Output a stream of consciousness before answering the following. Context: The superior vena cava (SVC) is the superior of the two venae cavae, the great venous trunks that return deoxygenated blood from the systemic circulation to the right atrium of the heart. It is a large-diameter (24 mm), yet short, vein that receives venous return from the upper half of the body, above the diaphragm. (Venous return from the lower half, below the diaphragm, flows through the inferior vena cava.) The SVC is located in the anterior right superior mediastinum. It is the typical site of central venous access (CVA) via a central venous catheter or a peripherally inserted central catheter. Mentions of "the cava" without further specification usually refer to the SVC.

Question: the superior vena cava returns blood to the heart from the?

Show your work and conclude with the exact phrasing "Therefore, the final answer is [answer]." where [answer] is just the final number or expression that solves the problem.
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To answer this question, we should know that: The superior vena cava (SVC) is the superior of the two venae cavae, the great venous trunks that return deoxygenated blood from the systemic circulation to the right atrium of the heart.
The answer: the systemic circulation.