How blood flows through the heart

Think of the heart as two pumps sitting side by side. Each side has an upper atrium, which receives blood, and a lower ventricle, which is the main pumping chamber. That gives four chambers in total.

The right side sends blood through the pulmonary circuit: to the lungs to pick up oxygen and back. The left side sends blood through the systemic circuit: out to nearly every tissue in the body and back. The two sides beat together, but after birth a wall called the septum keeps their blood separate.

Follow one drop of oxygen-poor blood coming back from the body:

1. The superior and inferior venae cavae empty it into the right atrium.
2. It passes the tricuspid valve into the right ventricle.
3. The right ventricle pumps it through the pulmonary semilunar valve into the pulmonary trunk, which splits into the left and right pulmonary arteries.
4. In the lungs it picks up oxygen.
5. The pulmonary veins bring it back to the left atrium.
6. It passes the mitral (bicuspid) valve into the left ventricle.
7. The left ventricle pumps it through the aortic semilunar valve into the aorta, and on to the body.

Notice the pattern on each side: atrium, atrioventricular valve, ventricle, semilunar valve, big artery.

Why is the left ventricle wall so much thicker? The right ventricle only has to push blood through the pulmonary circuit, which is shorter and offers less resistance. The left ventricle has to generate far more pressure to drive blood through the entire systemic circuit, so its muscle is much thicker and stronger. If a question asks which chamber works hardest, the answer is the left ventricle.

Valves are one-way doors that open and close because of pressure differences. Blood always moves from higher pressure to lower pressure, and a valve swings shut when pressure on the far side becomes higher.

The atrioventricular (AV) valves, tricuspid and mitral, sit between each atrium and ventricle. Their flaps are tied by cords called chordae tendineae to papillary muscles in the ventricle wall. When the ventricle contracts, those muscles tighten the cords so the flaps cannot flip back into the atrium.

The semilunar valves, pulmonary and aortic, sit at the exits of the ventricles and have no cords; they close when blood starts to fall back toward the ventricle.

You can hear this happen. The first heart sound, "lub," is the AV valves closing as the ventricles start to contract. The second sound, "dub," is the semilunar valves closing as the ventricles relax. An unusual extra sound from turbulent blood flow is called a murmur.

No. An artery is any vessel that carries blood away from the heart, and a vein carries blood toward the heart. Direction defines them, not oxygen.

In the systemic circuit, arteries do carry oxygen-rich blood. The pulmonary circuit is the exception. The pulmonary arteries carry oxygen-poor blood away from the heart to the lungs, and the pulmonary veins carry oxygen-rich blood back to the heart. Arteries also have thicker walls than veins because they handle higher pressure, while many veins have valves that help blood return to the heart.

Heart muscle has its own built-in wiring, the conduction system. The signal follows this route:

1. SA (sinoatrial) node in the wall of the right atrium fires first. It is the heart's natural pacemaker.
2. The signal spreads across the atria, and they contract.
3. AV (atrioventricular) node holds the signal for about a tenth of a second. This pause lets the atria finish pushing blood into the ventricles.
4. Bundle of His (AV bundle) carries the signal down the wall between the ventricles.
5. Right and left bundle branches split toward each ventricle.
6. Purkinje fibers spread the signal through the ventricle muscle, so the ventricles contract from the bottom (apex) upward.

Heart muscle works nonstop, so it needs its own dedicated blood supply, the coronary circulation. The left and right coronary arteries branch off the base of the aorta and supply the heart muscle. After use, most of this blood drains into the coronary sinus, a vein that empties into the right atrium.