The single, unified circulatory system is effectively divided into two distinct circuits that work in tandem: the pulmonary circulation and the systemic circulation. These two loops are linked by the heart, which acts as a double pump, processing blood sequentially through both circuits without ever mixing the blood from the two different pathways. Understanding the difference between these two circulatory paths reveals how the body manages both gas exchange and nutrient delivery simultaneously.
The Pulmonary Circuit
The pulmonary circuit is a short, specialized loop dedicated to processing blood through the lungs. It begins when deoxygenated blood, returned from the body, is collected in the right side of the heart. The right ventricle pumps this oxygen-poor blood into the pulmonary artery.
The pulmonary artery is unique because it is the only artery in the body that carries deoxygenated blood away from the heart. This vessel branches, leading the blood into the capillary network surrounding the air sacs, or alveoli, within the lungs. The core function of this circuit is external respiration, where the blood releases carbon dioxide for exhalation and absorbs a fresh supply of oxygen.
Once gas exchange is complete, the now-oxygenated blood collects in the pulmonary veins. These veins carry the oxygen-rich blood back to the left atrium, completing the pulmonary circuit.
The Systemic Circuit
The systemic circuit is the body’s extensive network responsible for distributing oxygenated blood to every tissue and organ. Its journey starts from the left side of the heart, which receives the freshly oxygenated blood from the lungs. The powerful left ventricle pumps this blood out through the aorta, the body’s largest artery.
The aorta branches into a complex tree of progressively smaller arteries and arterioles, directing blood flow to all regions of the body. As blood travels through this system, it eventually reaches the microscopic capillaries embedded within the tissues. Here, the blood performs its primary function of internal respiration, delivering oxygen and nutrients to the cells while simultaneously collecting cellular waste products and carbon dioxide.
After the exchange, the deoxygenated blood moves from the capillaries into small venules, which merge to form larger veins. These major veins ultimately converge into the two largest veins in the body, the superior and inferior vena cavae, which empty the deoxygenated blood back into the right atrium of the heart.
Key Distinctions in Function and Pressure
The primary difference between the two circuits lies in their scale and the resulting pressure required to operate them. The pulmonary circuit is a short loop confined to the heart and lungs, designed as a low-resistance pathway for gas exchange. Because of this low resistance, the right ventricle only needs to generate a mean arterial pressure of approximately 5 to 15 mmHg to push blood through the lungs effectively.
The systemic circuit, in contrast, is a high-resistance network that supplies blood to the entire body. To overcome this distance and resistance, the left ventricle must pump with considerably more force. The systemic circulation operates under a much higher mean arterial pressure, typically around 93 mmHg, which is about six to seven times greater than the pressure in the pulmonary circuit.
The difference in purpose also dictates the vessel structure in each circuit. Pulmonary vessels have thinner walls and less smooth muscle, which contributes to their high compliance and low resistance. Systemic arterioles, however, contain significant amounts of smooth muscle, allowing the body to precisely regulate resistance and redirect blood flow to organs based on immediate needs.