Module Two 12.7 How do our bodies keep internal conditions constant?

Homeostasis

The conditions inside the body must be controlled within narrow limits. This is called homeostasis. These include water content, ion content, body temperature and blood glucose concentration. The thermoregulatory centre is the part of the brain that monitors and controls body temperature. The pancreas monitors and controls blood glucose concentration. It produces a hormone called insulin that reduces blood glucose levels. Diabetes is a disease which can be caused by insufficient insulin.

Removing waste products

Waste products must be removed from the body. If they are not removed, they will increase in concentration and may interfere with the chemical reactions in the body or damage cells.

Waste products that must be removed include carbon dioxide and urea.

waste product	why is it produced?	how is it removed?
carbon dioxide	it is a product of aerobic respiration	through the lungs when we breathe out
urea	it is produced in the liver when excess amino acids are broken down	the kidneys remove it from the blood and make urine, which is stored in the bladder temporarily

Water enters the body through food and drink. It is also a product of aerobic respiration in cells. If the amount of water in the body is wrong, cells can be damaged because too much water enters or leaves them.

Controlling blood glucose

The blood glucose concentration is controlled to provide cells with a constant supply of energy

The pancreas and insulin

The pancreas monitors and controls the concentration of glucose in the blood. It produces a hormone called insulin.

Insulin causes glucose to move from the blood into cells. This lowers the blood glucose concentration if has become too high. This can happen after eating a meal rich in carbohydrates (for example sweets, potatoes, bread, rice or pasta).

Diabetes

Diabetes is a disease in which the concentration of glucose in the blood is not controlled properly by the body. In type 1 diabetes, the pancreas does not produce enough insulin. This can lead to high levels of glucose in the blood, which can be fatal.

Banting and Best

You should be able to evaluate experimental data from Banting and Best's experiments, which led to insulin being discovered.

Two doctors, Frederick Banting and Charles Best, made an extract from the pancreas in 1921. The extract had anti-diabetic properties. They successfully tested their extract on diabetic dogs. The first tests in humans were carried out in 1922 and were a success, too. Some patients in a diabetic coma even recovered. The extract contained the hormone insulin.

Diabetes

There are 2 types of treatment for diabetes:

• Careful monitoring of food intake, with particular care over carbohydrates (which are digested into glucose); and
• Injecting insulin into the blood before meals. The extra insulin causes glucose to be taken up by the liver and other tissues. Cells get the glucose they need for respiration, and the blood glucose concentration stays normal

Temperature regulation

Human enzymes work best at 37ºC, so the body's temperature is controlled. A part of the brain called the thermoregulatory centre monitors and controls body temperature. It gathers information as nerve impulses from temperature receptors in:

• the brain – these are sensitive to the temperature of the blood flowing there; and
• the skin – these are sensitive to skin temperature.

Sweating

Sweating is one way to help cool the body. We sweat more in hot conditions, so more water is lost from the body then. It must be replaced through food or drink to maintain the balance of water in the body. Ions such as sodium ions and chloride ions are also lost when we sweat. They must be replaced through food and drink: if the body's ion content is wrong, cells can be damaged.

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

When we get too hot:

• Sweat glands in the skin release more sweat. This evaporates, removing heat energy from the skin.
• Blood vessels leading to the skin capillaries become wider (they dilate), letting more blood flow through the skin and more heat to be lost.

Too cold

When we get too cold:

• 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 hairs on the skin also help to control body temperature. They lie flat when we are warm but rise when we are cold. The hairs trap a layer of air above the skin, which helps to insulate the skin against heat loss.

From warm to cool

If the loss of heat from your body is greater than heat production, your body temperature will fall. In order to prevent this, the following changes occur:

• Your blood is directed away from your skin, and more to the central part of your body to maintain the temperature of those parts,
• Your skin becomes cool as a result of this,
• You get goose flesh, which increases the insulation capacity of your skin as rough surfaces lose less heat than smooth ones,
• You might shiver which activates the muscles in order to produce additional heat,
• You might adjust your behaviour - you might move to a warmer space, put on clothing and change your posture to preserve heat.

From cool to warm

When you move from a cool to a warmer environment, your body temperature will increase as:

• More blood is directed to the surface of your body (skin),
• Your skin surface is warmed as a result (and a result of radiant heat from the surrounding air,
• Your heart rate and pulse rate increases,
• You may start to sweat,
• You may adjust your behaviour by moving to cooler areas, adjusting your clothing, reducing activity and changing posture.