Each living being has to adjust itself and regulate its body functions according to circumstances. If we have to move out in extreme cold, we would put on warm clothes. If our blood does not have enough glucose, we would feel hungry; if there is enough salt in our blood we would feel thirsty. It is fairly easy to say that we act and react according to circumstances. But how do we react, and how do we become angry? Let us try to analyze an event. Suppose, your eyes saw a snake moving towards you, your eyes reported this to the brain. Accordingly, the brain informed a particular region of the brain- the hypothalamus.
The latter asked a ductless gland, pituitary, to release a chemical compound (hormone), which would through the blood vessel reach another ductless gland, the adrenal gland, this gland would then release a hormone which would increase the blood pressure and the rate of respiration. All this would cause excitement and you would start running to safety. When your sense organ reports to your brain that there is no snake behind you, your brain accordingly sends the second message through the same route to the glands and tissues not to work harder anymore and you slowly return to the relaxed state.
While studying the endocrine glands you will notice that in some cases what one hormone does is just the opposite of another hormone. One hormone accelerates the function of a particular organ, but the other hormone puts a brake on it. This system of opposing effects leads to a proper control and balance in the working of the organs. When there is too much of acceleration of the effected organ, the latter sends a message back to the endocrine gland asking it to stop secreting the hormone. This is a kind of feedback information which serves to bring about a steady state or a stable state. This steady state of body functions is called homeostasis.