Veins take blood back to the heart. which gas would be most abundant in a vein that takes blood from muscle cells in the arm back to the heart?
carbon dioxide
oxygen
nitrogen
hydrogen

The right answer is carbon dioxide.

In anatomy, a vein is a blood vessel that allows the transport of blood from the periphery (organs or tissues) to the heart (venous return):

* from the lungs to the heart: the pulmonary veins carry blood rich in oxygen so that it is redistributed in the body in the systemic circulation.

* from the other organs (like the muscle in the arm) to the heart: the other veins carry oxygen-depleted and carbon dioxide-rich blood so that it is reoxygenated by the lungs into the pulmonary circulation.

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