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  • Created on: 10-04-14 18:40

Removal of waste and water control

The conditions inside our body must be very carefully controlled if the body is to function effectively. Waste is constantly being generated in the body and must be removed in order to stop waste levels becoming toxic. Water and mineral ion content must also be kept constant for our cells to work effectively. This is the role of the kidneys. Those who suffer from kidney failure cannot control their water and mineral ion levels, and must therefore undergo kidney dialysis or have a kidney transplant.

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Removing waste products

Waste products are constantly being produced by the body and must therefore be excreted. If they are not, they will increase in concentration and may interfere with chemical reactions or damage cells. Waste products that must be removed include carbon dioxide and urea.

Production and removal of waste products

Waste product Why is it produced? How is it removed? Carbon dioxide It is a product of aerobic respiration Through the lungs when we breathe out Urea

It is produced in the liver when

excess amino acids are broken down

The kidneys remove it from the blood and make urine - which is temporarily stored in the bladder

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Water balance

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 the water and mineral ion content in our body is maintained at the correct level. This is an example of homeostasis. If the water and ion content was to change, this would cause too much water to move into or out of cells - leading to them becoming damaged.

Our body must maintain a balance between the water we take in and the water we lose. This is done by the kidneys.

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How is the 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, which makes 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.

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The role of the kidney

Each kidney contains over one million microscopic filtering units called nephrons. Each nephron is made of a tubule and is responsible for ‘cleaning’ the blood by removing urea and excess water and mineral ions.

This process takes place in stages:

Stage 1: Filtration

As blood passes through the capillary at the start of the nephron, small molecules are filtered out and pass into the nephron tubule. These small molecules include glucose, urea, ions and water. However, large molecules, such as blood proteins, are too big to fit through the capillary wall and remain in the blood.

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Stage 2: Selective reabsorption

Having filtered out small molecules from the blood - many of which are essential to the body - the kidneys must reabsorb the molecules which are needed, while allowing those molecules which are not needed to pass out in the urine. Therefore, the kidneys selectively reabsorb only those molecules which the body needs back in the bloodstream.The reabsorbed molecules include:

  • all of the glucose which was originally filtered out

  • as much water as the body needs to maintain a constant water level in the blood plasma

  • as many ions as the body needs to maintain a constant balance of water and mineral ions in the blood plasma

The reabsorption of water takes place by osmosis. The reabsorption of glucose and mineral ions - from the nephron to the blood capillary - takes place by active transport. The cells which make up the wall of the nephron are adapted by having a folded membrane (providing a large surface area) and a large number of mitochondria (to supply the energy for active transport).

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Stage 3: The formation of urine

The molecules which are not selectively reabsorbed (the urea and excess water and ions) continue along the nephron tubule as urine . This eventually passes down to the bladder.

In carrying out these processes, the kidney is able to fulfil its functions of regulating the water and ion balance of the blood plasma, as well as keeping the level of urea low

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Kidney dialysis

Kidney failure has serious consequences as it means that the water and ion balance cannot be regulated, and the levels of toxic urea build up in the body. This would ultimately be fatal if not treated.

One method of treatment is kidney dialysis. In this procedure, patients are connected to a dialysis machine which acts as an artificial kidney to remove most of the urea and restore/maintain the water and ion balance of the blood.

How dialysis works

‘Dirty’ blood (high in urea) is taken from a blood vessel in the arm, mixed with blood thinners to prevent clotting, and pumped into the machine. Inside the machine - separated by a partially permeable membrane the blood flows in the opposite direction to dialysis fluid, allowing exchange to occur between the two where a concentration gradient exists.

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Dialysis fluid contains:

  • a glucose concentration similar to a normal level in the blood

  • a concentration of ions similar to that found in normal blood plasma

  • no urea

As the dialysis fluid has no urea in it, there is a large concentration gradient - meaning that urea moves across the partially permeable membrane, from the blood to the dialysis fluid, by diffusion. As the dialysis fluid contains a glucose concentration equal to a normal blood sugar level, this prevents the net movement of glucose across the membrane as no concentration gradient exists. And, as the dialysis fluid contains an ion concentration similar to the ideal blood plasma concentration, movement of ions across the membrane only occurs where there is an imbalance.

  • If the patient’s blood is too low in ions , they will diffuse from the dialysis fluid into the blood, restoring the ideal level in the blood.

  • If the patient’s blood is too high in ions , the excess ions will diffuse from the blood to the dialysis fluid.

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Dialysis summary

The overall effect of this is that the blood leaving the machine and returning into the patient’s arm will have:

  • greatly reduced levels of urea – it is ‘cleaned blood’

  • no overall change in blood glucose levels

  • the correct water and ion balance maintained or restored (with only excess ions removed)

Kidney dialysis requires highly specialised and expensive machinery. The patient must be connected to this machinery 2-3 times a week for periods (on average) of between 4-6 hours at a time. As the filtration only works when they are connected, kidney patients must monitor their diet carefully in between dialysis sessions. They need to avoid eating foods with a high salt content or a high protein content as excess amino acids are broken down into urea. So although dialysis is a life-saving treatment, it does have a significant effect on a person’s lifestyle.

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Kidney transplants

Kidney transplantation is an alternative method for treating kidney failure. This procedure involves implanting a kidney from an organ donor into the patient’s body to replace the damaged kidney.

As with all cells, the donor kidney cells will have protein antigens on their surface. Antigens are unique to each of us (with the exception of identical twins), and allow our body to identify our own cells from those of potential pathogens.

Differences in the antigens of the donor kidney cells and those of the patient receiving the transplant would mean that the patient’s immune system would quickly form antibodies against the kidney cell antigens, and would ultimately destroy the kidney. This is known as organ rejection.

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Precautions against rejection

Two precautions can be taken to reduce organ rejection:

  1. Tissue typing - only giving the kidney to patients who have antigens that are very similar to the antigens of the donor kidney. This can lead to long waits for a transplant for many kidney patients while compatible donors become available - during which time patients must undergo dialysis.

  2. Immuno-suppressant drugs – these drugs must be taken by transplant patients for the rest of their lives. They suppress the immune system, greatly reducing the immune response against the donor kidney. The negative effect of this is that it also suppresses the immune response against pathogens which enter the body, increasing the risk of getting infections.

Even with these two precautions, most donor kidneys will only survive for an average period of 8-9 years before the patient will require a further transplant or a return to dialysis.

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Transplants versus dialysis

The table below shows some of the pros and cons for both dialysis and kidney transplants

Advantages Disadvantages Kidney transplants

  • Patients can lead a more normal life without having to watch what they eat and drink
  • Cheaper for the NHS overall
  • Must take immune-suppressant drugs which increase the risk of infection
  • Shortage of organ donors
  • Kidney only lasts 8-9 years on average
  • Any operation carries risks

Kidney dialysis

  • Available to all kidney patients (no shortage)
  • No need for immune-suppressant drugs
  • Patient must limit their salt and protein intake between dialysis sessions
  • Expensive for the NHS
  • Regular dialysis sessions – impacts on the patient’s lifestyle
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Temp control

Our body temperature must be controlled within a very narrow range so that our body can function properly. A constant core temperature of around 37ºC needs to be maintained. The thermoregulatory centre of the brain triggers changes in effectors, such as sweat glands and muscles, in order to constantly balance our temperature gains and temperature losses.

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Maintaining body temperature

Temperature control is the process of keeping the body at a constant core temperature close to 37°C.Our body can only stay at a constant temperature if the heat we generate is balanced and equal to the heat we lose.Although our core temperature must be close to 37ºC , our fingers and toes can be colder. This is because energy is transferred from the blood as it travels to our fingers and toes.

How our body maintains a constant temperature

Temperature receptors in the skin detect changes in the external temperature. Sensoryand relay neurones transmit this information as impulses to the thermoregulatory centre of the brain – the area of the brain responsible for monitoring and controlling temperature. The thermoregulatory centre also has temperature receptors which detect changes in the temperature of the blood flowing through the brain. In the event of a change in temperature away from 37oC, the thermoregulatory centre sends electrical impulses to effectors (predominantly in the skin) which bring about responses that correct the temperature back to 37oC.

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When the body is too cold:

  • The blood vessels supplying the skin capillaries constrict, causing less blood to flow nearer the surface of the skin, the skin to become pale in appearance, and a reduction of heat loss.
  • The body shivers - the twitching of muscles generates additional heat as their contraction causes the muscles to respire thus releasing energy to warm the body.

When the body is too hot:

  • The blood vessels supplying the skin capillaries dilate causing more blood to flow nearer the surface of the skin, the skin to become red in appearance, and an increase in heat loss.

  • The body sweats - which increases heat loss due to the large amount of heat energy required to evaporate the water.

Note that we sweat more in hot conditions, so more water is lost from the body. This water must be replaced through food or drink to maintain the balance of water in the body. Ions such as sodium ions and chloride ions are also lost when we sweat. They must be replaced through food and drink.

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Blood sugar control

The concentration of glucose in our blood is important and must be carefully regulated. This is done by the pancreas, which releases hormones that regulate the usage and storage of glucose by cells. Type 1 diabetics are unable to make sufficient quantities of one of these hormones – insulin - and must therefore control their blood sugar levels by injecting insulin, as well as by carefully controlling their diet and exercise levels.

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Controlling rising blood sugar

It is important that blood glucose level is kept within a narrow range due to its importance as an energy source for respiration - but also because of the effects it could have in causing the movement of water into and out of cells by osmosis

Having eaten a meal containing sugars or starch (eg sweets, potatoes, bread, rice or pasta), the starch and large sugars are digested down into glucose and absorbed across the small intestine wall into the bloodstream. This triggers a rise in blood glucose concentration.

The pancreas monitors and controls the concentration of glucose in the blood. In response to an increase in blood glucose level above the normal level, the pancreas produces a hormone called insulin which is released into the bloodstream.

Insulin causes glucose to move from the blood into cells, where it is either used for respiration or stored in liver and muscle cells as glycogen. The effect of this is to lower the blood glucose concentration back to normal.

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There are two main types of diabetes:

  • Type 1 which usually develops during childhood

  • Type 2 which is usually develops in later life

This syllabus focuses on Type 1 diabetes - which is caused when the pancreas does not produce enough insulin. The body is therefore unable to lower blood sugar level when it rises too high.

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Controlling Type 1 diabetes

Sufferers of Type 1 diabetes can help to control their blood glucose level by being careful with their diet (eating foods which will not cause big spikes in their blood sugar level) and by exercising (which can lower blood glucose levels due to increased respiration in the muscles).

However, Type 1 diabetics must also inject insulin to control their blood glucose level. This requires a person to conduct a blood test to provide a reading of their blood glucose level (using a blood glucose meter), from which they can then work out the dose of insulin they are required to inject.

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Traditionally, diabetics have had to inject themselves with multiple injections of insulin throughout the day to try to regulate their blood sugar level.

However, some diabetics now wear an insulin pump. This supplies insulin continuously at low levels and can be programmed to adjust the supply at meal times or times of exercise.

Injecting insulin multiple times throughout the day


  • Equipment is cheaper
  • More discrete as needles, insulin and blood glucose monitor are easy to conceal


  • Uses more insulin
  • Does not control blood glucose levels as well, leading to more swings in blood glucose levels (which can lead to health effects).
  • Requires more careful control of diet and exercise
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Insulin pump;


  • Allows the delivery of more precise volumes of insulin – and therefore offers better control of blood glucose level

  • Reduced risk of long-term effects of diabetes (due to better control

  • Uses less insulin


  • Equipment is more expensive

  • Pump may be uncomfortable to wear and may present problems for some activities, eg contact sports
  • Users may have to do more blood glucose tests per day to identify if pump is working effectively

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