1

Summary

What is the change in flow between systolic and diastolic phases specifically in the muscular arteries? There is a lot of information on how pressure changes between these phases (see graph) as blood travels throughout the body. I highly doubt flow dips down to 0 just based on the inertia of the blood and also the elasticity of the arteries providing a "second beat".

2
  • 3
    What is the source of the diagram?
    – Carey Gregory
    Jan 16 at 19:29
  • 2
    Answering my question isn't optional. Publishing other people's work without attribution is a violation of SE policies and copyright law. If it's your own work, just add a sentence saying so. Otherwise, the diagram will be removed.
    – Carey Gregory
    Jan 19 at 0:27

2 Answers 2

3

Flow is roughly constant once you get past the arterioles. You can see that on the figure you provided, no other source is needed: there's no pulsatile pressure there, just constant pressure differences, so flow is going to be constant there.

Note that pressures we talk about are almost always actually pressure differences, for example when we talk about a "normal" blood pressure being 120/80 mmHg, we're ignoring the ~760 mmHg of atmospheric pressure and just reporting the pressure difference with the outside. Pressure differences are what move things, so that's what important. Knowing this, pressure has to be zero in the vena cava because in this figure it's being used as the reference pressure; by definition the pressure difference with itself is zero. That doesn't mean there's no flow there, but it means there's no pressure gradient to make blood flow from the vena cava to the vena cava. Considering the vasculature is mostly a closed system, total flow at any cross-section through that system has to be the same as everywhere else in the system, especially once you get past the pulsatile arteries. Don't look at "60 mmHg" here and think oh, that must mean blood is flowing faster there, think about where it's going, too: it's not skipping from muscular artery to vena cava or the outside world (if it did, yes it would come squirting out at high speed), it's moving into more distal arteries and arterioles that are only slightly lower pressure. There's a gradual pressure gradient along the whole system.

2
  • I think for flow, the overall lumen, ie. the summarised area of cross-sections of all respective blood vessels, is of just the same importance. It is basically a bell-curve in that diagram (smallest vessels = biggest lumen altogether) if memory serves, so flow should be the invert of that, actually (highest/fastest in aorta and vena cava, gradually slower towards the mid of that x-axis). Also a reason for higher blood pressure with adipose people, ie. has to be higher in bigger vessels to keep the minimal flow in the additional small vessels. Or am I missing something? Jan 18 at 19:58
  • @PhilipKlöcking There's velocity and volume... Volume has to be constant for a cross section across the whole vasculature (e.g. all the arteries, all the veins) unless something's leaking out. Velocity is going to be proportional to the slope of the pressure drop (if the x-axis is corrected to be actual rather than arbitrary distance): fast flow where pressure is dropping quickly == smaller total cross section. That's typically in the arterioles, another way we'd describe that physiologically is to say that the arterioles control the total resistance.
    – Bryan Krause
    Jan 18 at 20:10
3

General relative flow velocity in different vessel types

I found a nice image here which gives a general idea of several haemodynamic parameters over different vessel types:

enter image description here

Thus, this source says that the lowest velocity is in capillaries with about 0.05 cm/s or 0.0005 m/s (mind, this figure is for a dog with a heart pump rate of about 2l/min but gives you a ballpark number).

But, certainly, this is not constant over a heart cycle, is it? And that is why your question is quite interesting.

Towards the heart of your question

As the linked article points out:

[The flow] is strongly pulsatile, as a consequence of the alternation between ejection and filling [ie. systolic and diastolic] phases during the cardiac cycle.

That answers one part, ie. of course the flow velocity is underlying constant change. But, how much difference is there, approximately?

The very detailed article stops at saying that the peak velocity in the aorta (obviously during systolic phase) is about 1 m/s and shows different velocity change patterns dependent on various variables. It also has the following, general remark to offer:

In reality, velocity profiles in arteries are highly variable and affected by several fluid mechanical factors including the no-slip condition at vessel walls, effects of fluid inertia, the strongly time-varying flow, the curvature and non-uniform diameter of vessels, and the presence of bifurcations.

But there are loads and loads of measured dynamic flow velocity and flow volume diagrams out there. They all (as, as a random example, this paper has some flow diagrams for different arteries) show that

  1. The peak is shortly after the beginning of the systolic phase
  2. During the cycle, the flow not only comes to a halt but there is actually a small retrograde flow as well in many vessels. This happens at both the beginning and the end of the systolic phase

enter image description here

DAo = Descending Aorta

Therefore, to answer your question: yes, the blood flow is not constant in arteries (nor in venes, for that matter, but often for other reasons), comes to a halt, and there is even a small amount of backflow in each cycle when it changes from systolic to diastolic and again when it changes from diastolic to systolic.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.