Definition of Kidney
One of a pair of organs located in the right and left side of the abdomen which clear "poisons" from the blood, regulate acid concentration and maintain water balance in the body by excreting urine. The kidneys are part of the urinary tract. The urine then passes through connecting tubes called "ureters" into the bladder. The bladder stores the urine until it is released during urination.

The kidneys remove waste products from the blood and produce urine. As blood flows through the kidneys, they filter waste products, chemicals, and unneeded water from the blood. Urine collects in the middle of each kidney, an area called the renal pelvis. Urine then drains from the kidney through a long tube, the ureter, to the bladder, where it is stored.

The kidneys also make substances that help control blood pressure and regulate the formation of red blood cells.
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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.


What you need to know

Regions of the kidney

How the kidney works

You should:

be able to label a diagram of the kidney;

explain what is meant by excretion;

explain how a nephron works;

understand the part played by the kidney in osmoregulation.

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.

  1. 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

  1. 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.

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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

  1. 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!

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  1. 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.

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  1. 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.

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