Following the Heart’s Evidence Trail

Inside a Working Heart

1Press two fingers gently against the inside of your wrist, and you may feel a pulse. Each pulse is a pressure wave traveling through an artery after the heart contracts. The heart is a hollow, muscular organ, not a container that simply stores blood. Its repeated contractions move blood through connected vessels, bringing oxygen and nutrients to body tissues and carrying away wastes.

2Inside the heart are four chambers: two upper atria and two lower ventricles. The atria receive blood returning to the heart. Their contractions help move blood into the ventricles, which then contract to send blood out of the heart. Between beats, the chambers relax and fill again. A muscular wall separates the right and left sides, keeping their blood from mixing in a healthy heart.

3The right side receives blood returning from the body after tissues have used some of its oxygen. The right ventricle pushes this blood through the pulmonary arteries toward the lungs. In the lungs, blood releases carbon dioxide and picks up oxygen. It returns through the pulmonary veins to the left atrium. Thus, blood leaving the right ventricle has another stop before it can deliver fresh oxygen to body tissues.

4From the left atrium, blood moves into the left ventricle. This chamber pumps it into the aorta, the large artery leading to the body. The left ventricle has a thicker muscular wall than the right ventricle because it must generate greater pressure for the body’s circulation. Eventually, blood returns to the right atrium, completing a route that includes both the lungs and the rest of the body.

5Muscle power alone would not make this route work properly. Valves open when pressure pushes blood forward and close when blood begins to move backward. They do not actively pull blood through the heart; they help keep it moving in one direction. Electrical signals provide another kind of control. Normally, a group of specialized cells in the right atrium starts a signal that spreads through the heart, coordinating contractions so the atria contract before the ventricles.

6During exercise, working muscles use oxygen more quickly. The heart usually responds by beating faster and pumping more blood with each beat, increasing the amount delivered each minute. However, counting beats alone does not reveal how much blood moves with every contraction. The blood vessels matter, too: arteries carry blood away from the heart, and veins return it. Even a strong pump depends on open pathways, properly working valves, and coordinated signals to keep blood circulating.