Hormonal coordination in humans

The endocrine system is composed 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.

Compared to the nervous system, the effects are slower but act for longer. That comparison is asked for constantly, so hold both halves of it.

The pituitary gland in the brain is a 'master gland' which 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.

Blood glucose concentration is monitored and controlled by the pancreas.

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

You should be able to interpret graphs showing the effect of insulin on blood glucose in people with and without diabetes. Read them the same way each time: find the meal, find how high the glucose rises, and find how long it takes to come back down. In a person without diabetes the peak is lower and the return is quicker, because insulin is released and acted on immediately.

Obesity is a risk factor for Type 2 diabetes, and questions here often ask for a recommendation as well as an explanation. Evaluating that means using the data on the relationship rather than assumptions about individuals, and recognising the social and ethical side: diet is shaped by cost, time and what food is available locally, so "tell people to eat better" is a weaker recommendation than one that changes what is easy to buy. Blame is not a treatment plan.

Water leaves the body by three routes, and only one of them is controlled.

• Via the lungs during exhalation. • From the skin in sweat, which also loses ions and urea. • Via the kidneys in the urine, which removes excess water, ions and urea.

There is no control over water, ion or urea loss by the lungs or the skin. You lose water when you breathe whether you can spare it or not, which is why the kidneys have to do all the adjusting.

Osmotic changes in body fluids act directly on cells. If body cells lose or gain too much water by osmosis they do not function efficiently: in fluid that is too dilute water enters and the cell swells, and in fluid that is too concentrated water leaves and the cell shrinks. Either way the reactions inside are disturbed, which is why water balance is regulated rather than left to chance.

The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water. Selective is the important word: everything small is filtered out first, and what the body still needs is taken back.

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

When you are given a table of glucose, ions and urea before and after filtration, the pattern is always the same: glucose disappears from the urine entirely because it is all reabsorbed, urea is concentrated because none is, and ions are somewhere in between depending on what the body needs.

Translating between the table and the bar chart is examined in both directions, and each form makes a different thing obvious. The table gives you the exact figures to calculate with. The chart shows the pattern at a glance: one bar disappearing entirely, one growing slightly, one growing enormously.

Reading the chart, say what each pair of bars means rather than describing their heights. Glucose falls to zero because all of it is reabsorbed. Urea rises steeply because none is reabsorbed while water is, so what remains is concentrated. Ions rise only somewhat, because some are reabsorbed according to what the body needs.

People who suffer from kidney failure may be treated by organ transplant or by a mechanical device — the dialysis machine. Evaluating the two against each other is a standard question, and the specification frames it exactly that way: mechanical device or transplant. In dialysis the patient's blood flows on one side of a partially permeable membrane, with dialysis fluid on the other. The fluid contains the normal concentrations of glucose and ions but no urea, so urea diffuses out of the blood down its concentration gradient, while glucose and useful ions do not — there is no gradient for them to move down. Dialysis has to be repeated regularly because it does nothing between sessions.

During puberty reproductive hormones cause secondary sex characteristics to develop.

Oestrogen is the main female reproductive hormone, produced in the ovary. At puberty eggs begin to mature and one is released approximately every 28 days — this is called ovulation.

Testosterone is the main male reproductive hormone, produced by the testes, and it stimulates sperm production.

Fertility can be controlled by hormonal and non-hormonal methods, and you should be able to evaluate them. The distinction the exam wants is which ones use hormones and which physically stop the sperm and egg meeting.

An evaluation weighs the evidence — how reliable each method is — against its implications, and the specification names four kinds: personal, social, economic and environmental. Condoms are the only method in the list that also protects against sexually transmitted infections, which is often the deciding point.

This is also a place where science does not settle the question. Whether a method is acceptable depends on religious belief, on a person's health and circumstances, and on whether they want children later. Science can say how reliable a method is; it cannot say which one a person should choose. Saying that explicitly, and then reaching a decision based on the evidence and the arguments, is what these questions are looking for.

Hormones are used in modern reproductive technologies to treat infertility. The simplest version is giving FSH and LH in a 'fertility drug' to a woman, after which she may become pregnant in the normal way.

In Vitro Fertilisation (IVF) goes further, and the four steps are worth learning in order:

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.

Although fertility treatment gives a woman the chance to have a baby of her own, the specification is direct about the drawbacks: it is very emotionally and physically stressful, the success rates are not high, and it can lead to multiple births, which are a risk to both the babies and the mother.

Evaluate it from both perspectives the exam names. For the patient: the chance of a child they could not otherwise have, against repeated stress, cost, and a real chance of failure. For the doctor: how much intervention is justified, at what odds, and the ethical question of what happens to embryos that are not used. Microscopy techniques had to develop before IVF was possible at all, which is a neat example of one science enabling another.

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, from the thyroid gland, stimulates the basal metabolic rate — the rate of the body's chemical reactions at rest. It plays an important role in growth and development.

The difference between them is a good illustration of the endocrine system's range: adrenaline is a short, sharp response to a situation, thyroxine a steady background setting.

Thyroxine levels are controlled by negative feedback. When thyroxine in the blood falls below normal, the pituitary releases more of the hormone that stimulates the thyroid, so more thyroxine is produced. When thyroxine rises above normal, that stimulating hormone is inhibited and less thyroxine is made.

The general shape is the thing to learn, because it recurs: too high triggers what lowers it, too low triggers what raises it, and the level oscillates around a norm rather than sitting perfectly still.