I am reading now the book which states that the Q (blood flow per unit of time) of the brain is 13% while the Vo2 (relative Oxygen used per unit of time) is 21%. Now my question is if the Vo2 of the brain (21%) is taken from the entire blood or from the part the brain gets (Q=13%)? It is not clear on the book. I've attached the picture with the explanations.

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  • $\begingroup$ What do $Q$ and $Vo2$ stand for? $\endgroup$ – Remi.b Nov 19 '16 at 16:29
  • $\begingroup$ Q stands for blood flow per minute, and Vo2 stands for relative Oxygen used per minute - as it's written on the chart. I can assume that the letter Q is for Quantity and Vo2 is for Volume of o2 (oxygen). $\endgroup$ – Alleged Biologist Nov 19 '16 at 16:38

TL; DR 21% of the whole body.

The VO2 numbers add up to 100%. You are looking at the way six major compartments of the body split the oxygen consumption. The brain takes 21% of the whole oxygen brought into the body by the arteries.

The Q numbers add to 100% too. The brain takes 13% of the blood coming out of the heart.

The picture is meant to contrast high-flow and high-metabolism organs. The kidneys take 20% of all the aortic blood, but use only 7% of the oxygen consumed in the body. The coronaries take 4% of the blood leaving the ventricles, but use a large 11% of the oxygen take in by the whole body.

  • $\begingroup$ Thank you. Now I understand it (I also calculated all the vo2 in the chart and I got 100% and that proofs your things). But now I don't understand why there is not a relation or correlation between the Q and VO2, it means that it can be Q of 4% while the vo2 is 11% (almost 3 times than the Q!). Then my exact wondering is how could it be that a very small Q can supply a big quantity of vo2. By the way, What does it mean "TL; DR" (in the beginning of your answer)? $\endgroup$ – Alleged Biologist Nov 19 '16 at 20:35
  • $\begingroup$ TL:Dr means "too long ; didn't read", a frequent rebuttal on the Net for those who make too long an en explanation. It has been taken up in preemptive defense by those who write long explanations, like me. The heart and brain use a lot of oxygen because they do intensive work, which require intensive burning of energetic substrates. The kidney is not a burner, but it needs a lot of blood in order to do its job - filtering -, effectively. Neither organ takes up all of the oxygen in the blood, but heart muscle and brain are making more of an effort compared to the kidney. $\endgroup$ – nvja Nov 20 '16 at 3:08
  • $\begingroup$ Thank you for your comment. I think that I didn't explained my question well. I've been asking, how could it be that a small amount of blood (e.g. in the heart 4%) can supply a big quantity of oxygen (vo2=11%)? $\endgroup$ – Alleged Biologist Nov 20 '16 at 12:44
  • $\begingroup$ When blood leaves the brain or the kidney, it is not fully depleted of O2. Blood leaving the left heart has a hemoglobin saturation with O2 of nearly 100%. (Almost every Hb molecules carries the maximum, four O2 molecules.) Blood returning from the body through veins, mixed in the right atrium, has a saturation of about 75%. (That is, the average Hb molecule has three O2 molecules still bound to it.) Roughly speaking, the various organs take up only one of the 4 molecules of O2. Brain is striving a bit harder; brain venous blood saturation is about 70%. Kidney vein Hb O2 saturation is ~80%. $\endgroup$ – nvja Nov 21 '16 at 5:22

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