Understanding Blood Pressure: Causes and Effects

[Cheman]

Blood Pressure...

What exactly is blood pressure? What causes it? (ie - on molecular level)
Also, why is blood pressurehigh in arteries but low in capillaries and veins?
Thanks in advance.

 

[Dooga Blackrazor]

I'll try and tell you what I know. I'm not sure about molecular levels.

Blood pressure is the force exerted on walls an artery by the beating of the heart. That's the definition I received in class.

Arteries have elastic tissue which would effect the pressure. Blood would cause the arteries to dilate, increasing pressure. That's a guess. Veins don't have the elastic tissue.

Capillaries are the site of fluid exchange/balance so they would have a balanced pressure. 50% blood / 50% osmotic pressure (water) pressing against each other to achieve a stable environment. I only know basic Biology so someone else can probably explain everything in more detail or inform you if I'm incorrect in things I've said.

 

[Jikx]

also:

Typically veins are generally wider than arteries, and therefore the pressure is lower. Capillaries are tiny vessels, so repsent high resistance to the incoming blood, so the rate of blood flow through will be quite low. All blood from the artery eventually passes through capillaries and into the vein. There are a lot of capilliaries, so the amount of blood passing, while the same as arteries, will have a much decreased flow by sheer number. I suppose this would make sense if pressure proportional to rate of flow, which makes sense, but I'm not sure is correct.

On a molecular level.. people have begin to have calcified arteries as they grow older, which (i think) restricts blood flow and overall elasticity. High blood pressure results because of the increased rate of flow of blood needed. Also things like high sodium levels in the blood, which causes high blood pressure because it also draws a large volume of water via osmosis. Somethign like that.. :/

 

[Gokul43201]

I don't mean to be blunt or dismissive, but the pressure in the veins/arteries has nothing to do with (a) the thickness/elasticity of the wall tissue, (b) the diameter of the arteries/veins, or (c) the number of (branches of) arteries/veins.

The answer is simply that the blood pumped from the heart travels first through the arteries before getting to the veins. The heart is just like a hydraulic (diaphragm) pump that creates a large pressure difference. The blood flows from the higher pressure side to the lower pressure side. Along the way, it loses pressure due to impedance (friction) from the vessel walls, branches, etc. Since the arteries lead out of the heart, the pressure is high in them. By the time the blood travels through the arteries and gets to the veins, there is a significant drop in pressure. And by the time the blood gets back to the heart, the pressure is low enough that the heart needs to pump on it (raise the pressure) so it can go round again.

Check out : http://distance.stcc.edu/AandP/AP/AP2pages/vessels/arteries.htm

 

[Moonbear]

Actually, Gokul, diameter of the vessels and their elasticity (which is also related to diameter) are related to blood pressure. This is Bernoulli's principle. Any standard physiology textbook will explain how it relates to blood pressure. And any standard physics textbook will explain how it relates in general to hydraulics (same volume of fluid per time being sent through a larger diameter pipe/tube/blood vessel will have a lower pressure than if it is sent through a smaller diameter pipe/tube/blood vessel). For example, think of your standard garden hose. Turn on the water and you get a stream of water coming out of the hose. Now, put your finger over the end and narrow the diameter of the hose (or use a nozzle of some sort to narrow the opening), and you get a higher pressure stream passing through that smaller diameter opening.

Blood pressure is a mechanical phenomenon, not a molecular one.

 

[Gokul43201]

By reducing the diameter, you only increase the flow velocity. This is a result of non-compressibility (continuity requires v.A = const) of the fluid. In fact, if you apply Bernoulli's principle (which is just energy conservation), then reducing the diameter increases the velocity (and hence the \rho v^2 term) which in turn causes the pressure to decrease. However, the Bernoulli equation does not consider energy lossed due to impedance in the pipes.

Reducing the diameter results in increasing the pressure DROP across that section of pipe, by increasing its impedance.

Q=K \delta P(d^n), n=3..4 depending on the flow; 3 for laminar flow and 4 for turbulent, I think.

Putting a reducer or nozzle into the end of a length of garden hose only results in a higher velocity of flow...the pressure is actually less. You can actually verify this by measuring Q, the volume of water coming out per unit time (fill a bucket and see how long it takes).

 

[hitssquad]

Protein crosslinking and blood pulse pressure

I don't mean to be blunt or dismissive, but the pressure in the veins/arteries has nothing to do with (a) the thickness/elasticity of the wall tissue

Alteon is developing a drug, ALT-711, which increases elasticity of blood vessels by breaking protein crosslinks. Protein crosslinking in the body expresses itself partly in the form of stiffer blood vessels, and stiffer blood vessels result in a wider separation between systolic (the blood pressure during an instant wherein the heart is pumping out blood) and diastolic (the blood pressure during an instant wherein the heart is filling with blood) pressure readings. The degree of separation between systolic and diastolic pressure is called pulse pressure. Generally, a larger pulse pressure comes in the form of both higher systolic pressure, and lower diastolic pressure (just like an old, dried-out balloon will present more resistance to further filling than a young balloon will at a state of high fullness, and less resistance than a young balloon at a state if low fullness - i.e., old, crosslinked skin not only doesn't stretch very well, but also sags instead of snapping back into place).

Pumping blood into a crosslinked blood vessel is like hammering against concrete. The impact is more intense (and the instantaneous pressure is higher) than when hammering a sofa cushion.

 

[Gokul43201]

hitssquad, sure that makes sense. But what about the average pressure? Stiffer walls make for sharper pressure pulses - that would only be natural. The systolic pressure goes up and the diastolic pressure goes down. So it seems that the average pressure remains nearly the same. Even if there is an effect on the average pressure, I'd imagine it's small compared to the effect of the position along the hydraulic path.

We are looking for the dominant factor here, and the first 2 paragraphs of http://distance.stcc.edu/AandP/AP/AP2pages/vessels/arteries.htm support my argument.

 

[Cheman]

So what exactly causes the blood pressure then? Is it the fact that cells, etc hit the walls or that liquids are incompressable and therefore have to push through the arteries?
Thanks. :-)

 

[chroot]

Pressure (any kind) is always due to the constituent particles of the fluid being in motion, and thus having momentum. When the particles collide with the walls of the container (the blood vessel walls in this case), they impart momentum to the walls, pushing them away.

- Warren

 

[TenNen]

What exactly is blood pressure? What causes it? (ie - on molecular level)
Also, why is blood pressurehigh in arteries but low in capillaries and veins?
Thanks in advance.

http://search.yahoo.com/search?fr=fp-pull-web-t&p=What+is+blood+pressure
That link should give you the answer and many more, I guess the fourth link have something you need...