Question regarding Fluids and Bernoulli's Equation

[NeveStash]

In recently studying for the MCAT I find myself confused with difficulty finding clear cut information regarding this online.

I was doing a practice problem for physics regarding fluids in a tube which bottlenecks at some point, it was observed at the bottle neck pressure decreased and velocity increased through this bottleneck based on bernoulli's equation. I wondered how this idea of pressure/velocity applied to blood circulation. I was under the impression vasoconstriction of blood vessels increased blood pressure as well as velocity of the blood to circulate it throughout the body faster which I find contradicting after this practice problem. If someone could please help break it down for me it would be greatly appreciated. TIA

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[Cornfed101]

Bernoulli's equation is only valid with the following assumptions:

1. The static pressure and velocity (or flow rate) at some location is constant between the two cases being compared.
2. The flow then enters a section of the arteries where the cross-sectional area increases. Bernoulli’s alone would predict an increase in static wall pressure for the larger cross-section. (This is not entirely true - see assumption 3.)
3. No frictional losses (ie: permanent pressure loss).

The biggest offender here is #3. A real system of blood vessels has a lot of friction and you would expect this to increase as the radius decreases. Also keep in mind that Bernoulli's is just a little snapshot. It is not meant to be used for before and after comparisons.

Here is an excellent explanation from Q_Goest on physics forums (post #15)

 

[NeveStash]

Thanks Cornfed, fantastic link.