eli5: Why is it that a heart beats in pairs of beats?
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The human heart has four distinct chambers in it, separated by tissue membranes and valves. Deoxygenated blood returns from the body via the great veins (vena cava) into the right atrium, and is then pumped into the right ventricle when the atrium contracts. When the right ventricle contracts, the blood is forced through the pulmonary artery into the lungs where it can offgas waste carbon dioxide and uptake oxygen. The freshly oxygenated blood returns from the lungs through the pulmonary vein into the left atrium, which is relaxing while the right ventricle is contracting, so the blood has somewhere to go. When the left atrium contracts, blood is forced into the left ventricle, and when the left ventricle contracts, blood is forced out to the body via the great arteries, eventually to return to the right atrium. Both atria contract simultaneously while the ventricles fill, and both ventricles contract simultaneously while the atria fill. The characteristic “lub dub” sound of a heartbeat corresponds to the relatively weak contraction of both atria (the low pressure chambers), followed by the powerful contraction of both ventricles (the high pressure chambers).
It’s how the heart is “designed” so to speak. The smaller first beat moves the blood into the ventricles, which are the bigger chambers. Then the second beat is the muscle around those bigger chambers sending the blood down the arteries. After the big beat it takes a moment to recover and take in what is going to be the next pulse.
To compare it to something, you could use a slingshot. The first beat is pulling the shot back, and the big beat is letting it fly.
Human hearts have 4 chambers to them: two top ones, called atria, and two bottom ones, called ventricles. Your atria receive blood and pass it along to your ventricles, your ventricles take the blood from the atria and push it hard enough to get all the way to where it’s going and back. At the exit of each chamber there is a valve. Think of them as one way doors; the blood can keep moving the way it’s supposed to, but it can’t go the wrong way (unless the valve is damaged).
The left atrium gets blood from your lungs, freshly filled with oxygen, and passes it along to the left ventricle, which sends it all over your body. The blood then reaches the right atrium, at which point it needs to go to the lungs to leave it’s CO2 and get more oxygen, so your right ventricle sends it there. In order to keep blood exiting and entering the heart at the same rate, both atria squeeze blood to the ventricles at the same time, and both ventricles squeeze blood out of the heart at the same time. Your atria squeeze first, your ventricles squeeze second.
The sound you can hear is the sound made by the valves at each exit closing. You hear a “lub” when the valves between the atria and ventricles close, as the ventricles start to squeeze. You then hear a “dub” when the valves at the exit if the ventricles close, as the ventricles start to relax.
The heart is composed of for chambers, two atria (the top part of the heart) and two ventricles (the bottom part). Both atria contract or beat first forcing blood into the ventricles which contract soon after. While this generates a single pulse if felt at any pulse point such as the wrist or neck, if listening to the chest what can be heard is the “lub dub” associated with a heart beat as the chambers contract in sequence.
The heart is also divided into two halves, each with one atria and one ventricle. The right side sends blood to the lungs to get oxygenated and the left side sends the oxygenated blood to the body.
This is a very simplified explanation, hope it helps.
First some anatomy/physiology: The heart has four chambers and four major valves. Starting with deoxygenated blood returning to the heart, it takes the following path: right atrium, right ventricle via the tricuspid valve, pulmonary artery via the pulmonic valve, lung, left atrium via the pulmonary vein, left ventricle via the mitral valve, and finally the aorta via the aortic valve. Blood at this point is oxygenated and flows to tissues and organs throughout the body. After delivering oxygen it then returns to the right atrium and the cycle repeats itself.
The heart is pump and like any pump, it’s effectiveness is dependent on forward flow of blood and preventing back-flow (or regurgitation in medical speak). This why the heart valves are so important; when certain chambers are contracting, some valves need to be closed to prevent backflow while others need to be open to allow for forward flow.
The “lub-dub” of a heart beating is actually the valves snapping shut. The first sound (or “lub”) is the tricuspid and mitral valves snapping shut to prevent backflow into the right and left atria, respectively. The second sound (or “dub”) is the pulmonic and aortic valves snapping shut to prevent backflow into the right and left ventricles, respectively.
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