Continuity, Bernoulli's equation and blood pressure

[cthai7236]

According to the continuity equation Av1=Av2-- blood going from the arteries to the capillaries will have a lower velocity going into the capillaries (since capillaries have a larger cross sectional area). But if the velocity of blood in capillaries is slower then by Bernoulli's equation the blood pressure exerting on the walls should be higher than in the arterioles?

[mcat_premed3832]

The first problem is the identification of variables in the continuity equation. Consider what cross sectional area (A) means. It compares, for example, a huge pipe (you take a cross section which would be the shape of a circle; then the area is pi r squared) compared to a small pipe (small radius, therefore small A).

In other words, capillaries have a very tiny A compared to arteries. What you were referring to (I think) is that capillaries have a massive surface area compared to other vessels.

I hope that helps.