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���˿���ostasis

The conditions inside our body must be carefully controlled if it is to function effectively. ���˿���ostasis 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 are controlled during homeostasis. The human body maintains the temperature that enzymes work best, which is around 37°C. 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 thermoregulatory centre.

Too hot

When we get too hot:

• Sweat glands in the skin release more sweat. The sweat evaporates, transferring heat energy from the skin to the environment.

Blood vessels leading to the skin capillaries become wider - they dilate - allowing more blood to flow through the skin, and more heat to be lost to the environment. This is called vasodilation.

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. Blood vessels, which lead to the skin capillaries, become narrower - they constrict – which allows less blood to flow through the skin and conserve the core body temperature. This is called vasoconstriction.

The hairs on the skin also help to control body temperature. The hairs lie flat when we are warm, and rise when we are cold.

If we are too cold nerve impulses are sent to the hair erector muscles which contract. This raises the skin hairs and traps a layer of insulating air next to the skin.

Skin hairs lie flat when we are hot and stand upright when we are cold

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)
• blood flowing in the skin capillaries

Vasoconstriction and vasodilation

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.