Control of Plasma Osmolality

What is the primary function of the loop of Henle in the nephron?

To generate an osmotic gradient in the medulla that allows concentration of urine.

What is reabsorbed in the descending limb of the loop of Henle?

Water only.

What is reabsorbed in the ascending limb of the loop of Henle?

Na⁺, K⁺, and Cl⁻ (ions), but not water.

What happens to the osmolarity of the filtrate in the descending limb?

It becomes more concentrated as water is reabsorbed.

What happens to the osmolarity in the ascending limb?

It becomes hyposmotic as solutes are reabsorbed but not water.

How does ADH affect the collecting duct?

It increases water permeability, allowing water reabsorption and production of concentrated urine.

What does aldosterone do in the distal tubule and collecting duct?

Increases Na⁺ reabsorption, contributing to fluid and electrolyte balance.

What does the counter-current multiplier refer to?

Solute transport that creates the medullary gradient in the loop of Henle.

What does the counter-current exchanger refer to?

Water and solute exchange in the vasa recta that maintains the osmotic gradient.

Where does filtrate become hyposmotic?

In the thick ascending limb of the loop of Henle.

Where is water and solute reabsorption hormonally regulated?

In the distal tubule and collecting duct.

What is the range of final urine osmolarity?

50 to 1200 mOsM depending on hydration and ADH levels.

How do nephron cells vary by function?

• PCT: Many mitochondria (active transport)

• Loop of Henle: Varying permeability

• Collecting duct: Hormone-responsive fine-tuning

Where is the osmolarity of interstitial fluid highest?

At the bottom of the loop of Henle (≈1200 mOsM).

What allows urine to become concentrated in the collecting duct?

The presence of the medullary osmotic gradient and ADH-regulated water permeability.