Respiratory System Anatomy

The cells of the human body run on oxygen much like a car runs on gas. There are parts of the cell called mitochondria that use the oxygen to generate ATP, the energy currency of cells. How do the cells get their oxygen? The body takes in air rich in oxygen through the nose and mouth. It then travels down through the air tubes (the pharynx and larynx), through the trachea, the bronchial tubes, and into the alveolar spaces. The alveolar spaces are millions of tiny air sacs that have a very thin wall. The alveolar capillary rests immediately on the other side of the alveolar wall. Oxygen readily transfers from the alveolar space through the thin alveolar wall into the oxygen-depleted red blood cells in the capillary. The oxygen-rich blood then returns to the heart and is pumped throughout the body. Waste in the form of carbon dioxide transfers from the blood into the alveolar space and is exhaled from the body.

Lung Anatomy

The lungs are the body's major organs of respiration. The two vital parts that make up 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., the heart and large vessels, windpipe). The lungs are shaped rather like an upside-down butterfly. The top, or apex, of each lung 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, although the upper lobe of the right lung contains another triangular subdivision known as the middle lobe. The right lung is larger and heavier than the left lung, which is somewhat smaller in size because of the position of the heart. 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 the years.

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 brings 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 glass 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. These 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 more than 23 paired subdivisions. At every branching, the number of airways increases greatly. The entire structure, from the trachea to the smallest bronchioles, resembles an upside-down branching tree, with the branches getting smaller and smaller as they get further from the trunk (the tracheobronchial tree).

The tracheobronchial tree conducts, humidifies, and heats 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. A watery "mucous blanket" - a gel-like liquid - covers and is moved by the cilia and aids the lungs' self-cleaning. Coughing triggers a high-speed flow of air that 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 passages of alveolar ducts). The alveolar sacs are tiny, thin-walled, cup-shaped structures (250–300 mm2) are lined with a detergent-like substance known as surfactant, which reduces surface tension and prevents them from collapsing during breathing. Another substance in the alveolar wall, elastin, applies an elastic force to "shrink" the alveoli during exhalation. There are approximately 3,000,000 alveoli in the lungs.

Anatomy of the Respiratory Muscles

Air is inhaled by the active contraction of the respiratory muscles. The major respiratory muscle is the diaphragm, a large muscular partition that separates the chest cavity from the abdominal cavity. It lies beneath the lungs and is dome shaped, with the apex of the dome in approximation with the lower portion of the lungs. During inhalation, the diaphragm contracts downward and away from the lungs, creating more space in the chest cavity. This lowers the pressure in the chest cavity relative to the pressure outside the body, and air moves into the lungs.

The diaphragm receives help from accessory muscles. The muscles in the neck that attach to the upper part of the sternum and clavicles, such as the sternocleidomastoid muscle, and the muscles between the ribs, called the intercostal muscles, increase the diameter of the chest wall, which helps lower the pressure in the chest. Intercostal muscles contract and shrink the chest wall during forced exhalation.

Anatomy of the Blood Vessels

Oxygen-poor blood is brought back to the lungs by the pulmonary artery. The pulmonary artery divides into branches that parallel the bronchial tubes and 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 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.

Anatomy of the Lymphatic System

Lymphatic vessels are structures that drain lymph, 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 surface lymphatic vessels are located beneath the pleura (thin, serous covering of the lungs), whereas the deep-set vessels follow the blood vessels and extend along the bronchi. Both sets 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.

Anatomy of the Nerves

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 - e.g., the sympathetic nerves of the trunk and the pneumogastric (tenth cranial, or vagus) nerve. The nerves of the lung contain small, knotty masses called ganglia. The diaphragm receives its nerve supply from the phrenic nerves, which descend from the upper neck.

Publication Review By: the Editorial Staff at

Published: 01 Jun 2000

Last Modified: 11 Apr 2013