Given a scientific passage and an answer, generate a question for the given answer.

[Q]: Passage: The combined gas law relates pressure, volume, and temperature of a gas. Answer: combines gas law
[A]: Which law relates the pressure, volume, and temperature of a gas?

[Q]: Passage: Pulmonary circulation is the part of the circulatory system that carries blood between the heart and lungs (the term pulmonary means "of the lungs"). It is illustrated in Figure below . Deoxygenated blood leaves the right ventricle through pulmonary arteries, which transport it to the lungs. In the lungs, the blood gives up carbon dioxide and picks up oxygen. The oxygenated blood then returns to the left atrium of the heart through pulmonary veins. Answer: oxygen
[A]: While in the lungs, blood gives up carbon dioxide and picks up what element before returning to the heart?

[Q]: Passage: The main organs of the cardiovascular system are the heart and blood vessels. Both organs contain valves. Valves also are found in plumbing systems. They can be turned on or off to control the flow of water. Answer: heart
[A]:
The main organs of the cardiovascular system are the blood vessels and what else?