To sustain life, the body must produce sufficient energy. Energy is produced by burning molecules in food, which is done by the process of oxidation (whereby food molecules are combined with oxygen). Oxidation involves carbon and hydrogen being combined with oxygen to form carbon dioxide and water. The consumption of oxygen and the production of carbon dioxide are thus indispensable to life. It follows that the human body must have an organ system designed to eliminate carbon dioxide from the circulating blood and absorb oxygen from the atmosphere at a rate rapid enough for the body’s needs, even during peak exercise. The respiratory system enables oxygen to enter the body and carbon dioxide to leave the body.
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 windpipe (trachea) is the largest airway. The trachea 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 (or 2/100 of an inch) 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 relax or contract, thus changing airway size.
Thousands of small air sacs (alveoli) are at the end of each bronchiole. Together, the millions of alveoli of the lungs form a surface of more than 100 square meters (1111 square feet). 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.