The respiratory system starts at the nose and mouth and continues through the airways and the lungs. Air enters the respiratory system through the nose and mouth and passes down the throat (pharynx) and through the voice box, or larynx. The entrance to the larynx is covered by a small flap of tissue (epiglottis) that automatically closes during swallowing, thus preventing food or drink from entering the airways.
The largest airway is the windpipe (trachea), which branches into two smaller airways: the left and right bronchi, which lead to the two lungs. Each lung is divided into sections (lobes): three in the right lung and two in the left lung. The left lung is a little smaller than the right lung because it shares space in the left side of the chest with the heart.
The bronchi themselves branch many times into smaller airways, ending in the narrowest airways (bronchioles), which are as small as one half of a millimeter across. The airways resemble an upside-down tree, which is why this part of the respiratory system is often called the bronchial tree. Large airways are held open by semiflexible, fibrous connective tissue called cartilage. Smaller airways are supported by the lung tissue that surrounds and is attached to them. The walls of the smaller airways have a thin, circular layer of smooth muscle. The airway muscle can dilate or constrict, thus changing airway size.
At the end of each bronchiole are thousands of small air sacs (alveoli). Together, the millions of alveoli of the lungs form a surface of more than 100 square meters. Within the alveolar walls is a dense network of tiny blood vessels called capillaries. The extremely thin barrier between air and capillaries allows oxygen to move from the alveoli into the blood and allows carbon dioxide to move from the blood in the capillaries into the air in the alveoli.
The pleura is a slippery membrane that covers the lungs as well as the inside of the chest wall. It allows the lungs to move smoothly during breathing and as the person moves. Normally, the two layers of the pleura have only a small amount of lubricating fluid between them. The two layers glide smoothly over each other as the lungs change size and shape.