Nephron Function (Edexcel IGCSE Biology): Revision Note

Formation of urine

• The human body is constantly producing metabolic waste (eg. urea)

• The kidneys filter the blood to remove waste products, forming urine

• However it’s important that useful substances (eg. glucose) or too much water, don’t end up in urine

• Three main processes occur in the nephron to form urine:

  • 1. Ultrafiltration

  • 2. Selective reabsorption of glucose

  • 3. Reabsorption of water

Ultrafiltration

• Ultrafiltration is the first stage of urine formation

• Ultrafiltration occurs in the glomerulus

• The walls of the capillaries of the glomerulus and the Bowman’s capsule are separated by a thin membrane that acts like a filter (or sieve)

  • Small molecules can pass through the membrane but large molecules cannot

• Blood in the glomerulus is under high pressure, forcing small molecules such as water, glucose and urea out of the blood into the Bowman’s capsule

• Larger molecules in the blood (eg. proteins) and blood cells are too big to pass through the membrane, so they remain in the blood in the capillaries 

• The fluid that collects in the Bowman’s capsule is called the glomerular filtrate

• Substances present in the glomerular filtrate are:

  • Water

  • Glucose

  • Urea

  • Ions (salts)

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

Selective Reabsorption

• After ultrafiltration, the glomerular filtrate passes through the proximal convoluted tube (PCT)

• The body does not want to excrete (lose) any glucose, so all of the glucose present in the glomerular filtrate is selectively reabsorbed in the PCT

• This process is called selective reabsorption because glucose, a useful substance, is reabsorbed back into the bloodstream while waste products like urea remain in the filtrate

  • Remember glucose is required for respiration, as well as maintaining a blood glucose concentration

• Glucose is reabsorbed from the filtrate in the PCT into the capillaries by active transport, which moves glucose against its concentration gradient. 

• The cells of the PCT contain many mitochondria to provide the energy (ATP) needed for active transport of glucose

Diagram showing reabsorption in the nephron

Reabsorption of Water

• The final step for urine formation is reabsorption of water, which is part of the process of osmoregulation — maintaining a water levels in the body

• After selective reabsorption of glucose in the PCT, the filtrate passes through the rest of the nephron: the loop of Henle, distal convoluted tubule (DCT), and collecting duct  

• Some water reabsorption from the filtrate occurs in the loop of Henle, but the majority of water reabsorption occurs from the collecting duct

• Water is absorbed into the blood from the collecting duct by osmosis. The amount of water absorbed from the filtrate is dependent on the concentration of the hormone ADH (antidiuretic hormone), which is released by the pituitary gland in the brain:

  • More ADH → more water reabsorbed, producing concentrated urine

  • Less ADH → less water reabsorbed, producing dilute urine

Table summarising ultrafiltration, selective reabsorption & water absorption