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The kidneys and body balance
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How the kidney works
Each kidney contains about a million tiny structures called nephrons. A nephron starts as a cup-shaped Bowman's capsule which becomes a renal tubule. The two sections of the nephrons extend across the two different regions of the kidney:
  • the Bowman's capsules are found within the outer cortex region
  • the renal tubules run from the cortex into the darker medulla
The blood's journey

Placed end to end, the nephrons of one kidney would stretch about 8 km. A branch of the renal artery supplies the nephrons with blood. In the Bowman's capsule, the artery splits up into a network of capillaries called the glomerulus. These capillaries join up again on leaving the capsule. This blood vessel splits once more into a second set of capillaries that are wrapped around the renal tubule.
rollover diagram of nephron
Picture 4. Diagram of the paths through the nephron of blood and glomerular fluid. Roll over here to restore the whole diagram.
Step 1. Blood flows into the glomerulus from the renal artery

Step 2. High blood pressure in the glomerulus forces water, glucose, urea and salts through the capillary wall and into the tubule. Protein molecules and blood cells are too big so they remain in the blood.

Step 3. Blood leaves the glomerulus and flows to the capillaries that surround the renal tubule.

Step 4. The fluid called the glomerular filtrate passes along the tubule.

Step 5. All of the glucose and most of the water and salts are absorbed back into the blood in the nearby capillaries – to ensure there’s the right concentration in the blood.

Step 6. Urea and other unwanted substances stay dissolved in the glomerular filtrate. They pass down the tubule and eventually reach the bladder.

Step 7. Blood flows out of the kidney to the renal vein.

Water regulation is controlled by Anti-Diuretic Hormone (ADH), for more information see Hormones and their effects.

Diagram of nephron for Q 2
Picture 5. See question 2.

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Question 2
a) When blood leaves the capsule, the concentration of some substances has changed. Give four substances whose concentration has changed and say what has happened to the concentration.

b) What causes substances to pass out of the blood into the Bowman's capsule?

c) Look at picture 5. Click on the picture where you would expect to find the greatest concentration of glucose in the renal tubule.

d) As the glomerular filtrate passes along the tubule, most of the water is absorbed back into the blood. How does this affect the concentration of urea?