(2) The three functions of the kidneys (and mention of associated liver function)

The kidneys are bean-shaped organs found in the abdomen, one on either side of the aorta.

The kidneys are really important in control of ...

(i) Water control

The kidneys help to regulate the water content of the body by controlling how much water is reabsorbed and how much is lost in urine.

Water is lost through the skin in sweating and the lungs when breathing out, but these losses cannot be controlled by any organ in the body, including the kidneys.

We also lose water when the kidneys remove urea in urine.

So, this water loss must be balanced by water we consume in food and drinks.

But, the kidney will excrete excess water too.

Water balance is strongly related to ion concentrations in the body fluids.

(ii) Ions and small molecules concentration control

Ions such as sodium, iron and calcium are taken into our body in food and drinks and then reabsorbed into the bloodstream.

If the ion content of the body is wrong. it can upset the balance between ion concentration and water and too much water can be absorbed into cells by osmosis - so the wrong balance can lead to cell damage e.g. extreme dehydration from lack of water OR excess hydration (caused by some illegal drugs) and both situations can be fatal.

Important ions like sodium and chloride, are lost in sweat, which tastes salty as a consequence, but this situation is not regulated by the body.

Hence the importance of the kidneys in maintaining water AND ion balance in the body.

The correct amount of ions are reabsorbed and excess ions are removed in urine.

So, the kidneys are also an important high pressure 'filtration' system for removing unwanted materials and waste products out of the blood passing through the kidneys, which would otherwise cause harmful effects on the body if not removed.

The kidneys filter out these substances as the blood through - hence the process name of filtration.

Glucose, ions and other useful substances and appropriate amount of water are re-absorbed back into the blood stream - this process is called selective reabsorption.

This process is essential to adjust the levels of ions to their appropriate concentration.

(iii) Urea removal

What else does the kidney remove to maintain the right balance of various substances?

After the filtration, the principal waste substance removed by the body in the urine is urea from the blood.

Urea is made from excess nitrogen, often from amino acids from metabolised food and urea is less toxic than ammonia.

Neither proteins or amino acids can be stored in the body, so excess amino acids are converted into fats and carbohydrates, but these molecules do not contain nitrogen, so the nitrogen part of amino acids must removed in some way.

This process, called deamination, happens in the liver and produces the waste product ammonia.

However, ammonia is toxic and is converted to urea and transported to the kidneys where it is filtered out and excreted in urine.

See (3) Details on how the kidney works in conjunction with the urinary and blood systems

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