19.3: Lower Respiratory Tract

Lower Respiratory Tract

Once air makes its way into the larynx it then continues down the trachea (commonly known as the windpipe) which ultimately splits into two main stems towards the right and left lungs, respectively: the right primary bronchus and the left primary bronchus. The right primary bronchus is wider than the left primary bronchus and it is also has a more vertical angle than the left primary bronchus. Due to these structural differences, if a foreign object is accidently inhaled, it is more likely to become lodged in the right primary bronchus than the left primary bronchus.

The large airways are lined mostly by pseudostratified ciliated columnar epithelium with mucus-secreting goblet cells, respiratory mucosa. Mucus helps trap particles and then the ciliated columnar cells beat rhythmically to move the mucus and any trapped particles from the air towards the pharynx where they can either be spit out or swallowed. This mechanism is known as the mucociliary escalator.

Above: (Left) The larynx, trachea, large bronchi, and lungs (only part shown). (Top right) Cross section of the trachea, located anterior to the esophagus. (Bottom right) The respiratory mucosa lining the inside of most of the large airways and creates the mucociliary escalator.

Above: Pseudostratified ciliated columnar epithelia that line the inside of large airways and are responsible for creating the mucociliary escalator.

Above: (Left) The trachea, magnified by 10x. The trachea, continuing inferiorly from the larynx, lies anterior to the esophagus. The patency of the trachea and primary bronchi respiratory organs is maintained by C-shaped cartilage rings, whose openings face the esophagus. The two main lobes of the thyroid gland are located at the lateral boundaries of the trachea. (Right) The walls of the trachea and primary bronchi are identical, resembling the pattern typical for most respiratory passageways. Longitudinally oriented elastic fibers are numerous in the lamina propria, sometimes forming an obvious elastic lamina. Bundles of interlacing smooth muscle lie external to the elastic fibers. The trachea and primary bronchi are extrapulmonary and are part of the conducting portion of the respiratory system. Magnified by 100x.

The bronchi are the largest airways of the lungs and they branch again and again becoming smaller and smaller. At each branch point, they get a different name. The primary bronchi branch into secondary bronchi which branch into tertiary bronchi. The first three generations of bronchi are all pretty wide and have cartilage rings for support. After the first three generations of bronchi though the airwaves get more narrow called bronchioles meaning "little bronchi" and these can stay open without the need for cartilage air is conducted through smaller and smaller bronchioles for about 15 to 20 generations and collectively these are known as conducting bronchioles. These conducting bronchioles are also lined by ciliated columnar cells. Mucus-secreting goblet cells are present as well as a new type of cell called a club cell. These club cells secrete glycosaminoglycans which protect a bronchial or epithelium, that can also transform into ciliated columnar cells so they help regenerate and replace damaged cells, if needed.

Lower Respiratory Tract Structure	Location	Function
alveolar duct	thin-walled passages at the end of respiratory bronchioles leading to alveolar sacs containing alveoli	conducting air to/from the alveolar sacs and alveoli
alveolar macrophages	mobile cells in the alveoli	big eater cells that patrol alveoli for debris and engulf it to remove it (dust, bacteria, etc.)
alveolar sac	a structure resembling a bunch of grapes off of an alveolar duct; contains alveoli	conducting air to/from alveoli; assemblage of alveoli that allow gas transfer with the pulmonary capillaries wrapped around the outside of alveoli
alveoli	spherical thin-tissue structures assembled into bunches (alveolar sacs)	assemblage of alveoli that allow gas transfer with the pulmonary capillaries wrapped around the outside of alveoli
bronchial tree	successive branches of airways wrapped in cartilage; primary bronchi (first branches), secondary bronchi (second branches), and tertiary bronchi	conducting air to/from bronchioles
bronchioles	airway branches off of tertiary bronchi in the lungs; smaller passages than bronchi; have a layer of smooth muscle; have cilia; lead to terminal bronchioles which lead to respiratory bronchioles; divide into 2-10 alveolar ducts	conducting air to/from alveolar ducts
lungs	paired organs within pleural cavities	conduct air and enable gas exchange to occur between the air in the lungs and blood in the pulmonary capillaries
parietal pleura	serous membranes forming the outer layers of the pleural cavities	forms outer aspects of the pleura cavities; produces pleura fluid to lubricate the pleural cavities
trachea	tube between the larynx and the primary bronchi; wrapped in tracheal cartilage to keep airways open	conducting air to/from the lungs
type I alveolar cell	form about 95% of alveoli surfaces; simple squamous epithelial cells	gas exchange with capillaries
type II alveolar cell	very small cuboidal cells in the alveoli	produce and secrete pulmonary surfactant to maximize gas exchange
visceral pleura	serous membranes on the external surface of the lung	forms inner aspects of the pleura cavities; produces pleura fluid to lubricate the pleural cavities