Homeostasis
The conditions inside our body must be very carefully controlled if the body is to function
effectively. Homeostasis is the maintenance of a constant internal environment. The nervous
system and hormones are responsible for this.
One example of homeostasis is the concentration of carbon dioxide in the blood being carefully
controlled. Here are some of the other internal conditions that are regulated:
Body temperature
This is controlled to maintain the temperature at which the body’s enzymes work best, which is
usually 37°C.
Blood sugar level
This is controlled to provide cells with a constant supply of glucose for respiration. It is
controlled by the release and storage of glucose, which is in turn controlled by insulin.
Water content
This is controlled to protect cells by stopping too much water from entering or leaving them.
Water content is controlled by water loss from:
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the lungs - when we exhale
the skin - by sweating
the body - in urine produced by the kidneys
Negative feedback
Homeostatic control is achieved using negative feedback mechanisms:
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if the level of something rises, control systems reduce it again
if the level of something falls, control systems raise it again
Negative feedback flowchart
Regulating body temperature
The human body is designed to function most efficiently at 37ºC. If you become too hot or too
cold, there are ways in which your body temperature can be controlled.
Too hot
When we get too hot:
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Sweat glands in the skin release more sweat. The sweat evaporates, removing heat
energy from the skin.
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.
Too cold
When we get too cold:
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Muscles contract rapidly - we shiver. These contractions need energy from respiration,
and some of this is released as heat.
Blood vessels leading to the skin capillaries become narrower - they constrict - letting
less blood flow through the skin and conserving heat in the body.
The skin
The hairs on the skin also help to control body temperature. They lie flat when we are warm, and
rise when we are cold. The hairs trap a layer of air above the skin, which helps to insulate the
skin against heat loss.
The hypothalamus is the part of the brain which monitors the body's temperature. It receives
information from temperature-sensitive receptors in the skin and circulatory system.
The hypothalamus responds to this information by sending nerve impulses to effectors to
maintain body temperature. For example, if we become too cold, the hair erector muscles
contract. This raises the skin hairs and traps a layer of air next to the skin.
Skin hairs lie flat when we are hot and stand upright when we are cold
Negative feedback mechanisms control body temperature. They include the amount of:
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shivering (rapid muscle contractions release heat)
sweating (evaporation of water in sweat causes cooling)
blood flowing in the skin capillaries
Negative feedback in temperature regulation
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Vasoconstriction and vasodilation
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These diagrams show the processes that take place when vasoconstriction and
vasodilation occur.
The amount of blood flowing through the skin capillaries is altered by vasoconstriction
and vasodilation
Regulating blood glucose
Glucose is needed by cells for respiration. It is important that the concentration of glucose in the
blood is maintained at a constant level. Insulin is a hormone - produced by the pancreas - that
regulates glucose levels in the blood.