Circumferential strips of pulmonary vessel wall were obtained at necropsy from the major arterial and venous branches at the lung hilum in patients aged 7-87 years. The extensibility of these strips was measured using the tension balance method of Harris et al. (British Heart Journal, 1965, 27, 651-659). The vessels were then bisected, and half of each strip was submitted for structural analysis using morphometric methods on paraffin sections stained to show the collagen, elastin, and muscle content. The other halves of the formalin-fixed vessel strips were examined chemically to determine their collagen content by estimation of the total hydroxyproline content. The thickness of the vessel media was measured microscopically on all of the sections examined. Quantitative measurements were made on 42 arteries and 37 veins. Contrary to expectation, there was a steady fall in medial collagen content with increasing age in arteries and veins. The decrease in collagen content was similar in the morphometric and chemical studies and was statistically significant. The thickness of the vessel media did not change significantly with age. The pulmonary artery and vein strips were less extensible in the older age groups, the main change occurring in the elastic phase of the vascular stress/strain curves. It is suggested that changes in the elastic tissue at a molecular and lamellar level are responsible for the increasing stiffness of pulmonary vessels rather than changes in the medial collagen content.
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