E1.1 Osmoregulation QB

Question 1

DSE 2017
Explain why there is no change in the solute concentration of the fluid inside the tubule as the fluid flows from point A (Bowman’s capsule) to point B (first coiled tubule).

Answer 1

• Some solutes in the fluid inside the tubule (e.g. glucose, sodium ion, amino acids) are reabsorbed from the first coiled tubule (between point A and point B) back into blood in the surrounding blood capillaries by diffusion and active transport. (1)
• Hence, water potential of the blood surrounding the first coiled tubule decreases / water potential of the fluid in the first coiled tubule increases. (1)
• As a result, water is also reabsorbed from the fluid in the first coiled tubule down the water potential gradient by osmosis. (1)
• The proportion of water reabsorbed equals the proportion of solute re-absorbed between point A and point B, so the solute concentration remains the same as the fluid flows from point A to point B. (1)

Question 2

Explain how most of the water is reabsorbed regardless of hormonal control.

Answer 2

As most of the useful substances in the glomerular filtrate are reabsorbed into the blood by diffusion and active transport (1)
The water potential of blood is much lower than the remaining fluid in the kidney tubules (1)

Hence, there is a net flow of water from the filtrate in the kidney tubules to the blood in the blood capillaries by osmosis. (1)

Question 3

State one similarity and one difference between the functioning of the artificial membrane in the dialysis machine and that of the wall of nephrons.

Answer 3

Similarity: both the dialysis membrane and the wall of the nephron allow small molecules such as urea and other waste materials to pass through but not plasma proteins and blood cells (1)
Difference: the cells of the wall of the nephron actively reabsorb useful substances (e.g. glucose) into the blood, while the dialysis membrane has no such function