The human body is a miraculous machine, and many of its functions are automatically programmed, such as breathing, food digestion and maintaining body temperature.
People maintain a core body temperature of 37 degrees Celsius through a series of automatic checks and balances triggered by a gland in the brain, called the hypothalamus. As the body temperature rises or falls, the hypothalamus sends messages to the body to minimise heat loss to keep warm or to maximise heat loss to cool off.
There are three primary ways the body reacts to external temperature changes to maintain a constant internal temperature.
Constricting or dilating vessels
There are tiny blood vessels, called capillaries, beneath the skin. Those vessels not only bring blood to the skin, they exchange heat. Because the skin is in contact with the environment, heat or cold can travel through it and affect the temperature of the capillaries. The blood in these vessels then travels through the body and transfers that temperature change to larger blood vessels, the venules and arterioles. The venules and arterioles become veins (which take blood to the heart) and arteries (which take blood away from the heart) and continue exchanging heat, which ultimately changes the core temperature of the body.
To regulate the body's temperature when it is cold, capillaries, arteries, arterioles and veins constrict or get smaller. This causes less blood to flow near the skin, therefore, causing less blood to be cooled by the environment. Observe the pale, cool-looking skin of someone who is cold. There is no warm, red blood racing to the surface, and the colouration and temperature of the skin have changed in response to the vessel constriction.
Likewise, when the body is hot, the blood vessels dilate or open. This allows more blood to flow near the skin, where it can be cooled by the environment. Observe, for example, someone who has been exerting herself in the heat of summer; notice her flushed skin and red cheeks. The blood has travelled to the surface of the skin and is pooled in an area where it is cooled.
Goose bumps — Piloerection
The second temperature maintenance tool the body has is goose bumps. When the body is cold, tiny muscles beneath the skin stand up. Each tiny muscle, called the arrector pilli, is attached to a hair follicle, and when the muscle stands up, the hair stands up in a process called piloerection. (Pilo, in Latin for hair, and erect, in Latin, for stiffen or stand). This causes the hair to actually trap more heat beneath it. When the body is overheated, these same muscles cause the body hair to flatten so that more heat can leave the body.
Sweating and Shivering
Sweating occurs when the body is overheated. Sweat is excreted through sweat glands in the skin. Cooling occurs when the sweat evaporates. This cooling helps to quickly regulate skin temperature.
When the body is faced with regulating its temperature in the cold, it shivers. Shivering is a contraction of muscles. These muscle contractions cause heat to be generated within the body.