The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, that consists of the heart and blood vessels (from Greek kardia meaning heart, and from Latin vascula meaning vessels). The circulatory system has two divisions, a systemic circulation or circuit, and a pulmonary circulation or circuit. Some sources use the terms cardiovascular system and vascular system interchangeably with the circulatory system.

The network of blood vessels are the great vessels of the heart including large elastic arteries, and large veins; other arteries, smaller arterioles, capillaries that join with venules (small veins), and other veins. The circulatory system is closed in vertebrates, which means that the blood never leaves the network of blood vessels. Some invertebrates such as arthropods have an open circulatory system. Diploblasts such as sponges, and comb jellies lack a circulatory system.

Blood is a fluid consisting of plasma, red blood cells, white blood cells, and platelets that is circulated around the body carrying oxygen and nutrients to the tissues, and waste materials away. Circulated nutrients include proteins and minerals, other components transported are gases such as oxygen, and carbon dioxide, hormones, and hemoglobin; providing nourishment, help in the immune system to fight diseases, and in maintaining homeostasis by stabilizing temperature and natural pH.

In vertebrates, complementary to the circulatory system is the lymphatic system. This system carries excess plasma filtered from the capillaries as interstitial fluid between cells, away from the body tissues in an accessory route to return the excess fluid back to the blood circulation as lymph. The passage of lymph takes much longer than that of blood. The lymphatic system is a subsystem that is essential for the functioning of the blood circulatory system; without it the blood would become depleted of fluid. The lymphatic system works together with the immune system. Unlike the closed circulatory system, the lymphatic system is an open system. Some sources describe it as a secondary circulatory system.

The circulatory system can be affected by many cardiovascular diseases. Cardiologists are medical professionals which specialise in the heart, and cardiothoracic surgeons specialise in operating on the heart and its surrounding areas. Vascular surgeons focus on disorders of the blood vessels, and lymphatic vessels.