How Does the Blood Circulate?

The Circulatory System

Because blood 'circles' through the body, the system, including your heart and your blood vessels, that moves your blood is called the circulatory system. The circulatory system includes the pulmonary circulation, the circle through the lungs where blood releases wastes and becomes oxygenated, and the systemic circulation, where the freshly oxygenated blood is delivered to the rest of the body through the network of blood vessels. At the center of this system is the heart, the organ that keeps the circulatory system running.

Pulmonary Circulation and the Heart

First, blood without oxygen enters the right part of the heart (the right atrium through the vena cava) and is then pumped into the right ventricle. The familiar "ta-thump" of the heart illustrates this two part cycle--the heart first fills itself on the first part of the heartbeat, and then contracts violently to force blood out at pressure, which makes the louder second part of the heartbeat. The cardiac muscle propels it all. From the right ventricle, the blood is pumped into the pulmonary arteries, one for each lung, and the blood becomes oxygenated. The oxygenated blood then leaves the lungs by the pulmonary veins. This oxygen-rich blood enters the left atrium, and is pumped through the bicuspid valve into the left ventricle, ready to be pumped to the body.

Systemic Circulation

Blood rich in oxygen is pumped out of the left ventricle through the aortic semilunar valve, into the aorta. The aorta is thick and large, because it has to accept the high amount of pressure the heart puts out to the rest of the body. The aorta splits off into major arteries, some of which go through to the upper body while others go below the diaphragm. Blood moves through smaller and smaller vessels, from arteries to arterioles and eventually to capillaries, some of which are so thin that red blood cells must enter single file. Venules collect the blood after it loses oxygen in the capillaries, and carries the fluid into veins. The veins, in turn, make their way back to the right side of the heart, and the process (first pulmonary circulation, then systemic circulation) repeats itself.

References

• The Franklin Institute: Circulatory System
• The cardiovascular system
• A moving diagram of blood circulation