Anatomy of the Respiratory System
The lungs are the body's major organs of breathing (respiration). The lungs are located on each side of the chest within the rib cage. They are separated by the heart and other contents of the mediastinum—the tissues and organs of the middle chest (e.g., large blood vessels, windpipe). The lungs are shaped like an upside-down butterfly. The top of each lung (called the apex) extends into the lowest part of the neck, just above the level of the first rib. The bottom, or base, of each lung extends down to the diaphragm, which is the major breathing-associated muscle that separates the chest from the abdominal cavity.
Each lung is divided into upper and lower lobes. The right lung is larger and heavier than the left lung, which is somewhat smaller in size because of the position of the heart. The upper lobe of the right lung contains another triangular subdivision known as the middle lobe. At birth, the lungs are pinkish-white in color; however, with age, the lungs darken to gray or mottled black because of deposits of carbon and other particles that are inhaled over time.
The root connects the lungs to the heart and the trachea (windpipe). Each root is made up of a main stem bronchus (large air passage connecting the windpipe to the right or left lungs), pulmonary artery (major artery that delivers oxygen-poor blood back to the right or left lungs), pulmonary vein (major vein receiving oxygen-rich blood from the lobes of the right or left lungs), the bronchial arteries and veins, as well as nerves and lymphatic vessels.
A clear, thin, shiny covering (known as the serous coat, or pleura) covers the lungs. The inner, visceral layer of the pleura is attached to the lungs and the outer, parietal layer is attached to the chest wall. Both layers are held in place by a film of pleural fluid in a manner similar to two microscope slides that are wet and stuck together. Beneath the pleura is a layer of elastic fibers that span the lung surface and extend down into its subdivisions.
The trachea splits into right and left main stem bronchi. The main stem bronchi are the major air passages from the trachea to the lungs and are similar to the trachea in tissue composition. The main stem bronchi enter each lung and progressively branch off into paired subdivisions throughout the entire organ (called the tracheobronchial tree).
The tracheobronchial tree serves to conduct, humidify, and heat air that is breathed in, or inspired. At its endpoints, the tracheobronchial tree connects with the blood vessels. The lining of the tracheobronchial tree is composed of columnar epithelium (column-shaped surface cells) and glands that produce mucus and serous (clear plasma) fluid. The cilia (hair-like projections on columnar epithelium) move in a constant, beating motion to cleanse the airways of foreign bodies and infectious organisms. In normal lungs, the cilia are covered by a watery "mucous blanket"—a gel-like liquid that is moved by the cilia and aids the lungs' self-cleaning.
Lungs that have been damaged by smoking or other toxic exposures often have defective or missing cilia and show other abnormalities in the tissue lining. Coughing triggers a high-speed flow of air, which mobilizes the mucous blanket. The sputum produced by such mobilization contains mucus, nasal secretions, and saliva.
The essential tissue of the lung—lung parenchyma—is made up of clusters of spongy air sacs called lobules. There are about 130,000 primary lobules in each lung. Each lobule is approximately 3.5 millimeters in diameter and contains about 2,200 alveoli (air sacs and ducts). Tracheobronchial branches that are larger than 1 millimeter in diameter and have connective tissue coverings are called segmental bronchi.
The smallest subdivisions, which are less than 1 millimeter in diameter and do not have connective tissue coverings, are called bronchioles. The final branches of the bronchioles are called terminal bronchioles. The bronchioles end in irregular, swollen projections known as alveolar ducts (terminal branches composed of special gas-exchanging tissue) and alveolar sacs (blind passage of an alveolar duct). The fluid that lines the alveolar regions contains a detergent-like substance known as surfactant, which reduces surface tension within the alveoli and keeps them from collapsing during breathing.
Oxygen-poor blood is brought back to the lungs by means of the pulmonary artery. The pulmonary artery divides into branches that parallel the bronchial tubes and it ends in a network of pulmonary capillaries (tiny blood vessels) within the walls of the small air passages and alveoli of the lungs.
The pulmonary veins carry oxygen-rich blood away from the lungs. They begin in the pulmonary capillaries, unite to form larger branches (e.g., the left and right superior and inferior pulmonary veins), and eventually lead into the left atrium of the heart. The heart then pumps the oxygenated blood out to the body parts via the aorta (the great artery arising from the left ventricle of the heart).
The bronchial arteries are blood vessels that branch off from the aorta to supply blood and nutrition for the lung itself and the bronchial tubes. The bronchial vein begins at the root of the lung and receives blood from vessels near the bronchial arteries.
Lymphatic vessels are structures that drain lymph. Lymph is the clear, yellowish fluid containing lymphocytes (white blood cells that fight disease) from the tissues of the body. The lungs have two sets of lymphatic vessels—a surface, or superficial set, and a deep set. The superficial lymphatic vessels are located beneath the pleura (thin, serous covering of the lungs), whereas the deep set follow the blood vessels and extend along the bronchi. Both sets of lymphatic vessels end at the root of the lungs, within the bronchial glands.
Two or three efferent (outward-leading) vessels travel up the trachea (windpipe) to the base of the neck, where they cross the trachea and esophagus (tube that passes from the mouth to the stomach). These vessels end at either the thoracic duct (passage that empties a large amount of lymph and lymph-related compounds into the blood) on the left side or the lymphatic duct on the right.
The lungs receive their nerve supply from the anterior (front) and posterior (back) nerve networks, called pulmonary plexuses. These plexuses are offshoots of larger nerves, such as the sympathetic nerves of the trunk and the pneumogastric (tenth cranial, or vagus) nerve. The nerves of the lung contain small, knot-like masses known as ganglia.