gland secretions are affected in a similar way, although those abnormalities
donotoccur in all cases. In the case of the lungs, the loss of the mucociliary
escalator action results in susceptibility to opportunistic lung infections.
187.The answer is c.(Junqueira, pp 349–357. Kumar, pp 481–483. Moore
and Persaud, Developing, pp 107, 247–248, 251–252.)Differentiation of
type II pneumocytes (shown in the electron micrograph) occurs late in ges-
tation and is, therefore, incomplete at birth in premature infants. Those
newborn “premies” have a deficiency of surfactant because of the immatu-
rity of the type II pneumocytes. The deficiency of surfactant inhibits nor-
mal expansion of the alveoli and results in idiopathic respiratory distress
syndrome [(RDS); hyaline membrane disease]. The lecithin/sphingomyelin
ratio is a test that can be performed on a sample of amniotic fluid obtained
by amniocentesis. It is used to determine whether the type II pneumocytes
are mature and are synthesizing and secreting surfactant. Maternally
administered glucocorticoids may be used to induce surfactant production
prior to birth, and exogenous surfactant may be given intratracheally to
premature infants to reduce the severity of RDS.
The surfactant is produced by type II pneumocytes in the lung and is
stored in the form of lamellar bodies (the whorls seen in the electron
micrograph). Surfactant consists of an aqueous layer, or hypophase, that
contains proteins and mucopolysaccharides. That layer is covered by a
functional layer of phospholipid that consists predominantly of dipalmi-
toyl phosphatidylcholine (lecithin). The release of lamellar bodies by exo-
cytosis is followed by their general unraveling to form tubulomyelin
figures. The tubulomyelin consists of a crisscross lipid bilayer that covers
the type II pneumocytes. Surfactant-associated proteins (SAP) stabilize sur-
factant, activate surfactant recycling, enhance surfactant-induced reduc-
tion of surface tension, and possess antiviral and antibacterial activities.
Turnover occurs by both endocytosis (type I and II pneumocytes) and
phagocytosis (macrophages); 90% of surfactant is recycled.
The blood-air barrier is formed by the type I pneumocyte, the capillary
endothelial cell, and their fused basal laminae.
G2P2 refers to two pregnancies and two children.
188.The answer is e.(Kumar, pp 723–727. Kasper, pp 1508–1511. Junqueira,
p 349.)The teenage patient is suffering from an asthmatic attack, probably
allergen-induced. Mast cells are a key player in this airway disease. Mast cells
Respiratory System Answers 297