How is Na⁺ reabsorbed?

• A. by active transport using ATP
• B. by receptor-mediated endocytosis
• C. by diffusion
• D. by osmosis
• E. by facilitated diffusion

Answer: (A)

Verified step by step guidance

1. Understand the role of Na+ (sodium ions) in the body, particularly in maintaining fluid balance and nerve function.
2. Recognize that Na+ reabsorption primarily occurs in the kidneys, specifically in the nephron, which is the functional unit of the kidney.
3. Identify the mechanism of active transport, which involves the movement of ions against their concentration gradient using energy in the form of ATP. This is crucial for Na+ reabsorption.
4. Differentiate active transport from other processes such as diffusion, osmosis, and facilitated diffusion, which do not require energy input and typically involve movement along the concentration gradient.
5. Conclude that Na+ reabsorption in the kidneys is primarily achieved through active transport, utilizing ATP to move Na+ ions from the filtrate back into the blood.