Title: IGCSE Biology Revision : Homeostasis

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When you exercise, there are significant effects of exercise on the heart, lungs, and other major body systems . This provides a considerable test to the body in maintaining homeostasis both during and after exercise.......

What is homeostasis ?

Homeostasis is the maintenance of a constant internal environment.

For any homeostatic control to occur there must exist the following :

- a stimulus which is a change in the internal environment,
- a receptor which can detect the stimulus,
- an automatic or self-regulatory corrective mechanism, which bring about a negative feedback

The roles of hormones and negative feedback

Many of the mechanisms that are used for homeostasis involve hormones.

Hormones are chemical messengers that are carried in the blood stream. They are released by glands and pass to their target organ. Hormones take longer to have an effect than nerves but their responses usually last longer.

Many of these control mechanisms work by negative feedback. This means that if the levels change too much, a hormone is released and this brings the change back to the normal level.

Homeostasis (Video 1 )

Why is the control of blood sugar important?

It is essential that the sugar or glucose level of the blood is kept constant. If the glucose level becomes too low then our cells will not have enough to use for respiration. If it is too high then glucose may start to pass out in the urine.

Insulin is the hormone that controls the level of glucose in the blood.

When glucose levels are too high, more insulin is made. The insulin converts excess glucose into glycogen to be stored in the liver. Some people have diabetes and do not produce enough insulin naturally. They need regular insulin injections in order to make sure that the level of glucose in their blood is controlled. They also need to control their diet carefully.

How is blood sugar controlled ?

It is essential that the sugar level of the blood is kept constant.
If the sugar level too low then cells will not have enough to use for respiration.
If the sugar level is too high then glucose may start to pass out in the urine.

Insulin is the hormone that controls the level of glucose in the blood.

When glucose levels are too high, more insulin is made.
The insulin converts excess glucose into glycogen to be stored in the liver.

Some people have diabetes and do not produce enough insulin.
They need regular insulin injections in order to control the level of glucose in their blood.
They also need to control their diet carefully.

What happens if blood sugar is too high ?

- 1. Insulin injected by pancreas
- 2. Glucose absorbed by tissues
- 3. Glucose absorbed by liver
- 4. Blood glucose reduced

What happens if blood sugar is too low ?

- 1. Insulin not injected by pancreas
- 2. Less glucose absorbed by tissues
- 3. Less glucose absorbed by liver
- 4. Blood glucose increased

Blood sugar homeostasis and diabetes (video)

Why is it important for body temperature to be controlled ?

It is important to keep our body temperature at about 37 °C.  Enzymes work best at this temperature. The blood temperature is monitored by the brain and if it changes from 37 °C, and a number of changes occur.

How is temperature controlled ?

1. Body is too hot

If you feel too hot you need to lose heat at a faster rate, since your body temperature is in danger of rising.

You body does this by :

- sweating – Heat is absorbed by the water from our body.  Thew body loses heat by evaporation of sweat.
- vasodilation – This is done by blood capillaries near the surface of the skin becoming wider . This action allows more blood to flow near the surface.  Heat is transfered to the air since the blood is warmer than the surrounding air

How is temperature controlled ?

2. Body is too cold

When we feel too cold we are in danger of losing heat too quickly and cooling down. This means we need to conserve our heat to maintain a constant 37 °C. We do this by:

- shivering – rapid contraction and relaxation of body muscles. This increases the rate of respiration and more energy is released as heat
- vasoconstriction – blood capillaries near the skin surface get narrower and this process reduces blood flow to the surface. The blood is diverted to deeper within the body to conserve heat.
- sweating less

Temperature Regulation in the Human Body  (Video)

Homeostasis

Q: What are the responses to decreased body temperature?
- shivering and vasoconstriction in blood vessels near the skin surface (correct)
- shivering and vasodilation in blood vessels near the skin surface
- sweating and vasodilation in blood vessels near the skin surface
- sweating and vasoconstriction in blood vessels near the skin surface

Q: Which process keeps conditions such as temperature and glucose concentration constant inside the body ?
- excretion
- homeostasis (correct)
- secretion
- vasodilation

Homeostasis

Q: The diagram shows skin temperature of a human when exposed to warm air and then exposed to cold air.  What causes the observed change in skin temperature on exposure to cold air ?
- less blood flowing just below the skin (correct)
- less blood going to the heart and lungs
- more blood flowing just below the skin
- more blood going to the heart and lungs

Homeostasis

Q: The diagram refers to the control of water concentration in the blood. Why is this a negative feedback system ?
- It decreases the amount of water in the blood.
- It increases any change in the amount of water in the blood.
- It increases the amount of water in the blood.
- It reverses any change in the amount of water in the blood (correct)
- It increases any change in the amount of water in the blood.

Homeostasis

Q: What helps heat retention in the human body ?
- actively secreting sweat glands
- dilated skin blood vessels
- fat in and under the skin (correct)
- relaxed hair erector muscles

Q: The diagram shows some nerve pathways involved in temperature control of the human body.  Which part of the nervous system does X represent ?
- cerebrum
- hypothalamus (correct)
- medulla
- pituitary gland

Homeostasis

Q: The graph shows changes in a person’s body temperature plotted against time. What causes the change in temperature between X and Y ?
- increased air temperature
- increased evaporation of sweat (correct)
- reduced blood flow through surface capillaries
- shivering

Homeostasis

Q: Which response to a drop in body temperature does not involve muscle contraction ?
- blood vessels narrowing
- hairs standing up
- shivering
- reduced sweating (correct)

Homeostasis

Q: A finger that touches a hot object is quickly taken away from the source of heat. What is the role of relay neurones in this response ?
- to carry nerve impulses within the spinal cord (correct)
- to generate impulses in the receptors of the finger
- to link the sense organs to the sensory neurones
- to pass nerve impulses out to the neurones