Respiratory System, in anatomy and physiology, organs that deliver oxygen to the circulatory system for transport to all body cells. Oxygen is essential for cells, which use this vital substance to liberate the energy needed for cellular activities. In addition to supplying oxygen, the respiratory system aids in removing of carbon dioxide, preventing the lethal buildup of this waste product in body tissues. Day-in and day-out, without the prompt of conscious thought, the respiratory system carries out its life-sustaining activities. If the respiratory system’s tasks are interrupted for more than a few minutes, serious, irreversible damage to tissues occurs, followed by the failure of all body systems, and ultimately, death.
While the intake of oxygen and removal of carbon dioxide are the primary functions of the respiratory system, it plays other important roles in the body. The respiratory system helps regulate the balance of acid and base in tissues, a process crucial for the normal functioning of cells. It protects the body against disease-causing organisms and toxic substances inhaled with air. The respiratory system also houses the cells that detect smell, and assists in the production of sounds for speech.
The organs of the respiratory system extend from the nose to the lungs and are divided into the upper and lower respiratory tracts. The upper respiratory tract consists of the nose and the pharynx, or throat. The lower respiratory tract includes the larynx, or voice box; the trachea, or windpipe, which splits into two main branches called bronchi; tiny branches of the bronchi called bronchioles; and the lungs, a pair of saclike, spongy organs. The nose, pharynx, larynx, trachea, bronchi, and bronchioles conduct air to and from the lungs. The lungs interact with the circulatory system to deliver oxygen and remove carbon dioxide.
The flow of air from outside of the body to the lungs begins with the nose, which is divided into the left and right nasal passages. The nasal passages are lined with a membrane composed primarily of one layer of flat, closely packed cells called epithelial cells. Each epithelial cell is densely fringed with thousands of microscopic cilia, fingerlike extensions of the cells. Interspersed among the epithelial cells are goblet cells, specialized cells that produce mucus, a sticky, thick, moist fluid that coats the epithelial cells and the cilia. Numerous tiny blood vessels called capillaries lie just under the mucous membrane, near the surface of the nasal passages. While transporting air to the pharynx, the nasal passages play two critical roles: they filter the air to remove potentially disease-causing particles; and they moisten and warm the air to protect the structures in the respiratory system.
Air leaves the nasal passages and flows to the pharynx, a short, funnel-shaped tube about 13 cm (5 in) long that transports air to the larynx. Like the nasal passages, the pharynx is lined with a protective mucous membrane and ciliated cells that remove impurities from the air. In addition to serving as an air passage, the pharynx houses the tonsils, lymphatic tissues that contain white blood cells. The white blood cells attack any disease-causing organisms that escape the hairs, cilia, and mucus of the nasal passages and pharynx. The tonsils are strategically located to prevent these organisms from moving further into the body. One tonsil, called the adenoids, is found high in the rear wall of the pharynx. A pair of tonsils, the palatine tonsils, is located at the back of the pharynx on either side of the tongue. Another pair, the lingual tonsils, is found deep in the pharynx at the base of the tongue. In their battles with disease-causing organisms, the tonsils sometimes become swollen with infection. When the adenoids are swollen, they block the flow of air from the nasal passages to the pharynx, and a person must breathe through the mouth.
Air moves from the pharynx to the larynx, a structure about 5 cm (2 in) long located approximately in the middle of the neck. Several layers of cartilage, a tough and flexible tissue, comprise most of the larynx. A protrusion in the cartilage called the Adam’s apple sometimes enlarges in males during puberty, creating a prominent bulge visible on the neck.
While the primary role of the larynx is to transport air to the trachea, it also serves other functions. It plays a primary role in producing sound; it prevents food and fluid from entering the air passage to cause choking; and its mucous membranes and cilia-bearing cells help filter air. The cilia in the larynx waft airborne particles up toward the pharynx to be swallowed.Food and fluids from the pharynx usually are prevented from entering the larynx by the epiglottis, a thin, leaflike tissue. The “stem” of the leaf attaches to the front and top of the larynx. When a person is breathing, the epiglottis is held in a vertical position, like an open trap door. When a person swallows, however, a reflex causes the larynx and the epiglottis to move toward each other, forming a protective seal, and food and fluids are routed to the esophagus. If a person is eating or drinking too rapidly, or laughs while swallowing, the swallowing reflex may not work, and food or fluid can enter the larynx. Food, fluid, or other substances in the larynx initiate a cough reflex as the body attempts to clear the larynx of the obstruction. If the cough reflex does not work, a person can choke, a life-threatening situation. The Heimlich maneuver is a technique used to clear a blocked larynx (see First Aid). A surgical procedure called a tracheotomy is used to bypass the larynx and get air to the trachea in extreme cases of choking.