Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Cardiovascular System
Blood Vessels of the Human Body

Blood circulates through The Human Body via two closed vascular systems known as the pulmonary and systemic circuits (Fig. 88, see color insert).

The Pulmonary Circuit. The pathway of blood from the right ventricle through the pulmonary Arteries, capillaries, and Veins to the left atrium is called the pulmonary circuit, or lesser Circulation. Its primary function is to oxygenate the blood and eliminate carbon dioxide from the body. From the right ventricle, deoxygenated blood enters a large vessel called the pulmonary trunk, which divides into the right and left pulmonary arteries. Within the Lungs, these branches subdivide further and form dense capillary networks that wrap around the pulmonary alveoli (air sacs). Gas exchange occurs across the alveolar walls: oxygen diffuses from the alveoli into the blood, while carbon dioxide moves in the opposite direction, from the blood into the alveoli. As deoxygenated blood flows through the pulmonary capillaries, it becomes saturated with oxygen and is transformed into oxygenated blood. This oxygen-rich blood then flows from the capillaries into venules, which merge in each lung to form two Pulmonary veins that empty into the left atrium.

The Systemic Circuit. The pathway of blood from the left ventricle of The Heart to the right atrium is known as the systemic circuit, or greater circulation. Upon contraction, the left ventricle pumps oxygenated blood into the aorta, the largest artery in the body. Branches of the aorta extend to all Tissues and Organs, ultimately branching into extensive capillary beds. Through the systemic capillaries, oxygen, nutrients, and Hormones are delivered to all body Cells and Tissues, while Metabolic waste products, including carbon dioxide, pass from the cells and tissues into the capillaries. Consequently, blood carrying metabolic waste becomes deoxygenated. Capillaries converge into venules, which subsequently merge to form smaller and eventually larger veins. Deoxygenated blood from the trunk, lower limbs, and the abdominal and thoracic cavities returns to the right atrium via a large vessel called the INFERIOR VENA CAVA. Simultaneously, deoxygenated blood from the HEAD, neck, and upper limbs drains into the right atrium through the SUPERIOR VENA CAVA.