What is the key function of the heart?

The key function of the heart is to circulate blood throughout the body. This vital activity maintains the flow of blood, which carries oxygen, nutrients, hormones, and waste products to and from cells and tissues. The heart acts as a pump, ensuring that oxygen-rich blood is delivered to the body's organs and muscles while simultaneously facilitating the return of oxygen-poor blood to the lungs for oxygenation.

The other options touch on important processes related to cardiovascular function but do not pinpoint the heart's primary role. For instance, while the circulation of nutrients is crucial, it is a process facilitated by the blood rather than a direct function of the heart itself. Oxygenation of blood takes place in the lungs, where carbon dioxide is exchanged for oxygen; this process is separate from the heart’s pumping action. Lastly, the heart does not control metabolism directly; instead, metabolism is regulated by various hormones and enzymes throughout the body that depend on the cardiovascular system to deliver necessary substances. Thus, focusing on the heart's main role as a blood-circulating organ accurately defines its key function.