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I was going through my university lecture on kidney structure and function, and am slightly confused because of a statement made by the lecturer. In the lecture he states that (I am paraphrasing what he said for copyright reasons); ''If 180L of blood plasma is filtered through the glomerular capillaries per day, and if 42 L is the total blood volume in the human body, so it is clear that the kidneys filter the blood plasma a total of 5 times per day (approx.). So the high GFR value of 180L/day is fundamental when it comes to the removal of soluble metabolic waste products like urea (every urea molecule lasts around 4 to 5 hours within the blood before it is removed).''

My confusions:

  1. Is the total blood volume 42 L, I thought it was around 4 to 5 L and of that approximately 3 L is plasma? Is my lecturer confusing blood volume with the total amount of fluid within the body, which I believe is equal to 42 L?
  2. If my assumption of there being 3 L of plasma comprising the total blood supply in the average human body, is it correct that the kidneys filter the blood plasma approximately 60 times per day instead of 5 times as he suggests?
  3. If the blood plasma is filtered around 60 times per day, does that mean that each molecule of urea lasts in the blood for approximately 24/60= 0.4 hours = 24 minutes, in contrast to the 4 to 5 hours he suggests in the lecture?

Any help would be truly appreciated!

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