Label I and II.

Outline the function of III.

Estimate the content of glomerular filtrate and urine of a healthy person by completing the following table.

Explain the role of the medulla and the collecting duct of the kidney in the maintenance of the water balance in blood.

I: glomerulus;
II: (descending limb of) loop of Henle;
(both needed)

III: selective re-absorption of glucose/minerals/amino acids/water/ useful substances;
absorption by active transport/using ATP of glucose/minerals/ amino acids/useful substances;

Award [1] for each correct row.

collecting duct has water channels/aquaporins/is permeable to water;
high solute concentration of medulla / medulla is hypertonic;
reabsorption of water allows excretion of concentrated urine (antidiuresis);
secretion of ADH/vasopressin increases permeability of collecting duct to water / vice versa;

Many candidates were able to name the glomerulus and the loop of Henle in (a)(i).

Part (a) (ii) is another question where brief answers did not always gain the mark. A simple statement that glucose is reabsorbed in the proximal convoluted tubule was not enough. Two types of qualification were needed; either that reabsorption is selective or that nutrients are reabsorbed by active transport.

For part (b) there were some comments from teachers suggesting that knowledge of actual concentrations of solutes in glomerular filtrate and urine should not be expected. The question did not expect concentrations to have been memorized. Both of the marks could be gained if the candidate knew that protein is not filtered out of the blood, but that urea is, and that glucose is all reabsorbed but urea becomes more concentrated as water is reabsorbed from the filtrate.

There was some confusion among candidates in part (c) about the meaning of the word medulla in this question. A common error was to assume that it meant the medulla oblongata and ascribe a role in the maintenance of water balance to this part of the brain. In fact the question referred to the medulla of the kidney, with the hypothalamus and pituitary gland as the regulatory centres. There were some excellent answers to this question showing a very secure understanding of kidney physiology.