For P, blood flows from body to the heart to the gills. This means that oxygen has reached the body first for respiration to occur, which leads to carbon dioxide formation in the blood. Hence, blood flows to the gills so that the carbon dioxide can be removed from the body to take in oxygen, via breathing.
For Q, blood flows from heart to the body, which is from the lungs. This means the blood already has more oxygen so that it can flow to the body for respiration in the various body parts to occur.