The respiratory system consists of the lungs, a pair of elastic organs housed in the chest cavity, and the air passages leading to them. The air inhaled into the lungs provides oxygen to cells throughout the body. Air forced out of the lungs removes carbon dioxide from the body.
Air enters the respiratory system through the nose or mouth. It then travels through the larynx (voice box) and into the trachea (windpipe). At about the middle of the chest, the trachea divides into two tubes, the right and left bronchi. The right bronchus carries air to the three lobes of the right lung. The left bronchus supplies air to the two lobes of the left lung.
Inside the lungs the bronchi divide into smaller branches called bronchioles, which eventually empty into thousands of minute sacs called alveoli. The alveoli are surrounded by thin- walled capillaries. The air in the alveoli passes through to the blood cells within the capillaries. At the same time, carbon dioxide from these blood cells passes into the alveoli. From the alveoli, air containing carbon dioxide travels out of the lungs and is exhaled through the nose or mouth.