The Cardiovascular System
The cardiovascular system includes the heart, blood, and the blood vessels.
Heart: the pump that moves the blood to the organs, tissues, and cells of the body.
Blood: delivers oxygen and nutrients to cells; removes carbon dioxide and waste products made by cells.
Blood Vessels: circulate blood from the heart to the rest of the body and back through a complex network of arteries, arterioles, capillaries, venules, and veins. The entire vessel network in one human body extends approximately 60,000 miles: far enough to circle the earth more than two times!
Circulation: the one-way distribution of blood throughout the body. Arteries carry oxygen-rich blood away from the heart, and veins carry oxygen-poor blood back to the heart. The 20 major arteries branch into smaller arterioles, which further branch into capillaries. Most capillaries are thinner than a hair, allowing only one blood cell to travel through at a time. In the capillaries, blood cells deliver oxygen and nutrients to the adjacent tissue cells, pick up carbon dioxide and other waste, and then move on through to the wider venules. Venules join to form veins, and the veins return the blood cells to the heart and lungs.
When the cardiovascular system is used and challenged regularly, it becomes efficient. Cardiovascular exercise performed with the large muscle groups radically develops the intricate, web-like network of arterioles, capillaries, and veins. This process is called capillary proliferation. When this occurs, cells (especially muscle cells) have oxygen immediately available virtually any time it is needed. This is one of the advantages of achieving cardiovascular fitness.