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Controlling Blood Water Potential

Biology: AQA A Level, topic 6.4.9

Glomerular Filtrate

The first stage in osmoregulation is the formation of the glomerular filtrate. This process takes place in the Bowman's capsule. The steps involved are:

1) Pressure filtration

1) Pressure filtration

  • The branch of capillary that enters the glomerulus is much wider than the branch that exits the glomerulus. This creates a high blood pressure in the glomerulus.
  • The high blood pressure causes the fluid and its solutes (e.g. glucose, amino acids) in the blood to be forced out of the capillary.
  • This is called pressure filtration.
2) Capillary endothelium

2) Capillary endothelium

  • The fluid flows through the pores in the capillary endothelium.
3) Basement membrane

3) Basement membrane

  • Then the smaller molecules filter through slit pores in the basement membrane. This is a mesh of collagen fibres and glycoprotein.
  • Most proteins and all blood cells are too big to pass through the slit pores.
4) Podocytes

4) Podocytes

  • The substances finally pass between the epithelial cells of the Bowman's capsule.
  • The epithelial cells, called podocytes, have finger-like projections that the substances can flow between.
5) Glomerular filtrate

5) Glomerular filtrate

  • The fluid that has filtered from the capillaries to the Bowman's capsule is called the glomerular filtrate.
  • The filtrate contains:
  • Water.
  • Amino acids.
  • Urea.
  • Glucose.
  • Inorganic ions.

Proximal Convoluted Tubule

Most of the substances in the glomerular filtrate are selectively reabsorbed into the bloodstream in the proximal convoluted tubule (PCT). The steps involved are:

1) Sodium-potassium pumps

1) Sodium-potassium pumps

  • Na+ ions are actively transported out of the PCT epithelial cells and into the blood by sodium-potassium pumps.
  • K+ ions are also transported into the epithelium.
2) Co-transporter proteins

2) Co-transporter proteins

  • Active transport of Na+ ions causes the concentration of Na+ ions inside the epithelial cells to decrease.
  • Na+ ions in the filtrate diffuse into the epithelial cells (down their concentration gradient) through co-transporter proteins.
  • Co-transporter proteins allow glucose and amino acids to be transported into the epithelial cells along with the Na+ ions.
3) Reabsorption of glucose and amino acids

3) Reabsorption of glucose and amino acids

  • As glucose and the amino acids are co-transported into the PCT epithelial cells, their concentration increases inside the cells.
  • Glucose and the amino acids diffuse down the concentration gradient into the blood.
  • Blood pressure is relatively high so that the substances in the blood are carried away quickly. This maintains a steep concentration gradient.
4) Reabsorption of water

4) Reabsorption of water

  • The movement of Na+ ions, glucose and amino acids into the bloodstream causes the water potential to decrease in the blood and increase in the PCT.
  • Water in the PCT diffuses into the blood through osmosis.
  • Any substances that are not reabsorbed are excreted as waste.

Loop of Henle

The loop of Henle creates a region of low water potential and high sodium concentration in the medulla of the kidney. This allows water to be reabsorbed in the collecting duct. The steps involved are:

1) Top of the ascending limb

1) Top of the ascending limb

  • Na+ ions are actively transported out of the top of the ascending limb into the surrounding tissue fluid in the medulla.
  • This causes the solute concentration of the medulla to increase and the water potential to decrease.
  • The ascending limb is impermeable to water. This means water inside the tubule cannot diffuse out.
2) Bottom of the ascending limb

2) Bottom of the ascending limb

  • Na+ ions diffuse out of the bottom of the ascending limb into the medulla.
  • This further increases the solute concentration of medulla.
3) The descending limb

3) The descending limb

  • The descending limb is permeable to water. This means that water inside the tubule can diffuse out because there is a lower water potential in the medulla.
  • The water is reabsorbed by the bloodstream.
4) Reabsorption of water

4) Reabsorption of water

  • The overall effect of the descending and ascending limb is to create a high solute concentration and low water potential in the tissue fluid surrounding the collecting duct.
  • This causes the water inside the collecting duct to diffuse into the surrounding tissue fluid by osmosis.
  • The water is then reabsorbed into the bloodstream.
5) Osmoregulation

5) Osmoregulation

  • The volume of water that is reabsorbed into the bloodstream depends on the permeability of the collecting duct.
  • The permeability of the collecting duct varies according to the water potential of the blood.
  • If the water potential is high, the collecting duct is less permeable and less water is absorbed in the blood.
  • If the water potential is low, the collecting duct is more permeable and more water is absorbed in the blood.

Test yourself on Controlling Blood Water Potential

  • Which of the following can NOT filter through the basement membrane?

  • Stages in Formation of the Glomerula Filtrate

  • What are the steps involved in selective reabsorption?

  • What happens to the substances that are not reabsorbed by selective reabsorption?

  • Which of the following statements about selective reabsorption are true?

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