1. Excretion and Homeostasis
Strand 2
Unit 5
Pages

2. The kidneys maintain homeostasis by:
Regulating the of water content of the body through the action of the hormone ADH.
Controlling the pH of the blood
Removing various cellular wastes and substances which are in excess.

3. What is homeostasis?
A process that maintains the composition of body fluids (body and tissue fluids) within normal limits/ranges.
Concentration
Acidity
temperature

4. First we have a look at how the water balance in the body is controlled.
7 Marks

5. Shortage of water in the body
Blood water concentration will be low.

6. Shortage of water in the body
Osmoreceptors situated in the hypothalamus are stimulated.

7. Shortage of water in the body
Receptors then stimulate the pituitary to secrete more ADH into the blood.

8. Shortage of water in the body
ADH is carried by the blood to increase the permeability of the walls of the distal convoluted tubules and collecting ducts.

9. Shortage of water in the body
This causes more water to pass osmotically from the tubules and the ducts into the medulla and then into the blood.

10. Shortage of water in the body
The amount of water in the blood increases

11. Shortage of water in the body
Less water remains in the tubules

12. Shortage of water in the body
Concentrated urine is excreted

13. Excess water in the body
Blood water concentration will be high.

14. Excess water in the body
Osmoreceptors situated in the hypothalamus are stimulated.

15. Excess water in the body
Receptors then stimulate the pituitary to secrete less ADH into the blood.

16. Excess water in the body
This reduces the permeability of the walls of the distal convoluted tubules and collecting ducts.

17. Excess water in the body
This causes less water to pass osmotically from the tubules and the ducts into the medulla and then into the blood.

18. Excess water in the body
The amount of water in the blood decreases

19. Excess water in the body
More water remains in the tubules

20. Excess water in the body
Dilute urine is excreted

21. The role of aldosterone in maintaining the salt balance of the blood.
7 Marks

22. Salt balance
The hormone aldosterone is secreted from the adrenal cortex of the adrenal gland.

23. Salt balance
Aldosterone regulates the salt concentration of the blood and tissue fluid.

24. Salt balance
The regulation of the salt content of the blood and tissue fluid is related to the sodium pump mechanism and the recovery of water from the distal convoluted tubule and collecting duct.

25. Salt balance Aldosterone level Sodium level
When the sodium level in the tissue fluid and blood drops, more aldosterone is secreted into the bloodstream.
Aldosterone level
Sodium level

26. Salt balance
Aldosterone stimulates the active re-absorption of sodium ions from the filtrate into the blood.

27. Salt balance
Water also moves into the blood from the filtrate.

28. Salt balance
Whenever solutes are absorbed, water will follow by osmosis.

29. Copy down the following table and complete it as part of your homework.

30. WATER CONTENT OF BLOOD
NORMAL
BRAIN
KIDNEY
Too much water
drunk
Salt eaten or much
sweating occurs

31. Homework
Activity 3 Page 254

32. Video Time
How your kidneys work.
What your pee says about you.