The heart actually consists of two separate pumps. The right side of the heart pumps blood to the lungs through the pulmonary circulation so that gas exchange, uptake of oxygen and elimination of carbon dioxide can take place. The left side of the heart pumps blood to the rest of the tissues of the body through the systemic circulation. In this way, oxygen and nutrients are delivered to the tissues to sustain their activities and carbon dioxide and other metabolic waste products are removed from the tissues. In both circulations, blood vessels of the
, arteries and arterioles, carry blood away from the heart and toward the tissues. The arterioles deliver blood to the
where the exchange of substances between the blood and the tissues takes place. From the capillaries, blood flows into the vessels of the
, veins and venules, which carry blood back to the heart. The human heart begins pumping approximately 3 weeks after concep-
tion and must continue this activity without interruption all day, every day, for an entire lifetime. In a typical individual, this means the heart pumps over 100,000 times per day and propels about 2000 gallons of blood through almost 65,000 miles of blood vessels. This function of the heart will be discussed here as well as in the following chapter on cardiac output. The function of the blood vessels will be considered in the chapter on the circulatory system.