Circulatory system

Lore & Background

The circulatory system consists of the heart, blood vessels, and blood. In vertebrates, it is a closed system, meaning blood never leaves the network of vessels. It has two major divisions: the pulmonary circulation, which carries deoxygenated blood to the lungs and returns oxygenated blood to the heart, and the systemic circulation, which delivers oxygenated blood to the body and returns deoxygenated blood to the heart. The systemic circulation includes macrocirculation and microcirculation. The heart has four chambers: left atrium, left ventricle, right atrium, and right ventricle. Blood vessels include arteries, veins, and capillaries. The great vessels are the large arteries and veins connected to the heart. The lymphatic system is a complementary, open subsystem that returns excess interstitial fluid as lymph to the blood circulation and works with the immune system. Blood is a fluid containing plasma, red blood cells, white blood cells, and platelets. It carries oxygen, nutrients, hormones, and gases, and helps maintain homeostasis. The circulatory system can be affected by cardiovascular diseases, treated by cardiologists, cardiothoracic surgeons, and vascular surgeons.

Reader's Guide

The circulatory system is fundamental to vertebrate physiology, enabling the transport of oxygen, nutrients, and waste throughout the body. Its closed nature in vertebrates contrasts with the open systems of many invertebrates. The division into pulmonary and systemic circuits allows efficient oxygenation and distribution. The complementary lymphatic system prevents fluid depletion and supports immunity. Understanding this system is critical for diagnosing and treating cardiovascular diseases, which are a major focus of medical specialties such as cardiology and vascular surgery. The system's structure—from the heart's four chambers to the extensive capillary network—illustrates the complexity required to sustain life. Its study remains central to biology and medicine.

Did You Know?

• An average adult contains five to six quarts (roughly 4.7 to 5.7 liters) of blood, accounting for approximately 7% of total body weight.
• The lymphatic system is an open system, while the blood circulatory system is closed in vertebrates.
• The hepatic portal vein is a portal vein that branches into a second capillary system in the liver.