10.3: Systemic and Pulmonary Circuits

Blood vessels play a fundamental role in establishing two primary circulatory circuits, each with distinct roles and varying volumes of blood involved (Figure 1). The Systemic Circuit, comprising systemic arteries and veins, is tasked with the transport of oxygen-rich blood to nourish the body's diverse tissues. As it proceeds, the oxygen content of this blood diminishes as a significant portion of the oxygen carried by the arteries is dispensed to meet the metabolic demands of the body's cells. Systemic veins then return this partially deoxygenated blood back to the heart. Conversely, the Pulmonary Circuit specializes in a unique role. It is composed of arteries responsible for carrying oxygen-depleted blood exclusively to the lungs. This journey culminates in the lungs, where gas exchange occurs, replenishing the oxygen content of the blood. Pulmonary veins are the next link in this circuit, returning oxygenated blood from the lungs to the heart.¹

Approximately 5-6 liters of blood circulate through each of these circuits per minute, ensuring a constant flow of blood. In spite of their structural and functional differences, arteries and veins exhibit common features that underpin their collective purpose within the circulatory system.

​Figure 1 | Cardiovascular Circulation  | ² The pulmonary circuit moves blood from the right side of the heart to the lungs and back to the heart. The systemic circuit moves blood from the left side of the heart to the head and body and returns it to the right side of the heart to repeat the cycle. The arrows indicate the direction of blood flow, and the colors show the relative levels of oxygen concentration.

---