The proportion of parenchyma appears to be well conserved, from 85% in human lung ( 84) to 86% in mouse lung ( 87). On average parenchyma comprises 84% of lung anatomic volume (range 77-87%) ( 84, 87, 125, 175, 244, 245, 300, 343), leaving a relatively small fraction of lung volume for extra-alveolar structures, including extra-alveolar blood vessels.
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The parenchyma includes vessels < 20-25 μm in diameter, predominantly capillaries, whereas non-parenchymal structures include vessels and airways which exceed 20-25 μm in diameter. Morphometric approaches have been used to quantitate the fraction of overall anatomic lung volume (which includes lung tissue, intravascular volume and intra-airway volume) which can be defined as either parenchyma or non-parenchyma ( 124). In contrast, the diameter of capillaries that populate the alveolar septal walls tends to decrease with lung inflation ( 37, 94, 183). Extra-alveolar vessels are tethered to lung parenchyma and distend and/or lengthen with lung inflation ( 48, 173). Intra-acinar vessels are associated with respiratory bronchioles, alveolar ducts and alveolar walls, i.e., airways involved in gas exchange. Pulmonary vessels can be divided into 1) those external to (pre-acinar) vs those within the respiratory acinus of the lung (intra-acinar), 2) those external to (extra-alveolar) vs those within the alveolar compartment, and 3) those upstream of (pre-capillary) vs those distal to the alveolar capillary bed (post-capillary). The relationship of the pulmonary vasculature to surrounding lung tissue provides additional context for compartmentalization. Rather than define pulmonary conduit or microvessels based on an arbitrary selection of diameter ranges, extra-alveolar arteries or veins are subclassified based on structural features such as the presence and number of elastic lamina and the degree of muscularity ( 58, 115).
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As in systemic vascular beds, the pulmonary vasculature is composed of three vascular compartments connected in series: arteries, capillaries and veins. The architecture of the normal pulmonary vasculature is engineered to ensure a high compliance, low resistance network that provides an extensive surface area for gas exchange. Finally, the need for rigorous quantitative approaches to study of vascular structure in lung is highlighted. Data from studies utilizing vascular casting, light and electron microscopy, as well as models developed from those data, are discussed. These issues for pulmonary vascular structure are compared, when data is available, across species from human to mouse and shrew. This review is multifaceted, bringing together information regarding 1) classification of pulmonary vessels, 2) branching geometry in the pulmonary vascular tree, 3) a quantitative view of structure based on morphometry of the vascular wall, 4) the relationship of nerves, a variety of interstitial cells, matrix proteins, and striated myocytes to smooth muscle and endothelium in the vascular wall, 5) heterogeneity within cell populations and between vascular compartments, 6) homo- and heterotypic cell-cell junctional complexes, and 7) the relation of the pulmonary vasculature to that of airways. Contributions to structure (and thus potentially to function) from cells other than endothelial and smooth muscle cells as well as those from the extracellular matrix should be considered. Although in general this vasculature is thin-walled, structure is nonetheless complex. Step_Input = 5 + step(10,3) produces the pattern shown below.The pulmonary vasculature is comprised of three anatomic compartments connected in series: the arterial tree, an extensive capillary bed, and the venular tree. The smth3 function will return the value of comp3 in the structure and equations shown below.įlow1 = (input-comp1)*3/averaging (1/day)įlow2 = (comp1-comp2)*3/averaging (1/day)įlow3 = (comp2-comp3)*3/averaging (1/day) If you do not specify an initial value initial, they assume the value to be the initial value of input. They return the value of the final smooth in the cascade.
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smth3 does this by setting up a cascade of three first-order exponential smooths, each with an averaging time of averaging/3. The smth1, smth3 and smthn functions perform a first-, third- and nth-order respectively exponential smooth of input, using an exponential averaging time of averaging, and an optional initial value initial for the smooth. Initial (optional): the value of the result when the function first applies
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N ( smthn only): order of the material smoothing Averaging: time over which to smooth the input value