Systemic veins drain deoxygenated blood from the tissues and return it to the right atrium of the heart. The coronary sinus, located in the posterior coronary sulcus, is a large systemic vein. In addition to smaller tributaries, the coronary sinus receives blood from three major channels: the posteriorly located middle and small cardiac veins and the anteriorly located great cardiac vein. The coronary sinus receives almost all the venous blood from the myocardium and empties into the right atrium. The vena cavae are the two large systemic veins in the human body. The superior vena cava is named so, as it collects blood from organs above the diaphragm, except the air sacs of the lungs, and transports it to the superior part of the right atrium. The inferior vena cava, the largest systemic vein in the body, drains organs below the diaphragm and empties into the inferior part of the right atrium. During pregnancy, this vein can get compressed by the enlarging uterus, causing edema of the ankles and feet.