循環器系の解剖学

The circulatory system has three divisions: the coronary circuit, which supplies blood to the heart muscle itself; the pulmonary circuit, which carries blood between the heart and lungs for gas exchange; and the systemic circuit, which delivers oxygenated blood to the entire body. Each division plays a critical role in maintaining circulation and oxygen delivery.

Deoxygenated blood from the body enters the right atrium, then flows into the right ventricle through the tricuspid valve. The right ventricle pumps this blood through the pulmonary valve into the pulmonary artery, which carries it to the lungs. This pathway ensures deoxygenated blood reaches the lungs for gas exchange and transport of oxygen.

The left ventricle has a much thicker wall because it must pump oxygenated blood throughout the entire body via the aorta, requiring significantly more force than the right ventricle. The right ventricle only needs to pump blood the short distance to the lungs. This structural difference reflects the greater workload and pressure demands of systemic circulation.

The coronary circuit is a network of arteries and veins surrounding the heart tissue in a crown-like pattern that supplies oxygenated blood directly to the heart muscle. Although the heart pumps blood throughout the body, it cannot extract oxygen from the blood passing through its chambers, so it requires its own dedicated blood supply to function properly.

Arteries are thick-walled vessels that withstand high pressure from the heart's pumping action. They branch into smaller arterioles, which further divide into capillaries with extremely thin walls designed for gas exchange. This progressive reduction in vessel size and wall thickness allows blood to slow down and oxygen and nutrients to diffuse into surrounding tissues.

Veins collect deoxygenated blood from capillaries and merge into progressively larger vessels, including the jugular vein from the head, superior vena cava from the upper body, and inferior vena cava from the lower body. All converge back to the right atrium. Veins have thin, flexible walls because blood returns at much lower pressure than in arteries.

In double circulation, blood passes through the heart twice: first through the right side to the lungs for oxygenation, then through the left side to the body. This two-pass system allows complete gas exchange in the lungs before oxygen-rich blood is pumped to tissues, ensuring efficient oxygen delivery and carbon dioxide removal throughout the body.