How does the heart pump blood?

The heart is a muscular pump about the size of a fist, divided into four chambers: two upper atria and two lower ventricles. Blood returning from the body enters the right atrium, drops into the right ventricle, and is pushed to the lungs to pick up oxygen. Oxygen-rich blood comes back to the left atrium, fills the left ventricle, and is driven out through the aorta to the rest of the body. Each beat has two phases: the chambers relax and fill, called diastole, then contract and eject, called systole. Four one-way valves snap shut to stop blood from flowing backward, and their closing produces the familiar lub-dub you hear through a stethoscope. This cycle repeats without pause, all day, every day.

The heart is really two pumps working side by side, and it runs on its own electricity. The right side handles the short loop to the lungs; the left side, with a thicker muscular wall, drives blood through the whole body against much higher pressure. Crucially, the heartbeat begins inside the heart itself. A cluster of cells called the sinoatrial node acts as a natural pacemaker, firing electrical impulses that sweep across the atria and then, after a brief delay at the atrioventricular node, race down specialized fibers to squeeze the ventricles from the bottom up. This built-in rhythm is why a heart can keep beating even when removed from the body. Nerves and hormones do not start the beat; they only speed it up or slow it down as the body's needs change.

This design explains much of modern heart medicine. A blood pressure reading captures the two phases directly: the higher systolic number is the force during contraction, the lower diastolic number the pressure while the heart refills. An electrocardiogram traces the electrical wave from the pacemaker across the muscle, letting doctors spot rhythm problems. When the natural pacemaker or its wiring fails, an implanted electronic pacemaker can take over. Heart attacks happen when a coronary artery, one of the vessels feeding the heart muscle itself, becomes blocked, starving part of the pump of oxygen. Heart failure means the muscle can no longer push enough blood to meet the body's needs. Each of these conditions traces directly back to how the pump and its electrical system are built to work.

Myth: the heart sits on the left side of the chest. Reality: it lies roughly in the center, behind the breastbone, tilted slightly to the left. Myth: the brain tells the heart to beat. Reality: the heart generates its own electrical signal and will beat on its own; the nervous system only adjusts the pace. Myth: oxygen-poor blood is blue. Reality: blood is always red, just darker red when low on oxygen, and veins only look bluish through the skin. Myth: the heart rests between beats. Reality: it never stops, beating roughly 100,000 times a day and around 2.5 billion times over an average lifetime, doing its only refilling in the split-second pause within each cycle.