6.4.10 The Function of the Nephron

1. Ultrafiltration

• Blood enters the kidney via the renal artery under high pressure.
• The artery branches into the afferent arteriole, leading to a capillary network called the glomerulus.
• Water and small molecules (e.g., glucose, ions, urea) are forced out into the Bowman's capsule, forming the glomerular filtrate.
• Large molecules, such as proteins and blood cells, remain in the blood as they are too large to pass through.
• High pressure in the glomerulus is maintained because the efferent arteriole leaving the glomerulus is narrower than the afferent arteriole entering it.

2. Selective Reabsorption

• Occurs mainly in the proximal convoluted tubule (PCT).
• Glucose, amino acids, and some salts are reabsorbed into the blood.
• Process involves co-transport:
  1. Sodium ions are actively transported from the PCT epithelial cells into the blood.
  2. This creates a low sodium concentration in the epithelial cells.
  3. Sodium ions move into the epithelial cells from the tubule lumen via facilitated diffusion, bringing glucose or amino acids with them.
  4. Glucose then diffuses into blood capillaries.
• Water follows by osmosis due to the concentration gradient created by solute reabsorption.

3. The Loop of Henle

• Acts as a counter-current multiplier to concentrate solutes in the medulla, aiding water reabsorption.
  • Ascending limb:
  • Actively transports sodium and chloride ions out into the interstitial space using ATP.
  • Impermeable to water, so water remains inside.
  • Descending limb:
  • Permeable to water, so water moves out into the interstitial space by osmosis where the solute concentration is high.
  • At the hairpin bend, the filtrate reaches its lowest water potential.
• Sodium ions diffuse out at the bottom of the ascending limb due to the steep concentration gradient.

4. Distal Convoluted Tubule (DCT) & Collecting Duct

• DCT fine-tunes the concentration of solutes in the filtrate through selective reabsorption.
• Water moves out of the collecting duct by osmosis, following the high solute concentration gradient in the medulla.
• As the duct descends deeper into the medulla, the ion concentration increases, allowing more water to leave.

Role of ADH (Antidiuretic Hormone):

• Osmoreceptors in the hypothalamus detect changes in blood water potential.
• If water potential decreases: 5. Osmoreceptors shrink, stimulating the release of ADH from the posterior pituitary gland. 6. ADH binds to receptors on collecting duct cells, activating the enzyme phosphorylase. 7. Vesicles containing aquaporins fuse with the cell membrane, increasing the duct's permeability to water. 8. Urea permeability also increases, further reducing water potential in the medulla, drawing more water out by osmosis.
• This allows more water to be reabsorbed into the blood, reducing water loss in urine.

Summary of Key Functions:

• Ultrafiltration: Filters blood to remove small molecules into the nephron.
• Selective Reabsorption: Recovers glucose, salts, and water back into the blood.
• Loop of Henle: Concentrates solutes in the medulla to aid water reabsorption.
• ADH Regulation: Controls water reabsorption in the collecting duct to maintain hydration.