What Is the Heart and Circulation?

Some ancient philosophers believed that the heart was the foundation of our soul. Today we recognize the heart for its true function, that of a muscular pump. The adult heart is about the size of its carrier’s fist and weighs about one-half pound (Figure 2.9). It serves to pump blood through thousands of miles of blood vessels to all regions of our body. Blood leaves the heart through arteries on route to tissue throughout the

body. Arteries feed into smaller arterioles and subsequently tiny capillaries, which then thoroughly infiltrate tissue. Most blood vessel mileage is attributable to capillaries. These blood vessels are so numerous in tissue
that nearly every cell in our body will have a capillary right next to it or very close. This is like having one river (artery) flowing into town that branches to the extent whereby every house has its own little stream (capillary).
 

Our heart is made up of muscle, nerves, and connective tissue and can beat more than two billion times during a lifetime.
 

As blood reaches the end of the capillaries and the tissue has been properly served, the blood will then drain into larger venules. The venules will eventually drain into larger veins, which ultimately return blood to the heart. This is like the streams draining into larger steams, which then drain back into the larger river. Quite simply, our blood serves as a delivery system. It delivers oxygen, nutrients, and other substances to cells throughout our body. At the same time, blood also serves to remove the waste products of cell metabolism, such as carbon dioxide and heat from our tissue. Capillaries are the actual sites of exchange of substances between our cells and the blood.
 

Our heart consists of four chambers (two atria and two ventricles), left and right. The left half, consisting of the left atrium and ventricle, serves to receive oxygen-rich blood returning from the lungs and to pump it to all the tissues throughout the body. The right half of the heart, consisting of the right atrium and ventricle, serves to receive oxygen-poor blood returning from tissue throughout our body and to pump it to the lungs. Therefore, our heart functions as a relay station for moving blood throughout our body in one large loop, hence the term circulation.