Respiratory system: structure

You breathe about 12 to 20 times a minute when you are resting, without thinking about it. Each breath follows a path of connected passages, and each part of the path has a job.

Air enters through the nose or mouth. The nose is the better route: hairs and sticky mucus trap dust, and the air is warmed and moistened. The air then passes through the pharynx (throat) and the larynx (voice box), which holds the vocal cords. A small flap called the epiglottis folds over the entrance to the airway when you swallow, so food goes into the esophagus and not into your lungs. Air continues into the trachea (windpipe), a tube held open by rings of cartilage. Its lining has tiny hairlike cilia that sweep mucus and trapped dust upward and out of the lungs.

The trachea splits into two bronchi, one for each lung. Inside the lungs the bronchi divide again and again into narrower bronchioles, like the branches of an upside-down tree. At the ends of the smallest branches are clusters of tiny air sacs called alveoli, where gases enter and leave the blood. The right lung has three lobes, while the left lung has two, leaving room for the heart.

The lungs cannot move by themselves because they contain no muscle. Instead, breathing depends on the diaphragm, a large, dome-shaped muscle under the lungs, and on the muscles between the ribs. When you inhale, the diaphragm contracts and flattens, and the rib muscles lift the ribs. The chest cavity gets bigger, which lowers the air pressure inside the lungs. Higher-pressure outside air then flows in. When you exhale, the muscles relax, the chest cavity gets smaller, the pressure in the lungs rises, and air flows out. A thin, slippery membrane called the pleura surrounds each lung and lets it slide smoothly as the chest moves.

Your brainstem controls the rhythm of breathing automatically. It responds to the amount of carbon dioxide in your blood: more carbon dioxide makes you breathe faster. You can also take control for a while, as when you hold your breath, but the automatic system soon takes over. Smoking damages the cilia and the airways, which is why smokers cough and get more lung infections.

Common mistakes: thinking the lungs are muscles that squeeze air, and thinking the air we breathe goes into the stomach. Next lesson: what happens at the alveoli.