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Our kidneys do a grand job
removing the toxic waste products of metabolism. This process is called excretion. Our
kidneys produce urine which contains urea, excess salts and excess water.
Index
What you need to know
Regions of the kidney
How the kidney works
You should:
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be able to label a diagram of the kidney;
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explain what is meant by excretion;
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explain how a nephron works;
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understand the part played by the kidney in
osmoregulation.
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You need to know about the
general structure of the kidney and how it works, so let's start with a diagram to show
the regions of the kidney. The three main regions are called the cortex, medulla and
pelvis. Can you label this diagram. Jot down what you think the parts are called, A to F
and then click on each letter in turn to reveal the truth.
You should be able to name all the parts labelled A
to F. Click on any one of the letters to find out more about that part.
Renal Vein
This has a large diameter and a thin wall. It
carries blood away from the kidney and back to the right hand side of the heart. Blood in
the kidney has had all its urea removed. Urea is produced by your liver to get rid of
excess amino-acids.
Blood in the renal vein also has exactly the right
amount of water and salts. This is because the kidney gets rid of excess water and salts.
The kidney is controlled by the brain. A hormone in our blood called Anti-Diuretic Hormone
(ADH for short) is used to control exactly how much water is excreted.
Back to the Kidney Diagram
Renal Artery
This blood vessel supplies blood to the kidney from
the left hand side of the heart. This blood must contain glucose and oxygen because the
kidney has to work hard producing urine. Blood in the renal artery must have sufficient
pressure or the kidney will not be able to filter the blood.
Blood supplied to the kidney contains a toxic
product called urea which must be removed from the blood. It may have too much salt and
too much water. The kidney removes these excess materials; that is its function.
Back to the Kidney Diagram
Pelvis
This is the region of the kidney where urine
collects. If you are very unlucky, you may develop kidney stones. Sometimes the salts in
the urine crystallise in the pelvis and form a solid mass which prevents urine from
draining out of the medulla of the kidney. You will need treatment: see your doctor.
Back to the Kidney Diagram
Ureter
This one is easy peasy: the ureter carries the urine
down to the bladder. It does this 24 hours per day, but fortunately the urine can be
stored in a bladder so that it is not necessary to wear a nappy!
Back to the Kidney Diagram
Medulla
The medulla is the inside part of the kidney. It is
shown in green in the diagram, but in real life it is a very dark red colour. This is
where the amount of salt and water in your urine is controlled. It consists of billions of
loops of Henlé. These work very hard pumping sodium ions. ADH makes the loops work harder
to pump more sodium ions. The result of this is that very concentrated urine is produced.
The opposite of an anti-diuretic is a
"diuretic". Alcohol and tea are diuretics.
Back to the Kidney Diagram
Cortex
The cortex is the outer part of the kidney. This is
where blood is filtered. We call this process "ultra-filtration" or "high
pressure filtration" because it only works if the blood entering the kidney in the
renal artery is at high pressure.
Billions of glomeruli are found in the cortex. A
glomerulus is a tiny ball of capillaries. Each glomerulus is surrounded by a
"Bowman's Capsule". Glomeruli leak. Things like red blood cells, white blood
cells, platelets and fibrinogen stay in the blood vessels. Most of the plasma leaks out
into the Bowman's capsules. This is about 160 litres of liquid every 24 hours.
Most of this liquid, which we call
"ultra-filtrate" is re-absorbed in the medulla and put back into the blood.
Back to the Kidney Diagram
Here is a diagram of
the kidney showing a nephron broken up into six parts. Each part has a specific function.
You can click on a letter to find out what that part does.
Here is a diagram of a nephron
put back together again. You can click on a green or red area to find out what happens
there. Ultra-filtration happens in the red area, and re-absorption happens in the green
areas.
Glomerulus and
Bowman's Capsule
This is where ultra-filtration takes place.
Blood from the renal artery is forced into the glomerulus under high pressure. Most of the
liquid is forced out of the glomerulus into the Bowman's capsule which surrounds it. This
does not work properly in people who have very low blood pressure.
Back to the diagram of a
nephron
Proximal
Convoluted Tubules
Don't worry about remembering the name for
your GCSE biology. Jolly good though if you can. Proximal means "near to" and
convoluted means "coiled up" so this is the coiled up tube near to the Bowman's
capsule.
This is the place where all that useful
glucose is re-absorbed from the ultra-filtrate and put back into the blood. If the glucose
was not absorbed it would end up in your urine. This happens in people who are suffering
from diabetes.
Losing glucose in your urine is bad news;
how much did those chips at lunchtime cost you?
Back to the diagram of a
nephron
Loop of Henlé
This part of the nephron is where water is
reabsorbed. Kidney cells in this region spend all their time pumping sodium ions. This
makes the medulla very salty; you could say that this is a region of very low water
concentration. If you remember the definition of osmosis, you will realise that water will
pass from a region of high water concentration (the ultra-filtrate and urine) into a
region of low water concentration (the medulla) through cell membranes which are
semi-permeable.
Back to the diagram of a
nephron
Distal Convoluted
Tubules
Don't worry too much about the name. Distal
means "distant" so it is at the other end of the nephron from the Bowman's
capsule. This is where most of the salts in the ultra-filtrate are re-absorbed.
Back to the diagram of a
nephron
Collecting Duct
Collecting ducts run through the medulla
and are surrounded by loops of Henlé. The liquid in the collecting ducts (ultra-filtrate)
is turned into urine as water and salts are removed from it. Although our kidneys make
about 160 litres of urine every 24 hours, we only produce about ˝ litre of urine.
It is called a collecting duct because it
collects the liquid produced by lots of nephrons.
Back to the diagram of a
nephron
So now you know how your kidney
works !
Now have a go with my "REVISION TOYs".
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