The Kidney

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.


  1. What you need to know;
  2. Regions of the kidney;
  3. How the kidney works.

You should:

  1. be able to label a diagram of the kidney;
  2. explain what is meant by excretion;
  3. explain how a nephron works;
  4. 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.

A:  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.

B:  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.

C:  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.

D:  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!

E:  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.

F:  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.

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.

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?

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.

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.

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 ½ litre of urine.

It is called a collecting duct because it collects the liquid produced by lots of nephrons.