It is formed by the left and right brachiocephalic veins, (also referred to as the innominate veins) which receive blood from the upper limbs and the head and neck, behind the lower border of the first right costal cartilage. The azygous vein (which receives blood from the rib cage) joins it just before it enters the right atrium, at the upper right front portion of the heart. It is also known as the cranial vena cava in animals.
No valve separates the superior vena cava from the right atrium. As a result, the (right) atrial and (right) ventricular contractions are conducted up into the internal jugular vein and, through the sternocleidomastoid muscle, can be seen as the jugular venous pressure. In tricuspid valve regurgitation, these pulsations are very strong.