Small Blood Vessels

Observations of living vascular beds have contributed anatomical information that is significant for understanding normo- and pathophysiology. M. H. Knisely's 1940 classification of vascular beds according to whether tissue or blood is nourished or acted upon is functionally important (1). Common significant anatomical features include cylindrical capillaries, tapered, cone-shaped arteries and veins, arterial branching at obtuse angles, and distensibility allowing emboli to plug arterioles three fourths to two thirds their own diameters. Small pulmonary blood vessels have significant variants. Small arteries and veins lie proximate to alveoli, appear to taper very rapidly, have many precapillary and capillary branches leaving at 90 deg, and form extensive anastomoses. Variants are seen in the bat's wing, with 90-deg branching arterioles containing a muscular ring at the orifice; and in the very thin walls of cerebral vessels. Small pulmonary vessels, being in series with the rest, are potentially traps for all particulate matter originating in the vascular system. Two current examples with such pathophysiologic significance are the intrapulmonary events in somatic burns and "Da Nang lung."