OCR Categories Archives: B2: Keeping Healthy

B2.4 How do our bodies keep a healthy water balance?

B2.4 How do our bodies keep a healthy water balance?

HOMEOSTATIS is the maintenance of a constant internal environment – It is achieved by balancing bodily inputs and outputs, using the NERVOUS SYSTEM and HORMONES to control the process. Examples if things that the body keeps the same are:

  • Body temperature at 37˚C
  • The amount of water inside our body

Automatic control systems throughout the body maintain a range of factors at steady levels and that this is required for cells to function properly.

For homeostasis to work, these control systems need to have:

  • RECEPTORS to detect changes in the environment
  • PROCESSING CENTRES to receive information and coordinate responses automatically
  • EFFECTORS to produce the response

NEGATIVE FEEDBACK ensures that, in any control system, changes are reversed and returned back to the set level. For EXAMPLE when receptors detect that the temperature in the body has increased above a certain level, the processing centre (brain) sends signals to the effectors (in this case sweat glands) to produce sweat to cool down the body. This process is where the steady state of the body is adjusted to reverse the change.

For the cells of our body to work properly, it is important that their water contained is maintained at the correct level. This means our body must maintain a balance between the water we take in and water we lose – Water is INPUT (gained) from drinks, foods and respiration and it is OUTPUT (lost) through sweating, breathing, faeces and the excretion of urine.

The KIDNEYS play a vital role in balancing levels of water, waste and other chemicals in the blood.

The brain monitors water content constantly and causes the kidney to adjust the concentration and volume of urine produced-

  • 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 the urine becomes more DILUTE

The amount of water that needs to be reabsorbed depends on a number of factors:

  • External temperature
    • High → Concentrated urine
    • Low → Dilute urine
  • Level of exercise
    • High → Concentrated urine
    • Low → Dilute urine
  • Fluid intake
    • High → Dilute urine
    • Low → Concentrated urine
  • Salt intake
    • High → Dilute urine
    • Low → Concentrated urine

The concentration of urine is controlled by a hormone called ADH, which is released into the bloodstream by the PITUITARY GLAND.

When the level of water in the blood is too low, ADH is released and this causes concentrated urine to be produced. This is because the hormone causes the kidney to make MORE PERMEABLE, allowing water to be reabsorbed.

When the level of water in the blood is too high, ADH is NOT released. The kidney becomes LESS PERMEABLE and this causes dilute, watery urine to be produced.

Effects of Alcohol on water balance:

  • Alcohol causes the kidneys to produce a greater volume of more dilute urine. This can lead to DEHYDRATION
  • This is because alcohol suppresses (restrains) ADH production


Effect of Ecstasy on water balance:

  • Ecstasy causes the kidneys to produce smaller volume of less dilute urine. This can result in the body having TOO MUCH water
  • Ecstasy increases ADH production which means the kidneys reabsorb water


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B2.3 What factors increase the risk of heart disease?

B2.3 What factors increase the risk of heart disease?


The HEART is a muscular organ in the circulatory system. It beats automatically, pumping blood around the body to provide cells with oxygen and dissolved food for RESPIRATION. The blood removes carbon dioxide and water as waste products. 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 from the rest of the body enters the RIGHT ATRIUM of the heart. It then moves into the RIGHT VENTRICLE before being pumped to the lungs. When the oxygenated blood returns to the heart, it enters the LEFT ATRIUM. It then moves into the LEFT VENTRICLE before being pumped to the rest of the body.


The heart is a DOUBLE PUMP in the circulatory system because blood returns twice.

Arteries, Veins and Capillaries

CAPILLIRIES allow food and oxygen to diffuse to cells while waste is diffused from cells.

Narrow, thin-walled vessels – only one cell thick – that allow them to effectively perform their function

The heart rate can be measured by taking the pulse. If it is too fast or too slow, then it could indicate problems.

Another, more accurate way of checking how hard your heart is working is through measuring BLOOD PRESSURE. This records the pressure of the blood on the walls of the artery and it results from two forces:

  • SYSTOLIC pressure from the heart as it contracts and pumps blood into the arteries and through the circulatory system (HIGHER VALUE)
  • DIASTOLIC pressure from the force of the arteries as they resist the flow when the heart relaxes (LOWER VALUE)


High blood pressure is the biggest concern as it increases the risk of a heart attack- but low blood pressure can be dangerous too. However ‘NORMAL’ measurements for factors such as heart rate and blood pressure are always given as ranges because individuals vary.

Blood vessels called the CORONARY ARTERIES supply blood to the heart muscles. If they become blocked, a HEART ATTACK can happen

A heart attack can happen because:

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

HEART DISEASE is an abnormality of the heart that can lead to a HEART ATTACK. It is usually caused by LIFESTYLE and/or GENETIC FACTORS, not by infection. Lifestyle factors that can increase the risk of heart disease include:

  • Poor diet
  • Stress
  • Cigarette smoking
  • Misuse of drugs (e.g. Ecstasy, cannabis, nicotine and alcohol)

Reducing the risk when provided with lifestyle and genetic data:

  • Regular moderate exercise
  • Stop smoking
  • Reduce salt intake in diet
  • Monitor cholesterol levels (and use cholesterol-reducing drugs and foods if necessary
  • Maintain a healthy body weight

As heart disease is a big killer worldwide, studies continue to try to identify what factors cause it. These are called EPIDEMIOLOGICAL STUDIES and they try to identify whether a factor present in a large number of sufferers is the cause.  In addition, there are more genetic studies taking place to identify the genes responsible for heart disease.

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B2.2 What are vaccines and antibiotics and how do they work?

B2.2 What are vaccines and antibiotics and how do they work?


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. If the body is re-infected by the same microorganism, memory cells produce antibodies quickly so that the microorganism is destroyed before damage is done. This is how vaccination works:

Injection of vaccine


A safe form of the diseases – cause microorganism is injected into the body


Immune response triggered

Although the microorganism is safe, the antigens on its surface still cause the white blood cells to produce specific antibodies


Memory cells remain in body

Long after the vaccination, memory cells patrol the body. If the disease-causing microorganism infects the body again, the white blood cells can attack it very quickly.


In order to prevent an EPIDEMIC of a disease in a population it is important that as many as individuals as possible are vaccinated. If more than 95% of the population is vaccinated then the unvaccinated will be protected too because the risk of coming in contact with an infected person will be very small.


There is no guarantee that all vaccines and drugs (medicines) are risk free. People have GENETIC DIFFERENCES, so they may react to a vaccine or a drug in different ways – these are called SIDE EFFECTS.

ANTIMICROBIALS are chemicals that kill, or inhibit bacteria, fungi and viruses. ANTIBIOTICS are a type of antimicrobial that are only effective against bacteria but NOT viruses


Over a period of time, bacteria can become RESISTANT to antimicrobials.

MUTATIONS (random changes) can take place in the genes of microorganisms. This leads to new strains of bacteria and fungi that are no longer affected by the antimicrobial. These reproduce and pass on the resistance – as a result, the antimicrobial is no longer effective.


To prevent resistance to antimicrobials increase:


  • Doctors should only prescribe them when completely necessary
  • Patients should always complete a course of antibiotics, even if they are feeling better.



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B2.1 How do our bodies resist infection?

B2.1 How do our bodies resist infection?

MICROORGANISMS are organisms that are too small to see with the naked eye. They include BACTERIA, VIRUSES and FUNGI. They can be beneficial to us (e.g. the bacteria that live in our intestines can produce certain vitamins) or they can cause us harm (e.g. bacteria that cause food poisoning).

PATHOGENS are microorganisms that cause infectious diseases. BACTERIA and VIRUSES are the main PATHOGENS.

Symptoms of an infectious disease are caused by damage done to cells by microorganisms or the poisons (TOXINS) they produce.

In the correct conditions (with warmth, moisture, nutrients) bacteria can multiply rapidly. The human body provides ideal conditions for microorganisms to grow. In the body, there is water, oxygen, food and heat, as well as different pH levels.


The form of growth is known as EXPONENTIAL GROWTH. It follows the formula:

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

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 called ACTIVE IMMUNITY. The WHITE BLOOD CELLS have key functions in this. White blood cells are part of the body’s immune system and can:

  • Destroy pathogens by engulfing and digesting them
  • Produce antibodies to destroy pathogens
  • Produce antitoxins to neutralise the toxins released by the microbe

Each microorganism has its own markers made out of protein on its surface – these markers are called ANTIGENS. ANTIBODIES recognise microorganisms by the ANTIGENS that they carry on their surface therefore a different ANTIBODY is needed to recognise each different type of microorganism.

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 very fast for the particular microorganism and destroy it before you feel unwell. This is described as being IMMUNE to a disease.

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