Pressure question (physics +bio)

[IlyaR]

From an idea fluid, pressure decreases as speed increases.

Taking the heart as an example: The aorta has a much smaller area than the capillaries, thus if blood behaved as an ideal fluid, the capillaries would have a slower speed (Q=AV) and would have a higher pressure.

The thing is, this seems intuitively wrong to me, shouldn't the Aorta have a higher pressure in order to force blood throughout the circulatory system?

Similarly, the Venae Cavae have the lowest pressure, but it seems to contradict Q=AV, and Bernoulli's principle. It makes perfect sense to me that the end of the fluid circuit should have the lowest pressure, but I can't understand why when comparing with ideal fluids. Ex: Aorta has 4 times less area, thus has the quickest velocity, shouldn't it have the lowest pressure (I know this doesn't make sense)

Thanks!

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[gettheleadout]

The continuity equation for branching vessels is Q = Av_avg = ∑A_nv_{n, avg}

The aorta effectively branches out to become the capillaries, and because the total cross-sectional area A of the capillaries is greater than that of the aorta (by a great deal), the average flow velocity in an individual capillary vessel can be much lower.

 

[IlyaR]

I think I worded my question incorrectly. What I'm having difficulty grasping is dP=QR while keeping Q=AV in mind.

Basically, what is more important in a REAL fluid.

Ex: The area of the capillaries is a LOT more than the aorta. So, with Q=AV, It would stand to reason that the velocity of the capillaries is significantly lower than the aorta. From Bernoulli's principle, this would mean that the pressure of the capillaries is more than in the aorta.

From what I understand, the pressure is actually greatest in the Aorta, and dissipates while travelling through the circulatory system, with the Venae Cavae ultimately having the lowest pressure. This makes absolute sense to me, but it seems like it completely ignores Bernoulli's principle.

The only thing that I can think of is that the drag in the capillaries lowers the velocity significantly (wouldn't this increase the pressure even more), but even then, it can't possible be slower than the blood in the aorta since then fluid buildup would occur.

Sorry if I'm being confusing

 

[IlyaR]

If anyone else was confused or wondering the same thing, this thread actually explains it very well

http://forums.studentdoctor.net/showthread.php?t=648822