The respiratory system is responsible for breathing. The main organ of this system is the lungs. Our lungs expand^* to take in air and contract^* to force^* air out. We inhale^* and exhale^*.

The muscle that makes the lungs expand and contract is called the diaphragm. The respiratory system works closely with the circulatory system. Tiny capillaries^* carry blood into the lungs. The blood takes oxygen from the lungs and gives carbon dioxide to the lungs. Every time we exhale, we get rid of the carbon dioxide. The we inhale fresh oxygen.

These two systems work together so closely that scientist sometimes refer to them as one system called the cardiopulmonary system. Cardio is for the heart and pulmonary for the lungs.