2.75B Ultrafiltration (Edexcel IGCSE Biology)

• In the kidney, blood is filtered before the key substances are reabsorbed back into the blood again
• The process occurs in the following order:

1. 1. Ultrafiltration
   2. Selective reabsorption of glucose
   3. Reabsorption of water and salts

Ultrafiltration

• Arterioles branch off the renal artery and lead to each nephron, where they form a knot of capillaries (the glomerulus) sitting inside the cup-shaped Bowman’s capsule
• The capillaries get narrower as they get further into the glomerulus which increases the pressure on the blood moving through them (which is already at high pressure because it is coming directly from the renal artery which is connected to the aorta)
• This eventually causes the smaller molecules being carried in the blood to be forced out of the capillaries and into the Bowman’s capsule, where they form what is known as the filtrate
• This process is known as ultrafiltration
• The substances forced out of the capillaries are glucose, water, urea, salts
• Some of these are useful and will be reabsorbed back into the blood further down the nephron

Diagram showing the process of ultrafiltration

Components of Filtrate Table