Pressures in the cardiovascular system

Definitions & Key takeaways

In the human body, the heart is the pump, the arteries are pressure reservoirs and conduits, the arterioles are resistance vessels that control distribution, the capillaries are exchange sites, and the veins are conduits and blood reservoirs. Due to the varying degrees of compliance and resistance, blood pressures are not equal throughout the cardiovascular system. The mean arterial pressure falls as blood moves away from the heart to the periphery. This is because as blood flows downstream through many blood vessels, each of those vessels offers a bit of resistance, which adds up and reduces the blood pressure.

Chapters:

Watch video only

Introduction0:00–0:21

When we talk about pressures in the cardiovascular system, we’re talking about blood pressure. Pressures in different parts of the cardiovascular system aren’t equal and these differences in pressures keep the blood moving from high pressure areas leaving the heart like the arteries to low pressure areas like the veins.

Mean Pressure0:21–1:27

Actually, the pressure curve looks a little more like this, and fluctuates in the arteries depending on which part of the cardiac cycle it’s in, with these peaks being systole, and these low points being diastole -That being said, this original line is the average of these fluctuations, or the mean arterial pressure.
Now, since systole takes up about a third of a single cardiac cycle, and diastole takes up the remaining 2/3 of the cycle, we can calculate the mean arterial pressure at any time by the equation mean arterial pressure.
MAP = (⅓) SBP + (⅔) DBP So, for example, if systolic blood pressure is 120 mm of mercury, which is the unit we use for blood pressure, and diastolic blood pressure is 80 mmHg, plugging the numbers in, this becomes: [⅓ (120) + ⅔ (80)] or a mean arterial pressure of roughly 93 mmHg.

Pulse Pressure1:27–2:11

Another value we can look at is the pulse pressure, which is the difference between the systolic and the diastolic blood pressures.
In this scenario the pulse pressure would be 120mmHg - 80mmHg or 40mmHg. If we replace the SBP with (DBP + PP) in the last equation we get: MAP = (⅓) (DBP + PP) + (⅔) DBP Which after distributing we get: MAP = DBP + (⅓) PP Now, looking at these fluctuations on the arterial side, there’s a couple important things to notice.

Dicrotic Notch2:11–3:01

First of all, on the downswing of the curve, there’s a sharp sharp pressure drop followed by a rise again forming what’s called the dicrotic notch or incisura.
As blood is ejected out into the aorta, pressure rises quickly, and then as a tiny amount of blood flows back into the ventricle, and causes the valve to snap shut and the pressure to fall.
That snapping shut of the valve causes it to recoil back, which causes a brief increase in pressure of aorta, and then finally the pressure falls as the aorta settles and the heart relaxes.

Arteries and Arterioles3:01–5:48

A second interesting thing to notice is that the pulse pressure in the large arteries downstream of the aorta is larger than those in the aorta themselves!
That’s because the pressure from blood travels a bit faster than blood itself. To understand that idea - think of the molecules and cells in the blood like Newton’s cradle, and while they move together, they bump into each other and transmit that pressure wave faster than the group can move as a whole, meaning that the pressure wave actually increases the pressure downstream.
Also, the pressure waves bounce off the branch points in the arteries, which causes them to reflect back and increase the pressure in the arteries even more.
Alright, so it should seem a little weird that both systolic pressure and pulse pressure are higher in the downstream arteries, since we know that blood wants to move from a high pressure area to a low pressure area, but one thing to remember is that in the large arteries the diastolic pressure is lower, and remember that it’s the mean arterial pressure that drives blood flow, which this is mostly affected by diastolic pressure, which is highest in the aorta.
As an example, let’s say the aorta’s SBP is 115 and DBP is 85, meaning PP is 30. That said, the MAP is 95 mmHg.
Now, if the large arteries’ SBP is 120 and DBP is 80, with a PP of 40, the MAP ends up being 93, which is lower than in the aorta like we’d expect!
Okay, so as we move out from the arterial tree, the pressure gets lower and lower as the arteries branch out into smaller and smaller arteries and then arterioles and then capillaries.
As the blood move across the arterioles the pressure falls from about 80 mmHg to 30 mmHg. As the vessels branch into smaller and smaller arterioles, resistance goes up, and since resistance goes up, and according to the equation flow rate equals change in pressure or pressure initial minus pressure final, divided by resistance, with increased resistance, the pressure final or downstream pressure must decrease, which increases the change in pressure and keeps flow rate constant.
Across the capillaries, the pressure drops from 30 mmHg down to about 10 mmHg. So the pressure drop is greater across the arterioles then the capillaries, even though capillaries are even smaller, which means even more resistance.

Capillaries5:48–6:21

The pressure drop is less, though, because there are so many capillaries that run in parallel to one another, that it helps to reduce total resistance, since total resistance for vessels in parallel is less than the resistance in any individual vessel.

Venules and Veins6:21–6:58

After the capillaries, blood goes into the venules and then the larger veins. At this point, the pressure drops aren’t great because the blood vessels have a larger and larger radius and have a high compliance and can expand to hold more blood.
Finally blood makes it to the vena cavae, and pressure is roughly 4 mmHg and right atrial pressure is around 2 mmHg. Even though there are only small differences in pressure, it’s enough to move blood back to the heart, even when it’s going against gravity.

Arteriosclerosis6:58–8:18

Various clinical situations can cause pressures in the system to go up or down, and that affects all of the downstream pressures as well.
As an example, let’s take a closer look at the normal aortic pressure curve, and then two situations that often alter aortic pressures.
The first is arteriosclerosis, which is a chronic condition where the arteries become stiff and less compliant, a bit like a rubber hose turning into a lead pipe over time.
The baseline diastolic pressure usually doesn’t change, but the extra volume of blood that exerts pressure on the walls during systole, results in enormous increases in systolic pressure because the walls are so stiff.
The high systolic pressure with the unchanged diastolic pressure means that there is also an increased pulse pressure and therefore an increased mean arterial pressure.
The second example is aortic stenosis - a narrowing of the aortic valve which increases its resistance. With increased resistance, the change in pressure must increase, which means there’s a decrease in pressure after the valve.
The diastolic pressure remains unchanged, but there’s a decrease systolic pressure, decreased pulse pressure, and decreased mean arterial pressure.

Pulmonary Circulation8:18–9:05

Alright, so this whole bit we just talked about is the systemic circulation, but once the blood gets the right atrium it enters the right ventricle and pulmonary circulation.
So here in the pulmonary arteries, the pressure is 25 mm Hg during systole and 8 mm Hg during diastole, which means that the mean arterial pressure in the pulmonary artery is 25 (⅓) + 8 (⅔) = 14 mm Hg.
That pressure drops to 10 mm Hg in the capillaries, to 8 mm Hg in the pulmonary vein, then to 2-5 mm Hg in the left atrium.
All right, as a quick recap: vascular pressures drive blood flow because the blood always wants to move from high pressure to lower pressure.

Review9:05–9:38

The reason that blood pressure falls as the blood moves away from the heart is that blood has to get through many blood vessels, each of which typically offers a bit of resistance.
There are pulsations -- pressure fluctuations -- in the aorta, that get bigger in the downstream arteries and then disappear in the arterioles, but mean arterial pressure always falls as the blood goes downstream.