In renal physiology, what is the primary mechanism by which water is reabsorbed from the filtrate back into the blood in the renal tubules of the kidney?

Introduction / Context:
The kidneys filter large volumes of plasma and then reabsorb most of the water so that the body does not lose excessive fluid. Understanding how water reabsorption occurs in renal tubules is important for grasping how urine concentration and volume are regulated. This question focuses on the main mechanism that drives water movement from tubular fluid back into the blood.

Given Data / Assumptions:
- Renal tubules reabsorb most of the filtered water.
- Solutes such as sodium and glucose are actively transported or cotransported in tubular cells.
- Water can follow solutes where permeability and osmotic gradients exist.

Concept / Approach:
Water reabsorption in the kidney is primarily a passive process driven by osmotic gradients. When solutes, especially sodium ions, are actively transported out of the tubular fluid into the interstitial fluid and then into peritubular capillaries, they create a higher solute concentration outside the tubule. Water then moves by osmosis through water channels such as aquaporins or across permeable membranes toward the higher solute concentration. There is no direct active pump for water in the renal tubule; instead, active transport of solutes sets up the conditions for passive osmotic flow of water.

Step-by-Step Solution:
Step 1: Recognize that water movement across a semipermeable membrane in response to solute concentration differences is called osmosis. Step 2: Recall that in the proximal tubule and other nephron segments, sodium and other solutes are reabsorbed by active transport and cotransport mechanisms. Step 3: Understand that these solute movements increase osmolarity of the interstitial fluid relative to the tubular fluid. Step 4: Note that water then follows passively by osmosis, moving from areas of lower solute concentration to areas of higher solute concentration. Step 5: Conclude that osmosis is the primary mechanism of water reabsorption in the renal tubules.

Verification / Alternative check:
Physiology textbooks state that approximately two thirds of filtered water is reabsorbed in the proximal convoluted tubule, where solute reabsorption is very high. They emphasize that water reabsorption is passive and follows solutes osmotically, not by direct active pumping. The role of antidiuretic hormone in increasing water reabsorption in the collecting ducts by inserting aquaporin channels further supports the importance of osmosis.

Why Other Options Are Wrong:
Option A: Solvent drag describes solutes moving with water flow, not water moving as a result of its own active or passive transport, so it is not the main mechanism of water reabsorption.
Option C: There is no known active transport pump that directly uses ATP to move water across renal tubular cells; water movement is passive and driven by osmotic gradients.
Option D: Cotransport with sodium ions is a mechanism for reabsorbing solutes such as glucose and amino acids, but water is not cotransported as a solute; it moves separately by osmosis.

Common Pitfalls:
A common misunderstanding is to assume that because sodium is actively transported, water must also be actively transported. In reality, only solutes are actively pumped, and water follows passively. Another pitfall is ignoring the role of permeability; some segments, like the descending limb of the loop of Henle, are highly water permeable, while others are not, which affects where osmosis can occur.

Final Answer:
The correct answer is osmosis, in which water moves passively across membranes down its concentration gradient because water reabsorption from the renal tubules into the blood is driven by osmotic gradients set up by solute transport.