EXCRETORY SYSTEM
The kidneys maintain our body's water balance by controlling the
water concentration of blood plasma. The kidneys also control salt
levels and the excretion of urea. Water that is not put back into the
blood is excreted in our urine.
THE KIDNEYS
Humans have two kidneys. They are bean-shaped organs, approximately
11.5cm long, that are situated in the abdominal cavity, just below the
ribcage, one on either side of the spine.
Location of kidneys in abdominal cavity
One of the main jobs of the kidney is to filter the waste out of the
blood. How does the waste get in your blood? Well, your blood delivers
nutrients to your body. Chemical reactions occur in the cells on your body to
break down the nutrients. Some of the waste is the result of these chemical
reactions. The waste has to go somewhere; this is where the kidneys come
in.
As the blood passes through the kidneys, all the small molecules are filtered
out of the blood.
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EXCRETORY SYSTEM
This includes molecules of:

water

salt

glucose

urea (a waste product from the breakdown of proteins)
The kidneys then reabsorb all of the glucose and as much water and salt as
the body needs, putting them back into the blood. This leaves some water
and salt, and all of the urea, which is now called urine. The urine passes
from the kidneys to the bladder, where it is stored prior to being excreted
from the body.
The kidneys also do control the body’s water balance:
HOW IS THE WATER BALANCE MAINTAINED?
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 their water
content is maintained at the correct level. This means our body must
maintain a balance between the water we take in and the water we lose.
This is done by the kidneys
The kidneys maintain our water balance by producing urine of different
concentrations.
The level of water in the blood plasma can vary depending on:

External temperature - when it is hot, we sweat more and lose
water, thereby making 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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EXCRETORY SYSTEM
THE STRUCTURE OF THE RENAL SYSTEM
Urine is produced in the kidneys from water and wastes extracted from the
blood. The rest of the urinary system is concerned with the storage and
ducting of the urine to the outside of the body - Figure 01.
A kidney consists mostly of about one million tiny kidney units called
nephrons.
bbc document
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EXCRETORY SYSTEM
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What else do kidneys do?
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The urine is made of.......
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The measurement of some of the processes that take place in our
organism is important to know whether we are healthy and our
systems are functioning correctly. If “abnormal” values are obtained
when processes such as the production of urine in the kidneys or the
frequency of the heart beat are measured, it can indicate a health
problem.
We will now do some calculations of these processes.
Knowing that each day we expel approximately 1.5 litres of urine:
a) What quantity of urine is produced per minute?
b) The maximum quantity of urine that the bladder can retain is 700
cm 3 . Knowing that when the bladder contains 300 cm 3 we feel the
urge to urinate, how many times will we have to urinate a day?
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