The human circulatory system is organized into distinct, yet interconnected, pathways that manage blood flow efficiently.
Pulmonary Circulation
Pulmonary circulation directs blood between the heart and the lungs. Deoxygenated blood enters the right atrium, then moves into the right ventricle. From there, it is pumped into the pulmonary artery, which branches to carry it to both lungs.
Within the lungs, pulmonary arteries divide into capillaries surrounding tiny air sacs called alveoli. Here, gas exchange occurs: carbon dioxide is released from the blood, and oxygen diffuses into it. Once oxygenated, blood collects into venules and then into larger pulmonary veins, which transport it back to the left atrium, completing the pulmonary circuit.
Systemic Circulation
Systemic circulation distributes oxygenated blood from the heart to the body’s tissues and organs, excluding the lungs. Oxygenated blood from the lungs enters the left atrium, then flows into the left ventricle. This chamber pumps the blood into the aorta, the body’s largest artery.
The aorta branches into smaller arteries that extend throughout the body, delivering blood to various organs, muscles, and tissues. These arteries further divide into arterioles and then into capillaries, where oxygen and nutrients are delivered to cells. Concurrently, capillaries collect carbon dioxide and other waste products. The deoxygenated blood then flows into venules, which merge to form larger veins, ultimately returning to the right atrium via the superior and inferior venae cavae.
Comparing the Two Circulations
The pulmonary and systemic circulations serve distinct purposes. Pulmonary circulation focuses on gas exchange, transporting deoxygenated blood to the lungs to receive oxygen and release carbon dioxide. Systemic circulation’s primary function is to deliver oxygen and nutrients to all body tissues while collecting waste products.
In the pulmonary circuit, arteries carry deoxygenated blood, and veins carry oxygenated blood. The pulmonary artery carries blood from the heart to the lungs, while pulmonary veins return oxygenated blood to the heart. Conversely, in systemic circulation, arteries carry oxygenated blood away from the heart, and veins return deoxygenated blood to the heart.
The organs involved in each circuit also differ. Pulmonary circulation primarily involves the heart and lungs, facilitating blood flow exclusively between these two. Systemic circulation encompasses the heart and all other body tissues and organs, distributing blood throughout the entire body.
Differences in blood pressure are notable. Systemic circulation operates under higher pressure, around 120/80 mmHg in the arteries, to ensure blood reaches all distant parts of the body. Pulmonary circulation is a lower-pressure system, with arterial pressures ranging from 11-20 mmHg at rest. This lower pressure is due to the shorter distance blood travels and the high compliance of pulmonary vessels.
How They Work Together
Pulmonary and systemic circulations are integral parts of a single, continuous circulatory system. Blood flows sequentially from one circuit to the other, forming a complete loop.
After blood completes its journey through the systemic circuit and returns to the right side of the heart, it enters the pulmonary circuit for reoxygenation. Once oxygenated in the lungs, blood re-enters the left side of the heart, ready for the systemic circuit. This continuous flow ensures oxygen and nutrients are supplied to all body cells, and waste products are removed. The effective functioning of one circuit relies on the other, creating an interdependent system that supports overall bodily functions.