving information from all parts of the body and giving . appropriate signals to the glands. This feedback system is very much like a thermostat that controls temperature in a machine.