Do the atria Contract?

If so, show me a detail medical reason for this phenonmenon. Why doesn't the Atria have a valve to prevent posterior flow if the atria do contract. Where does the blood go? What is the mechanism of atrial contraction ?> Please give a most detailed as answer as possible. I call for to proove this to a fellow classmate who refuses to believe the atria contract, he says its not possible because if the atria contract blood will flow support as there is no valve that sepeates the atra from the vena cavas.
Answers:
In most people they do. The atrial kick improve the blood flow into the ventricles. People in chronic atrial fibrillation, of course, don't have that pre-eminence, and may lose as much as 15% of their cardiac output as a result.
They do contract, but not as strongly as the ventricles. The blood is directed down into the ventricles both by gravity, and by the direction of the muscular contraction. The nerves spread down the heart, away from the top, and there are gap junctions between cell that allow action potentials to pass between cells. This results within a slightly asynchronous contraction from the top spreading down, pushing the blood in the right direction. As well as this, the pressure is lower in the atria than the ventricles, and the pressure within the vena cava is higher, so blood does not tend to flow back in that direction. Valves are superfluous. You are definitely right, the atria do contract! Source(s): Vet student.
The atria do contract to push the blood into the ventricles. This is to prevent backflow from the more powerful ventricle pressure. There is some blood flow from the atria into the veins, and this final pressure is detectable in the jugular veins.

The atria inlets don't need valve, since the veins have valves as it is. Therefore, the support pressure will not force the blood into the veins very far.

As to the mechanism, it's alike as for the ventricles.

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