The blood that the vena cava vein takes to the heart is full of carbon dioxide. It has much less oxygen than (systemic) arterial blood. The right side of the heart pushes the venous blood into the pulmonary artery. The pulmonary artery takes blood to the lungs. In the lungs, the blood goes through the "pulmonary capillary bed". (The capillaries that are in the lungs). Here it gets more oxygen. It also drops off carbon dioxide. (This is the opposite of what happens in capillary beds in the rest of the body. In the systemic circulation blood drops off oxygen and picks up carbon dioxide).