respiratory tract respiratory tract the respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. the respiratory tract is lined with respiratory epithelium as respiratory mucosa. respiratory tractconducting passagesdetailssystemrespiratory systemidentifiersfma265130anatomical terminology air is breathed in through the nose to the nasal cavity, where a layer of nasal mucosa acts as a filter and traps pollutants and other harmful substances found in the air. next, air moves into the pharynx, a passage that contains the intersection between the oesophagus and the larynx. the opening of the larynx has a special flap of cartilage, the epiglottis, that opens to allow air to pass through but closes to prevent food from moving into the airway. from the larynx, air moves into the trachea and down to the intersection known as the carina that branches to form the right and left primary (main) bronchi. each of these bronchi branches into a secondary (lobar) bronchus that branches into tertiary (segmental) bronchi, that branch into smaller airways called bronchioles that eventually connect with tiny specialized structures called alveoli that function in gas exchange. the lungs which are located in the thoracic cavity, are protected from physical damage by the rib cage. at the base of the lungs is a sheet of skeletal muscle called the diaphragm. the diaphragm separates the lungs from the stomach and intestines. the diaphragm is also the main muscle of respiration involved in breathing, and is controlled by the sympathetic nervous system. the lungs are encased in a serous membrane that folds in on itself to form the pleurae – a two-layered protective barrier. the inner visceral pleura covers the surface of the lungs, and the outer parietal pleura is attached to the inner surface of the thoracic cavity. the pleurae enclose a cavity called the pleural cavity that contains pleural fluid. this fluid is used to decrease the amount of friction that lungs experience during breathing. structure complete respiratory system the respiratory tract is divided into the upper airways and lower airways. the upper airways or upper respiratory tract includes the nose and nasal passages, paranasal sinuses, the pharynx, and the portion of the larynx above the vocal folds (cords). the lower airways or lower respiratory tract includes the portion of the larynx below the vocal folds, trachea, bronchi and bronchioles. the lungs can be included in the lower respiratory tract or as separate entity and include the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli. the respiratory tract can also be divided into a conducting zone and a respiratory zone, based on the distinction of transporting gases or exchanging them. the conducting zone includes structures outside of the lungs – the nose, pharynx, larynx, and trachea, and structures inside the lungs – the bronchi, bronchioles, and terminal bronchioles. the conduction zone conducts air breathed in that is filtered, warmed, and moistened, into the lungs. it represents the 1st through the 16th division of the respiratory tract. the conducting zone is most of the respiratory tract that conducts gases into and out of the lungs but excludes the respiratory zone that exchanges gases. the conducting zone also functions to offer a low resistance pathway for airflow. it provides a major defense role in its filtering abilities. the respiratory zone includes the respiratory bronchioles, alveolar ducts, and alveoli, and is the site of oxygen and carbon dioxide exchange with the blood. the respiratory bronchioles and the alveolar ducts are responsible for 10% of the gas exchange. the alveoli are responsible for the other 90%. the respiratory zone represents the 16th through the 23rd division of the respiratory tract. from the bronchi, the dividing tubes become progressively smaller with an estimated 20 to 23 divisions before ending at an alveolus. upper respiratory tract details of upper respiratory tract. the upper respiratory tract can refer to the parts of the respiratory system lying above the sternal angle (outside of the thorax), above the vocal folds, or above the cricoid cartilage. the larynx is sometimes included in both the upper and lower airways. the larynx is also called the voice box and has the associated cartilage that produces sound. the tract consists of the nasal cavity and paranasal sinuses, the pharynx (nasopharynx, oropharynx and laryngopharynx) and sometimes includes the larynx. lower respiratory tract parts of the lower respiratory tract. tracheamainstem bronchuslobar bronchussegmental bronchusbronchiolealveolar ductalveolusthe lower respiratory tract or lower airway is derived from the developing foregut and consists of the trachea, bronchi (primary, secondary and tertiary), bronchioles (including terminal and respiratory), and lungs (including alveoli). it also sometimes includes the larynx. the lower respiratory tract is also called the respiratory tree or tracheobronchial tree, to describe the branching structure of airways supplying air to the lungs, and includes the trachea, bronchi and bronchioles. trachea main bronchus (diameter approximately 1 – 1.4 cm in adults) lobar bronchus (diameter approximately 1 cm) segmental bronchus (diameter 4.5 to 13 mm) subsegmental bronchus (diameter 1 to 6 mm) conducting bronchiole terminal bronchiole respiratory bronchiole alveolar duct alveolar sac alveolus at each division point or generation, one airway branches into two smaller airways. the human respiratory tree may consist on average of 23 generations, while the respiratory tree of the mouse has up to 13 generations. proximal divisions (those closest to the top of the tree, such as the bronchi) mainly function to transmit air to the lower airways. later divisions including the respiratory bronchiole, alveolar ducts, and alveoli, are specialized for gas exchange. the trachea is the largest tube in the respiratory tract and consists of tracheal rings of hyaline cartilage. it branches off into two bronchial tubes, a left and a right main bronchus. the bronchi branch off into smaller sections inside the lungs, called bronchioles. these bronchioles give rise to the air sacs in the lungs called the alveoli. the lungs are the largest organs in the lower respiratory tract. the lungs are suspended within the pleural cavity of the thorax. the pleurae are two thin membranes, one cell layer thick, which surround the lungs. the inner (visceral pleura) covers the lungs and the outer (parietal pleura) lines the inner surface of the chest wall. this membrane secretes a small amount of fluid, allowing the lungs to move freely within the pleural cavity while expanding and contracting during breathing. the lungs are divided into different lobes. the right lung is larger in size than the left, because of the heart's being situated to the left of the midline. the right lung has three lobes – upper, middle, and lower (or superior, middle, and inferior), and the left lung has two – upper and lower (or superior and inferior), plus a small tongue-shaped portion of the upper lobe known as the lingula. each lobe is further divided up into segments called bronchopulmonary segments. each lung has a costal surface, which is adjacent to the ribcage; a diaphragmatic surface, which faces downward toward the diaphragm; and a mediastinal surface, which faces toward the center of the chest, and lies against the heart, great vessels, and the carina where the two mainstem bronchi branch off from the base of the trachea. the alveoli are tiny air sacs in the lungs where gas exchange takes place. the mean number of alveoli in a human lung is 480 million. when the diaphragm contracts, a negative pressure is generated in the thorax and air rushes in to fill the cavity. when that happens, these sacs fill with air, making the lung expand. the alveoli are rich with capillaries, called alveolar capillaries. here the red blood cells absorb oxygen from the air and then carry it back in the form of oxyhaemaglobin, to nourish the cells. the red blood cells also carry carbon dioxide (co2) away from the cells in the form of carbaminohemoglobin and release it into the alveoli through the alveolar capillaries. when the diaphragm relaxes, a positive pressure is generated in the thorax and air rushes out of the alveoli expelling the carbon dioxide. microanatomy