6.4.11 Control of Blood Water Potential

Structure of the Kidney

The kidney has a specialised structure to perform its regulatory functions:

• Outer fibrous capsule: Protects the kidney.
• Cortex: Contains Bowman's capsules, convoluted tubules, and blood vessels.
• Medulla: Contains loops of Henle, collecting ducts, and blood vessels.
• Renal pelvis: Collects urine and transfers it to the ureter.

The Process of Water Reabsorption

1. Ultrafiltration

• Blood enters the kidney through the renal artery, under high pressure.
• It passes into the afferent arteriole and then into a network of capillaries called the glomerulus.
• Small molecules such as water, glucose, ions, and urea are filtered into the Bowman's capsule, forming the glomerular filtrate.
• Large proteins and blood cells remain in the blood as they are too large to pass through.
• High pressure is maintained as the efferent arteriole exiting the glomerulus is narrower than the afferent arteriole.

2. Selective Reabsorption

• All glucose and necessary ions are reabsorbed in the proximal convoluted tubule (PCT).
• Reabsorption involves co-transport:
  1. Sodium ions are actively transported into the blood, creating a low sodium concentration in the epithelial cells.
  2. Sodium ions move into the cells from the tubule lumen via facilitated diffusion, bringing glucose or amino acids along.
  3. Glucose diffuses into blood capillaries.
• Water follows by osmosis due to the concentration gradient created by solute reabsorption.

3. The Loop of Henle

• Functions as a counter-current multiplier to concentrate solutes in the medulla, enabling water reabsorption:
  • Ascending limb:
  • Actively transports sodium and chloride ions into the medulla.
  • Impermeable to water, so water remains inside.
  • Descending limb:
  • Permeable to water, so water moves out into the medulla via osmosis due to the high solute concentration.
  • At the hairpin bend, the filtrate has its lowest water potential.
  • Sodium ions naturally diffuse out of the loop here.

4. Distal Convoluted Tubule (DCT) and Collecting Duct

• In the DCT, further selective reabsorption occurs.
• Water moves out of the collecting duct by osmosis due to the medulla's high ion concentration.
• The collecting duct's permeability to water is regulated by the hormone ADH (antidiuretic hormone).

Role of Hormonal Control in Water Reabsorption

Action of ADH:

1. Osmoreceptors in the hypothalamus detect changes in blood water potential.
2. If water potential falls, the osmoreceptors shrink, triggering the release of ADH from the posterior pituitary gland.
3. ADH travels in the blood to the kidneys, where it binds to receptors on the collecting duct cells.
4. This activates the enzyme phosphorylase, causing vesicles containing aquaporins to fuse with the cell surface membrane.
5. Aquaporins increase water permeability, allowing more water to be reabsorbed into the blood.

Urea and Water Reabsorption:

• Urea permeability also increases, further reducing water potential in the medulla.
• This enhances water movement out of the collecting duct by osmosis.

Summary of Key Steps in Regulation:

1. Filtration: Water and small molecules are filtered in the glomerulus.
2. Reabsorption: Glucose, ions, and water are selectively reabsorbed in the tubules.
3. Concentration: The loop of Henle creates a gradient for water reabsorption.
4. Hormonal Control: ADH adjusts collecting duct permeability to maintain hydration.