31. The circulatory and respiratory systems work together. The blood that returns to the heart through the veins is depleted of oxygen. First it is pumped to the lungs.
32. Small sacs with thin walls allow gases to be transferred from the blood
33. Oxygen is taken in from the air as carbon dioxide is released
34. Fresh (oxygenated) blood returns to the heart to be pumped back out to the body through the arteries.