Here's a problem to solve: Which best describes how the human circulatory and respiratory systems interact?

Among the 4 following options, which is the correct answer?

- A: The heart removes waste built up in the lungs.
 
- B: Oxygen enters the bloodstream directly from the nose.
 
- C: Oxygen is transferred to the bloodstream as blood passes by the lungs.
 
- D: The heart pumps air into the blood through the lungs.
 