Why is the velocity (linear speed, i. e. distance traveled per unit time) slow in an individual capillary, but the total flow (volume per unit time per unit surface area) over all the body’s capillary beds is the same as in the relatively huge aorta?

There is no short answer.

Explanation:

The blood flow speed in the aorta is about 30cm/s whereas in the capilaries it is around 1-1,5 mm/s. The reason why this is happening is because the capilaries are much smaller and thinner than the aorta and they are not as durable to those conditions. The total blood flow is the same but the rate is not so that the blood pressure can be kept in balance throughout the whole body.

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