The muscular organ that pumps blood around the body and is strengthened by regular cardio exercise is the heart.
The heart is a four-chambered pump. Its right side sends oxygen-poor blood to the lungs, while its left side sends oxygen-rich blood through the aorta to the rest of the body. Valves keep blood moving in one direction, and electrical signals coordinate each heartbeat.
Regular aerobic exercise can improve cardiovascular fitness by helping the heart pump more efficiently. It can also support healthier blood pressure and circulation. The heart is not the same as the lungs: the lungs exchange oxygen and carbon dioxide, while the heart circulates the blood that carries those gases.
The heart begins beating early in human development, at roughly three weeks after conception. Its rhythm is normally initiated by pacemaker cells in the sinoatrial node, often called the heart's natural pacemaker.