Anatomy and Physiology II Practice Test

What structure plays a crucial role in the concentration of urine?

1. Proximal convoluted tubule

2. Loop of Henle

3. Distal convoluted tubule

4. Collecting duct

The correct answer is: Loop of Henle

The Loop of Henle plays a crucial role in the concentration of urine through its unique structure and function. This segment of the nephron creates a concentration gradient in the medulla of the kidney, which is essential for reabsorbing water and concentrating urine. As filtrate flows down the descending limb of the Loop of Henle, water is reabsorbed into the hyperosmotic medullary interstitium due to the high osmolarity, creating more concentrated filtrate. Conversely, when the filtrate ascends the thick ascending limb, sodium and chloride ions are actively transported out of the tubule, but this segment is impermeable to water. This movement further dilutes the filtrate, allowing the medullary interstitium to remain hypertonic. This countercurrent multiplier system is vital for establishing the osmotic gradient that allows water to be reabsorbed in the collecting duct, leading to the production of concentrated urine. This mechanism is key for maintaining fluid balance and is influenced by the hormone vasopressin (ADH), which promotes water reabsorption in the collecting ducts based on the filtrate's osmolarity. Other segments, like the proximal convoluted tubule, are important for reabsorption of nutrients