Pressures in the cardiovascular system
By the time blood reaches venules and veins, pressure is (less/more) than 10 millimeters of mercury.
Content Reviewers:Rishi Desai, MD, MPH, Tanner Marshall, MS, Tanner Marshall, MS, Yifan Xiao, MD, Andrea Day
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.
Actually, the pressure curve looks a little more like this, and fluctuates in the arteries depending on 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:
MAP = (⅓) SBP + (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. 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.
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.