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Water and wastes
1/ The excretory system
a/ Structure
b/ Function
In the kidney there are about a million small structures called nephrons.
Nephrons are responsible for blood filtration (i.e. the removal of wastes) and the control of water and salt balance in the body.
Structure of the nephron
Function of the nephron
Blood enters the kidney by the renal artery.
It contains:
- red blood cells
- water
- proteins
- glucose
- urea (poisonous substance)
Two processes are taking place in the nephron.
1. Filtration.
The blood enters the glomerulus from the renal artery. Small molecules are filtered out of the blood forming the glomerulate filtrate into the Bowman’s capsule. These small molecules are:
- Urea
- Glucose
- Water
- Salt
2. Re-absorption of useful small molecules.
When the glomerulate filtrate reaches the tubule, glucose water and salt are re-absorbed into the blood. Urea and excess water enter the collecting duct which takes this liquid (i.e. urine) to the ureter and finally reaches the bladder where it is temporarily stored.
2/ Control of water balance
• 70% of the body consists of water
• It is very important that the level is kept constant.
• This achieved when we gain and lose water every day:
Gain of water
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Loss of water
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Drink
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Urine
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Food
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Faeces
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Body chemistry
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Metabolic reactions
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• The volume of water reabsorbed into the capillaries, is controlled by the hormone called Anti Diuretic Hormone – ADH.
• ADH is produced by the pituitary gland in the brain.
• The pituitary gland detects how much water is in the blood and alters the amount of ADH released in response to this.
4/ The implications of the damage of kidneys by accidents or disease.
When the kidneys are damaged:
1- the body cannot remove urea and other poisonous substances
2- the body cannot regulate blood’s water content
There are two ways to deal with the removal or lost of kidneys:
Transplant
Benefit : normal life.
Limitations: organ can be rejected + drugs need to betaken to prevent rejection. There is a shortage of organs.
Kidney machine
Benefit: no drugs need to be taken, the machines available if money is available.
Limitations: the diet is restricted, patients must live near a machine, it is time consuming.
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