What constitutes approximately 90% of lung parenchyma?
What the hell is parenchyma?
Alveoli constitute 90% of lung parenchyma.
Parenchyma is the term for designating the functional portion of an organ. Other supporting structures and tissues are called stroma.
What creates about 95% of the alveolar surface area? What makes up the rest? Distinguish the two.
Type I pneumocytes make up 95% of alveolar surface area as simple squamous epithelium.
Type II pneumocytes make up the remainder of alveolar surface area and they synthesize and secrete pulmonary surfactant.
What is the function of surfactant and what is the role of type II pneumocytes after a lung injury?
Surfactant reduces the surface tension on alveoli.
Type II cells are mitotically active and renew the alveolar surface via differentiation into type I pneumocytes which is important for healing after a lung injury.
What is the pathological significance of interalveolar pores (of Khon)?
Interaveolar pores of Khon are small round to oval holes in the alveolar walls that allow collateral air passage between acini. These holes also facilitate the spread of infection.
Why is progressive surfactant accumulation in the fluid-filled lungs before birth critical for neonatal respiration? When is the mature number of pulmonary alveoli attained?
Without a high enough amount of surfactant in the fluid-filled lungs, the aveoli of a baby will not be able to resist collapse during expiration due to the high surface tension. This is called respiratory distress syndrome (RDS).
The mature amount of pulmonary alveoli is attained by ten years of age.
Why are premature infants susceptible to respiratory distress syndrome and how is it treated?
Premature infants are susceptible to RDS because they have not had enough time to produce enough surfactant to sufficiently reduce the surface tension on their alveoli, making them susceptible to collapse during expiration. Most of the surfactant is produced in the last two weeks of a normal term.
The treatment for this is artificial surfactant and treating the mothers with glucocorticoids to stimulate surfactant production.
What structures share an interalveolar septum?
Adjacent alveolar sacs are separated by an interalveolar septum.
Where do pulmonary capillaries reside and what makes up the blood-air barrier?
Pulmonary capillaries reside within the elastic fiber-rich stroma of the interalveolar septa.
The blood-air barrier is made up by type I pneumocytes and pulmonary capillary endothelium that are only separated by a basement membrane.
How thick is the blood-air barrier and what does it enable?
The blood-air barrier is less than 1 micrometer thick and it provides a short distance for oxygen into the bloodstream and carbon dioxide into the alveolar air. It also allows vast exposure of the pulmonary circulation to filtered ambient air.
What type of defense do alveoli have against pathogens? Describe this defense.
Macrophages are normal residents of the alveolar lumens, so unfiltered pathogens are readily phagocytized.
Macrophages are motile cells derived from monocytes whose precursors arise in bone marrow. They migrate across the walls of pulmonary capillaries to the interalveolar septa. After they remove debris from the alveoli, they move up the bronchial tree, where they are carried by cilia and are eventually swallowed or expectorated (coughed or spit out) with mucus.
What are heart failure cells?
Heart failure cells are basically swollen alveolar macrophages that contain ingested hemosiderin from phagocytizing erythrocytes that have escaped into pulmonary alveolar spaces. Heart failure cells are commonly seen in sputum and are associated with certain types of heart disease such as congetive heart failure.