Atrium (heart) 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 that term is still used to describe this chamber in, for example, the "Mollusca", but in humans that name is
Heart failure with preserved ejection fraction atrio-ventricular valves open (mitral valve at left side and tricuspid valve at right side) and the blood passes from the atria into the ventricles. First, ventricles are filled by a pressure gradient but near the end, atria contract (atrial kick) and force more blood to pass into ventricles. Atrial contraction is responsible for around 20% of the total filling blood volume. (In atrial fibrillation, this additional 20% filling volume is lost and the patient may experience systolic heart failure symptoms). Complete left ventricular filling is essential to maintain maximum cardiac output. Left ventricular filling is dependent upon ventricular relaxation and