Roughly how much blood plasma do the kidneys filter through their glomeruli each day, most of which is immediately reabsorbed back into the blood?
- About 1.5 liters
- About 18 liters
- About 45 liters
- About 180 liters
Why D? And why not the others?
Correct answer: D. About 180 liters
Each kidney's roughly one million glomeruli filter blood plasma at a combined rate of about 180 liters a day, more than thirty times an adult's entire blood volume passing through the filtering units in a single day. Almost all of that filtrate, more than 99 percent of it, is reabsorbed back into the bloodstream as it travels along the renal tubules, recovering water, glucose, and essential ions, which is why the body ultimately excretes only about one to one-and-a-half liters as actual urine. The 1.5-liter figure is itself a real and correct number, but it describes the final urine output at the end of the whole process, not the much larger volume filtered at its start. The 18-liter and 45-liter figures both undershoot the true filtration rate by roughly an order of magnitude or more.
Source: American Physiological Society: How Does the Kidney Filter Plasma?; NIH-indexed renal filtration and reabsorption physiology