Key Terms

• alveoli: Air sacs in the lungs that provide the surface for gas exchange between the air and capillaries.
• pulmonary circulation: The part of blood circulation which carries oxygen-depleted blood away from the heart, to the lungs, and returns oxygenated blood back to the heart.
• systemic circulation: The part of blood circulation that carries oxygenated blood away from the heart, to the body, and returns deoxygenated blood back to the heart.

The cardiovascular system is composed of two circulatory paths: pulmonary circulation, the circuit through the lungs where blood is oxygenated; and systemic circulation, the circuit through the rest of the body to provide oxygenated blood. The two circuits are linked to each other through the heart, creating a continuous cycle of blood through the body.

Pulmonary Circulation

Pulmonary circulation is the movement of blood from the heart to the lungs for oxygenation, then back to the heart again. Oxygen-depleted blood from the body leaves the systemic circulation when it enters the right atrium through the superior and inferior venae cavae. The blood is then pumped through the tricuspid valve into the right ventricle. From the right ventricle, blood is pumped through the pulmonary valve and into the pulmonary artery. The pulmonary artery splits into the right and left pulmonary arteries and travel to each lung.

At the lungs, the blood travels through capillary beds on the alveoli where gas exchange occurs, removing carbon dioxide and adding oxygen to the blood. Gas exchange occurs due to gas partial pressure gradients across the the alveoli of the lungs and the capillaries interwoven in the alveoli. The oxygenated blood then leaves the lungs through pulmonary veins, which returns it to the left atrium, completing the pulmonary circuit. As the pulmonary circuit ends, the systemic circuit begins.

Alveoli: A diagram of the alveoli, showing the capillary beds where gas exchange with the blood occurs.

Pulmonary circuit: Diagram of pulmonary circulation. Oxygen-rich blood is shown in red; oxygen-depleted blood in blue.

Systemic Circulation

Systemic circulation is the movement of blood from the heart through the body to provide oxygen and nutrients to the tissues of the body while bringing deoxygenated blood back to the heart. Oxygenated blood enters the left atrium from the pulmonary veins. The blood is then pumped through the mitral valve into the left ventricle. From the left ventricle, blood is pumped through the aortic valve and into the aorta, the body’s largest artery. The aorta arches and branches into major arteries to the upper body before passing through the diaphragm, where it branches further into the illiac, renal, and suprarenal arteries which supply the lower parts of the body.

The arteries branch into smaller arteries, arterioles, and finally capillaries. Gas and nutrient exchange with the tissues occurs within the capillaries that run through the tissues. Metabolic waste and carbon dioxide diffuse out of the cell into the blood, while oxygen and glucose in the blood diffuses out of the blood and into the cell. Systemic circulation keeps the metabolism of every organ and every tissue in the body alive, with the exception of the parenchyma of the lungs, which are supplied by pulmonary circulation.

The deoxygenated blood continues through the capillaries which merge into venules, then veins, and finally the venae cavae, which drain into the right atrium of the heart. From the right atrium, the blood will travel through the pulmonary circulation to be oxygenated before returning gain to the system circulation, completing the cycle of circulation through the body. The arterial component of systemic circulation the highest blood pressures in the body. The venous component of systemic circulation has considerably lower blood pressure in comparison, due to their distance from the heart, but contain semi-lunar valves to compensate. Systemic circulation as a whole is a higher pressure system than pulmonary circulation simply because systemic circulation must force greater volumes of blood farther through the body compared to pulmonary circulation.