The right atrium is the upper cavity of the heart which lies on its right side. It is separated from the left atrium by the interatrial septum. Like the other three chambers, it is lined with endocardium, which is a thin membrane composed of endothelial tissue. It is irregularly cuboidal in shape, with the apex forming an anteriorly directed ear-shaped portion, which is called the auricle of the right atrium, which is shaped like a flattened cone. The apex of this cone is directed to the left, towards the pulmonary trunk.
The right atrium is communicated with the right ventricle through the tricuspid valve, which consists of three muscular flaps that open and close to regulate the flow of deoxygenated blood from the right atrium into the right ventricle. The superior and inferior venae cavae, the coronary sinus, and small veins of the heart proper enter the right atrium. The coronary sinus collects the blood from most of the veins of the heart, emptying it into the right atrium.
The superior vena cava, which drains deoxygenated blood from the head and upper limbs, enters the right atrium at the junction of superior and anterior walls by means of the opening of the superior vena cava (ostium vena cava superioris). The inferior vena cava, which drains blood from the lower limbs and abdomen, enters this cavity at the junction of the superior and posterior walls through the opening for the inferior vena cava (ostium vena cava inferioris).
The posterior wall of the right atrium contains the sinoatrial node, which is a collection of specialized cardiac fiber, acting as a pacemaker. It generates electro-chemical impulses for the autonomic contraction of the heart muscle. Meanwhile, the left wall of the right atrium (interatrial septum) holds the atrioventricular node, which relays electro-chemical impulses from the sinoatrial node to the ventricles.
Below, the human heart, with its different parts, showing its chambers, which include the right atrium
