Hormonal coordination in humans

The human endocrine systemSpec 4.5.3.1

The endocrine system is made of glands which secrete chemicals called hormones directly into the bloodstream. The blood carries the hormone to a target organ where it produces an effect.

The pituitary gland in the brain is a ‘master gland’. It secretes several hormones into the blood in response to body conditions. These hormones in turn act on other glands to stimulate other hormones to be released to bring about effects.

Hormones from these glands that you need in this topic: the pituitary gland releases FSH and LH, the pancreas releases insulin, the ovaries release oestrogen and the testes release testosterone.

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Control of blood glucose concentrationSpec 4.5.3.2

Blood glucose concentration is monitored and controlled by the pancreas.

If the blood glucose concentration is too high, the pancreas produces the hormone insulin. Insulin causes glucose to move from the blood into the cells. In liver and muscle cells excess glucose is converted to glycogen for storage.

1. Blood glucose concentration rises (for example after a meal).
2. The pancreas detects this and produces insulin.
3. Insulin causes glucose to move from the blood into the cells.
4. In the liver and muscle cells, excess glucose is converted to glycogen.
5. Blood glucose concentration falls back to the normal level.

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Type 1 and Type 2 diabetesSpec 4.5.3.2

Type 1 diabetes is a disorder in which the pancreas fails to produce sufficient insulin. It is characterised by uncontrolled high blood glucose levels and is normally treated with insulin injections.

In Type 2 diabetes the body cells no longer respond to insulin produced by the pancreas. A carbohydrate controlled diet and an exercise regime are common treatments. Obesity is a risk factor for Type 2 diabetes.

Interpreting graphs

You may be given graphs showing blood glucose over time for a person with diabetes and a person without. In a person without diabetes, blood glucose rises after a meal and then falls back to the normal level as insulin is released. In a person with Type 1 diabetes who has not taken insulin, the glucose rises higher and stays high or falls much more slowly. If insulin is injected, the glucose falls.

Obesity increases the risk of Type 2 diabetes, so questions may ask you to evaluate information on obesity and diabetes and make recommendations. Think about social and ethical issues as well as the science, for example the cost to health services and whether people should be encouraged to change their diet.

Water balance and the effect of osmosis on cellsSpec 4.5.3.3Triple only

The body loses water in several ways.

• Water leaves the body via the lungs during exhalation.
• Water, ions and urea are lost from the skin in sweat.
• Excess water, ions and urea are removed via the kidneys in the urine.

There is no control over water, ion or urea loss by the lungs or skin. Only the kidneys can adjust the amounts lost, which is why they are central to controlling the water level.

Effect of osmotic changes on cells

Water moves into and out of cells by osmosis. If body cells lose or gain too much water by osmosis they do not function efficiently. If the fluid around the cells is too dilute, water moves into the cells; if it is too concentrated, water moves out of the cells.

The kidneys and ureaSpec 4.5.3.3Triple only

The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water. The waste urea and the excess water and ions leave in the urine.

1. Blood is filtered in the kidney. Water, glucose, ions and urea pass out of the blood.
2. Useful substances are selectively reabsorbed back into the blood: all of the glucose, some ions and some water.
3. The rest, including urea, excess ions and excess water, passes out as urine.

You do not need to know the other parts of the urinary system, the structure of the kidney or the structure of a nephron.

This kind of table can be shown as a bar chart. The glucose bar drops to zero in urine because all of it is reabsorbed. The urea bar is much higher in urine because water is reabsorbed while urea is not.

ADH and water levelSpec 4.5.3.3Triple onlyHigher tier

The water level in the body is controlled by the hormone ADH, which acts on the kidney tubules. ADH is released by the pituitary gland when the blood is too concentrated. It causes more water to be reabsorbed back into the blood from the kidney tubules. It does this by making the kidney tubules more permeable to water.

This is controlled by negative feedback: when the water level returns to normal, less ADH is released.

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Kidney failure: dialysis and transplantSpec 4.5.3.3Triple only

People who suffer from kidney failure may be treated by organ transplant or by using kidney dialysis.

How dialysis works

1. Blood from the patient flows into a dialysis machine.
2. The blood flows next to a partially permeable membrane. On the other side is dialysis fluid.
3. Urea and excess ions diffuse from the blood into the dialysis fluid. Glucose and other useful substances stay in the blood, because the dialysis fluid contains these at the same concentration as normal blood.
4. The cleaned blood returns to the patient.

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Hormones in human reproductionSpec 4.5.3.4

During puberty, reproductive hormones cause secondary sex characteristics to develop.

At puberty eggs begin to mature in the ovaries, and one is released approximately every 28 days. This is called ovulation.

Hormones in the menstrual cycle

Higher tier

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ContraceptionSpec 4.5.3.5

Fertility can be controlled by a variety of hormonal and non-hormonal methods of contraception.

Evaluating methods

• How effective it is when used correctly.
• Whether it can have side effects (hormonal methods can).
• Whether it is reversible. Sterilisation is permanent; the others are not.
• Whether it also protects against sexually transmitted infections. Condoms do; hormonal methods do not.
• How easy it is to use correctly, and how often it must be used or replaced.

Some issues around contraception cannot be answered by science alone. Personal, social, economic and religious views also affect whether and how people use contraception.

The use of hormones to treat infertilitySpec 4.5.3.6Higher tier

Hormones are used in modern reproductive technologies to treat infertility. A ‘fertility drug’ contains FSH and LH given to a woman. She may then become pregnant in the normal way.

In vitro fertilisation (IVF)

1. The mother is given FSH and LH to stimulate the maturation of several eggs.
2. The eggs are collected from the mother and fertilised by sperm from the father in the laboratory.
3. The fertilised eggs develop into embryos.
4. At the stage when they are tiny balls of cells, one or two embryos are inserted into the mother’s uterus (womb).

Evaluating fertility treatment

Fertility treatment gives a woman the chance to have a baby of her own. However:

• it is very emotionally and physically stressful
• the success rates are not high
• it can lead to multiple births, which are a risk to both the babies and the mother.

Adrenaline, thyroxine and negative feedbackSpec 4.5.3.7Higher tier

Adrenaline

Adrenaline is produced by the adrenal glands in times of fear or stress. It increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for ‘flight or fight’.

Thyroxine

Thyroxine from the thyroid gland stimulates the basal metabolic rate. It plays an important role in growth and development.

Negative feedback

Thyroxine levels are controlled by negative feedback. If the thyroxine level in the blood becomes too high, less thyroxine is released, so the level falls. If it becomes too low, more is released, so the level rises. A change in the level is reversed to bring it back to normal.

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