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It is important that the body’s internal environment is controlled. For example, the amount of carbon dioxide in the bloodstream must be carefully controlled.
Maintaining a constant internal environment is called homeostasis. The nervous system and hormones are responsible for this. Here are some of the other internal conditions that are controlled:
Blood sugar level
This is controlled to provide cells with a constant supply of energy. The blood sugar level is controlled by the release and storage of glucose, which is in turn controlled by a hormone called insulin.
Body temperature
This is controlled to maintain the temperature at which enzymes work best, which is 37°C. Body temperature is controlled by:
controlling blood flow to the skin
The body’s water content
This is controlled to protect cells by stopping too much water from entering or leaving them. The process is called osmoregulation.

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Controlling water content of the body
This is controlled to protect cells by avoiding too much water entering or leaving them. Water content is controlled by water loss from:
the lungs when we exhale--
the skin by sweating-
the body, in urine produced by the kidneys///

///Ion (salts) content of the body
This is controlled to protect cells by avoiding too much water entering or leaving them. Ion content is controlled by loss of ions from:
the skin by sweating--
the body, in urine produced by the kidneys

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Controlling body temperature
Human enzymes usually work best at 37ºC, which is human body temperature. This can be measured in several places, including the ear, finger, mouth and anus.
There are various ways to measure body temperature, including using a clinical thermometer, heat-sensitive strips, digital probes or thermal imaging cameras.
Extremes of body temperature are dangerous:
high temperatures can cause dehydration, heat stroke and death if untreated
low temperatures can cause hypothermia and death if untreated
Control mechanisms
The body’s temperature is monitored by the brain. If you are too hot or too cold, the brain sends nerve impulses to the skin, which has three ways to either increase or decrease heat loss from the body’s surface:
Hairs on the skin trap more warm air if they are standing up, and less if they are lying flat. Tiny muscles in the skin can quickly pull the hairs upright to reduce heat loss, or lay them down flat to increase heat loss.
If the body is too hot, glands under the skin secrete sweat onto the surface of the skin, to increase heat loss by evaporation. Sweat secretion stops when body temperature returns to normal.

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Blood vessels supplying blood to the skin can swell or dilate - vasodilation. This causes more heat to be carried by the blood to the skin, where it can be lost to the air. Blood vessels can shrink down again - vasoconstriction. This reduces heat loss through the skin once the body’s temperature has returned to normal.
Muscles can also receive messages from the brain when you are cold. They respond by shivering, which warms you up.

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Thermoregulation - Higher tier

If you become too hot or too cold, there are several ways in which your temperature can be controlled. They involve sweating, shivering, skin capillaries and hairs.
Too hot
sweat glands in the skin release more sweat when we get too hot. This evaporates, removing heat energy from the skin.
vasodilation occurs. Blood vessels leading to the skin capillaries become wider (dilate) allowing more blood to flow through the skin, and more heat to be lost.
Too cold
muscles contract rapidly and we shiver when we're cold. These contractions need energy from respiration, and some of this is released as heat.
vasoconstriction occurs - blood vessels leading to the skin capillaries become narrower (constrict) letting less blood flow through the skin and conserving heat in the body.
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.

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Hormones are chemicals secreted by glands in the body. Different hormones affect different target organs.
The bloodstream transports hormones from the glands to the target organs. Bodily reactions to hormones are usually slower and longer lasting than nervous reactions.
Move the mouse over the different glands to see what they do. You need to know the locations of the pancreas, ovaries and testes. You should also know which hormones they produce.

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Blood glucose regulation

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

Glucagon – Higher tier
The pancreas releases another hormone, glucagon, when the blood sugar levels fall. This causes the cells in the liver to turn glycogen back into glucose which can then be released into the blood. The blood sugar levels will then rise.

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