Explain the process of ultrafiltration.

The diagram below shows part of the human kidney. The arrow shows the direction of blood flow.

Compare the composition of the fluids found in the regions labelled I and II by giving one difference and one similarity.

Difference:        ............................................................           
Similarity:        ............................................................          

blood (in the glomerulus) under high pressure caused by difference in diameter of (afferent and efferent) arterioles;
fluid plasma and small molecules forced into kidney tubule/Bowman’s capsule/ through fenestrations/basal membrane;
which prevent larger molecules/blood cells from passing through;

difference: fluid at II has less urea/glucose/oxygen/salts/ions/water;
similarity: fluid at II has same (amount of) proteins/blood cells as fluid at I;
Accept converse for both marking points.

Many candidates had general knowledge of ultrafiltration but did not express it clearly and concisely. They mentioned high pressure without explaining the causes or that there was filtration of some substances and not others, but again without an explanation.

Many candidates lost marks here for lack of precision in their answers. To get marks, they had to refer to amounts, or the composition of the fluids in zones I and II. Many said there was no glucose or urea in II instead of smaller concentrations. Others said there were blood cells or large proteins in both, but did not state that there were present in the same amounts.