The circulation and respiratory system are closely linked together. It started in the respiratory system forms your noise and mouth. With you inhale, you inhale mainly oxygen and to trachea down the trachea. The trachea goes to the bronchus that goes into the lungs, the air you breathe isn't clean so there are small hairs called cilia that traps them and push them up the trachea. Inside the lung are the alveoli, which is the main area where oxygen and carbon dioxide is exchanged. The oxygen goes into one tube inside the alveoli, which leads to the red blood cells, while carbon dioxide leaves the blood cells down another tube and out of the mouth when you exhale. A major muscle in the respiratory system is the diaphragm. When you inhale the diaphragm flattens so your lungs will be able to expand.