The Heart

The heart is the source of power, the pump that drives blood through the network of blood vessels throughout the body. The heart and blood vessels together form the cardiovascular system. In fact, the heart consists of two pumps that serve two separate systems of blood vessels: the pulmonary circulation, which goes through the lungs, and the systemic circulation, which goes through the rest of the body (see Figure 3.2).


Figure 3.2 The pulmonary circulation and the systemic circulation

The heart is a large four-chambered muscular bag on the left side of the chest.

Because of its four-chamber design, the heart can serve both circuits at once, by using its two pumps simultaneously push blood from one circuit through one-half of its structure and blood from the other circuit through its other half (see Figure 3.3).


Figure 3.3 Anatomy of the heart.

The muscular part of a heart is called the myocardium ("myo-" means "muscle" and "-cardium" means "of the heart"). The heart muscles are very special because they keep beating (contracting and relaxing) spontaneously throughout our whole lives without any conscious decision from us to make them beat.

The Blood Vessels

There are three types of blood vessels:

a. Arteries are subjected to higher blood pressure than any other vessels.They carry blood away from the heart. The volume and the rate of blood flow in the arteries vary with the pumping force of the heart. Arteries have layers of muscular and elastic tissue in their walls, which allows the vessels to expand with the contraction of the heart, and contract again as the heart refills with blood. The smallest arteries are arterioles while the largest artery in the body is aorta. All arteries except pulmonary artery carry oxygenated blood.
b. Capillaries are the smallest blood vessels in the body, and it is the site where the exchange of gases takes place.
c. Veins are collecting vessels that return blood to the heart. Veins have valves to prevent the blood from running backwards or pooling. The smallest veins are called venules, and the largest one is vena cava. All veins except pulmonary veins carry deoxygenated blood.


Figure 3.4 Diagram of the different types of blood vessels.
Last modified: Tuesday, 28 February 2017, 7:01 PM