Why Do Blood Pressure Levels Vary Throughout the Cardiovascular System?

[jaechung]

Hi, I am studying about blood pressure. And I have some questions.
The blood pressure in the left ventricle is about 105mmHg, and that in the right atrium is about 0 to 2 mmHg,
Why do pressures decrease through our vasculature?
And what kind of pressures are the pressures in the blood? Is it just "relative pressure?" In the blood, what is the total, atmospheric, and gauge pressure?
Please help me understand this concept easily and clearly.
Thanks in advance.

 

[BillTre]

The left ventricle of the heart provides a pulsitile pressure to force the flow through most of arteries. It therefore has a high and low pressure for each heart beat cycle.
The pressure pulse pushes blood through the vessels, raising the blood pressure at those peripheral locations.

Liquid flow is similar to electrical flow: Flow = Pressure/Resistance :: Current = Voltage/Resistance
Smaller vessels have more resistance.
As the blood flows out, the arteries branch to smaller arteries. This spreads out the volume of flowing blood which decreases its local effects on pressure.
In addition, there is compliance in arteries (more so in veins). This means the vessels can expand to take in more blood volume while having a smaller than expected pressure increase. On the other hand, the vessels will contract back toward their normal size providing some additional pressure later in the heart beat cycle that would be expected from just the heart beat. This effects the local blood pressure by reducing the highs, increasing the lows, and widening the the overall pressure pulse.

Another way you could think about this is in the rate of flow. All the blood coming out of the heart goes through the aorta (first vessel after the heart). As the vessels split up so the blood can go through out the body the cross-sectional area of all the arteries increases (compared to the aorta) and so the flow rate (of the single heart beat goes down).

I think blood pressure is relative to atmospheric pressure. In the doctor's office blood pressure is measured by detecting the heart beat downstream of a constriction made by an air filled cuff. The pressure in the blood vessels have to exceed the pressure in the cuff for the beat to be detected on the other side of it. Different (high or low level components) can be detected at different pressures as the cuff pressure is reduced.

After going through the capillaries, there is very little blood pressure to drive blood flow back to the heart. Muscle contractions add local pressure on veins by squeezing the blood along, combined with valves that only permit the blood to flow in only one direction.

 

[berkeman]

I think blood pressure is relative to atmospheric pressure.

That does appear to be correct. Especially since atmospheric pressure is 760mm of Hg, it would be weird if the BPs we measure on patients were absolute. We'd get squished...

Here is a related paragraph from Wikipedia:

https://en.wikipedia.org/wiki/Millimeter_of_mercury

Use in medicine and physiology
In medicine, pressure is still generally measured in millimeters of mercury. These measurements are in general given relative to the current atmospheric pressure: for example, a blood pressure of 120 mmHg, when the current atmospheric pressure is 760 mmHg, means 880 mmHg relative to perfect vacuum.

 

[Fra]

The question is answerrd already but to add a detail to the flow = pressure / resistance note.

The typical relation from the point of view of the heart is

MAP - CVP = SVR × CO

CO = cardiac output in liter/min
SVR = Systemic (total) Vascular Resistance
MAP - CVP = the pressure DIFFERENCE between arterial output and venous return. This is the "pump pressure"

Further CO = HeartRate x StrokeVolume

The regulation of flow are both under central and local control and modulates both the heart and the vascular resistance in interplay.

So while we have a simple "ohms law analogy" the full regulatory implementation here is extremely complex as if you change one one parameter the remainig regulatory system may partially adapt or compensate!

/Fredrik