Superior vena cava

The superior vena cava is a large diameter, short vein that carries deoxygenated blood from the upper half of the body to the heart's right atrium. It is located in the anterior right superior mediastinum.

It is formed by the left and right brachiocephalic veins, which receive blood from the upper limbs, head and neck. The azygos vein joins the superior vena cava just before it enters the right atrium at the upper right anterior portion of the heart. 

No valve divides the superior vena cava from the right atrium. As a result, pressure from contractions of the right side of the heart is conducted up into the internal jugular vein and can be measured as the jugular venous pressure through the sternocleidomastoid muscle.