Homeostasis

Question 1

What happens to the sodium ions in the PCT?

Answer 1

1. Sodium ions are actively transported into the blood capillaries, reducing the Na+ concentration in the epithelial cells lining the PCT
2. Na+ diffuses from the PCT lumen into the epithelial cells, down its concentration gradient
3. Na+ is co-transported with substances like amino acids and glucose into the epithelial cells
4. These reabsorbed molecules can then diffuse into blood capillaries

Question 2

What happens to the filtrate as it moves up the ascending limp of the loop of Henle?

Answer 2

1. Sodium and chloride ions are actively pumped out of the filtrate into the surrounding tissues of the medulla
2. This creates a low water potential in the medulla
3. As the ascending limb is impermeable to water, no water leaves the filtrate
4. Towards the top of the ascending limb, the water potential of the filtrate progressively increases as more ions are actively transported out

Question 3

What happens when osmoreceptors detects a decrease in water potential in the blood?

Answer 3

1. They produce ADH
2. ADH is transported to the posterior pituitary gland and secreted into the bloodstream
3. Blood transports ADH to the kidneys
4. ADH binds to receptor proteins on the cells lining the DCT and collecting duct
5. This stimulates second messanger cAMP which activates a cascade leading to the phosphorylation and integration of aquaporin molecules into the cell surface membrane
6. This increases the permeability of the cells to water
7. More water is reabsorbed from the filtrate into the blood
8. Smaller volume of more concentrated urine is produced

Question 4

Outline how a pregnancy test works

Answer 4

1. Wick is soaked in urine, which if a person is pregnant will contain hCG
2. Mobile monoclonal antibodies attached to coloured beads bind to hCG to form an antibody-hCG complex
3. Urine carries this complex to a window with immobilised monoclonal antibodies that will only bind to the antibody-hCG complex
4. This forms a coloured line or pattern indicating pregnancy
5. Other immobilised antibodies bind to mobile antibodies with or without hCG, forming a control line