Problem 4 - Campbell Biology

Question

The high osmolarity of the renal medulla is maintained by all of the following except \begin{equation}\begin{array}{l}{\text { (A) active transport of salt from the upper region of the }} \\ {\text { ascending limb. }} \\ {\text { (B) the spatial arrangement of juxtamedullary nephrons. }} \\ {\text { (C) diffusion of urea from the collecting duct. }} \\ {\text { (D) diffusion of salt from the descending limb of the loop }} \\ {\text { of Henle. }}\end{array}\end{equation}

Step-by-Step Solution

Answer

Option (D) diffusion of salt from the descending limb of the loop of Henle.

Step 1: Understand the question

The question asks which mechanism is NOT involved in maintaining the high osmolarity of the renal medulla. You need to identify the incorrect option.

Step 2: Review the role of active salt transport

Option (A) describes the active transport of salt from the upper region of the ascending limb of Henle. This is a key process in maintaining the osmolarity gradient because the ascending limb is impermeable to water and actively transports NaCl into the medulla.

Step 3: Examine the spatial arrangement of nephrons

Option (B) mentions the spatial arrangement of juxtamedullary nephrons. Juxtamedullary nephrons have long loops that extend deep into the medulla, which is crucial for creating and maintaining the osmolarity gradient.

Step 4: Analyze the diffusion of urea

Option (C) involves the diffusion of urea from the collecting duct. Urea diffuses into the medulla, contributing to the high osmolarity of the medulla, especially deep in the inner medulla.

Step 5: Assess salt diffusion in the loop of Henle

Option (D) suggests the diffusion of salt from the descending limb of the loop of Henle. However, the descending limb is largely impermeable to salts; it primarily allows water to leave and does not contribute salts to the medulla.

Step 6: Identify the incorrect process

Based on the examination of each process, the diffusion of salt from the descending limb (Option D) is not a mechanism that maintains the high osmolarity of the renal medulla.

Key Concepts

Active Transport

Active transport plays a vital role in maintaining the osmolarity of the renal medulla. One of the key areas where this occurs is in the ascending limb of the loop of Henle. Here, cells actively pump sodium chloride (NaCl) from the tubular fluid into the surrounding interstitial fluid. This process requires energy, usually derived from ATP, because it moves NaCl against its concentration gradient. Since the ascending limb is impermeable to water, this active transport of salt significantly contributes to the osmolarity gradient in the medulla. This osmolarity gradient is crucial for the kidney's ability to concentrate urine, especially under conditions of low water availability.

• Active transport of NaCl takes energy
• Occurs in the ascending limb of the loop of Henle
• Contributes to the osmolarity gradient in the renal medulla

Juxtamedullary Nephrons

Juxtamedullary nephrons are specialized nephrons located near the boundary of the renal cortex and medulla. These nephrons have longer loops of Henle that extend deep into the medulla. This structural arrangement is essential for creating and maintaining the high osmolarity of the renal medulla. Because of their deep loops, these nephrons facilitate the counter-current multiplication system. This system relies on a gradient of osmolarity that increases from the cortex to the inner medulla, allowing for the concentration of urine.

• Located near the boundary of the cortex and medulla
• Have longer loops of Henle
• Essential for the counter-current multiplication system

Urea Diffusion

Urea plays a significant role in maintaining the osmolarity gradient of the renal medulla. Once urine is formed, it flows through the collecting ducts where urea can diffuse back into the interstitial fluid of the medulla. This happens mainly in the inner medulla and helps to increase the osmolarity in the deeper parts of the medulla. By contributing to the osmotic gradient, urea assists in drawing water out of the collecting ducts, thus concentrating the urine.

• Occurs mainly in the collecting ducts
• Significantly contributes to the osmolarity of the inner medulla
• Helps in concentrating urine

Loop of Henle

The loop of Henle consists of descending and ascending limbs, each playing distinct roles in regulating the osmolarity of the renal medulla. The descending limb is highly permeable to water but impermeable to salts, allowing water to leave the tubular fluid and enter the interstitial fluid, thus concentrating the fluid inside the tube. Conversely, the ascending limb is impermeable to water but actively transports NaCl into the interstitial fluid. By selectively allowing substances to enter or leave, the loop of Henle creates a steep osmolarity gradient required for the kidney's function.

• Descending limb: permeable to water, impermeable to salts
• Ascending limb: impermeable to water, actively transports NaCl
• Creates an osmolarity gradient essential for renal function