Blood Pressure

[puffylover]

- BP increases with constriction (of arteries? what vessels can constrict again?)
- BP increases with increased heart rate

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[IntelInside]

BP generally increases with heart rate. I know arteries and veins can constrict not sure about capillaries (probably can). As you constrict the vessels blood pressure is going to rise because of the increased resistance within the vessels.

 

[UCB05]

Veins and arteries both relax and contract. Generally it's the function of the more muscular, less elastic arteries which constrict to increase systemic resistance and blood pressure. Veins are more elastic and their main function is to act as a blood reserve. Contraction of veins shuttles more blood out of the venous system and into active circulation.

 

[joshto]

BP generally increases with heart rate. I know arteries and veins can constrict not sure about capillaries (probably can). As you constrict the vessels blood pressure is going to rise because of the increased resistance within the vessels.

doesnt this go against Bernoulli's Law?

I was under the impression that Vasoconstriction=Smaller Area=Increased Velocity (according to Continuity), but, then, according to Bernoulli, High Velocity should have low pressure? Why is this not so?

 

[sheldon09]

Blood flow doesn't follow Bernoulli's.

Capillaries have the smallest cross sectional area and the lowest pressure.

Arteries have medium cross sectional area and have the highest pressure.

Veins have the largest cross sectional area and medium pressure (lower than arteries, higher the capillaries).

As you can see, the area/pressure relationship described by Bernoulli's does not apply to blood vessels.

Hope that helps.

 

[IntelInside]

Yes but as I constrict the the veins or arteries, there is more resistance so the overall blood pressure is going to increase.

 

[Charles_Carmichael]

Yes but as I constrict the the veins or arteries, there is more resistance so the overall blood pressure is going to increase.

Yes, systemic vasoconstriction would increase the mean arterial pressure (MAP).

dP = Q x R, where dP is the change in pressure, Q is the blood flow, and R is the peripheral resistance.

One of the initial responses to hemorrhage is vasoconstriction. This mechanism tries to maintain MAP by compensating for the loss in blood volume.

 

[puffylover]

what's the equation that relates BP with resistance again?

nvm. someone just answered my question. thanks!

 

[Charles_Carmichael]

Blood flow doesn't follow Bernoulli's.

Capillaries have the smallest cross sectional area and the lowest pressure.

Arteries have medium cross sectional area and have the highest pressure.

Veins have the largest cross sectional area and medium pressure (lower than arteries, higher the capillaries).

As you can see, the area/pressure relationship described by Bernoulli's does not apply to blood vessels.

Hope that helps.

This is not true. Veins have lower pressure than capillaries. As the blood travels through the blood vessels (starting from the aorta and ending in the right atrium) it's constantly losing pressure due to the resistance of the blood vessels. Think of pressure as potential energy. As blood flows from the beginning of a blood vessel to the end of it, it's losing energy due to the friction (ie. resistance) of the vessel.

The changes in pressure across different vasculature of the CV system is due to the loss of energy to resistance (which is affected by how "open" blood vessels are).

Hope this helps.

 

[RogueUnicorn]

if veins had higher pressure than capillaries blood would flow backwards.

 

[fizzgig]

the bernoulli comment: higher velocity does reduce pressure on what the fluid goes BY, so if you just measure the pressure exerted on the walls of the pipe, then smaller pipe, higher veloc, lower pressure.

the pressure felt by stopping the blood stagnant is different (greater). this transfers all the motion energy to potential energy (stored in the height of the column of liquid). it involves a tube sticking off the pipe that curves in an L shape, open end facing the midline of the pipe flow.

might also help:
http://www.cvphysiology.com/Hemodynamics/H012.htm

 

[BJJ]

huuuuuuuuuuu

 

[puravida85]

You guys are posting some really confusing information about blood.

Bernoulli's equation is only good for an ideal flow. I wold not consider the blood system an ideal flow for many reasons.

As far as continuity goes, it definitely still applies (Q= Av). If the area increases, velocity will definitely decrease to maintain constant flow rate (controlled by the heart's contraction). When we talk about capillaries in general, it is the addition of the many capillaries' cross sectional area together creates a large amount of area), so blood moves really slow because of Q = Av. In fact, EK says that the cross sectional area of the capillaries is much larger than the arteries or veins.

Of course if you think of a single lonesome capillary, velocity will generally be fast due to a small cross sectional area (Q = Av). And please stop trying to relate Bernoulli's equation with blood. You can in fact think about it, but it won't be helpful because we're speaking in terms of physiology and not physics.

 

[RogueUnicorn]

You guys are posting some really confusing information about blood.

Bernoulli's equation is only good for an ideal flow. I wold not consider the blood system an ideal flow for many reasons.

As far as continuity goes, it definitely still applies (Q= Av). If the area increases, velocity will definitely decrease to maintain constant flow rate (controlled by the heart's contraction). When we talk about capillaries in general, it is the addition of the many capillaries' cross sectional area together creates a large amount of area), so blood moves really slow because of Q = Av. In fact, EK says that the cross sectional area of the capillaries is much larger than the arteries or veins.

Of course if you think of a single lonesome capillary, velocity will generally be fast due to a small cross sectional area (Q = Av). And please stop trying to relate Bernoulli's equation with blood. You can in fact think about it, but it won't be helpful because we're speaking in terms of physiology and not physics.

no, bernoulli's equation works with any flow.

 

[joshto]

why is there no consensus on this issue

 

[RogueUnicorn]

there is, but some are slightly misinformed as to what bernoulli's principle actually is (just a restatement of energy conservation)

 

[Pisiform]

Blood Pressure increases when arteries, veins, arterioles, venules constrict. When they constrict, they increase Peripheral Resistance. So, blood pressure Increase.

BUT,

When we exercise, blood pressure Increase even though arterioles dilate (less Peripheral Resistance). This is because Blood Pressure is related to Cardiac Output and Peripheral Resistance both. Here, Cardiac Output (which is product of stroke volume and heart rate) increase far more than Peripheral Resistance could decrease. So, blood pressure increase.

Guys, if I am wrong, please correct me .

 

[Pisiform]

Blood Pressure increases when arteries, veins, arterioles, venules constrict. When they constrict, they increase Peripheral Resistance. So, blood pressure Increase.

BUT,

When we exercise, blood pressure Increase even though arterioles dilate (less Peripheral Resistance). This is because Blood Pressure is related to Cardiac Output and Peripheral Resistance both. Here, Cardiac Output (which is product of stroke volume and heart rate) increase far more than Peripheral Resistance could decrease. So, blood pressure increase.

Guys, if I am wrong, please correct me .

and as far as Principal of Continuity is concerned, the smaller the vessel the greater the flow/velocity.

This is also true for Capillaries but in this case, we take into account the entire cross-section of capillaries and not a single capillary. That's why, speed in capillary is slow.

(Don't think that Bernoullis Equation works here, only Principal of continuity)

 

[MintJulep]

Bernouille's equation refers to ideal fluid flow only - it doesn't factor in viscosity, laminar/turbulent flow.

Acutal blood flow cannot be modeled with Bernouille's law. Maybe Poiseulle's (however, I think actual blood flow is even more complicated than that).

 

[Charles_Carmichael]

Bernouille's equation refers to ideal fluid flow only - it doesn't factor in viscosity, laminar/turbulent flow.

Acutal blood flow cannot be modeled with Bernouille's law. Maybe Poiseulle's (however, I think actual blood flow is even more complicated than that).

Blood flow can be approximated by the Poiseuille-Hagen law, which takes viscosity into account as well. Can't remember the exact equation off the top of my head though.