kidneys are bean-shaped organs found in the abdomen, one on either side
of the aorta.
The kidneys are really important in
control of ...
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.
Ions and small molecules
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
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
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.
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
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.