Heart

Heart muscles squeeze and relax in a repeating pattern called the heartbeat. About the size of a fist, the heart sits behind the sternum and slightly to the left. With every beat it pushes fresh, oxygen-rich blood to the body and returns used blood to the lungs for refilling.

The heart has four rooms called chambers. The two atria on top receive blood, and the two ventricles below pump it back out. Flap-like valves between the chambers act like doors that open only one way, so blood keeps moving forward and the familiar "lub-dub" sound stays even.

A natural pacemaker named the sinoatrial node sends electrical sparks that spread through the heart wall. These signals tell the atria to squeeze first and the ventricles second, creating a smooth rhythm. Doctors can read the pattern on an electrocardiogram to make sure the timing looks healthy.

Coronary arteries wrap around the heart and bring it oxygen. When these vessels stay clear, the heart muscle stays strong. Too much plaque, smoking, or high blood pressure can narrow the arteries and cause chest pain or even a heart attack, so healthy habits matter.

Movement, rest, and food all affect the heartbeat. Exercise tells the heart to beat faster and pump more strongly, while sleep slows it down so it can recover. Eating colorful fruits, vegetables, and whole grains helps blood vessels stay flexible. Wearing seat belts, learning CPR, and seeing doctors for checkups all protect this nonstop pump.