The vessel wall becomes thinner and the lumen diameter increases. The external diameter, however, remains unchanged for several hours, during which time the endothelial cells that are orientated along the direction of blood flow at birth appear to become more elongated (Fig 1). These findings are compatible with the vessels having been stretched at birth, possibly by the considerable negative intrapleural pressure generated during the first gasps for air. Similar structural changes are evident immediately after birth in the small muscular arteries of the respiratory unit, although they occur rather more slowly (Fig 2, a and b). Many small, “unopened” muscular arteries become recruited into the pulmonary circulation during the first four days of life. Structural adaptation to extrauterine life consists of changes in cell shape and position—the cells become thinner and “spread” within the vessel wall, with no reduction in the amount of pulmonary vascular smooth muscle. These changes may be facilitated by the lack of fixed interstitial connective tissue in the walls of the peripheral pulmonary arteries at birth and by the lack of nerve fibers of vasoconstrictor type.* In the pig, the nerves innervating the respiratory unit arteries contain the vasoconstrictor peptides neuropeptide tyrosine and tyrosine hydroxylase. Both nerve density and expression of neuropeptide increase rapidly during the first 2 weeks of life. paxil 40 mg
In addition to the classic explanations that structural adaptation is due to lung expansion and the release of hypoxic vasoconstrictor tone, we must now consider the role of the eicosanoids. The phospholipid leukotrienes LFC4 and LTD4 appear to act as pulmonary vasoconstrictors in the newborn lamb, while in the pig, the release of leukotriene-like material from the pulmonary arteries decreases toward term.- Prostacyclin dilates fetal pulmonary arteries, and the production of prostacyclin is stimulated by the onset of ventilation. Future investigations might include examination of cell adhesion molecules, junctional composition at birth, and the functional implications of affording a greater opportunity for interaction between an expanded endothelial surface area and blood mononuclear cells.
Figure 2. Electron micrographs of transverse sections through small muscular arteries, taken at same magnification. at stillborn; bt 5 min old. IEL = internal elastic lamina; L=lumen; j narrow process making contact with adjacent smooth muscle cells; * = spaces.
Category: Pulmonary Hypertension
Tags: pulmonary hypertension, pulmonary vascular, smooth muscle