[II] Biology - B2

Our bodies have a range of ways to defend us against the invasion of pathogens. The first line of defence is external: our skin with its ability to heal itself, our saliva and our tears. These body fluids, and also stomach acid, either wash away invading microorganisms or destroy them before they can do any damage.

If a pathogen gets past the external defences and into our bodies, then the internal system of defence, called the immune systen, starts to work. This system attacks pathogens by using several different types of white blood cell. 

These white blood cells are made up in the bone marrow. There are two different mechanisms used to destroy pathogens that enter the blood.

Some white blood cells engulf microorganisms and digest them.

Another type of white blood cell produces antibodies that recognise and destroy particular microorganisms.

White blood cells "engulf" and "digest" invading microrganisms but they don't "eat" them.

The white blood cells that engulf and digest microorganisms attack any invader. The response of the white blood cells that produce antibodies is targeted at particular pathogens.

Pathogens have proteins in their cell membranes that our immune systems recognise as foreign. These proteins are called antigens. Specific antibodies are made for each antigen by the white blood cells.

Antibodies attach to the antigens on the surface of the microorganism. This inactivates the microorganisms and makes them clump together. They can then be engulfed by other white blood cells or destroyed.

If the immune system has encountered a particular pathogen before, it can respond very quickly to the threat. The immune system recognises pathogens that it has encountered because the antigens in the pathogen's cell membrane stay the same.

If an antigen is recognised, cells of the immune system called memory cells are able to rapidly produce large numbers of antibodies to the pathogen and disable or destroy it before it s able to take hold in the body. When this occurs, the person is said to be immune to that pathogen.

The immune system is one of the ways that the body defends itself against disease. One of the parts of the immune system is the white blood cells that make antibodies to the proteins on the surface of pathogens. These proteins are called ANTIGENS. Special cells called memory cells are able to produce vast amounts of appropriate antibodies when they find an antigen the body has never met before. This is a huge advantage because the disease can be stopped before it has damaged the body, and often before the person even knows they have contracted it.

A VACCINE contains a safe form of the microorganism that causes a disease. Once the vaccine is in the body, the immune system attacks the vaccine and develops memory cells against the antigens it carries. These memory cells respond very quickly if they meet the real disease-causing pathogens.

Babies and children undergo a course of vaccinations in their first year and as they prepare to go to school. Vaccines usually contain a weak or non-living form of the pathogen to ensure that the vaccination itself does not make the patient ill. Some pathogens are very stable, with antigens that do not change much from year to year. The vaccines against these pathogens can stay the same for a long period of time. Other pathogens, such as the flu virus, change rapidly and new vaccines must be developed every year to make sure the body will recognise the altered antigens on the pathogen. A vaccination from the previous year would not protect someone. 

Epidemics can occur when diseases spread rapidly through populations. The flu epidemic of 1918 killed an estimated 50 million people. Because many diseases have become much less common as a result of widespread vaccination, some parents are now less concerned about whether or not their own children are vaccinated. In addition, people worry about how safe vaccines are. A scare that was begun by a very small study of 12 children publishes in the Lancet medical journal stirred up media interest in vaccine safety. The study and the doctor concerned have since been discredited, but many people stopped having their children vaccinated.

The danger of fewer children being vaccinated is that, if the disease were to begin to spread through the population, the unvaccinated children would be much more likely to contract the illness and to pass it on to other unvaccinated and otherwise susceptible people.

To avoid an epidemic, it is necessary for a high percentage of the population to be vaccinated. The population then has "herd immunity".

Childhood diseases like measles,mumps and rubella can cause particular problems for children, pregnant women and the elderly and can cause infertility, deafness and even death. The World Health Organisation states that measles is one of the leading causes of death among young children, even though a safe and cost-effective vaccination is available.

Scientists go to great lengths to make sure that vaccines and prescripton drugs are as safe as they can be. Prescription drugs and vaccinations, if used in the right way, are very safe.

Hardly anything is without some degree of risk, even if that risk is very small. However the perception of risk of some prescription drugs and vaccinations is higher than the actual risk. There are many reasons for this. Research has shown that people tend to be more concerned about risks from technology, such as vaccination or nuclear power, than they are about "natural risks", such as heart disease. Heart disease is the number one cause of death worldwide, yet millions of people do not take actions such as giving up smoking or maintaining a healthy weight to prevent it.

When a new medicine or vaccine is being developed, it is tested very carefully to make sure it is safe and to find out if there are any side effects. These are effects that are no wanted and may be harmful or unpleasant.

Some individuals may have more side effects from prescription drugs than others, due to genetic variation.

Vaccinations are extremely safe and hundreds of millions of children have benefitted from the immunity from disease that they provide. However, in a number of cases, a child may suffer a minor adverse reaction from the vaccination, such as a rash or mild fever. In very rare cases, the reaction may be more serious. Adverse reactions are carefully documented and followed up. Because of the huge number of children that have been vaccinated (estimated over 260million+), it is possible for scientists to be very confident of the safety of each vaccine. Vaccine and drugs that, despite clinical testing, seem to be generating an unusual number of adverse reactions, are quickly withdrawn.

ANTIMICROBIALS are a group of substances that are used to kill microorganisms or slow their growth. They are effective against bacteria, fungi and viruses. Anti-biotics, which are effective against bacteria but not against viruses, are a type of antimicrobial. Antibiotics transformed medicine when they first became available, allowing doctors to treat a variety of common and serious illnesses caused by bacteria, such as tuberculosis. There has been concern about microorganisms developing resistance to these chemicals. Resistance is thought to have come about due to the very wide use of antimicrobials. Resistance to antimicrobials has already led to some strains on bacteria that are difficult to control - most types of antimicrobial are ineffective against them. Anti-microbial resistant microbes are a particular problem when antimicrobials are used frequently, such as in hospitals.

The word antimicrobial refers to a group of chemicals that are effective in destroying microorganisms. There are many different types of antimicrobial, and not all of them are effective against all organisms. Antibiotics are effective against bacteria, antifungals against fungi and antivirals against viruses.

Antibiotics are effective against bacteria infection, but not against viruses. This means they are of no use against common viruses such as cold and flu, unless a bacterial infection develops. This is why doctors do not prescribe antibiotics for the flu. The drugs would have absolutely no effect.

Bacteria and fungi may develop resistance to antimicrobials. Resistant microorganisms are not easily killed by antimicrobial substances. The normal population of microorganisms contains come that are resistant to the antimicrobial. When we use the antiicrobial, all those that are susceptible will be destroyed, leaving the microorganisms that are more resistant to antimicrobials. These resistant microorganisms then reproduce, resulting in a more resistant population. The more the antimicrobial is applied, the more and more resistant the remaining microorganisms are. These extremely resistant microbes are known as superbugs.

Microbial resistance is not only caused by the use of antimicrobials for cleaning, but also when antibiotic drugs are not used correctly by patients. A patient who has an infection that has become resistant to antibiotics is in a very serious position and such infections can be fatal.

There are a number of measures that can be taken to reduce the rate at which antibiotic resistance develops. Antibiotics should only be prescribed if they are really needed, so people should never take antibiotics that are not meant for them. The course of antibiotics should be taken as directed, and patients should take all the tablets they are told to.

Bacterial resistance to antibiotics is a particular problem that scientists are working hard to resolve. As the number of types of antibiotic used has increased, so the incidence of antibiotic resistance has risen. Scientists call this a correlation. Further work has to be done to determine whether there is actually a "cause and effect" relationship when a correlation like this is found. In the case of antibiotic resistance, studies have found that there is a cause and effect relationship to this correlation. The use of antibiotics, particularly if they are not used correctly, actually causes antibiotic resistance to develop. Antibiotic resistance has led to some strains on bacteria that are extremely difficult to eradicate. Random changes (mutations) in the genes of microorganisms can allow them to survive the initial application of the antimicrobial. These microorganisms are able to reproduce, thus spreading the mutation for resistance.

Before new medicines, treatments and vaccinations are made available to everyone, they are very carefully tested. The initial stages of the testing involve trying out the substance on animals and on samples of human cells to see what the effects of the drug are and to check for toxicity (whether it has any poisonous effects). Medicines and vaccinations that are likely to cause severe side effects do not make it past this stage of testing.

If the drugs seem safe at this stage, they are then tested on humans in CLINICAL TRIALS. There are two main types of clinical trial. One type tests the drugs on healthy participants and closely monitors them for side effects to ensure that the drug is safe. The participants in clinical trials are usually paid for taking part. The other type of trial uses participants who are suffering from a condition and have consented to being part of a trial. The trial will test a new drug or treatment for how well it treats the disease as well as safety.

Clinical trials can be carried out in a single location or in clinics in several locations. For each trial there are very detailed instructions that explain the method that must be followed. This allows the results of trials in several different clinics to be combined.

When drug tests are carried out to compare an existing treatment with a new one, some participants receive the existing treatment - they are the CONTROL GROUP. The other participants receive the new treatment. Someetimes, drug tests are carried out that compare the effects of a drug against a PLACEBO. A placebo is a tablet or solution made to look just like the drug being tested, but without the active ingredient. In this case, the control group is given the placebo. 

There are ethical issues to consider when placebos are used in clinical trials. For example, should a placebo ever be used in a clinical trial on patients who are suffering from a disease when an effective treatment for that disease exists? There are guidelines in place to ensure patients are not disadvantaged by taking part in a clinical trial.

In an OPEN-LABEL trial, both researchers and patients know which drug the patient is receiving. A BLIND study is one in which the patients do not know which drug they are receiving, but the researchers do know. In a DOUBLE-BLIND trial, neither patients nor researchers know who is receiving the trial drug and who is receiving an existing treatment or placebo. Some trials look at the effects of a drug taken over a long period of time. This is important as some of the side effects may increase or appear over time, or the drug may become less effective as the body deals with it more effectively.

The cells in the body receive the nutrients and oxygen they need from the blood. The blood also removes waste products from the cells. The blood is pumped around the body through a system of specialised vessels by the heart. The heart and the blood vessels make up the CIRCULATORY SYSTEM. The heart is a specialised pump called a double pump.

Blood is pumped independently to the lungs and the rest of the body.

Arteries carry blood away from the heart.

The left side of the heart receives oxygenated blood afrom lungs and pumps it to the rest of the body.

The right side of the heart receives deoxygenated blood from the body and pumps this to the lungs, where it absorbs oxygen and gets rid of carbon dioxide.

Veins carry blood back to the heart.

There are three types of blood vessels in the body: arteries, veins and capillaries. Because the pressure of blood being forced from the heart as it pumps is high, arteries have very thick, elastic muscular walls to withstand the pressure. 

Gradually, the arteries branch into smaller and smaller vessels until blood cells can only just fit through. These tiny vessels are called capillaries. Because they are so narrow, the blood cells travel slowly and because they are squashed against the capillary walls, which are very thin, maximum transfer of substances such as oxygen can occur across the capillary walls.

Capillaries join together again into larger vessels now containing blood with the oxygen removed and with a higher amount of carbon dioxide and other waste products. These larger vessels carry blood back to the heart and are called veins. 

Because the blood is at low pressure, veins do not need thick walls like arteries. Veins contain valves to prevent the slow-moving blood going backwards or "pooling" in the lower parts of the body. As the large muscles of the body are moved, blood is squeezed through the veins and back to the heart. 

Heart muscle cells can be seen to twitch in very tiny embryos, even before the shape of the heart itself can be seen. Heart muscle works hard and needs a good blood supply. Blood is provided to the heart muscle itself by the CORONARY ARTERIES. These arteries are located over the surface of the heart muscle and provide the oxygen and nutrients it needs, while coronary veins remove waste products such as carbon dioxide.

Like all blood vessels, the coronary arteries around the heart can be partially or fully blocked by fatty deposits. These deposits can slow down or stop the blood flow to part of the heart, which can lead to a heart attack. 

A heart attack occurs if parts of the heart muscle are not getting sufficient blood supply and stop contracting (beating) in the way they should. How serious a heart attack is depends on where the blockage has occured.

Coronary heart disease is caused by the build up of fatty substances in the coronary arteries - the blood vessels that supply the heart with blood. Heart disease causes more deaths in the UK than any other disease. It is not caused by a microorganism, but mostly by the choices we make about how we live our lives.  

There are many things that you can do to keep your heart healthy, such as avoiding the main risk factors. Smoking is one of the main risk factors for heart disease. The carbon monoxide and nicotine in cigarette smoke causes hardening of the arteries, which can lead to them becoming blocked.

Other factor that increase your risk of suffering from heart disease are a poor diet that contains a lot of saturated fat, excessive alcohol consumption, and a lifestyle that causes stress. So you are more likely to suffer from a heart disease if you smoke, drink alcohol and have a high-fat diet, but not everyone who has these factors gets heart disease.

When factors are associated with increased risk of disease, we say there is a correlation between these factors and the disease. Smoking, a high-stress lifestyle, and a poor diet are all correlated with heart disease. Members of families that have a history of heart disease are also more likely to suffer from it themselves.

One of the lifestyle choices that can help prevent heart disease is regular exercise. It is thought that regular exercise strengthens the heart muscle and enables it to pump blood with less effort. Exercise can also help people to maintain a healthy weight and can reduce stress.

One of the reasons saturated fat causes heart disease is because of the cholesterol it contains. There are different types of cholesterol, and not all are correlated with heart disease; it is thought that exercise increases the levels of "good" cholesterol and decreases the level of "bad" cholesterol, thus reducing risk.

There are differences in the incidence of heart disease across the world due to different lifestyle factors. For example, in HICs such as UK and the USA, heart disease rates are high compared to less industrialised nations such as China and India. Heart disease rates in less industrialised countries are rising as these countries become wealthier. It is predicted that as citizens of these nations adopt a more "western" lifestyle and more of them take up smoking, heart disease rates in these countries will rise dramatically. 

Japan, although highly-industrialised, has one of the lowest rates of heart disease in the world. The diet in Japan includes more rice and fish and less meat and dairy products than the diet in countries with higher heart disease risk. Research has shown, however, that when Japanese citizens move to North America and change their lifestyle and diet, their heart disease rates begin to increase. Eventually they are nearly as high as the heart disease rates of people who were born in North America.

In some places, blood vessels which carry the blood throughout your body are close to the skin. In these locations, you can measure the HEART RATE by counting the pulses of the blood through the blood vessels; we call this "taking your pulse". The pulse rate is measured in beats per minute (bpm). The main places that the pulse can be felt are the wrists, the neck and either side of the groin.

The resting heart rate is that measured when a person is relaxed. Generally, a lower resting heart rate is an indicator of high fitness. A resting heart rate of around 70-100 for teenagers is normal, and a resting heartrate of 50-70 can indicate good fitness. When a person has high fitness, their heart beats more efficiently, pumping more blood around the body per beat.

When you exercise, the muscles need more oxygen and nutrients, and create more waste products to take away. In order to provide the muscles with what they need, the heart beats faster and so pumps the blood more quickly around the body when exercise takes place. As well as heart rate, blood pressure is an important indication of health. The blood pressure measurement records the pressure of the blood on the walls of the artery and is taken with a special device called a sphygmometer. Blood pressure is such an important indicator of health that insurance companies will sometimes refuse health insurance to cover those who have high blood pressure.

The blood pressure reading is measured in mm of mercury(mmHg) and is given as two figurs. The first (higher) figure is the pressure when the heart is contracting and the second (lower) figure is the pressure when the heart is relaxed.

There is a range or normal blood pressure readings, but very low or very high readings can be a cause for concern. High blood pressure often causes no symptons, but a number of health conditions such as stroke and heart attack are more likely if a person has high blood pressure. Low blood pressure can cause dizziness and fainting.

Blood pressure measurements record the pressure of the blood on the walls of the arteries. People with consistently high blood pressure have an increased risk of heart disease. This is true even in individuals who do not smoke, drink little alcohol and have a healthy diet. This is because high blood pressure can damage the blood vessels and cause them to narrow or harden, reducing the blood flow and making the heart work harder. High blood pressure can also damage the organs, particularly the kidneys, heart and the brain, making a heart attack or stroke more likely.

As with heart rate, blood pressure varies from person to person, and even in an individual it can differ depending on what they are doing and whether they are relaxed or anxious. The NHS states that the ideal blood pressure for a young healthy adult is 120/80.

In order to investigate whether a lifestyle factor (such as a high-fat diet or smoking) increases the chance of a person getting a disease, scientists compare samples of individuals who are "matched" on as many factors as possible, but differ only in the factor being investigated. Alternatively, individuals can be chosen at random so that other factors are just as likely in both samples. 

Studies such as these tend to involve large numbers of individuals and are known as EPIDEMIOLOGICAL STUDIES.

For example, doctors trying to find out whether smoking was a factor in the development of heart disease would study two large groups of people, those who already smoked and those who did not smoke, to see whether those who smoked were more likely to develop heart disease.

In studies like this, people are chosen who already smoke; it would be against the ethical code for such experiments to ask subjects to take up smoking in order to find out whether it affected the chances of getting heart disease. Some large-scale studies also look at the genes carried by individuals and whether this affects their risk of suffering from particular health problems.

For the body to function as it should, the inside of the body must be a stable environment, even though the environment outside changes regularly. For example, a person might be in a sauna or a cold swimming pool, but the body temperature and the amount of water in the body have to stay almost exactly the same in both environemtns. Our cells contain enxymes to keep us alive. These function best at certain temperatures and pH, and also at particular levels of water, salt and blood sugar. The maintenance of a steady internal state is called homeostatis. 

If the body's environment rises or falls too quickly, serious consequences or even death can occur.

The system that maintains homeostatis is in three parts:

1. The receptors that detect any change in the environment

2. The processing centres that receive the information and determine how the body systems will respond.

3. The effectors that produce the response.

An important part of the homeostatic control system is the reversal of changes that have put the body system off balance. This reversal helps the body back to the steady state it should be in. It is called NEGATIVE FEEDBACK.

The body's response to temperature changes is a good example of negative feedback. If the body temperature rises over 37oC, the receptors detect this and send a message to the processing centre in the brain. The processing centre then directs the effectors, in this case the blood vessels, to vasodilate (become slightly wider and closer to the skin surface) so that the heat can be lost through the skin and the body temperature drops.

Once the temperature has dropped back to normal, the receptor would detect this and pass the message to the processing centre, which would direct the effectors (the blood vessels) to return to their normal diameter.

less than 37oC means a raised body temperature is detected > the effectors cause vasodilation > heat lost from skin surface > body temperature falls > receptors detect body temperature > the hypothalamus in brain detects.

The amount of water in our bodies, like blood sugar, temperature and level of salts, is controlled by the mechanism of homeostatis. Homeostatis keeps the environment inside our bodies constant, no mateer what our external environemtn is like.

We take water into our bodies through eating and drinking, and water is also made through the process of respiration, when energy is released from food. Water is lost from our bodies when we sweat, when we breathe out, in our faeces and in the excretion of urine. Sometimes, such as when we exercise a lot, we need to drink fluid to replace the water we have lost through sweating and breathing. Doctors sometimes need to measure the fluids taken in and check the urine excreted by patients, to make sure that their water balance is being maintained.

All the cells in the body are bathed with a fluid called the BLOOD PLASMA. It is important that the blood plasma remains at the correct concentration, as the enzymes within the cells work best at a particular concentration of water and salts.

If the concentration is too high, then the body will absorb water. The concentration of the blood plasma can vary according to how much salt we have consumed in ourdiet, as well as how much fluid we have drunk.

The body cell's are affected if the concentration of the blood plasma varies. If the blood plasma is too dilute, the cells will absorb water until they burst. If the blood plasma is too concentrated, the cells will lose water until they become dehydrated. 

The kidneys are responsible for maintaining the levels of water, urea, salts and other chemicals in the blood. Urea is produced when proteins are broken down in the liver. It is poisonous to the body and so much be excreted. The kidneys are the effectors in the control system that maintains the optimim amount of water in the body.

The collecting ducts of the kidney pass through a region of the kidney that has very high concentration of salt. How much water can move through the walls of the collecting ducts varies, and is under the control of a hormone. That means that when the collecting ducts are permeable to water, most of the water passing through them will be re-absorbed into the body rather than passing into the bladder. When the collecting ducts are impermeable to water, all the water will pass into the bladder and be excreted by the body. As the amount of urea and other waste materials stays relatively constant despite the amount of water lost, when we have too little water and the body is conserving water, our urine witll be very dark and concentrated. Conversely, if we need to lose a lot of water, our urine will be very pale and dilute.

It is vital that the amount of water in the body, and so the concentration of the blood plasma, is kept within certain limits. This is to make sure that the body cells have exactly the environment that they need to function. Too little water, and the cells would become dehydrated. Too much water and the cells could begin to absorb water and burst. 

The concentration of the blood plasma is monitored by receptors in the body. The blood plasma becomes more concentrated if a person consumes a lot of salt, does not take in enough fluid, exercises hard or is in a hot environment. The kidneys respond to changes in the blood plasma by changing the concentration of the urine that is excreted.

Some recreational drugs can affect how the body responds to a change in the concentration of blood plasma. When alcohol is consumed, the body produces a lot of dilute urine, even when the blood plasma is concentrated. This can lead to dehydration, which os one of the causes of the after-effect of excessive alcohol intake, the "hangover". Dehydration can be damaging to health. The drug Ecstasy, has the opposite effect to alcohol. Even if the blood plasma is dilute, only small amounts of concentrated urine are produced. Taking ecstasy combined with drinking large quantities of water can lead to the blood plasma becoming too dilute, and the body's cells swelling up with water. If the tissues of the brain swell, this can cause seizures and bleeding to the brain, which can be fatal.

Hormones are chemicals that control many of the systems of the body. ANTI-DIURETIC HORMONE (ADH) is released into the bloodstream by the pituitary gland in the brain in response to changes in the concentration of the blood plasma. ADH acts upon the kidneys and causes them to reduce the amount of water lost in urine. 

The effects seen when alcohol or ecstasy are taken are due to the effects of these substances upon ADH.

 - Alcohol suppresses the action of ADH, leading to a greater volume of more dilute urine, even when the blood plasma has become too concentrated.

- Ecstasy increases ADH production, leading to a smaller volume of less dilute urine, even when the blood plasma has become too dilute.

A "diuretic" is a drug that increases the amount of urine produced. Anti-diuretic hormone, therefore, reduces the amount of urine produced.