I'm a bit confused regarding the relative areas of capillaries, veins, and arteries. If veins act as a reservoir and contain most of the blood, shouldn't they have the largest area? Also, capillaries are 1 cell thick, but they also have the lowest velocity. Wouldn't this mean capillaries have the largest area?
area of capillaries/veins/arteries
- Thread starter NVswmmr
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Capillaries have the most area. Its pretty simple. Capillaries carry blood to every cell in the body. Obviously they have the biggest surface area. Just like there are more regular streets than there are highways.
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You have to keep in mind that when area is brought into cardiovascular physiology equations, they're talking about total cross-sectional area, not surface area, not the cross-sectional area of a single vessel. The total cross-sectional area of the capillaries is the greatest; that's why the blood velocity is the slowest through capillaries (this is very important since this is the site of nutrient exchange and you want blood to slow down to allow proper exchange rather than rush past).
Veins act as a reservoir because they're extremely compliant.
C = dV/dP
where C is compliance, dP is the change in pressure, and dV is the change in volume. Veins have a very high compliance compared to arteries. That's why they can store large volumes of blood with minimal change in pressure (ie. a big increase in volume only increases the pressure slightly because when you rearrange the equation, you have compliance in the denominator, and compliance is huge for veins). Changes in compliance (for example, via vasoconstriction) are what cause the redistribution of blood.
Hope this helps.
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