Closed circulatory system in the context of "Cardiovascular system"

In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of the heart and blood vessels (from Greek kardia meaning heart, and 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 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. Many invertebrates such as arthropods have an open circulatory system with a heart that pumps a hemolymph which returns via the body cavity rather than via blood vessels. Diploblasts such as sponges and comb jellies lack a circulatory system.

The atrium (Latin: ātrium, lit. 'entry hall'; pl.: atria) is one of the two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular mitral and tricuspid heart valves.

There are two atria in the human heart – the left atrium receives blood from the pulmonary circulation, and the right atrium receives blood from the venae cavae of the systemic circulation. During the cardiac cycle, the atria receive blood while relaxed in diastole, then contract in systole to move blood to the ventricles. Each atrium is roughly cube-shaped except for an ear-shaped projection called an atrial appendage, previously known as an auricle. All animals with a closed circulatory system have at least one atrium.