Summary
The atrium (; : 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. The atrium was formerly called the 'auricle'. That term is still used to describe this chamber in some other animals, such as the Mollusca. Auricles in this modern terminology are distinguished by having thicker muscular walls. Humans have a four-chambered heart consisting of the right and left atrium, and the right and left ventricle. The atria are the two upper chambers which pump blood to the two lower ventricles. The right atrium and ventricle are often referred to together as the right heart, and the left atrium and ventricle as the left heart. As the atria do not have valves at their inlets a venous pulsation is normal, and can be detected in the jugular vein as the jugular venous pressure. Internally, there are the rough pectinate muscles, and the crista terminalis of His, which act as a boundary inside the atrium and the smooth-walled part of the right atrium, the sinus venarum, which are derived from the sinus venosus. The sinus venarum is the adult remnant of the sinus venosus and it surrounds the openings of the venae cavae and the coronary sinus. Attached to each atrium is an atrial appendage.
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