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The conditions inside our body must be carefully controlled to allow it to function effectively. Homeostasis is the maintenance of a constant internal environment in the body. The nervous system and hormones are responsible for controlling this.

The body control systems are all automatic, and involve both nervous and chemical responses. It has many important parts, including:

• Receptors detect a stimulus, which is a change in the environment, such as temperature change
• Coordination centres in the brain, spinal cord and pancreas. They receive information from the receptors, process the information and instigate a response.
• Effectors, such as muscles or glands create the response. Glands often release a hormone, which would restore the optimum condition again.

Body temperature

Body temperature is one of the factors that is controlled during homeostasis. The human body maintains the temperature that enzymes work best, which is around 37°C.

If body temperature increases over this temperature, enzymes will denature and become less effective at catalysing important reactions, such as respiration.

This process is controlled by the thermoregulatory centre, which is contained in the hypothalamus in the brain, and it contains receptors sensitive to the temperature of the blood. The skin also has temperature receptors and sends nervous impulses back to the hypothalamus.

The hypothalamus responds to this information by sending nerve impulses to effectors in the skin to maintain body temperature.

The skin

The skin contains three layers: The epidermis, dermis and a layer of fatty tissue.

Structures within these layers are involved in thermoregulation.

Too hot

When we get too hot:

• Sweat glands in the dermis release more sweat onto the surface of the epidermis. The sweat evaporates, transferring heat energy from the skin to the environment.

Too cold

When we get too cold:

• Skeletal muscles contract rapidly and we shiver. These contractions need energy from respiration, and some of this is released as heat. Nerve impulses are sent to the hair erector muscles in the dermis, which contract. This raises the skin hairs and traps a layer of insulating air next to the skin.

The control of body temperature is an example of a negative feedback mechanism. It regulates the amount of:

• shivering (rapid muscle contractions release heat)
• sweating (evaporation of water in sweat causes cooling)

Vasoconstriction and vasodilation - Higher

The amount of blood flowing through the skin capillaries is altered by vasoconstriction and vasodilation.

These diagrams show the processes that take place when vasoconstriction and vasodilation occur.

When the temperature is too high, different processes happen: Vasodilation, sweat production, which both transfer energy from skin to the environment, resulting in a cooling effect.