Osmoregulation
Joanna George-Johnson, Contributor

The kidneys control the amount of water in the blood regulating the amount of water lost in urine. Have you ever noticed that when you drink a lot of water, you tend to go to the bathroom a lot? Or when the time gets cold and you are not sweating a lot, your trips to the bathroom also are increased? These observations are the result of a mechanism that is controlled by the brain. This control of the amount of water in the blood is called osmoregulation.

As the blood circulates through the body, it passes an area in the brain called the hypothalamus. The hypothalamus detects the concentration of the blood as it passes by.

If the day is hot and there is excessive sweating, or there is an intake of excessive salts and drinking of not enough water, these events could cause the concentration of the blood to be high.

Retaining water in the body

If it is too high, the hypothalamus sends a message to the pituitary gland to secrete a hormone known as anti-diuretic hormone (ADH). This hormone will travel in the blood to the kidneys. An increased amount of ADH in the blood causes the distal convoluted tubules and collecting ducts more permeable to water. Therefore, more water is reabsorbed from the filtrate in the nephron. Therefore, only a small amount of urine is produced. This action helps to retain water in the body.

Conversely, if the day is cold, or there is excessive drinking of water, the concentration of the blood will be very diluted. Therefore, when the water in the blood is too high, the hypothalamus sends a message to the pituitary gland to stop secreting ADH.

Since there is decreased concentration of ADH in the blood, the distal convoluted tubules and the collecting ducts becomes less permeable to water - less water reabsorbed from the filtrate. Therefore, a large amount of dilute urine is produced. This action helps to remove the excess water in the blood.

Therefore, with the presence of ADH, the urine is:

• Dark in colour
• Of a small volume
• Has a very pungent odour

With the absence of ADH, the urine is:

• Lighter in colour
• Large volume
• Odour is not as pungent

Questions

1. Decide which oval goes below and above the rectangle in the centre of the diagram.

2. Name the sensor in the brain which detects the concentration of the water in the blood.

3. What is the target organ for ADH?

4. Name the endocrine gland that secretes ADH.

5. What will happen if the concentration of the water in the blood is too low?

Information technology teacher, Delpherine Daniels (right), assists Taneisha Brown, one of her students, with class work during school activities at Cumberland High School on February 12. -Anthony Minott/Freelance Photographer

Joanna George-Johnson teaches at Ardenne High School.