14.1.7—Nephron structure and function

Nephron segments are not equivalent tubes: their epithelial surfaces, transport capacity and water permeability differ. Those structural differences determine which substances can leave the filtrate, which are recovered to blood and how the filtrate is changed along its route.

Segment → structure/permeability → consequence:

• Proximal convoluted tubule: a brush border gives a large surface area, and many mitochondria provide ATP for active solute recovery; useful solutes are reabsorbed early.
• Descending limb of the loop of Henle: relatively permeable to water, so water can leave where the surrounding conditions allow; it is not equivalent to the ascending limb.
• Ascending limb: relatively impermeable to water and associated with ion movement, so it does not simply reabsorb water in the same way as the descending limb.
• Distal convoluted tubule: a later nephron segment where the filtrate composition can be adjusted before it enters the collecting duct.
• Collecting duct: its water permeability can be changed, so it determines how much water remains in the final urine; the hormone control of this change belongs to the osmoregulation card.

The nephron therefore uses structure as a control boundary: surface area and mitochondria support solute transport, contrasting loop permeability separates water and ion handling, and the collecting duct provides a final water-recovery checkpoint. The effects are segment-specific, not a single rule for the whole nephron.

The loop of Henle does not reabsorb water uniformly: its descending and ascending limbs have contrasting water permeability. This card explains segment structure and broad movement boundaries; it does not give the complete ADH/osmoregulation chain or any blood-glucose mechanism.