Nephron Function (Edexcel IGCSE Biology)

• The kidneys filter the blood before reabsorbing useful substances prior to waste excretion
• The process occurs in the following order:
  1. Ultrafiltration
  2. Selective reabsorption of glucose
  3. Selective reabsorption of water and salts

Ultrafiltration

• Arterioles branch off the renal artery and lead to each nephron, where they form a bundle 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 blood pressure
• This eventually causes smaller molecules present in the blood to be forced out of the capillaries and into the Bowman’s capsule, where they form the glomerular filtrate; substances present in the filtrate include:
  • glucose
  • water
  • urea
  • salts
• Some of the components of the filtrate are useful and will be reabsorbed back into the blood further down the nephron

During ultrafiltration small molecules are forced out of the glomerulus and into the Bowman's capsule due to high pressure

• Note that proteins and red blood cells are too large to be filtered during ultrafiltration so are not present in kidney filtrate
  • The layers of cells and membrane that separate the capillaries of the glomerulus from the Bowman's capsule act like a sieve, preventing the passage of larger molecules and cells
• This means that the presence of protein or blood in the urine is a sign of damage to the glomerulus or Bowman's capsule, e.g. due to kidney disease or unusually high blood pressure

• After the glomerular filtrate enters the Bowman’s Capsule, glucose is the first substance to be reabsorbed at the proximal (first) convoluted tubule
  • This takes place by active transport
  • The nephron is adapted for this by having many mitochondria to provide energy for the active transport of glucose molecules
• Reabsorption of glucose cannot take place anywhere else in the nephron as the gates that facilitate the active transport of glucose are only found in the proximal convoluted tubule
• In a person with a normal blood glucose level, there are enough gates present to remove all of the glucose from the filtrate back into the blood
• People with diabetes cannot control their blood glucose levels and they are often very high, meaning that not all of the glucose filtered out can be reabsorbed into the blood in the proximal convoluted tubule
  • As there is nowhere else for the glucose to be reabsorbed, it continues in the filtrate and ends up in the urine
  • This is why one of the first tests a doctor may do to check if someone is diabetic is to test their urine for the presence of glucose

Diagram showing reabsorption in the nephron

• As the filtrate drips through the Loop of Henle necessary salts are reabsorbed back into the blood by diffusion and active transport
• As salts are reabsorbed back into the blood, water follows by osmosis
• Water is also reabsorbed from the collecting duct in different amounts depending on how much water the body needs at that time

Diagram showing reabsorption in the nephron