TBL10 - Pulmonary Alveoli

Question 1

What do alveoli mainly constitute? What are other structures and supporting tissues generally called?

Answer 1

1) Alveoli constitute about 90% of lung parenchyma, which is the generic term designating the functional portion of an organ
2) Other structures and supporting tissues are generically called stroma; thus, all organs consist of parenchyma and stroma

Question 2

What is each pulmonary alveolus lined by? What else completes the alveolar epithelium?

Answer 2

1) Each pulmonary alveolus is lined by type I pneumocytes (i.e., simple squamous epithelial cells) that create about 95% of alveolar surface area
2) Intermittent, cuboidal shaped type II pneumocytes complete the alveolar epithelium

Question 3

What is the function of type II cells in the alveolar epithelium? Define the function of surfactant. Cite the role of type II cells after injury to the lung parenchyma

Answer 3

1) Type II cells synthesize and secrete pulmonary surfactant
2) Surfactant’s detergent-like property prevents collapse of alveoli by reducing surface tension, thereby facilitating alveolar inflation during inspiration
3) Many type II cells are mitotically active and renew the alveolar surface via differentiation into type I pneumocytes. This replicative potential is important for healing after lung injury, because the large surface area of type I cells makes them especially susceptible to damage

Question 4

What is the pathological significance of interalveolar pores (of Kohn)?

Answer 4

1) Five to eight acini make up a lobule. Collateral air passage occurs between acini via small round to oval holes in the alveolar walls, which known as interalveolar pores of Kohn
2) These openings also allow air to pass from one alveolus to another and may permit the spread of infection

Question 5

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?

Answer 5

1) Surfactant is particularly important for survival of the premature infant. When surfactant is insufficient, the air-water (blood) surface membrane tension becomes high, bringing great risk that alveoli will collapse during expiration
2) No se

Question 6

Why are premature infants susceptible to respiratory distress syndrome (RDS) and how is it treated?

Answer 6

1) Surfactant is particularly important for survival of the premature infant. When surfactant is insufficient, the air-water (blood) surface membrane tension becomes high, bringing great risk that alveoli will collapse during expiration
2) As a result, respiratory distress syndrome (RDS) develops. This is a common cause of death in the premature infant

Question 7

What do adjacent alveoli in the alveolar sacs share?

Answer 7

Adjacent alveoli in the alveolar sacs share a common wall designated the interalveolar septum

Question 8

How is the blood-air barrier formed along alveoli?

Answer 8

1) Pulmonary capillaries reside within the elastic fiber-rich stroma of the interalveolar septa
2) Along half of the capillary circumference, the endothelium and type I pneumocytes are separated only by a basement membrane and thereby form the blood-air barrier

Question 9

What are functions of the blood-air barrier?

Answer 9

1) The blood-air barrier (≤ 1µm thick) provides a short diffusion distance for O2 into the bloodstream and CO2 into the alveolar air
2) The barrier also allows vast exposure of the pulmonary circulation to filtered ambient air

Question 10

How are pathogens phagocytized in the alveoli? Define the origin of the macrophages and their course from the alveoli after phagocytosis of pathogens

Answer 10

1) Macrophages are normal residents of the alveolar lumens; thus, unfiltered pathogens are readily phagocytized
2) These motile cells are derived from blood monocytes
whose precursors arise in bone marrow. They migrate across the walls of pulmonary capillaries to the interalveolar septa. They undergo maturational division in the interstitium of the lung and then enter alveolar spaces to lie free in the lumina. After they remove debris from alveoli, they move up the bronchial tree, where they are carried by cilia and are eventually swallowed or expectorated with mucus

Question 11

What are heart failure cells?

Answer 11

1) In certain types of heart disease, such as congestive heart failure, erythrocytes from the bloodstream may escape into pulmonary alveolar spaces, where alveolar macrophages may phagocytose them
2) These swollen macrophages with ingested hemosiderin may be seen in sputum and are known as heart failure cells