Question Video: Recognizing the Process in the Kidney That Directly Contributes to the Regulation of Body Fluid Levels and the Structure in the Kidney Where the Majority of This Process Takes Place Biology

About 180 liters of plasma are filtered by the kidney every day. However, the average urine excreted per day is about 1.5 liters only. Which of the following is responsible for that? [A] The filtration process taking place in the nephric tubule. [B] The reabsorption process taking place in the nephric tubule. [C] The filtration process taking place in the collecting duct. [D] The reabsorption process taking place in the collecting duct.

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Video Transcript

About 180 liters of plasma are filtered by the kidney every day. However, the average urine excreted per day is about 1.5 liters only. Which of the following is responsible for that?

This question is pointing out that the amount of plasma filtered by the kidneys is much greater than the amount of urine that we excrete every day. And it’s asking us to correctly identify the part of the urine production process which is responsible for this notable difference. Well, let’s start by recalling that the kidney filters waste from our blood, which is then removed from our body in the form of urine. The nephron is the functional unit of the kidney. It’s a microscopic structure responsible for urine production. And this process takes place in three steps. The first step is filtration, in which almost all of the small molecules are filtered out of the bloodstream and into the tubule. The filtrate includes water, ions, nutrients, and waste products such as urea.

The second step in this process is called selective reabsorption. During this step, the nutrients, most of the water, and any of the ions that your body needs are returned to the blood stream from the filtrate. In the last step, anything that’s left in the tubule is considered to be urine. And several nephric tubules will empty into one collecting duct. The collecting ducts join into larger and larger vessels until they converge in the renal pelvis, where the urine leaves the kidney through the ureter. So let’s return to our answer choices and figure out which of these is responsible for the volume of filtrate being much larger than the volume of urine.

Choice (A) is, the filtration process taking place in the nephric tubule. Well, the filtration process is actually responsible for generating the filtrate. And it occurs in a structure called the glomerulus and not the nephric tubule. Our next choice is, the reabsorption process taking place in the nephric tubule. Well, the reabsorption step is where most of the water leaves the filtrate and reenters the bloodstream. And this does occur in the nephric tubule, so this seems correct, but let’s keep going.

The filtration process taking place in the collecting duct. Well, filtration occurs in the glomerulus, not in the collecting duct. And our last choice, the reabsorption process taking place in the collecting duct. Well, reabsorption occurs in the nephric tubule. The collecting duct just collects urine. So our correct answer is, the reabsorption process taking place in the nephric tubule.

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