The mechanism of water reabsorption by the renal tubules is ________.

• A. filtration
• B. osmosis
• C. facilitated diffusion
• D. active transport

Answer: (B)

Verified step by step guidance

1. Understand the function of the renal tubules: The renal tubules are responsible for reabsorbing water and solutes from the filtrate back into the bloodstream. This process is essential for maintaining fluid and electrolyte balance in the body.
2. Review the mechanism of osmosis: Osmosis is the passive movement of water molecules across a semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. This process does not require energy and is driven by the concentration gradient of solutes.
3. Identify the role of osmotic gradients in the kidneys: In the renal tubules, water reabsorption occurs due to osmotic gradients created by the active transport of solutes, such as sodium ions (Na⁺), into the interstitial fluid. This increases the solute concentration outside the tubules, drawing water out of the filtrate by osmosis.
4. Differentiate osmosis from other mechanisms: Filtration refers to the movement of substances under pressure, facilitated diffusion involves carrier proteins for passive transport, and active transport requires energy to move substances against their concentration gradient. Osmosis is distinct because it is a passive process specifically for water movement.
5. Conclude the mechanism: Based on the explanation, water reabsorption in the renal tubules occurs primarily through osmosis, driven by the osmotic gradients established by solute transport.