Clones are genetically identical individuals. Bacteria, plants and some animals can reproduce asexually to form clones that are genetically identical to their parent. Identical human twins are also clones: any differences between them are due to environmental factors.

Asexual reproduction only requires one parent, unlike sexual reproduction, which needs two. Since there is only one parent, there is no fusion of gametes, and no mixing of genetic information. As a result, the offspring are genetically identical to the parent, and to each other - so they are clones.

Plants

Asexual reproduction in plants can take a number of forms. Many plants developunderground food-storage organs that later develop into the following year’s plants. Potato plants and daffodil plants do this.

Daffodil bulb at start of season

Daffodil bulb at end of season

Strawberry plant with runners - stems growing sideways

Some plants produce side branches withplantlets on them. The Busy Lizzie plant does this. Others, such as strawberry plants, produce runners with plantlets on them.

Animals

Asexual reproduction in animals is less common than sexual reproduction but it does happen in sea anemones and starfish, for example.

Natural cloning

Twins are genetically identical because they are formed after one egg cell is fertilised but splits to form two embryos. They have the same genes. As the genes came from both parents they are not clones of either parent, but they are natural clones of each other.

Artificial cloning

It is possible to make clones artificially. The cloning of animals has many important commercial implications. It allows an individual animal that has desirable features, such as a cow that produces a lot of milk, to be duplicated several times.

There are two types of stem cells:

• adult stem cells - these are unspecialised cells that can develop into many (but not all) types of cells
• embryonic stem cells - these are unspecialised cells that can develop into any type of cell.

During the development of an embryo, most of the cells become specialised(cells with modifications to structure according to the task they have to perform). They cannot later change to become a different type of cell.

But embryos contain a special type of cell called stem cells. These embryonic stem cells can grow into any type of cell found in the body so they are not specialised. Stem cells can be removed from human embryos that are a few days old, for example, from unused embryos left over from fertility treatment.

Watch

You may wish to view thisBBC News item (2007) about a British research team that grew human heart valves from stem cells for the first time.

Watch

Here are some of the things stem cells could be used for:

• making new brain cells to treat people with Parkinson’s disease
• rebuilding bones and cartilage
• repairing damaged immune systems
• making replacement heart valves.

Therapeutic cloning

If you were to receive medical treatment with cells grown from stem cells, your body’s immune system would recognise the cells as foreign, and they would be rejected and die. But this would not happen if you received cells with the same genes as your own.

This could be done by cloning one of your cells to produce an embryo, then taking stem cells from this. This is called therapeutic cloning. Here are the steps involved:

1. nucleus taken out of a human egg cell
2. nucleus from a patient's cell put into the egg cell
3. egg cell stimulated to develop into an embryo
4. stem cells taken from the embryo
5. stem cells grown in a container of warm nutrients
6. stem cells treated to develop into required cell types.

Ideas about science - weighing up arguments

The cloning of human embryos is an issue which raises many questions and arguments. You need to be able to use your ideas about science to:

• Clearly state what the issues are. For example, some human embryos will be destroyed during the cloning process
• Summarise different views that might be held. For instance, some people think that destroying human embryos this way is murder. Others believe it is furthering our knowledge of science for the benefit of everyone
• Identify and develop arguments based on the idea that the right decision is the one that has the best outcome for the majority of the people involved. For example, even though a few embryos will be destroyed, many people will be free from suffering and cured of diseases that are currently incurable
• Identify and develop arguments based on the idea that certain actions arevery hard to justify because they are considered unnatural and wrong. For example, some people think that no matter what the benefits are of curing a disease, we should not clone embryos because destroying human life is always wrong.

Remember that these types of decisions are values, and raise questions that cannot be answered by science.

Embryo transplants

A developing embryo is removed from a pregnant animal at an early stage, before its cells have had time to become specialised. The cells are separated, grown for a while in a laboratory then transplanted into host mothers.

When the offspring are born, they are identical to each other and to the original pregnant animal. They are not identical to their host mothers because they contain different genetic information.

Fusion cell cloning

Fusion cell cloning involves replacing the nucleus of an unfertilised egg with one from a different cell. The replacement can come from an embryo. If it is from an adult cell, it is called adult cell cloning.

'Dolly the sheep' was the first mammal to be cloned using adult cell cloning. She was born in the UK in 1996 and died in 2003. Here is how she was produced:

1. An egg cell was removed from the ovary of an adult female sheep, and its nucleus removed
2. The nucleus from an udder cell of a donor sheep was inserted into the empty egg cell
3. The fused cell then began to develop normally, using genetic information from the donated DNA
4. Before the dividing cells became specialised, the embryo was implanted into the uterus of a foster mother sheep. The result was Dolly, who was genetically identical to the donor sheep.

The cloning process of 'Dolly the sheep'

The body’s first line of defence against harmful pathogens is the skin and stomach acid, the second is white blood cells. Vaccination involves exposing the body’s immune system to a weakened or harmless version of the pathogen in order to stimulate white blood cells to produce antibodies.

Pathogens

Pathogens are microorganisms that cause infectious disease. Bacteria and viruses are the main pathogens.

Bacteria

Salmonella bacterium cell

Bacteria come in many shapes and sizes, but even the largest are only ten micrometres long (ten-millionths of a metre).

Bacteria are living cells and can multiply rapidly in favourable conditions. Once inside the body, they release poisons or toxins that can make us feel ill.

Viruses

A hepatitis C virus showing DNA enclosed in a protein coat.

Viruses are many times smaller than bacteria. They are among the smallestorganisms known, consisting of a fragment of genetic material inside a protective protein coat.

Viruses can only reproduce inside host cells, damaging them when they do so. Once inside, they take over the cell and make hundreds of thousands of copies of themselves. Eventually, the virus copies fill the whole host cell and it bursts open. The viruses then pass out through the bloodstream, the airways, or by other routes.

In the correct conditions (with warmth, moisture, nutrients) bacteria can multiply rapidly. The human body can provide these conditions for bacteria to multiply, for example in a cut. Each bacterium splits into two up to every 20 minutes.

So, after one hour a single bacterium could have reproduced to give eight bacteria.

microorganisms reproduce quickly in the human body

When microorganisms enter the body, they release toxins. The toxins damage cells to cause the symptoms of the disease. The body’s first line of defence is its natural barriers which include:

• skin
• chemicals in tears
• chemicals in sweat
• stomach acid.

White blood cells

The body’s first line of defence is called passive immunity, which means preventing the pathogen from entering in the first place. If a pathogen manages to get into the body, the second line of defence takes over, which is calledactive immunity. The white blood cells have key functions in this.

Functions of the white blood cells

White blood cells can:

• ingest pathogens and destroy them
• produce antibodies to destroy pathogens
• produce antitoxins that neutralise the toxins released by pathogens.

A white blood cell ingesting disease-causing bacteria.

In a written examination, it is easy to get carried away with metaphors about invaders and battles: stick to the point. Note that:

• the pathogens are not the disease, theycause the disease.
• white blood cells do not eat the pathogens, they ingest them.
• antibodies and antitoxins are not living things, they are specialised proteins.

Memory cells

Memory cells are a type of white blood cell that can respond quickly when it meets a microorganism for the second time. They produce the right antibody for the particular microorganism and destroy it before you feel unwell. This is described as being immune to a disease.

Mutation rate

On average an adult human will catch two to four colds every year, and a child will catch even more (because their immune system is still developing. This happens for two main reasons:

• there are hundreds of different cold viruses
• the viruses have a high mutation rate

A high mutation rate means that the DNA of the virus frequently changes, so the human body has to destroy the virus using a different antibody each time, which makes it harder to develop immunity.

There are several types of white blood cell, each with a different function. But there are two main groups. The first of these groups surrounds and digests bacteria. They can pass easily through blood vessel walls into the surrounding tissue, and move towards pathogens or toxins.

The second produces antibodies to label microorganisms. Pathogens contain certain chemicals that are foreign to the body. These are called antigens. Each of this group of white blood cell carries a specific type of antibody - a protein with a chemical ‘fit’ to a certain antigen. When a white blood cell with the appropriate antibody meets the antigen, the white blood cell reproduces quickly to make many copies of the antibody that neutralises the pathogen.

People can be immunised against a pathogen through vaccination. Different vaccines are needed for different pathogens.

Vaccination involves putting a small amount of an inactive form of a pathogen into the body. Vaccines can contain:

• live pathogens treated to make them harmless
• harmless fragments of the pathogen
• toxins produced by pathogens
• dead pathogens.

These all act as antigens. When injected into the body, they stimulate white blood cells to produce antibodies to fight the pathogen.

The vaccine contains only a weakened or harmless version of a pathogen, which means that the vaccinated person is in no danger of developing the disease. Some people, however, may suffer a mild reaction. If the person later becomes infected with the pathogen, the required (white blood cells) are able to reproduce rapidly and destroy it.

Vaccines and boosters

Vaccinations in early childhood can offer protection against many serious diseases. Sometimes more than one vaccine is given at a time, like the MMR triple vaccine against mumps, measles and rubella.

Sometimes vaccine boosters are required because the immune response ‘memory’ weakens over time. Anti-tetanus injections may need to be repeated every ten years, for example.

Ideas about science - making decisions

There is often a conflict between a person’s right to decide what is best for themselves and their family and what is best for society as a whole. For example, some people used to think the MMR (measles, mumps or rubella) vaccine could cause autism in children. They decided not to risk letting their child have the vaccine and hoped they would not catch any of the three diseases. But this meant that as less and less children were vaccinated the diseases began to spread more easily and the number of cases began to increase. Therefore, a decision originally taken by a number of single individuals had big implications for society as a whole.

Vaccinations can never be completely safe because side-effect levels vary. So, when making a decision, these are some of the factors that should be considered:

• when fewer people are vaccinated, the number of cases of the disease increases
• the chance of falling seriously ill or dying from the disease may be far greater than the chance of experiencing a serious side-effect
• using a vaccine may be much cheaper than treating a very ill person.

Viruses

A hepatitis C virus showing DNA enclosed in a protein coat.

Some common diseases like influenza (flu) and the common cold are caused by viruses. These mutate quickly, and this changes their surface proteins. This makes it almost impossible to develop a permanent vaccineagainst them. A new flu vaccine has to be developed every year, after the strain has been analysed.

There is no vaccine for the common cold because the virus that causes it mutates far too quickly. By the time a vaccine could be developed, the virus would have changed its surface proteins and would no longer be recognised by the antibodies.

Policies

Watch

You may wish to view thisBBC News item (2006) about the recommendation that pregnant women should be offered the flu vaccine in winter.

Watch

The government has policies on vaccination which advises which stage in their life people should be vaccinated against different diseases. The policies and advice are updated as and when new scientific information becomes available.

Ideas about science - weighing up arguments

With respect to vaccination policies, you need to be able to:

• Clearly state the issue. For example, is the risk of suffering side-effects from the vaccination greater or less than the risk of catching the disease?
• Summarise different views that might be held. For example, some people used to think there was a risk of children developing autism when they had the MMR vaccine. Other people thought the MMR vaccine was safe and there was no risk of developing autism.
• Identify and develop arguments based on the idea that the right decision is the one that leads to the best outcome for the majority of people. For example, even though there may be a slight risk from being vaccinated, society as a whole will benefit because it will help to reduce the risk of the disease being passed on to other people.
• Identify and develop arguments based on the idea that certain actions are very hard to justify because they are considered unnatural or wrong. For example, most people think governments should not pass laws making vaccination compulsory, because that would take away our human right to freedom of choice.

Epidemics

During an epidemic, an infectious disease such as influenza spreads very quickly. Epidemics can be prevented if a high proportion of the population has been vaccinated. This reduces the number of people who are able to catch the disease and pass it on to others. The more infectious the disease, the higher the proportion of the population that must be vaccinated to prevent the epidemic.

Ideas about science - feasibility

With respect to vaccination policies, you need to be able to distinguish what canbe done, ie what is technically feasible, from what should be done. For example, smallpox is the only disease that has been eradicated from the planet by vaccination. This was possible because smallpox is spread by direct contact, and not through the air.

This made it possible to vaccinate enough people in the world to completely stop the disease from spreading.

Some other diseases are more infectious but if we could vaccinate a sufficient number of the world’s population we could, in theory, eliminate the disease. However, at the moment this is not technically feasible because we do not have enough vaccine, some areas of the world are at war and inaccessible, and some people would refuse to be vaccinated.

Antibiotics are substances that kill bacteria or prevent their growth. They do not work against viruses. It is difficult to develop drugs that kill viruses without damaging the body’s tissues.

Penicillin

A bacterium damaged and distorted by penicillin

The first antibiotic, penicillin, was discovered by Alexander Fleming in 1928. He noticed that some bacteria he had left in a Petri dish had been killed by naturally occurring penicillium mould.

Since the discovery of penicillin, many other antibiotics have been discovered or developed. Most of those used in medicine have been altered chemically to make them more effective and more safe for humans.

Over time, bacteria can become resistant to certain antibiotics: this is an example of natural selection. In a large population of bacteria, there may be some that are not affected by the antibiotic. These survive and reproduce, creating more bacteria that are not affected by the antibiotic.

Superbugs

Mutations in bacteria can result in them becoming resistant to antibiotics, turning the bacteria into a ‘superbug’. Superbugs can develop while a person is taking a course of antibiotics.

MRSA

MRSA is methicillin-resistant Staphylococcus aureus. It is very dangerous because it is resistant to most antibiotics. To slow down or stop the development of other strains of resistant bacteria, we should:

• always avoid the unnecessary use of antibiotics
• always complete the full course

Tuberculosis

Watch

You may wish to view this BBC News item (2007) about how drug-resistant strains of TB are putting European Union states at risk of a deadly outbreak.

Watch

Tuberculosis (TB), is a disease caused by a bacterium called Mycobacterium tuberculosis. Most people who are infected do not show any symptoms. But about 10 per cent go on to develop serious symptoms including shortness of breath, coughing, fever and even death.

Infected people without symptoms are usually given a course of one antibiotic. Those who show symptoms need a course of several antibiotics at once. This is to reduce the chance of strains of antibiotic-resistant bacteria emerging.

Development of resistance - Higher tier

The main steps in the development of resistance are:

1. Random changes or mutations occur in the genes of individual bacterial cells
2. Some mutations protect the bacterial cell from the effects of the antibiotic
3. Bacteria without the mutation die or cannot reproduce with the antibiotic present
4. The resistant bacteria are able to reproduce with less competition from normal bacterial strains

Drugs and their origins

Drugs are substances that cause changes to the body. Some can help the body, others can harm it.

Certain drugs can be extracted from natural sources and their existence has been recognised for a long time. For example, willow bark was used by the ancient Greeks to help cure fevers and pains. It was later discovered that the active ingredient was salicylic acid. This was modified by chemists into the substance we call aspirin, which is less irritating to the stomach than salicylic acid.

Three stages of testing drugs

New medical drugs have to be tested to ensure that they work, and are safe, before they can be prescribed. There are three main stages of testing.

1. The drugs are tested using computer models and human cells grown in the laboratory. Many substances fail this test because they damage cells or do not seem to work.
2. Drugs that pass the first stage are tested on animals. In the UK, new medicines have to undergo these tests. But it is illegal to test cosmetics and tobacco products on animals. A typical test involves giving a known amount of the substance to the animals, then monitoring them carefully for any side-effects.
3. Drugs that have passed animal tests are used in clinical trials. They are tested on healthy volunteers to check they are safe. The substances are then tested on people with the illness to ensure they are safe and that they work.

Medical drug trials

Watch

You may wish to view thisBBC News item (2006) about a drug trial that left six volunteers very ill.

Watch

Medical drug trials are not without risk. Sometimes severe and unexpected side-effects occur.

Most substances do not pass all of the tests and trials, so drug development is expensive and takes a long time.

It is important that the results of clinical trials are not influenced by the expectations of the people involved. So volunteers are put into two groups at random. Checks are done to make sure both groups have a similar gender balance and age range.

There are three main types of clinical trial: 'blind', 'double-blind' and 'open-label'. In blind and double-blind trials one group of volunteers, called the test group, receives the new drug. Another, the control group, receives the existing drug for that illness. If there is no existing treatment, the control group is given a fake drug that has no effect on the body. This is called a placebo. The researchers look for differences between the experimental group and the control group. In an open-label trial there are some differences.

Blind trials

In a blind trial, the volunteers do not know which group they are in but the researchers do. The problem is the researchers may give away clues to the volunteers without realising it. This is called observer bias; it can make the results unreliable.

Double-blind trials

In a double-blind trial, the volunteers do not know which group they are in, and neither do the researchers, until the end of the trial. This removes the chance of bias and makes the results more reliable. But double-blind trials are more complex to set up.

Open-label trials

In an open-label trial the patient and doctor both know the treatment. This type of trial happens when there is no other treatment and the patients are so ill that doctors believe they will not recover from their illnesses.

Placebos

Many doctors do not like giving a placebo to patients with a disease because they feel the patient will not benefit from taking a fake drug and will not get better. They do not think this is fair to the patient.

The heart requires its own constant blood supply in order to keep beating and this is delivered through the coronary arteries. Genetic and lifestyle factors can lead to the coronary arteries becoming blocked, and an increased risk of heart disease.

The circulatory system

The circulatory system

Blood carries oxygen and nutrients to the body’s cells, and waste products away from them. The circulatory system consists of:

• the heart, which is the muscular pump that keeps the blood moving
• the arteries, which carry blood away from the heart
• the veins, which return blood to the heart
• the capillaries, which are tiny blood vessels that are close to the body’s cells.

The diagram outlines the circulatory system. To make things clear, oxygenated blood is shown in red, and deoxygenated blood in blue.

The arteries carry blood from the heart while veins return blood to it. With both, their structure is related to their function.

Arteries

Blood in the arteries is under high pressure generated by the heart. The arteries have:

• thick outer walls
• thick layers of muscle and elastic fibres.

Veins

The blood in veins is under lower pressure than the blood in arteries. The veins have:

• thin walls
• thin layers of muscle and elastic fibres.

Unlike arteries, veins have one-way valves in them to keep the blood moving in the correct direction.

Cross-section of a vein

Capillaries

Capillary cross-section shows thin walls

The function of capillaries is to allow food and oxygen to diffuse to cells while waste is diffused from cells. Capillaries have thin walls - only one cell thick - that allow them to effectively perform their function.

Sometimes the heart has to work harder: for example, when it becomes clogged up with fatty deposits.

One way to check how hard the heart is working is to measure your pulse rate(usually taken on the inside of your wrist); this measures the number of times your heart beats per minute.

Another, more accurate way of checking how hard your heart is working is measuring blood pressure. This records the pressure put on the walls of the artery by the blood. The measurement is recorded as two numbers, for example 140/90. The higher number (140) show the pressure when the heart is contracting. The lower number shows the pressure when the heart is relaxing.

'Normal' measurements for blood pressure and heart rate are given within a range

High blood pressure is the biggest concern as it increases the risk of heart attack, but low blood pressure can sometimes be dangerous too.

The heart is a muscular organ. It keeps beating at about 70 times per minute. You can see how it pumps the blood to the lungs and the rest of the body by studying this animation.

The muscle cells in the heart need a constant supply of oxygen and nutrients, and for their waste products to be removed. So the heart requires its own blood supply in order to keep beating.

Blood vessels called the coronary arteries supply blood to the heart muscles. If they become blocked, a heart attack can happen.

Heart attacks

A heart attack can happen because:

1. Fatty deposits build up in the coronary arteries
2. A blood clot can form on a fatty deposit
3. The blood clot can block a coronary artery
4. Some heart muscle cells do not get the oxygen and nutrients they need
5. These cells start to die.

In the UK about 300,000 people have a heart attack every year.

Heart disease is not usually caused by microorganisms - it is usually caused by:

• genetic factors, which show as a family history of heart disease
• lifestyle factors.

Heart disease is more common in the UK than in non-industrialised countries, and many other industrialised nations. This is due to lifestyle factors including:

• smoking
• lack of regular exercise
• stress leading to a fast heart rate
• drinking a lot of alcohol
• poor diet
• misuse of drugs.

Watch

You may wish to view thisBBC News item (2006) about research showing that people with heart disease have arteries that are biologically up to 40 years older than they are.

Watch

A lack of exercise and a diet that is high in salt and saturated fat cause people to:

• become overweight
• have high blood pressure
• have high levels of cholesterol in their blood.

These factors contribute to an increased risk of heart disease.

Epidemiology is the study of factors affecting the health and illness of populations. Doctors and scientists carry out epidemiological studies to try to determine the lifestyle factors that could increase the chances of getting heart disease and other illnesses. When carrying out a study, it would be morally wrong to get people to smoke to find out if smoking causes lung cancer. However, there are people who already smoke and their health can be compared with that of non-smokers.

Watch

You may wish to view thisBBC News item (2005) about a study from Norway. It showed that smoking just one to four cigarettes a day almost triples the risk of dying from heart disease.

Watch

In a typical cohort study, researchers examine two groups of people: those with an illness, and those without. They can do this by:

• following the cohort’s progress over several months or years
• looking at their medical records.

For example, the health of smokers and non-smokers was followed for many years. It was found that, on average, smokers die ten years earlier.

Correlation and cause

Using the example of how lifestyle factors can affect the risk of heart disease, you need to be able to:

• Give an example of a correlation between a factor and an outcome. For example, wealthier nations have higher levels of heart disease
• Suggest factors that might increase the chances of an outcome, but not necessarily determine an outcome. For example, the more cigarettes a person smokes, the more likely they are to develop heart disease. However, some people who smoke never go on to develop heart disease
• Explain that individual cases do not provide convincing evidence for or against a correlation. For instance, we might hear someone say, "Old George smoked 60 cigarettes every day of his life and lived to be 90." However, we know that ‘Old George’ was just lucky, because smoking 60 cigarettes a day seriously reduces your chances of living into old age. A correlation exists between life expectancy and the number of cigarettes smoked each day
• Evaluate how a study is designed by commenting on the sample size and how well the sample is matched. For example, to show that eating fatty foods increases the chances of developing heart disease, we need to compare a large sample size of people who eat fatty food with an equally large sample size of people who do not. The two samples also need to be chosen at random, or matched as closely as possible for other factors such as age, sex and so on.

Higher tier

Using the example of how lifestyle factors can affect the risk of heart disease, you need to be able to:

• Explain why a correlation between a factor and an outcome does not necessarily mean that one causes the other. For example, wealthier nations have higher levels of heart disease. However, this does not mean that wealth is a cause of heart disease, even though a correlation between the two exists. To find the cause, we need to have more scientific data
• Use data to show that a factor does, or does not, increase the chances of an outcome. In the examination, this data may be presented in the form of tables or charts
• Identify a mechanism that could be a causal link between a factor and an outcome. For instance, cigarette smoke contains chemicals that, when breathed in, can increase blood pressure and lead to heart damage. The chemicals in the smoke are the mechanism of the causal link.

The scientific community

When scientific data has been collected, it is important that this data and any conclusions are reviewed by other scientists. This is called 'peer review'.

Using the example of how lifestyle factors can affect the risk of heart disease, you need to be able to:

• describe the peer review process. For instance, state that other scientists repeat the experiment to see if they get similar results and conclusions
• understand that new scientific claims are less reliable if they have not yet been evaluated by peer review. For example, remarkable claims are being made for 'superfoods', such as that they prevent heart disease and increase longevity. However, many of the claims have not yet been peer reviewed, so they should not be treated as totally reliable
• understand that if the results of an investigation have not been replicated by other scientists, they should be treated with suspicion.

Higher tier

Using the example of how lifestyle factors can affect the risk of heart disease, you need to be able to explain why scientists think it is important that a claim by one scientist should be replicated by other scientists. This is because the first set of results may contain errors. When scientific research is being paid for by a company with a vested interest, such a food company producing a food that is claimed to lower cholesterol, the results that support the claim need to be examined and replicated to make sure that they are correct.

Our bodies need to control things like body temperature and water level to keep them constant. The process of keeping things the same is called homeostasis.

Homeostasis

Cells in the body need certain conditions to work properly. Homeostasis is how the body keeps internal conditions the same.

Two examples of things that the body keeps the same are:

• body temperature at 37°C
• the amount of water inside our body.

Keeping these two the same is not always easy when the outside environment is changing constantly. But it is important so that all our cells function properly.

Strenuous exercise, or living in a hot or cold environment, affect our body temperature and water balance.

We have control systems in our bodies to maintain a steady state.

• first, we need receptors to detect when things such as temperature change
• then we need a processing centre to receive this information and coordinate our response.

Veins

The blood in veins is under lower pressure than the blood in arteries. The veins have:

• thin walls
• thin layers of muscle and elastic fibres
• Finally, we need effectors to produce a response that ensures our body temperature stays at 37°C.

Example

It is easier to understand how this works by using a model. Think of an incubator in a premature baby unit:

• The incubator needs sensors to monitor the temperature. It also requires a computer or processing centre to monitor and process the data from the sensors and switch the heater on or off.
• When the incubator is too cold, the heater switches on. When it is too hot, the heater switches off.
• In this way, an almost constant temperature is maintained within the incubator.

Negative feedback ensures that, in any control system, changes are reversed and returned back to the set level.

For example, negative feedback keeps our body temperature at a constant 37°C. If we get too hot, blood vessels in our skin become larger and we lose heat and cool down. If we get too cold blood vessels in our skin become smaller, we lose less heat and our body warms up. Negative feedback makes sure this happens.

The other factors also controlled in the body by negative feedback are:

• blood oxygen levels
• salt levels.

The kidneys maintain our body's water balance by controlling the water concentration of blood plasma. The kidneys also control salt levels and the excretion of urea. Water that is not put back into the blood is excreted in our urine.

Our bodies take in water from food and drinks. We even get some water when we respire by burning glucose to release energy.

We lose water in sweat, faeces, urine and when we breathe out (on a cold day you can see this water as it condenses into vapour).

For the cells of our body to work properly, it is important that their water content is maintained at the correct level. This means our body must maintain a balance between the water we take in and the water we lose. This is done by the kidneys.

How is water balance maintained?

The kidneys maintain our water balance by producing urine of different concentrations.

When the water level of our blood plasma is low,more water is reabsorbed back into the blood and the urine becomes more concentrated.

When the water level of our blood plasma is high, less water is reabsorbed back into the blood and our urine is more dilute.

The level of water in the blood plasma can vary depending on:

• External temperature - when it is hot, we sweat more and lose water thereby making the blood plasma more concentrated.
• Amount of exercise- if we exercise, we get hot and increase our sweating so we lose more water and the blood plasma becomes more concentrated.
• Fluid intake - the more we drink the more we dilute the blood plasma. The kidneys respond by producing more dilute urine to get rid of the excess water.
• Salt intake - salt makes the plasma more concentrated. This makes us thirsty and we drink more water until the excess salt has been excreted by the kidneys.

Drugs that affect water balance

Alcohol

Alcohol causes the kidneys to produce a greater volume of more dilute urine. This can lead to dehydration.

Ecstasy

Ecstasy causes the kidneys to produce a smaller volume of less dilute urine. This can result in the body having too much water.

The concentration of our urine is controlled by a hormone called ADH.

ADH is produced by the pituitary gland that is situated just below the brain. The pituitary gland monitors the concentration of the blood plasma. It releases ADH into the bloodstream, which travels in the blood to the kidneys.

How it works

The more concentrated the plasma, the more ADH is released into the blood. When the ADH reaches the kidneys, it causes them to reabsorb more water. This keeps more water in the body and produces more concentrated urine.

When the plasma is more dilute, less ADH is released into the bloodstream. This allows more water to leave the kidneys, producing more dilute urine.

This method of control is an example of negative feedback.

How alcohol and ecstasy affect ADH

Alcohol suppresses ADH production. This causes the kidneys to produce more dilute urine. It can lead to dehydration.

Ecstasy increases ADH production. This causes the kidneys to reabsorb water. It can result in the body having too much water.