Why does the pressure change at the narrowing of a tube

[seratia]

From reading textbooks, we just know that there is a pressure difference because the fluid speeds up. But WHY is there a pressure difference, why does the pressure decrease?

 

[russ_watters]

Welcome to PF!

Though it isn't always a popular view, I think it is easiest to understand the issue by recognizing that pressure doesn't change in a constriction, but rather only shifts between different types of pressure (static and dynamic/velocity pressure).

The opposing school of thought holds that "pressure" only refers to static pressure and velocity pressure isn't really pressure, but is kinetic energy per unit volume (which still has units of pressure). In this way you can say that "pressure" is converted to something like motion.

In either case, it is important to recognise that this situation (Bernoulli's principle) is a conservation of energy statement. The conversion maintains a constant total system energy.

 

[seratia]

Hi, thank you for the welcome.

Actually, I am in the medical field. The concept of blood pressure does not make sense tome at all. When I talk to other students about it they look at me like I am crazy. But honestly, "blood pressure" REALLY doesn't make sense. There are so many questions that I can ask about it.

I just decided to read physics textbooks and work my way from the ground up.

As a medical professional, I feel uncomfortable discussing blood pressure with patients when I don't fully understand it myself.

 

[russ_watters]

We actually get blood pressure questions a lot. One thing to remember about how Bernoulli's principle applies to blood pressure is that Bernoulli's principle applies to only one flow situation at a time, along a streamline. So, for example, if your vessels dilate, your system has been changed and you can't compare it to the previous condition via Bernoulli's.

 

[sophiecentaur]

But honestly, "blood pressure" REALLY doesn't make sense.

I can sympathise with that because Physics often seems to present descriptions 'the wrong way round' for the uninitiated. You worry about the word "pressure", but the word "gradient" needs to be added in order to account for the flow. Firstly, you know that you can 'feel' the pressure in an artery because it pushes against your finger and the pressurised air in the 'cuff' can stop the flow. Some pressure gradient is needed all the way round the circuit to move the blood in the right direction all the way round and back into the Atria. In a single tube circuit with a uniform diameter tube, the pressure would drop steadily, being highest on the way out of the heart and lowest inside the Atria. In a real body, the capillaries account for a lot of the pressure drop and you can't easily get a useful measure of the veinous pressure (it's different for different heights in the body. Experience (I guess) has shown that the pressure on the way into the pump is low enough not to affect things and that the 'excess' pressure in the arteries gives a good enough idea of the pressure drop across the capilliaries.
So. your measurement of 'Pressure' near to the elbow is near enough to the 'Pressure Difference' around the circuit. Makes sense?