Fluid found in which of the following structures is called urine?
a. Proximal convoluted tubule
b. Collecting duct
c. Minor calyx
d. Ureter
e. All of the above structures transport urine
f. Only the minor calyx and ureter transport urine
The correct answer and explanation is:
The correct answer is b. Collecting duct.
Urine is the final product of the filtration, reabsorption, and secretion processes in the kidneys. It is formed after blood plasma is filtered through the glomerulus, and the resulting filtrate passes through various structures in the nephron.
In the proximal convoluted tubule, the filtrate undergoes substantial reabsorption of water, ions, and small molecules such as glucose. The filtrate at this stage is not yet considered urine because it still contains substances that will be reabsorbed into the bloodstream.
As the filtrate moves through the loop of Henle, more water and salts are reabsorbed. The filtrate becomes more concentrated as it moves down the descending loop and more diluted as it ascends.
The distal convoluted tubule performs more fine-tuning, including the reabsorption of sodium and calcium ions, and the secretion of substances like potassium and hydrogen ions. While the substance is closer to urine, it is still not quite there yet.
The collecting duct is where the final modification of the filtrate occurs. Here, water reabsorption is regulated under the influence of antidiuretic hormone (ADH). Once the fluid reaches the collecting duct, it is technically urine. The collecting ducts merge to form the renal papilla, where the urine enters the minor calyx, which collects the urine from the collecting ducts and channels it toward the ureter.
While the minor calyx and ureter are involved in transporting urine, they do not alter the composition of the fluid. The ureter carries the urine from the kidneys to the bladder for storage, while the minor calyx simply collects it. Thus, urine is found in the collecting duct, as that is the final stage before the fluid exits the kidney.