Question

In which of the following segments is sodium NOT actively transported out of the nephron? A. Proximal convoluted tubule B. The thin segments of the loop of Henle C. Distal convoluted tubule D. Sodium is always actively transported out of the nephron.

Short answer

B. The thin segments of the loop of Henle.

Step-by-step solution

Identify Active Transport Locations

Determine the segments of the nephron where sodium is actively transported. Sodium is actively transported out in the proximal convoluted tubule, distal convoluted tubule, and some parts of the loop of Henle.

Examine the Thin Segments of the Loop of Henle

The thin segments of the loop of Henle are primarily involved in passive transport and reabsorption due to their permeability characteristics.

Compare the Options

Review the options: A. Proximal convoluted tubule (active transport occurs), B. The thin segments of the loop of Henle (passive transport occurs), C. Distal convoluted tubule (active transport occurs), D. Sodium is always actively transported (not true).

Conclusion

From the comparisons, it is clear that the correct answer is B. The thin segments of the loop of Henle, where sodium is NOT actively transported.

Key concepts

Active Transport

Active transport is a crucial process in the nephron for maintaining the body's salt and water balance. It refers to the movement of molecules across a cell membrane against their concentration gradient, requiring energy in the form of ATP.
Active transport in the nephron allows for efficient reabsorption of essential ions, particularly sodium (Na+), from the urine back into the blood.
During this process, specialized protein pumps on the cell membranes, known as sodium-potassium pumps (Na+/K+ ATPase), actively transport sodium ions out of the nephron cells into the surrounding blood vessels.
Key sites for active sodium transport include:

• Proximal convoluted tubule: Reabsorbs about 65% of sodium using active transport mechanisms.
• Distal convoluted tubule: Adjusts sodium reabsorption based on the body's needs.
• Ascending limb of the loop of Henle: Particularly the thick segment, where significant active sodium reabsorption occurs.

Understanding active transport mechanisms in these segments is vital in studies related to kidney function and overall fluid balance.

Nephron Segments

The nephron is the functional unit of the kidney, consisting of various segments that play distinct roles in filtration, reabsorption, and secretion.
Each segment is specialized to handle different tasks to ensure that waste is excreted, and necessary substances are conserved.

• Proximal convoluted tubule (PCT): Responsible for reabsorbing nutrients, water, and electrolytes like sodium. Uses active transport to move Na+ back into the blood.
• Loop of Henle: Divided into descending and ascending limbs, each with unique permeability characteristics. The thin segments are involved in passive processes, while the thick segments actively transport ions.
• Distal convoluted tubule (DCT): Further modifies the urine composition by reabsorbing sodium and water and secreting potassium and hydrogen ions.
• Collecting duct: The final adjustment phase in which urine concentration is fine-tuned.

Familiarizing oneself with each nephron segment helps in understanding how the kidney efficiently filters blood and maintains homeostasis.

Loop of Henle

The loop of Henle is a critical nephron structure that extends into the kidney's medulla, forming a hairpin loop divided into thin and thick segments.

• Descending limb: The thin segment here is highly permeable to water but not to sodium or chloride. This allows for water reabsorption through passive diffusion into the surrounding medullary interstitium.
• Ascending limb: Comprises thin and thick segments. The thin segment here does not reabsorb water but allows for passive transport of sodium and chloride. The thick segment actively transports sodium, potassium, and chloride out of the nephron via a transporter (Na+/K+/2Cl- co-transporter), without allowing the water to follow, making it impermeable to water.

The loop of Henle is essential in concentrating the urine and creating a hyperosmotic medullary environment that facilitates final water reabsorption in the collecting ducts.
Its function illustrates the intricate balance kidneys maintain in determining urine concentration and volume, reflecting the body's water and electrolyte balance.