Ask Dr. Cerebrum

“How much oxygen goes through the body capillary beds in one minute?” Dear Dr. Cerebrum: “How much oxygen goes through the body capillary beds in one minute?”

Answer I’m glad that you asked. I’ll explain the answer in two parts. First how many milliliters of blood pass through the capillaries, and second, how much oxygen is contained in this blood.

Answer…continued The amount of blood that passes through the body capillaries is the cardiac output (Q); equivalent to the HR X SV. For my example I will use a Q of 6 L/m or 6000 ml/min.

Answer…continued Next, we need to quantify the oxygen contained in this blood: The oxygen content (CaO2) formula tells us that the total amount of oxygen is equal to the number or grams of Hemoglobin X 1.34 X SaO2 plus the dissolved portion equal to the PaO2 X .003.

Answer…continued For our example, we’ll give the subject a Hb level of 14g, a PaO2 of 100 mmHg, and an SaO2 of 97%. CaO2 = (14)(1.34)(.97) + (100)(.003) 18.2 + .3 18.5 vol% The amount of oxygen in 100 ml of blood

Answer…continued That means that in each 100 ml of the blood there is 18.5 ml of oxygen. Back the cardiac output, 60 of these 100 ml units went through the capillaries in one minute because: (100 ml X 60) = 6000 ml/min* *the cardiac output (6 L/m)

Answer…continued If each of the 60 units contained 18.5 vol% of oxygen then: (60 X 18.5) = 1110 ml of oxygen went through the capillaries in one minute. Finally the answer!

Dear Dr. Cerebrum “Of the 1110 ml/min of oxygen that was available to the cells how much was actually used?”

Answer…continued Another good question. There’s a term for this, it’s called: VO2. The way to find this out, is to sample the blood after it’s been through the capillaries to find out what was not used.

Answer…continued The total amount of oxygen in the 100 ml of blood before the capillaries (arterial blood) is called CaO2 (as discussed in the last example). The total amount of oxygen in blood after the capillaries (venous blood) is called the CvO2.

Answer…continued CvO2 is calculated by substituting the values for venous blood into the content equation (note: Hb is the same). For our example, we’ll say the person has a PvO2 of 40 and a SvO2 of 75%

Answer…continued CvO2 = (14)(1.34)(.75) + (40)(.003) 14.1 + .12 14.22

Answer…continued The difference between the CaO2 and CvO2 represents what was used by the body in that single 100 ml of blood. 18.5 vol% - 14.2 vol % = 4.3 vol% This is called the Ca-vO2 or arterial –venous content difference

Answer…continued Of course, the question was: how much oxygen was used per minute, not what was used in that single 100 ml of blood To find this out multiply the arterial –venous content difference times the cardiac output. This tells you the oxygen consumption in ml/minute.

Answer…continued VO2 = [(Ca-vO2)(Q)] 10* 258 ml/min The Fick Equation formula: VO2 = [(Ca-vO2)(Q)] 10* 4.3 X 6 X 10 258 ml/min *10 corrects the units

[(Hb)(1.34)(SaO2)+(PaO2)(.003)] VO2 = [(Hb)(1.34)(SaO2)+(PaO2)(.003)] - [(Hb)(1.34)(SvO2)+(PvO2)(.003)] [(HR)(SV)] Long version of the Fick Equation