Ultrafiltration

In the syllabus and the exam

Ultrafiltration is examined as the first and most physically distinct stage of urine formation, driven purely by pressure rather than by any active, energy-requiring transport. Candidates must explain that blood entering the glomerulus is under high pressure because the afferent vessel supplying it is wider than the efferent vessel leaving it, and this pressure forces small molecules, water, glucose, amino acids, salts and urea, out through the capillary walls and the wall of Bowman's capsule into the nephron, while blood cells and large plasma proteins remain in the blood because they are too large to pass through. Exam questions frequently ask candidates to name the fluid formed by ultrafiltration, called the glomerular filtrate, and to explain why proteins are absent from it under normal conditions, testing understanding of the filtering mechanism rather than mere recall of the word. A common error is describing ultrafiltration as a selective process; selectivity based on what the body needs is actually the role of the following stage, reabsorption, whereas ultrafiltration itself is a size-based, non-selective sieving of blood plasma.

Example

High blood pressure in the glomerulus forces water, glucose, salts and urea out of the blood during ultrafiltration.

Related terms

NephronOsmoregulationHomeostasis

Source:SRC-DSKP-EN