Renal System 4: What is reabsorbed and where?

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Fluid Homeostasis

On average, the extracellular fluid, i.e. the peritubular fluid, has an osmolarity of 300mosm/L. This means it has a certain number of solute molecules per litre of water in the fluid. Below is a summary of how and where solutes and water are reabsorbed along the nephron system.

Proximal Convoluted Tubule

65% Na+ and 65% H2O

Sodium reabsorption provided by:
  • Epithelial Na channel (ENaC)
  • Co-transport with glucose (SGLT)
  • Exchange pump for Na+ and K+
Water reabsorption provided by:
  • Osmosis

Nephron Loop (Loop of Henle)

25% Na+ and 15% H2O

Sodium reabsorption provided by:
  • Co-transport with K+ and Cl- ions (thick ascending limb only)
  • Exchange pump for Na+ and K+
Water reabsorption provided by:
  • Osmosis (thin descending limb)
NOTE: The thick ascending limb has a high efficiency when pumping Na+ out of the tubular fluid alongside potassium and chloride ions. It is impermeable to water and decreases the tubular fluid concentration. The thin descending limb has a high efficiency when reabsorbing water and is impermeable to solutes. It causes an increase in the osmotic concentration of the tubular fluid.

Countercurrent Multiplication

Countercurrent multiplication occurs between the nephron loop and the vasa recta (surrounding capillary). The flow of tubular fluid is in the opposite direction to the blood. When solutes move out of the thick ascending limb, they are reabsorbed straight into the vasa recta. However, as the blood flows in the vasa recta towards the thin descending limb, remembering this flow is opposite to filtrate flow, the high solute concentration attracts the water removed from the tubule, getting the blood osmolarity back to 300mosm/L.

Countercurrent multiplication efficiently reabsorbs solutes and water before the tubular fluid reaches the DCT and collecting system. It also establishes a concentration gradient, permitting passive reabsorption of water from the tubular fluid in the collecting system (regulated by ADH).


Distal Convoluted Tubule + Collecting System

9% Na+ and 19% H2O

The distal convoluted tubule and collecting ducts are the main sites of hormonal control.

Sodium reabsorption is stimulated by aldosterone through Na channels and a Na+ and K+ exchange pump.

Water reabsorption in the DCT is stimulated by ADH and is through aquaporins.

Water reabsorption in the collecting ducts (medulla region) is also stimulated by ADH through aquaporins.

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