I cant find a average value for average velocity of blood flow in a capillary in

Question

I cant find a average value for average velocity of blood flow in a capillary in the brain. Why is that? Also, i tried calculating it by using Flow Rate Equation of Q=Av and used a diameter of 5um and flow rate of 5.0L/Min because that's the cardiac output of the aorta and that's an average size of capillary but got something bizarre. What did I do wrong? I'm still learning about circulation physiolology , not too knowledgeable yet. I cant find a average value for average velocity of blood flow in a capillary in the brain. Why is that? Also, i tried calculating it by using Flow Rate Equation of Q=Av and used a diameter of 5um and flow rate of 5.0L/Min because that's the cardiac output of the aorta and that's an average size of capillary but got something bizarre. What did I do wrong? I'm still learning about circulation physiolology , not too knowledgeable yet.

Explanation / Answer

Average velocity of blood in brain capillaries is 0.5mm to 1.5 mm/second

Link for answer source: https://www.sciencedirect.com/science/article/abs/pii/0026286281900844

There is no standard or absolute value. The above value is obtained by experiment and comparing that with mathematical model.

Now Q = Av or v= Q/A

You need to keep the unit in same manner for each variables.

mean diameter of brain capillaries range from 2um to 5um , so average is 3.5um = 3.5 x 10-6 meter = 3.5 x 10-4 cm. So radius(r) is 1.75 x 10-4 cm , r2 = 3.06 x 10-8 cubic cc.

Now there are several capillaries in brain and all of those total brain cappilaries combinedly receives 14% of total cardiac output (it is not that a single capillaries is receiving this 14% blood ), which is 700ml. Or you can get the total cross sectional area of total capillaries in human brain.

Cardiac Output is 5.0 litre in one minute = 5000 cubic cc/minute ,

Brain receives 14% of total cardiac output = 700 cubic cc or ml

Now we need to count how many capillaries are there in brain or total cross sectional area of total capillaries in brain to get velocity from that equation. I have not found any literature that indicate total numbers of brain capillaries or total cross sectional area of total brain capillaries. You are dividing total cardiac out by cross sectional area of single capillary. That is why you getting such big value.

To calculate velocity in capillaries in brain you need these three information:

1. what percentage of cardiac out is going to brain ( in human it is 700ml/minute)

2. Total number of capillaries in brain.

3. Cross sectional area of a single capillary.

Alternatively, instead of 2 and 3 you can get total cross sectional area of total capillaries in brain.

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