The circulatory system functions as a continuous transport network. This complex system is organized into two separate, interconnected circuits that operate simultaneously: the pulmonary circulation and the systemic circulation. The heart acts as the central engine, precisely routing blood between these two distinct pathways.
Pulmonary Circulation: The Gas Exchange Loop
The pulmonary circuit is a dedicated short loop that begins its journey carrying blood low in oxygen content. This blood collects in the right side of the heart, specifically entering the right ventricle after passing through the right atrium. The right ventricle then contracts, pushing this deoxygenated blood through the pulmonary valve and into the large pulmonary artery.
The pulmonary artery is unique among the body’s arteries because it carries blood depleted of oxygen away from the heart toward the lungs. This vessel quickly branches into smaller arteries that deliver blood to the delicate capillary networks surrounding the alveoli, the tiny air sacs within the lungs. It is within these capillaries that the primary function of the pulmonary circuit occurs: gas exchange.
Carbon dioxide diffuses out of the capillaries into the alveoli to be exhaled. Simultaneously, oxygen from inhaled air diffuses from the alveoli into the blood. This newly oxygenated blood flows into small pulmonary venules, which merge to form the pulmonary veins. These veins carry the oxygen-rich blood back to the left atrium of the heart, completing the gas exchange loop.
Systemic Circulation: Delivering Resources
The systemic circuit begins where the pulmonary circuit ends, starting with the newly oxygenated blood that has returned to the left atrium. From the left atrium, the blood moves into the left ventricle, the heart’s most powerful chamber. The left ventricle then forcefully pumps this oxygen-rich blood through the aortic valve and into the aorta, the largest artery in the entire body.
This high-pressure flow distributes blood through a vast network of systemic arteries and arterioles that branch out to supply virtually every tissue and organ outside of the lungs. The arteries narrow down into a dense network of capillaries within the body’s tissues, where the second type of exchange occurs. Here, oxygen and nutrients diffuse out of the blood and into the cells, fueling cellular metabolism.
The blood absorbs metabolic waste products, including carbon dioxide, from the tissues. The deoxygenated blood flows from the capillaries into venules, which merge into larger systemic veins. These veins eventually converge into the superior and inferior venae cavae. These largest veins return the oxygen-depleted blood to the right atrium of the heart.
Structural and Functional Comparison
The two circuits differ significantly in their structure and function, which dictates how the heart must power them. The pulmonary circuit is a short, low-resistance pathway designed solely to move blood between the heart and the adjacent lungs. This short distance means the pulmonary circulation operates at a low pressure, typically ranging from 5 to 15 mmHg.
Conversely, the systemic circuit is a much longer, high-resistance pathway that must propel blood to the extremities, the brain, and all other organs. To overcome this vast distance and resistance, the systemic circulation operates at a significantly higher pressure, with a mean arterial pressure around 93 mmHg and peak systolic pressures reaching about 120 mmHg. This difference in pressure is a primary structural distinction between the two circuits.
A major functional difference involves the composition of blood within the main vessels. In the systemic circuit, arteries carry oxygenated blood away from the heart, and veins carry deoxygenated blood back to the heart. This is reversed in the pulmonary circuit: pulmonary arteries carry deoxygenated blood to the lungs, and pulmonary veins carry oxygenated blood back to the heart.
The Heart’s Role as a Dual Pump
The heart acts as a double pump, with a muscular wall called the septum separating the right and left sides to keep the two blood circuits distinct. This separation ensures that oxygenated blood destined for the body does not mix with deoxygenated blood heading to the lungs. The right side of the heart is dedicated to the pulmonary circuit, while the left side powers the systemic circuit.
The ventricles, which provide the main pumping force, display anatomical differences reflecting the demands of their circuits. The right ventricle has thinner, less muscular walls, generating only the low pressure needed for the pulmonary circuit. In contrast, the left ventricle possesses a much thicker and more muscular wall. This robust structure allows it to generate the high pressure necessary to propel blood throughout the extensive systemic network.