The rejection substance of the cell, that is the carbon dioxide arrives by the blood until the alveolus, and makes the contrary process of oxygen, that is, passes since the blood to the alveolar wall and finally is returned to the air that we breath.  

Our body get air (carbon dioxide and oxygen) by nose, this air travels through the nostrils, the pharynx, the larynx, the trachea, the bronchi, the bronchiole to the alveolus, where gas is exchanged. In the alveolus, the breathing air goes to the blood. Next, the exchange of gases is produced between the alveolar air and the blood.

When the blood arrives to the lungs have a lot of carbon dioxide and a few oxygen. The oxygen of the breathing air goes to the alveolar walls, capillaries and finally to the blood. There, the haemoglobin (is founded in the reds globules) transports the oxygen to all the body cells where is going to pass inside of this.