Atrium (heart) of blood occurs. In birds, mammals, and some other reptiles (alligators in particular) the partitioning of both chambers is complete. Atrium (heart) The atrium is the upper chamber in which blood enters the heart. There are two atria in the human heart – the left atrium connected to the lungs, and the right atrium connected to the venous circulation. The atria receive blood, and when the heart muscle contracts they pump blood to the ventricles. All animals with a closed circulatory system have at least one atrium/auricle (humans have two atria). The atrium used to be called the "auricle", and
Crisscross heart and right ventricle are working together to supply blood to the pulmonary artery, similarly to how the left atrium and the left ventricle work simultaneously to supply blood to the aorta. During the process of the heart contracting and releasing the right atrium and left atrium contract at the same time, while the left ventricle and right ventricle relax. In opposition, when the left atrium and right atrium are relaxed the left ventricle and right ventricle contract pushing blood to either the aorta or pulmonary artery. In an anatomically correct heart the atria are smaller than the ventricles. The ventricles