The high osmolarity of the renal medulla is maintained by all of the following except a. active transport of salt from the upper region of the ascending limb. b. the spatial arrangement of juxtamedullary nephrons. c. diffusion of urea from the collecting duct. d. diffusion of salt from the descending limb of the loop of Henle.

Verified step by step guidance

1. Identify the function of each part of the nephron mentioned in the options, focusing on how they contribute to the osmolarity of the renal medulla.
2. Understand that the ascending limb of the loop of Henle is impermeable to water but actively transports salt into the medullary interstitium, thus increasing the medulla's osmolarity.
3. Recognize that juxtamedullary nephrons, with their long loops of Henle, are crucial for establishing a high osmolarity in the renal medulla due to their ability to transport salts and urea effectively.
4. Consider the role of the collecting duct, which is permeable to urea under the influence of certain hormones, allowing urea to diffuse into the medulla, contributing to its high osmolarity.
5. Evaluate the role of the descending limb of the loop of Henle, which is primarily permeable to water and not to salt, and how this affects the osmolarity of the renal medulla.

Key Concepts

Osmolarity in the Renal Medulla

Osmolarity refers to the concentration of solutes in a solution. In the renal medulla, high osmolarity is crucial for the kidney's ability to concentrate urine and conserve water. This is achieved through the countercurrent multiplication mechanism, which involves the interaction between different segments of the nephron, particularly the loop of Henle.

Loop of Henle Function

The loop of Henle consists of descending and ascending limbs that play a vital role in creating a concentration gradient in the renal medulla. The descending limb is permeable to water but not to salts, while the ascending limb actively transports salts out, contributing to the osmotic gradient necessary for water reabsorption in the collecting duct.

Juxtamedullary Nephrons

Juxtamedullary nephrons are a type of nephron located near the border of the renal cortex and medulla. They have long loops of Henle that extend deep into the medulla, which enhances the kidney's ability to concentrate urine. Their spatial arrangement is essential for maintaining the osmotic gradient that supports water reabsorption.