Question 1

Why do juxtamedullary nephrons produce dilute urine during high fluid intake?

Accepted Answer

Juxtamedullary nephrons produce dilute urine during high fluid intake because antidiuretic hormone (ADH) levels decrease, making the late distal convoluted tubule and collecting ducts impermeable to water. Although ions continue to be reabsorbed, water remains in the tubular lumen and is excreted as dilute urine, preventing fluid overload.

Question 2

What happens to filtrate osmolarity as it moves through the proximal convoluted tubule?

Accepted Answer

In the proximal convoluted tubule, filtrate osmolarity remains isosmotic with the surrounding renal cortex because both solutes and water are reabsorbed equally. This balanced reabsorption maintains the osmolarity of the filtrate at the same level as the interstitial fluid in the cortex.

Question 3

How does the ascending limb of the nephron loop contribute to dilute urine formation?

Accepted Answer

The ascending limb actively transports ions like sodium, potassium, and chloride into the interstitial fluid but is impermeable to water. As ions leave the filtrate without water following, the filtrate becomes progressively more dilute, reducing its osmolarity as it moves toward the distal convoluted tubule.

Question 4

What role does the descending limb play in establishing the osmotic gradient for dilute urine?

Accepted Answer

The descending limb is permeable to water but impermeable to ions. As the filtrate enters the renal medulla where interstitial fluid osmolarity is significantly higher, water passively exits the descending limb, increasing the filtrate's osmolarity and establishing the osmotic gradient necessary for subsequent ion reabsorption.

Question 5

Why does the early distal convoluted tubule continue to dilute the filtrate?

Accepted Answer

The early distal convoluted tubule reabsorbs ions but remains impermeable to water, similar to the ascending limb. This selective ion reabsorption without water reabsorption further reduces the filtrate's osmolarity, continuing the dilution process that began in the ascending limb of the nephron loop.

Question 6

How does low ADH during overhydration affect water reabsorption in the collecting duct?

Accepted Answer

During overhydration, low ADH levels decrease the permeability of the late distal convoluted tubule and collecting ducts to water. This prevents water reabsorption despite continued ion reabsorption, allowing excess water to remain in the tubular lumen and be excreted as dilute urine.

Question 7

What is the relationship between osmotic gradient and water movement in the descending limb?

Accepted Answer

The osmotic gradient between the renal medulla's high-osmolarity interstitial fluid and the filtrate in the descending limb drives passive water movement out of the tubule. This water reabsorption concentrates the filtrate while maintaining the medullary osmotic gradient essential for the countercurrent multiplier system.

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