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
Heart within the ventricles falls below the pressure within the aorta and pulmonary arteries, the aortic and pulmonary valves close. The ventricles start to relax, the mitral and tricuspid valves open, and the cycle begins again. Cardiac output (CO) is a measurement of the amount of blood pumped by each ventricle (stroke volume) in one minute. This is calculated by multiplying the stroke volume (SV) by the beats per minute of the heart rate (HR). So that: CO = SV x HR. The cardiac output is normalized to body size through body surface area and is called the cardiac index. The