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How microorganisms cause disease

Pathogens are microorganisms that make us feel ill and cause disease eg. bacteria and viruses. Pathogens have chemicals on their surface called antigens that antibodies recognise as being foreign. When MOs get into the body they reproduce quickly and cause symptoms of the disease. 


Diseases caused by bacteria are caused by the release of poisonous toxins eg. tetanus, salmonella and TB. Bacteria reproduce by dividing into two (asexual reproduction) known as binary fission. They reproduce rapidly in the ideal conditions of the human body. 


Diseases caused by viruses are caused when cells are damaged as the viruses reproduce. eg. flu, common cold, measles and chicken pox. Viruses need a host cell to 'hijack' the cell's mechanisms for making DNA and proteins (they always need a cell to live in as they can't live outside the body for very long). The copies of viruses are released in large numbers from the infected cell and go on to infect other cells.

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Our defence system

The body's external defences include skin, saliva, tears and stomach acid.

The body's internal defences is our immune system.

The immune system uses white blood cells to defend the body. They are made in the bone marrow and are found in several types. Some engulf and digest invading microorganisms, whilst others produce antibodies that recognise and destroy invading microorganisms. Different white blood cells carry different types of antibody on their surface. An antibody is specific to one antigenThe antibody corresponding to the antigen locks onto it and this causes the white blood cell to divide and produce many copies. Each white blood cell produces many antibody molecules that lock onto the invading cells.The antigen is then engulfed by the white blood cell.

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Defence system continued and vaccines

There are 3 different types of antibody:

-Ones that destroy the invading microorganism

-Ones that enable white blood cells to recognise the microorganism as being foreign

-Ones that cause the microorganism to clump together, meaning that it is easier for the white blood cells to engulf it. 

After the infection clears up, memory cells remain in the bloodstream that are able to reproduce large numbers of antibodies very quickly if the infection occurs again. At this point, the person is said to be immune to that particular pathogen.

Vaccines: A vaccine contains a safe form of a disease-causing microorganism so that you don't become ill after receiving it. Some pathogens do not change over the years so the vaccine can continue to be used. However, other pathogens such as the influenza (flu) virus change rapidly over time so new vaccines must be developed.An epidemic is when a disease spreads rapidly thorugh a population. To avoid this, vaccines among a large population are vital as this leads to 'herd immunity'. Widespread vaccinations done in this way has eradicated smallpox from the world and has also reduced childhood diseases like measles, mumps and rubella.

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Ways of reducing infection and resistant microorga

Any risks of vaccination must be considered against the benefits. Any vaccine generating an unusual number of adverse reactions is quickly withdrawn.

Other ways of reducing infection:

Antimicrobials are a group of substances that are used to kill microorganisms or slow down their growth. They are effective against bacteria, viruses and fungi.

Antibiotics are a type of antimicrobial which are effective against bacteria but NOT viruses. They allow doctors to treat illnesses cause by bacteria such as TB.

Studies show that some microorganisms are developing resistance against the antimicrobials as a result of their widespread and sometimes unecessary use. This problem can also arise if you do not finish your course of antibiotics. It has led to some strains of bacteria being very difficult to eradicate. These includeMRSA which is a problem in many hospitals.

Over a peirod of time, bacteria and fungi can develop resistance to certain antimicrobials (mutations sometimes cause this). Those that are resistant will survive the use of the antimicrobial and the resistance then spreads through the population of microrganisms. Resistant microorganisms are sometimes called superbugs. 

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Trialing new treatments

New medicines must be tested before they are made available to the general public.

1) Early stages of testing include using human cells grown in laboratories and animals.

2) Then, the drug is tested on a selection of humans, some of whom are healthy (to check for safety) and some who have the illness (to check effectiveness) in clinicals trials.

3) When the drug trials are carried out, the results are compared with those from a control group. Within the control group, some volunteers would have been given a placebo (a fake drug).

  • Open-label trial-both researchers and patients know which drug the patient is receiving.
  • Blind study-the pateinet doesn't know what drug they are receiving but the researcher does.
  • Double blind study-neither the pateint nor the researchers know what drug is being given.

Some trials investigate the drug's effects over a long period of time to check for side-effects or if the drug loses effectiveness.

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The heart and circulatory system

The circulatory system is made up of the heart, blood vessels and blood. The blood carries nutrients and oxygen to the body's cells and removes waste products from the cells. The blood is pumped around the body in blood vessels by the heart. The heart is a double-pump which means that one half of it is pumping oxygenated blood from the lungs to the body and the other side is pumping deoxygenated blood from the body back to the lungs. At the lungs, deoxygenated blood absorbs oxygen and gets rid of carbon dioxide. Coronary arteries run over the surface of the heart. They provide the heart with nutrients and oxygen it needs to contract, and remove waste products.


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Heart disease, heart rate and blood pressure

Coronary heart disease (CHD) is caused by the build up of fatty substances in the arteries. A healthy diet that is low in saturated fat lowers blood cholesterol and reduces the risk of heart disease. The lifestyle factors that increase someone's risk of this are:

  • Smoking cigarettes
  • Poor diet (high in saturated fats and salt)
  • Misuse of drugs (including excessive consumption of alcohol)
  • Stress
  • Family history of CHD

Heart rate is the number of pulses as blood passes through an artery close to the skin. It is measured in bpm. The misuse of drugs such as nicotine, alcohol and ecstasy, has a negative effect on health, including heart rate and increases the risk of heart disease.

Blood pressure measurements record the pressure of blood on the wall of an artery. People with consistently high blood pressure are more at risk of strokes and heart disease as it damages the arterey wall and puts them under strain. People with low blood pressure may faint or feel dizzy. Blood pressure is measured as millimetres of mercury (mm Hg) and given as two numbers; for example 110/80. (Higher value is when heart is contracting & lower value is when heart is relaxed).                   

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Epidemiological studies

Epidemiological studies are studies of the occurence of disease using large numbers of individuals. These studies have been carried out on the links of lifestyle factors and heart disease.

Studies are carried out:

  • on samples of individuals who are matched on as many factors as possible and differ in only the factor being investigated eg. smokers and non-smokers.
  • on individuals who are chosen at random.
  • that investigate whether the genes carried by individuals affects their risk of suffering from particular health problems.
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Homeostasis=the maintenance of a constant environment. It involves communication by the nervous and hormonal systems.

Temperature, pH and levels of sugar, water and salt must all be kept within narrow margins for the body to function properly.

The systems involved in homeostasis are in 3 parts:

  • Receptors detect chnages in the environment
  • Processing centres receive information and determine how the body will respond
  • Effectors produce a response

Negative feedback is one way the body can maintain a constant environemnt.

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Water balance

Water is taken in by drinking, eating and breathing in. Water is lost by urine, faeces, sweat and breathing out.

The water content in our bodies must be maintained to keep the body's cells bathed in blood plasma. If the blood plasma is too concentrated, the cells will lose water. If the blood plasma is too dilute, the cells will absorb water and burst.

Concentration of blood plasma is affected by external temperature, exercise levels, fluid intake and salt intake. The kidneys respond to a change in blood plasma by changing urine's concentration. They play a vital role in balancing water, salt and other substances in the blood plasma.

Some recreational drugs affect the water balance of the body: Alcohol causes the kidneys to produce a large volume of dilute urine and the body becomes dehydrated. Ecstasy, however, causes the kidneys to produce very small amounts of concentrated urine, causing cells to swell with water.

ADH is released by the pituitary gland in response to the changes in the concentration of blood plasma. The secretion of ADH is controlled by negative feedback. ADH acts upon the kidneys to reduce the amount of water lost in the urine.

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Nour Ghanem


very useful and helpful

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