Heart pumping: AAMC 6

[EnginrTheFuture]

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In AAMC 6, they ask essentially what the condition of the heart is when the blood pressure is lowest. The answer of these choices seems obvious because 3 of them are clearly ventricle systole.

What I don't get is the correct answer... is it possible for the atrium and the ventricle to be relaxed at the same time?

I thought the ventricle squeezed blood into arteries while the atrium relaxed (allowing it to fill)... Then the ventricle relaxed and the atrium squeezed to fill the ventricle.

Any insight into what goes on in the heart 🙂

 

[Dasypus]

You know the sound the heart makes, "lub-dup, lub-dup"? During the 'lub', the atria contract. During the 'dup', the ventricles contract. And during that comma pause between dup and the next lub, the heart rests and the atria fill passively.

Edit: that's not exactly right. The noise of that 'lub' is caused by the valves that shut the atria off from the veins that fill them slamming shut. And the 'dup' is made by the valves that stop blood from flowing back from ventricle to atrium slamming shut.

Anyhow, most of the time, nothing in the heart is contracting. It's filling.

 

[EnginrTheFuture]

So the ventricles contract as the atria are relaxed. The ventricles then relax and the atria stay relaxed/ continue to fill. Then the atrium contracts, delays ventricle contraction for a bit, relaxes... then ventricles contract again?

 

[Dasypus]

Start the loop from just after the ventricles stop contracting.

Pressure is high in the aorta and pulmonary arteries. Valves (aortic and pulmonary, go figure) are shut so there's no backflow. Now there's the long pause, during which the heart is relatively relaxed. Blood keeps flowing into the atria from pulmonary and systemic veins, because they've got lots of valves in them that keep things flowing the right way. The atria fill relatively easily, because they're relatively relaxed and expand to accept the blood.

Eventually, the SA node fires. The atria contract, sending blood into the ventricles. As they start contracting, the valves that connect them to the veins close to stop backflow (lub).

The signal gets hung up at the AV node for a bit. It's passed on, and the ventricles, which by now are pretty full with blood, begin to contract. The valves that connect them to the atria (tricuspid and bicuspid/mitral) slam shut (dup). Blood floods the pulmonary arteries and aorta.

Loop to top.

 

[Morsetlis]

The ventricle refills passively for most of the time. The atrial contraction gives it that last oomph near the end of ventricular refill.

Notice "rapid inflow" is before "atrial systole". The atria and ventricles cannot contract at the same time, but they can both be relaxed at the same time! Choice C describes ventricular systole, and choice D describes pulmonary systole, which occurs at the same time. (However, during inspiration, the aortic valve closes before the pulmonary valve, so there could be a split S2, and that also means left ventricular systole ends slightly before right ventricular systole.)