7.20 Selective Reabsorption in the Kidney

• The nephron is the functional unit of the kidney and is responsible for the formation of urine
• The process of urine formation in the kidneys occurs in two stages
  • Ultrafiltration
  • Selective reabsorption
• Ultrafiltration involves filtering small molecules from the blood at high pressure
  • This occurs between the glomerulus and the bowman's capsule
• Selective reabsorption allows the kidney to reabsorb useful small molecules into the blood

Selective reabsorption

• Many of the substances that pass into the glomerular filtrate are useful to the body
• These substances are therefore reabsorbed into the blood as the filtrate passes along the nephron
• This process is known as selective reabsorption since not all substances are reabsorbed
  • Reabsorbed substances include water, salts, glucose, and amino acids
• Most of this reabsorption occurs in the proximal convoluted tubule
  • Note that while water and salts are reabsorbed in the proximal convoluted tubule, the loop of Henle and collecting duct are also involved in the reabsorption of these substances

Selective reabsorption of useful substances occurs in the proximal convoluted tubule, the loop of Henle and the collecting duct

• The lining of the proximal convoluted tubule is composed of a single layer of epithelial cells which are adapted to carry out reabsorption in several ways
  • Microvilli
    • Microvilli are tiny finger-like projections on the surface of epithelial cells which increase the surface area for diffusion
  • Co-transporter proteins
  • Many mitochondria
  • Tightly packed cells

Adaptations for Selective Reabsorption Table

Molecules reabsorbed from the Proximal Convoluted Tubule

• Sodium ions (Na⁺) are transported from the proximal convoluted tubule into the surrounding tissues by active transport
• The positively charged sodium ions creates an electrical gradient, causing chloride ions (Cl^-) to follow by diffusion
• Sugars and amino acids are transported into the surrounding tissues by co-transporter proteins which also transport sodium ions
• The movement of ions, sugars, and amino acids into the surrounding tissues lowers the water potential of the tissues, so water leaves the proximal convoluted tubule by osmosis
• Urea moves out of the proximal convoluted tubule from a high to a low concentration by diffusion
• All of the substances that leave the proximal convoluted tubule for the surrounding tissues eventually make their way into nearby capillaries down their concentration gradients

Sodium, amino acids, and glucose are reabsorbed from the proximal convoluted tubule by an active process that involves co-transporter proteins.

The role of the loop of Henle

• Many animals deal with the excretion of the toxic waste product urea by dissolving it in water and excreting it
• While this method of excretion works well, it brings with it the problem of water loss
• The role of the loop of Henle is to enable the production of urine that is more concentrated than the blood, and to therefore conserve water
  • Note that it is also possible to produce urine that is less concentrated than the blood; this is important when water intake is high to prevent blood becoming too dilute
• The loop of Henle achieves this by the use of a countercurrent multiplier system
  • Countercurrent refers to the opposite directions of filtrate flow in the descending and ascending limbs of the loop of Henle
  • Multiplier refers to the steep concentration gradient that the loop of Henle is able to generate across the medulla

The process in the loop of Henle

• Sodium and chloride ions move out of the filtrate in the ascending limb of the loop of Henle into the surrounding medulla region, lowering its water potential
  • The movement of ions occurs by both diffusion and active transport
    • Diffusion takes place in the first part of the ascending limb
    • Active transport occurs in the second part of the ascending limb 
  • The ascending limb of the loop of Henle is impermeable to water, so water is unable to leave the loop here by osmosis
  • The water potential in the ascending limb increases as it rises back into the cortex due to the removal of solutes and retention of water
• The neighbouring descending limb is permeable to water, so water moves out of the descending limb by osmosis due to the low water potential in the medulla created by the ascending limb
  • The descending limb has few transport proteins in the membranes of its cells, so has low permeability to ions
  • The water potential of the filtrate decreases as the descending limb moves down into the medulla due to the loss of water and retention of ions
• The water and ions that leave the loop of Henle for the medulla make their way into the nearby capillary network

The loop of Henle acts as a countercurrent multiplier, maximising the reabsorption of water by creating a steep water potential gradient across the medulla