The loop of Henle and the counter-current multiplier — CCEA A2 Biology Practice Questions

Question 1

Explain how the loop of Henle enables the production of urine that is more concentrated than the blood plasma.

Accepted Answer

The ascending limb of the loop of Henle is impermeable to water, and sodium and chloride ions are actively transported out of it into the tissue fluid of the medulla. This lowers the water potential of the medulla tissue fluid. The descending limb is permeable to water, so as filtrate flows down it water leaves by osmosis into the medulla and the filtrate becomes more concentrated towards the hairpin. Because the two limbs run in opposite directions, a counter-current multiplier is set up that maintains a water potential gradient that becomes increasingly negative deeper into the medulla. The collecting duct passes back down through this medulla, so water moves out of the filtrate by osmosis along the gradient, through aquaporins whose number is controlled by ADH, and the urine leaving is more concentrated than the blood plasma.

Question 2

Describe how the body responds to a fall in the water potential of the blood.

Accepted Answer

Osmoreceptors in the hypothalamus detect the fall in the water potential of the blood. This stimulates the posterior pituitary gland to release more ADH into the blood. ADH increases the permeability of the collecting duct and distal convoluted tubule to water by causing more aquaporins to be inserted into the cell surface membranes. More water is therefore reabsorbed by osmosis into the blood, and a smaller volume of more concentrated urine is produced. The water potential of the blood rises back towards normal, which is negative feedback.

The loop of Henle and the counter-current multiplier: CCEA Biology practice questions

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Explain how the loop of Henle enables the production of urine that is more concentrated than the blood plasma.

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