What is the primary importance of the medullary extracellular fluid (ECF) osmolarity gradient in the kidney?

• A. It promotes active secretion of glucose into the urine.
• B. It increases the filtration rate at the glomerulus.
• C. It prevents the reabsorption of sodium ions in the nephron.
• D. It enables the reabsorption of water from the collecting ducts, allowing the production of concentrated urine.

Answer: (D)

Verified step by step guidance

1. Understand the role of the medullary extracellular fluid (ECF) osmolarity gradient: The kidney's medullary ECF osmolarity gradient is a concentration gradient of solutes (mainly sodium chloride and urea) in the interstitial fluid of the renal medulla. This gradient increases as you move deeper into the medulla, creating a hyperosmotic environment.
2. Recognize the function of the collecting ducts: The collecting ducts in the nephron pass through the medullary region of the kidney. The osmolarity gradient in the medulla allows water to be reabsorbed from the collecting ducts into the surrounding interstitial fluid, depending on the presence of antidiuretic hormone (ADH).
3. Understand the mechanism of water reabsorption: When ADH is present, aquaporins (water channels) are inserted into the walls of the collecting ducts, making them permeable to water. Water moves out of the ducts by osmosis due to the high osmolarity of the surrounding medullary interstitial fluid.
4. Relate the gradient to urine concentration: The reabsorption of water from the collecting ducts reduces the volume of water in the urine, concentrating it. This process is essential for conserving water in the body, especially in conditions of dehydration.
5. Eliminate incorrect options: The medullary ECF osmolarity gradient does not promote active secretion of glucose, increase glomerular filtration rate, or prevent sodium reabsorption. Its primary importance is enabling water reabsorption from the collecting ducts, allowing the production of concentrated urine.