Loop of Henle

What is the loop of Henle?

A U-shaped tubule within the nephron that starts in the renal cortex and descends into the renal medulla

What are the two main sections of the loop of Henle?

The descending limb and ascending limb

How does the width of the descending limb compare with the ascending limb?

The descending limb is narrower

What is the descending limb permeable to?

Water

What is the descending limb impermeable to?

Ions

What is the ascending limb permeable to?

Sodium and chloride ions

What is the ascending limb impermeable to?

Water

What is the main function of the loop of Henle?

To decrease the water potential in the medulla and allow concentrated urine to be produced

How does the loop of Henle decrease the water potential of the medulla?

By actively transporting ions out of the filtrate into the medulla

How does water leave the descending limb?

By osmosis into the interstitial space

What happens to the water potential of filtrate as it moves down the descending limb?

It decreases

Where does the filtrate reach its lowest water potential?

At the tip of the loop in the medulla

How is water lost from the descending limb reabsorbed into the blood?

By osmosis into surrounding capillaries

Why can sodium and chloride ions leave the ascending limb?

The ascending limb is permeable to these ions

Where do sodium and chloride ions move from the ascending limb?

Into the interstitial space of the medulla

How are sodium and chloride ions removed from the top of the ascending limb?

By active transport

What happens to the water potential of the medulla deeper into the kidney?

It becomes increasingly lower

What water potential gradient is established in the medulla?

The highest water potential is in the cortex and the lowest is deep in the medulla

How does the loop of Henle allow water to leave the collecting duct?

It establishes a low water potential in the surrounding medulla

How does water leave the collecting duct?

By osmosis into the interstitial space and then surrounding capillaries

Why can water continue to leave the collecting duct deep in the medulla?

The loop of Henle establishes a very low water potential in the surrounding interstitial space

What is the water potential of urine leaving the collecting duct?

Very low

What is the countercurrent multiplier?

A system in which the loop of Henle intensifies the salt gradient in the kidney medulla

Why is the system called countercurrent?

The filtrate flows in opposite directions in the descending and ascending limbs

How does the countercurrent multiplier concentrate urine?

It maintains a water potential gradient that draws water out of the collecting duct

Why would parallel flow reduce water reabsorption?

It would produce a weaker water potential gradient