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Vena cava is valve less. So, during atrial systole what prevents backflow of blood to them?

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Besides @Bryan Krause answer, there's another very important factor that prevents the blood from flowing back from the right atrium to the vena cava during systole.

Part of the muscle that constitutes the atrial walls also wraps around the site of entrance of the vena cava. Thus, during systole, the muscle contracts, closing (or almost closing) the passage and doing the job of a valve.

Nature!

EDIT: because of the high skepticism over what I have said in this answer, and having no references, I went to search for the class where I studied this, and here's the slide containing the information. Unfortunately, it is in Portuguese, but I will translate what it says, and you can check in any translator.

enter image description here

Venous myocardium: Cava Veins and Lung Veins

The veins that open in the heart don't have functional valves, meaning that the atrial systole could cause blood backflow through these veins.

Venous myocardium: The cardiac muscle tissue is prolonged through the vein walls that bring blood into the atrial cavities to the point of insertion of the fibrous pericardium.

Physiology: The venous myocardium contracts at the same time as the atrial wall, preventing the backflow of blood to the veins during atrial systole.

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    $\begingroup$ Do you have a citation? I don't believe that is accurate. If it does close at all, it certainly isn't enough to prevent blood flow in either direction. $\endgroup$ – Bryan Krause Apr 9 '17 at 22:59
  • $\begingroup$ @BryanKrause I have studied this in anatomy class, I can search later for a citation in the internet if you wish. It does close, and even if it doesn't close completely the passage, it turns it much thinner, meaning much less blood can pass through it an só the blood will go the other way - to the ventricle. It's both this system and the different pressures that you talked about that contribute to the prevention of backflow to the vena cava $\endgroup$ – Filipe Rocha Apr 9 '17 at 23:03
  • $\begingroup$ @BryanKrause this is just a drawing but you can see the atrial muscle also wrapping around the entrance of the superior vens cava goo.gl/images/io75Jw $\endgroup$ – Filipe Rocha Apr 9 '17 at 23:07
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    $\begingroup$ @FilipeRocha - I don't think hat shows what you think it shows... $\endgroup$ – anongoodnurse Apr 10 '17 at 3:47
  • $\begingroup$ @anongoodnurse what do you mean? $\endgroup$ – Filipe Rocha Apr 10 '17 at 8:48
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Pressure differences. Atrial contractions are much more gentle than ventricular ones, and in normal circulation the venous pressure in the vena cava often stays higher than the right atrium, even during atrial systole. If the pressure in the atrium does rise above the vena cava pressure at the peak of systole, there can be some backflow, although it is slight and brief in normal physiology.

The major veins are typically somewhat distended so they are constantly pushing blood into the right atrium. Blood continues to flow from the veins into the right atrium and from the right atrium into the right ventricle throughout ventricular diastole, including during most of atrial systole.

References:

Anderson, R. M. (1993). The gross physiology of the cardiovascular system. Robert M. Anderson.

Wexler, L., Bergel, D. H., Gabe, I. T., Makin, G. S., & Mills, C. J. (1968). Velocity of blood flow in normal human venae cavae. Circulation Research, 23(3), 349-359.

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  • $\begingroup$ Venous pressure increases during atrial systole, can we say it is due to backflow of blood? $\endgroup$ – Anubhav Goel Apr 10 '17 at 19:24
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    $\begingroup$ @AnubhavGoel I think it's more accurate to say it's because of briefly reduced blood flow to the heart rather than from blood actually flowing backward. Maybe think about a highway, if something happens that causes some cars to slow down on a congested road, you are going to get more congestion ("pressure") behind them, but no one has gone in reverse, just slowing down is sufficient. That said, I have certainly seen this abbreviated as a "backflow" when describing venous pressure during the cardiac cycle. From a strict sense I would say this is wrong (in normal physiology). $\endgroup$ – Bryan Krause Apr 10 '17 at 19:54
  • $\begingroup$ I agree with you but regurgitation of blood do take place. $\endgroup$ – Anubhav Goel Apr 21 '17 at 11:40
  • $\begingroup$ @AnubhavGoel It shouldn't happen (in the human) outside of pathology, because right atrial pressure doesn't exceed vena cava pressure. In cases of pathology, yes of course reverse flow can occur. Filipe's answer is wrong and based on a single slide with no further reference. $\endgroup$ – Bryan Krause Apr 21 '17 at 16:10
  • $\begingroup$ Look at my answer. $\endgroup$ – Anubhav Goel Apr 21 '17 at 17:30
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The atrial pressure when equal to that in the incoming vein will contain blood which will be propelled into the right ventricle without the need for any increased pressure [as does the ''muscular pump' in the body. In other words the right atrium is ' functionally a sophisticated valve'

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    $\begingroup$ Welcome to Biology.SE! Please elaborate on your answer by providing references. For more information on how to provide good answers on this site, please see: biology.stackexchange.com/help/how-to-answer. Thanks! 😊 $\endgroup$ – tyersome Jul 29 '19 at 20:44

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