Most common site of water absorption in nephrons is?

NORCET 5 Prelims - Sept 2023 (Shift-2)

Applied Physiology

Easy

Appeared in: NORCET 5 Prelims - Sept 2023 (Shift-2)

Explanation

The Proximal Convoluted Tubule (PCT) is the primary site for water reabsorption, handling approximately 65-70% of the water filtered by the glomerulus.
This process is known as obligatory water reabsorption, where water passively follows the active transport of solutes like sodium and glucose via osmosis.
The cells lining the PCT have a very high permeability to water, which facilitates this large volume of reabsorption.
The key differentiator is the sheer volume; the PCT reabsorbs more water than all other segments combined.

Option B: The Collecting Tubule (CT) is involved in the fine-tuning of water reabsorption, not bulk absorption. It reabsorbs a smaller, variable amount of water.
Option C: Similar to the collecting tubule, the Distal Convoluted Tubule (DCT) reabsorbs a relatively small and variable amount of water (5-10%) under the control of ADH.
Option D: While the descending limb of the Loop of Henle is permeable to water and reabsorbs about 15-20%, this volume is significantly less than that reabsorbed in the PCT.

Visual 1: Diagram of the Nephron. A labeled diagram showing the glomerulus, PCT, Loop of Henle, DCT, and collecting duct, with arrows indicating the percentage of water reabsorbed at each site.
Visual 2: Flowchart of Filtrate Processing. A flowchart illustrating the journey of filtrate, highlighting the major reabsorptive and secretory events in each segment of the nephron.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Renal Physiology: Water Reabsorption in the Nephron as background academic context rather than a clinical decision trigger.
Understanding that the PCT is the site of bulk reabsorption is fundamental to understanding renal physiology and the effects of certain medications.
Diuretics that act on the PCT, such as osmotic diuretics (e.g., mannitol), cause a significant increase in urine output by preventing this initial bulk reabsorption of water.
Damage to the PCT, as seen in acute tubular necrosis (ATN), can lead to severe fluid and electrolyte imbalances due to the failure of this massive reabsorptive process.

How to Approach the Question

First, identify the core physiological process in the question: water absorption in the nephron.
The keyword 'most common' is a cue to look for the location where the largest quantity or percentage of absorption occurs.
Recall the main segments of the nephron: PCT, Loop of Henle, DCT, and CT.
Compare the function of each segment. Remember that the PCT is the 'workhorse' for bulk reabsorption, while the distal segments are for 'fine-tuning'.
Quantify the reabsorption if possible. The PCT reabsorbs ~65-70%, which is clearly the majority.
Select the option corresponding to the segment with the highest reabsorption percentage.

Concept Tested & Keywords

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