Controlling Body Water

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Controlling Body Water

We have two kidneys, which are in your lower back just where your belt goes. Their job is to clean the blood by filtering out unwanted material such as urea, excess water, salt and ions.

They are wonderfully constructed organs and do some amazing work.

One job that they are involved in is reabsorbing excess water so that we don't dry out. But how do they do it?

kidney function

Blood enters the kidney through the renal artery. It is filtered and the 'clean' blood leaves via the renal vein. Any waste material leaves through the ureter, then to the bladder and the world outside!

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If you cut into a kidney you see two distinct parts, the dark red outer zone called the cortex and the lighter inner zone, the medulla.

If you then use a microscope and look at the cortex you begin to see lots of structures called nephrons. There are about 750,000 of them in each kidney.

If you look at the nephrons at an even greater magnification they look a bit like this:


At one end is a cup-like structure called the Bowman's capsule. It encloses a knot of capillaries called the glomerulus. These capillaries are leaky and small molecules get filtered out and end up inside the Bowman's capsule. This process is called ultrafiltration.


If nothing else happens then the materials, such as water and urea, will end up going all the way through the nephron, down the ureter, through the bladder and into the toilet!

However, sometimes the body needs to grab back chemicals such as water and glucose which are still useful. This happens when they move out of the fluid in the nephron back into the capillary network that twists around the nephron.

This process is called reabsorption. Reabsorption means that the useful chemicals are taken back into the blood out of the nephron. They do not end up in the urine and are not lost from the body.

It's a very cunning mechanism!

When there is too little water in the body the following sequence of events occurs:

too little water sequence

The chemical messenger between the brain and the kidney is the hormone ADH, Anti-Diuretic Hormone.

The important parts of the process involve:

  1. The hypothalamus in the brain, which detects the lower blood water content.
  2. The pituitary gland at the base of the brain, which releases the hormone ADH.
  3. The kidney, which reabsorbs the water.

In order to get back to the normal level of water in the blood we absorb more water from the digestive system, feel thirsty, and so drink more.

A similar sequence of events occurs when there is too much water in the body.

This time, some of the details are reversed from what they were when there was too little water.

Can you sort these into the correct order?

Drag the numbers from the top bar to put the following sentances in the correct order:

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