**Question:** Active resorption of sodium ion occurs in: March 2008

**Core Concept:** The process of active transport is a crucial mechanism in maintaining electrolyte balance in the human body, particularly sodium (Na+) ions. Active resorption refers to the transport of ions against their concentration gradient, requiring energy input from cells. This process occurs in specific organs or structures where the concentration of sodium ions is higher than expected.

**Why the Correct Answer is Right:**
Active resorption of sodium ions primarily occurs in the **nephrons** of the **kidney**. Nephrons are the functional units of the kidneys responsible for filtering blood, maintaining electrolyte balance, and producing urine. In the nephron, sodium is actively transported back into the bloodstream, ensuring that the body retains essential sodium and maintains osmotic pressure. This process is facilitated by specific proteins and enzymes, such as the sodium pump (Na+/K+ ATPase) and sodium channels.

**Why Each Wrong Option is Incorrect:**

A. **Neuroglial cells:** While neuroglial cells are essential components of the central nervous system, they do not participate in active transport of sodium ions.

B. **Smooth muscle cells:** These cells are found in the walls of blood vessels and gastrointestinal tract, but they do not have the necessary proteins and enzymes for active sodium resorption.

C. **Pancreatic beta cells:** These cells are responsible for insulin production and glucose regulation in the pancreas, not sodium ion resorption.

D. **Leukocytes (white blood cells):** Leukocytes are essential components of the immune system but do not actively transport sodium ions as part of their function.

**Clinical Pearl:** Understanding active transport mechanisms is crucial for maintaining electrolyte balance, osmotic pressure, and overall physiological homeostasis in the body. This concept is vital in understanding pathophysiological conditions such as hyponatremia (low serum sodium levels) and hypernatremia (high serum sodium levels), which can occur due to disorders affecting sodium transport mechanisms.

**Correct Answer:** D. **Chloride cells (Cl cells)**

Active resorption of sodium ions primarily occurs in the **Cl cells** (Cl for chloride cells). These cells are located in the **collecting ducts** of the nephron, where they actively transport sodium ions back into the bloodstream, contributing to osmoregulation and maintaining electrolyte balance. The sodium pump (Na+/K+ ATPase) is the primary enzyme involved in the active transport of sodium ions in Cl cells.

**Why Each Wrong Option is Incorrect:**

A. **Podocytes (glomerular capillary cells):** Podocytes are crucial for maintaining the integrity of the blood-filtration barrier in the glomerulus, not for sodium transport.

B. **Intercalated cells (H+/K+ ATPase):** Intercalated cells are responsible for the active transport of hydrogen ions (H+) rather than sodium ions (Na+) in the **intercalated tubules** within the kidney. Intercalated cells play a role in acid-base balance regulation, not sodium transport.

C.