# Blood Pressure/Velocity in Capillaries

Discussion in 'MCAT Study Question Q&A' started by Dr Gerrard, Jul 5, 2009.

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1. ### Dr Gerrard 7+ Year Member

Feb 16, 2009
Capillaries have the lowest blood pressure.

Capillaries have the lowest velocity.

How are both of these true?

Bernoulli's principle says the greater the velocity, the smaller the pressure.

Does statement 1 only take into consideration the pressure in a single capillary, while statement 2 takes into consideration the overall velocity of blood, or the total flow rate?

If not, how else can I account for this?

3. ### Doodl3s 5+ Year Member

417
3
Jun 17, 2009
NorthEast
I can't answer your questions specifically, but yes it is true that capillaries have the lowest blood pressure (practically non-existant) and the lowest velocity (again, its literally the movement of one RBC at a time, almost non-existant V)

First off, if you have the EK bio book, they explain this well.

the theory is that the capillaries overall split from the larger arteries SO MUCH SO that even though the capillaries are like 100 times smaller, theres like a 1000 times as many branches. SO in the end, the OVERALL DIAMETER of ALL THE CAPILLARIES added together IS HUGE compared to that of original artery diameter...

In other words, image a pipe splitting into multiple smaller pipes, however there are so many small pipes their overall Cross-sectional area may be ten times that of the original pipe. so pressure drops, and velocity drops.

4. ### Charles_CarmichaelModerator Emeritus 7+ Year Member

4,085
36
May 10, 2008
Think of the continuity equation: Q = Av, where Q is blood flow, A is the total cross-sectionaly area, and v is the velocity. If we rearrange it, we get v = Q/A. The capillaries have the greatest total cross-sectional area (the key word there is total, not the area of an individual capillary), so they have the lowest velocity. Also, think of it intuitively: do you want the blood to rush past the capillaries which are the only place where nutrient exchange between blood and tissue takes place? Or do you want it to slow down so you have time for exchange? You want it to be slow.

5. ### G1SG2 5+ Year Member

May 2, 2008
Bernoulli's principle applies to ideal fluids. Blood is not an ideal fluid, and so it doesn't entirely follow Bernoulli's principle.

6. ### matth87 2+ Year Member

289
1
May 8, 2009
Yup this is true as well.

This comes into play when your dealing with peripheral resistance and how it affects blood pressure.

Also another important point to note is.

Blood Pressure in the Capillaries is greater on the arteriole side which tends to push stuff such as water and other goodies into the tissue. This decreases the blood pressure and creates an osmotic gradient on the venule side. Stuff tends to get push back into the blood on the venule side. Although not everything is recaptured that is why we have the lymphatic system!

7. ### wanderer 5+ Year Member

1,980
28
Dec 14, 2008
The individual diameter of capillaries is very narrow, so that blood flowing through a capillary does frictional (negative) work on itself, and energy is not conserved, so that it loses both velocity and pressure.
Bernoulli's Law is based on the conservation of energy so it doesn't apply here.

8. ### Dr Gerrard 7+ Year Member

Feb 16, 2009
All right, so what happens with vasoconstrictors/vasodilators?

I know they decrease/increase the area, but what does each one do in terms of blood pressure and velocity?

I know vasodilators increase blood flow, but does it also increase pressure? Because an increase in pressure would normally correspond to a decrease in velocity.

Also, am I correct in saying that vasodilators/vasoconstrictors mostly act on arterioles?

EDIT: I just read that vasodilation decreases blood pressure. Vasoconstriction increases blood pressure. But I do not know what it does to velocity yet.

Last edited: Jul 7, 2009
9. ### matth87 2+ Year Member

289
1
May 8, 2009
Blood Pressure is based on two things
1) Cardiac Output
2) Peripheral Resistance

Cardiac Output
CO = SV(strokevolume-volume/beat) x HR (beats/min)
CO (volume of blood perm minute)
Increasing CO by increasing SV or HR will increase BP

Peripheral Resistance
Vasodialte = decrease resistance / decrease BP
Vasoconstrict = increase resistance / increase BP

Try not to thing about Blood Pressure and Blood Flow together
Dilation of an artery will increase blood flow.
Constriction of an artery will decrease blood flow.

For example when the sympathetic nervous system is activated it will constrict arteries that lead to the digestive organs. This will divert blood flow to the skeletal muscles. There are changes in blood pressure due to this but nothing drastic enough where we need to worry about it.