Respiratory System Terms


Respiratory System Terms


Acidosis: A state of abnormally low pH of arterial blood, which is often caused by an increase in the concentration of carbon dioxide, CO2 .


Alkalosis: A state of abnormally high pH of arterial blood, which is often caused by a decrease in the concentration of carbon dioxide, CO2 .


Alveoli: Microscopic air chambers that are the sites of gas exchange in the lungs; the respiratory bronchioles branch into the alveolar ducts, which terminate with the alveolar sacs containing the alveoli; their walls are composed of simple squamous epithelium and contain surfactant secreting cells and alveolar macrophages. Also called pulmonary alveoli.


Apnea: Absence of breathing.


Asphyxiation: The state of impaired or absent exchange of oxygen and carbon dioxide in the lungs, resulting in hypoxia and hypercapnia.


Asthma: An inflammatory disease of the lungs; airways become inflamed due to irritation, bronchioles constrict due to muscle spasms, and characterized by episodes of coughing, wheezing, dyspnea, and chest tightness.


Atmospheric pressure: The pressure exerted by the air in the atmosphere, which varies with weather and altitude, but its average value at sea level is 1 atm = 760 mm Hg (torr) = 29.92 inch Hg = 14.696 psi (lb/in2 ) = 101.325 kPa = 1013.25 millibars = 1013.25 × 106 dynes/cm2 . Also called barometric pressure.


Boyle's law: States that when the temperature is constant, the volume of a gas is inversely proportional to its pressure, and the relationship between initial and final pressures and volumes of a gas is: P1 V1 = P2 V2


Bronchi: The large branches from the trachea that convey air to and from the lungs, that have a fibrous coat with plates of hyaline cartilage, smooth muscle, and are lined with pseudostratified ciliated columnar epithelium with goblet cells; the trachea divides into the left and right main (primary) bronchi that enters the lungs, these branch to the lobar (secondary) bronchi that enter the lobes of the lungs, then branch to the segmental (tertiary) bronchi that enter the segments of the lobes; branching continues down to the small bronchi that have proportionally more smooth muscle and less cartilage, and are lined with ciliated simple columnar epithelium.


Bronchial tree: The trachea, bronchi, and their tree-like branching structures down to the terminal bronchioles.


Bronchioles: Air passageways smaller than 1 mm in diameter that branch from the smallest bronchi, down six subdivisions to the alveoli; they contain abundant smooth muscle and elastic fibers, but no cartilage in their walls, and are lined with simple cuboidal epithelium.


Bronchitis: A lung disorder where airways become inflamed, mucus production increases, and coughing brings up mucus and pus.


Carbonic acid / Bicarbonate buffer system: Chemical buffer that helps maintain pH homeostasis of the blood, which contains the weak acid, carbonic acid ( H2CO3 ), and its conjugate base, bicarbonate ion ( HCO3 ).


Carbonic anhydrase: Enzyme that catalyzes the addition of water to carbon dioxide, which forms carbonic acid.


Chronic obstructive pulmonary disease (COPD): Collective term for progressive, obstructive respiratory disorders; includes emphysema and chronic bronchitis.


Compliance of lung: The ease with which a lung may be expanded, which is a factor of lung distensibility and alveolar surface tension; measured as the change in lung volume divided by change in transpulmonary pressure.


Dalton's law: Each gas in a mixture of gases exerts a pressure proportionate to the percentage of the gas, and independent of the other gases present. Also called Dalton's law of partial pressures.


Dyspnea: Shortness of breath and difficulty in breathing.


Emphysema: A lung disorder characterized by fibrosis of the lungs; alveoli burst and fuse into enlarged air spaces, surface area for gas exchange is reduced, and victims become barrel-chested because of air retention.


Epiglottis: Elastic cartilage at the back of the throat that covers the opening of the larynx during swallowing.


Esophagus: The hollow muscular tube that connects the pharynx to the stomach.


Eupnea: Normal quiet breathing.


Henry's law: States that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of the gas that is in contact with the liquid.


Hilum of lung: A depression on the medial surface of each lung, where the main (primary) bronchus, blood vessels, nerves, and lymphatics enter or leave. Also called pulmonary hilum.


Hypercapnia: Abnormally high carbon dioxide, CO2 , levels in arterial blood, which often causes acidosis.


Hyperoxia: An increased amount of oxygen, O2 , in blood and tissues.


Hyperventilation: Increased depth and rate of breathing, usually resulting in hyperoxia, but also causes hypocapnia and alkalosis.


Hypocapnia: Abnormally low carbon dioxide, CO2 , levels in arterial blood, which often causes alkalosis.


Hypoventilation: Decreased depth and rate of breathing, which causes hypoxia, hypercapnia, and acidosis.


Hypoxia: Abnormally low oxygen, O2 , levels in blood and tissues.


Larynx: The voice box; the organ continuous with the trachea that is composed of mostly hyaline cartilage; its opening is the glottis which can be closed by the epiglottis during swallowing; it contains the vocal folds (true vocal cords) with muscles that control their position and tension; its largest structure is the thyroid cartilage that has an anterior projection called the laryngeal prominence (Adam's apple); it is lined with pseudostratified ciliated columnar epithelium with goblet cells.


Lungs: The organs of respiration in which blood is aerated; they are located in the pulmonary cavities on each side of the mediastinum; the right lung is larger and is divided into three lobes and 10 segments, while the left lung has two lobes and 9 segments.


Nasal cavity: The cavities on each side of the bony vertical partition (nasal septum), that extends from the external nares (nostrils) to the internal nares (choana) that open into the nasopharynx; it contains three nasal conchae, which are bony ridges that protrude from the lateral walls of the nasal cavity and increase air turbulence; it is mostly lined with pseudostratified ciliated columnar epithelium with goblet cells.


Paranasal sinuses: The paired air-filled cavities that lighten four skull bones and open into the nasal cavity; they are lined with pseudostratified ciliated columnar epithelium with goblet cells, and are located in the frontal, maxillary, ethmoid, and sphenoid bones.


Pleura: The double-layered serous membrane that protects the lung; the parietal pleura lines the wall of the pulmonary cavity, the visceral pleura covers the outer surface of the lungs, and the pleural cavity is a slit-like space between these pleural layers that is filled with lubricating pleural fluid.


Pharynx: The throat; a muscular tube that starts at the internal nares and runs partway down the neck, where it opens into the esophagus posteriorly and the larynx anteriorly; it is subdivided into the nasopharynx which is lined with pseudostratified ciliated columnar epithelium with goblet cells, and the oropharynx and laryngopharynx which are both lined with stratified squamous epithelium.


Pneumonia: The most common infectious cause of death in the United States, which causes alveoli to fill with thick fluid, making gas exchange difficult.


Pulmonary fibrosis: Fibrous connective tissue builds up in lungs, reducing their elasticity.


Respiratory centers: Nuclei in the reticular formation of the brain stem that coordinate respiratory movements; the ventral respiratory group (VRG) in the medulla oblongata generates the basic respiratory rhythm; the dorsal respiratory group (DRG) in the medulla oblongata integrates input from peripheral stretch and chemoreceptors; the pontine respiratory group (PRG) in the pons smooths and fine-tunes the breathing rhythms during activities like vocalization, exercise, and sleep.


Respiratory volumes and capacities: Air volumes that are used to assess a patient’s respiratory status; Tidal volume (TV) is the volume of air that enters and leaves the lungs in a single breath during regular breathing; Inspiratory reserve volume (IRV) is the amount of air that can be forcibly inspired after a tidal inspiration; Expiratory reserve volume (ERV) is the amount of air that can be forcibly expired after a tidal expiration; Residual volume (RV) is the amount of air that remains in the lungs after a forced expiration; Vital capacity (VC) is the total volume of exchangeable air, which is measured as the volume of air that can be forcibly expelled from the lungs after the deepest inspiration; Total lung capacity (TLC) is the maximum volume of air in the lungs after the deepest inspiration.


Surfactant: An alveolar secretion containing a mixture of phospholipids and lipoproteins that is produced by type II alveolar cells (pneumocytes); it reduces the surface tension of water molecules, thus preventing the collapse of the alveoli after each expiration; neonatal respiratory distress syndrome is common in babies born 27 weeks after conception or younger, due to lack of surfactant production.


Trachea: The windpipe; the air tube that extends from the larynx to the two main bronchi which branch from the carina at its bifurcation; it has an outer fibrous coat (adventitia), is reinforced by C-shaped rings of hyaline cartilage that are connected posteriorly to the trachealis muscle; it is lined with pseudostratified ciliated columnar epithelium with goblet cells.


Tuberculosis: A lung disease where infection spread by airborne bacteria; tubercles encapsulate bacteria, elasticity of lungs is reduced, and infected tissues and lymph nodes may eventually calcify.


Ventilation-Perfusion Coupling: Autoregulatory mechanisms that maintain a close match (coupling) between the amount of gas reaching the alveoli (ventilation), and the blood flow reaching the alveoli (perfusion) for efficient gas exchange.