The right atrium is a small upper cavity of the |heart| that holds about three-and-a-half tablespoons of ~blood~. It serves as the receiving chamber for all the venous ~blood~ (short of oxygen and laden with carbon dioxide) returning through the superior and |inferior vena cava|, and from many minute ~blood~ vessels that drain ~blood~ from the walls of the chamber itself. The right atrium is slightly larger than the left atrium, which is slightly more powerful. The walls of the right atrium are less than an eighth of an inch thick. Two layers of muscle form the wall. The superficial layer spans both atria, and the inner layer, composed of many small bundles, arches over the atrial cavity at right angles to the superficial layer. As the |heart| contracts (ventricular systole), the ~blood~ is pushed through the pulmonary valve into pulmonary circulation. As the |heart| relaxes (ventricular diastole), the ~blood~ exits the right atrium through the |tricuspid valve| to the right ~ventricle~. In the upper part of the right atrium there is a small patch of special |heart| tissue called the ~sinus~ ~node~ or the sinuatrial ~node~. It is the |hearts| pacemaker. It triggers the heartbeat and establishes its pace.
