If your blood temperature drop a 10th of a degree on a cold day, and I cause the adrenal glands and the pituitary to make sure the liver releases more blood sugar as fuel for muscles, which are the main furnaces of the body. I get you to start shivering so that heat will be produced by muscle activity. Sweat glands also shut down, detouring blood from body surfaces where it would become further chilled. If you are chilled enough, though, surface blood vessels will almost entirely shut down - and you will turn blue. I do one thing which is rather pointless when you are cold : I give you gooseflesh. This is a hangover from your furry ancestors. For them I used to tighten skin muscles to make their hair stand on end; it produced better insulation that way.
If your blood heats up as little as a 10th of a degree on a warm day, I go to work. I send messages to the pituitary gland and through the sympathetic nervous system to dilate surface blood vessels and open tens of thousands of sweat glands. The sweat cools the skin so as to get rid of the extra heat in your blood. At the same time I signal other brain areas to speed up breathing so that, you will pant - and thus carry away more heat.
I can trace my ancestry back 100 million years and I do many of the same jobs for you today that I have done since the earliest primitive creatures began to appear on earth. Take the matter of temperature control. Thanks to me, you can survive in Siberia when the temperature drops as low as -90 degree F., or in Libya when it climbs to 136 degree. In either place, I will keep your internal environment about a steady 98.6 degree. Let it vary by more than a few degrees either up or down, and you would be a goner.
I am quite unimpressive in appearance. I lie near the underside of the brain, just about in the centre of your head. I am pink and grey in colour and approximately the size of a small prune - a mere 1/300 of the mass of the brain. Yet I have a richer blood supply than any portion of the body, a highly developed sensing system and extensive direct and indirect nerve connections within the nervous system.
You never heard of me but I am the single most important group of cells in your body - on duty 24 hours a day even though most of the time you are not aware of what I am doing. My chief responsibility is maintaining equilibrium inside you. I inform other regions of the brain and body that their services are required. As a result of my constant monitoring, you knows when you are hungry, thirsty, hot or cold and how to react to anger or fear. In one way or another, I take part in just about everything you do. I am not bright like other parts of your brain. Thinking is not my business. I suppose you could call me the central switchboard of your body, a sort of coordinator for much of your nervous system and for your pituitary gland which is often called the master gland because of its influence on metabolism, growth and secondary sex characteristics, as well as other functions of the hormone system.