Systemic veins are crucial blood vessels that return deoxygenated blood from various body tissues back to the heart. There are three systemic veins that return deoxygenated blood to the heart, they are as follows.

The coronary sinus, the heart's principal vein, resides in the coronary sulcus on the heart's posterior aspect. This broad venous channel receives nearly all venous blood from the myocardium, the heart muscle. It is fed by three primary veins: the great cardiac vein, the middle cardiac vein, and the small cardiac vein. Some anterior cardiac veins drain directly into the right atrium, further aiding in the circulatory process.

Next, the superior vena cava (SVC), which is roughly 7.5 cm long and 2 cm in diameter, begins posterior to the right first costal cartilage, formed by the union of the right and left brachiocephalic veins. It ends at the level of the right third costal cartilage, where it empties its blood into the superior part of the right atrium. The SVC is responsible for draining blood from the head, neck, upper limbs, and thorax.

Lastly, the inferior vena cava (IVC) is the largest vein in the human body. Originating anterior to the fifth lumbar vertebra by the union of the common iliac veins, it ascends behind the peritoneum, piercing the caval opening of the diaphragm at the level of the eighth thoracic vertebra before entering the inferior part of the right atrium. The IVC drains the abdomen, pelvis, and lower limbs. Notably, during the later stages of pregnancy, the enlarging uterus often compresses the IVC, leading to temporary varicose veins and edema of the ankles and feet.