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Point:Counterpoint 1535 24. Prisk GK, Yamada K, Henderson AC, Arai TJ, Levin DL, Buxton RB, Hopkins SR. Pulmonary perfusion in the prone and supine postures in the normal human lung. J Appl Physiol 103: 883– 894, 2007. 25. Weibel ER. Fractal geometry: a design principle for living organisms. Am J Physiol Lung Cell Mol Physiol 261: L361–L369, 1991. 26. West GB, Brown JH, Enquist BJ. The fourth dimension of life: fractal geometry and allometric scaling of organisms. Science 284: 1677–1679, 1999. 27. West JB, Dollery CT. Distribution of blood flow and ventilation-perfusion ratio in the lung, measured with radioactive carbon dioxide. J Appl Physiol 15: 405– 410, 1960. 28. West JB, Dollery CT, Naimark A. Distribution of Blood Flow in Isolated Lung; relation to vascular and alveolar pressures. J Appl Physiol 19: 713–724, 1964. Robb Glenny Division of Pulmonary and Critical Care Medicine University of Washington e-mail: [email protected] REBUTTAL FROM DRS. HUGHES AND WEST J Appl Physiol • VOL 1. Anthonisen NR, Milic-Emili J. Distribution of pulmonary perfusion in erect man. J Appl Physiol 21: 760 –766, 1966. 2. Dawson CA. Dynamics of blood flow and pressure-volume relationship. In: The Lung: Scientific Foundations, edited by Crystal RG, West JB, et al. Philadelphia: Lippincott-Raven, 1997, p. 1503–1522. 3. Glazier JB, Hughes JMB, Maloney JE, West JB. Measurements of capillary dimensions and blood volume in rapidly frozen lungs. J Appl Physiol 26: 65–76, 1969. 4. Glenny RW, Robertson HT. Fractal modelling of pulmonary blood flow inhomogeneity. J Appl Physiol 70: 1024 –1030, 1991. 5. Glenny RW. Spatial correlation of regional pulmonary perfusion. J Appl Physiol 72: 2378 –2386, 1992. 6. Hughes JMB, Glazier JB, Maloney JE, West JB. Effect of lung volume on the distribution of pulmonary blood flow in man. Respir Physiol 4: 58 –72, 1968. 7. West JB, Dollery CT, Naimark A. Distribution of blood flow in isolated lung; relation to vascular and alveolar pressures. J Appl Physiol 19: 713–724, 1964. 8. Warrell DA, Evans JW, Clarke RO, Kingaby JP, West JB. Pattern of filling in the pulmonary vascular bed. J Appl Physiol 32: 346 –356, 1972. 9. West JB, Schneider AM, Mitchell MM. Recruitment in networks of pulmonary capillaries. J Appl Physiol 39: 976 –984, 1975. REBUTTAL FROM DR. GLENNY Drs. Hughes and West (6) argue convincingly that a vertical gradient of perfusion exists in human lungs. We agree completely and have never stated otherwise. The controversy, however, is about whether gravity is the most important determinant of regional pulmonary blood flow. They reason that gravity must be the single most important determinant of regional perfusion because numerous prior studies have observed a gradient vertical. This logic parallels the argument that the sun revolves around the earth because it is repeatedly seen traversing from one horizon to the other. While the observation is consistent with the hypothesis, it is not sufficient proof. When it was discovered that the earth spins about its axis, prior observations were not invalidated but rather reinterpreted. The realization that blood flow within horizontal planes is present, well structured, and stable over time, suggests a reinterpretation of prior studies of pulmonary blood flow distribution. None of the studies cited in their Point (6) can determine if gravity is the most important determinant of regional pulmonary blood flow. To settle this issue, high spatial resolution measurements of perfusion must be obtained under varying conditions of posture or gravity while tracking the same lung regions across conditions. Although this kind of study is not currently possible in humans, the requisite study has been performed in baboons (2). The baboon is a biped that has similar lung anatomy, muscularization of pulmonary vessels, hydrostatic gradients, and pulmonary arterial pressures found in humans and therefore meets the criteria set by Drs. Hughes and West (4, 6). Baboons were studied in the upright, supine, prone, and head-down postures. Blood flow to each lung region was described by its vertical position in the lung and a structural component that remained stable across changes in posture. Vertical gradients of perfusion were observed in all postures, corroborating prior observations in humans (5, 8). Multiple stepwise linear regression was used to estimate the relative importance of the gravitational and structural components of blood flow. As expected, gravity had its largest influence in the upright posture. However, structure was more than twice as important as gravity in determining regional pulmonary blood flow (59 vs. 25%). 104 • MAY 2008 • www.jap.org Downloaded from http://jap.physiology.org/ by 10.220.33.5 on May 6, 2017 The areas of agreement in this debate are twofold: 1) mean pulmonary blood flow per horizontal slice [per unit volume or, more accurately, per unit density or “alveolus” (1,6)] increases in the gravity direction, independent (for the most part) of postural orientation, 2) there is considerable inhomogeneity of blood flow within a slice of minimal vertical height. Heterogeneity, whether random or ordered, is inevitable as the spatial resolution increases, just as the length of the shoreline increases indefinitely as the sample size gets smaller. Suffice to say that there are two extremes, 1) a well-expanded, vasodilated lung with low PVR and a mean Ppa level below the top of the lung, and 2) a low volume vasoconstricted lung with high PVR and a Ppa level greater than the lung’s vertical height. The effects of gravity will be prominent in the former and largely obscured in the latter. The core of the debate is about the mechanisms responsible for pulmonary blood flow heterogeneity. There has been acceptance (for more than 40 yr) that mean pulmonary blood flow per horizontal slice is influenced by the relations between Ppa, Palv, and Pv (zones I, II, and III) (7), provided the lung is well expanded and PVR is normal. But what determines heterogeneity at the same horizontal level? Between acini, or larger domains, the Seattle group maintains that the architecture of the vascular tree (presumably, arterial) determines local vascular resistance and blood flow by, for example, uneven diameters of the two daughter vessels at a bifurcation. Glenny (5) maintains that this is not random, but forms part of the fractal design of the lung (4). Against this hypothesis is 1) the arterial system contributes only 40% to total PVR (2), and 2) the compliance of the arterial tree means diameters are not fixed, but respond to changes of smooth muscle tone or intra- or extravascular pressures (themselves influenced by gravity from distortion of the lung by its own weight and by other intrathoracic structures). Distal to the arterial tree, at the alveolar septal level, recruitment and distension of vessels occurs (3), independent of arteriolar domains (8), as input pressure increases and exceeds critical opening pressure (9); in the lung periphery, input pressures are a function of vertical height and gravity. So, in subtle ways, gravity may interact with compliant vascular structures to influence local PVR within as well as between horizontal lung slices. REFERENCES Point:Counterpoint 1536 The perspective that the geometry of the vascular tree is more important than gravity in determining regional blood flow requires that other concepts of the gravitational model be reconsidered. While the zones of the lung must exist due to physical laws, they do not need to be vertically stacked in the lung (3). The fractal model predicts that all three zones can exist within the same horizontal plane. The concept that the shared effects of gravity cause matching of regional ventilation and perfusion, however, cannot explain efficient gas exchange in the face of large heterogeneities of ventilation (1) and perfusion within isogravitational planes or during sustained microgravity (7). REFERENCES 1. Altemeier WA, McKinney S, Glenny RW. Fractal nature of regional ventilation distribution. J Appl Physiol 88: 1551–1557, 2000. 2. Glenny RW, Bernard S, Robertson HT, Hlastala MP. Gravity is an important but secondary determinant of regional pulmonary blood flow in upright primates. J Appl Physiol 86: 623– 632, 1999. 3. Glenny RW, Robertson HT. Regional differences in the lung. In: Complexity in Structure and Function of the Lung, edited by Hlastala MP and Robertson HT. New York: Marcel Dekker, 1998, p. 461– 481. 4. Hughes JM. Pulmonary blood flow distribution in exercising and in resting horses. J Appl Physiol 81: 1049 –1050, 1996. 5. Hughes JM, Glazier JB, Maloney JE, West JB. Effect of lung volume on the distribution of pulmonary blood flow in man. Respir Physiol 4: 58 –72, 1968. 6. Hughes JM, West JB. Counterpoint: Gravity is not the major factor determining the distribution of blood flow in the healthy human lung. J Appl Physiol; doi:10.1152/japplphysiol.01092.2007a. 7. Prisk GK, Guy HJ, Elliott AR, West JB. Inhomogeneity of pulmonary perfusion during sustained microgravity on SLS-1. J Appl Physiol 76: 1730 –1738, 1994. 8. West JB, Dollery CT. Distribution of blood flow and ventilation-perfusion ratio in the lung, measured with radioactive carbon dioxide. J Appl Physiol 15: 405– 410, 1960. Downloaded from http://jap.physiology.org/ by 10.220.33.5 on May 6, 2017 J Appl Physiol • VOL 104 • MAY 2008 • www.jap.org