The Geometry of the Pulmonary Microcirculation

Abstract

In contradistinction to extensive knowledge of the pulmonary airway and factors involved in ventilation, the physiologic mechanisms which regulate blood flow through the lung are incompletely understood. Questions to be answered concerning the pulmonary circulation are most fundamental and numerous.1Although planned and even accidental physiologic observations often lead to investigation of structure, understanding of function should logically follow precise knowledge of morphology. Despite considerable detailed information of the anatomy and pathologic anatomy of the several components of the pulmonary circulation, there is, in fact, limited available information on the basic architecture of the pulmonary micro circulation. 2-5 Such knowledge could serve as the base for application of hydrodynamic theory through consideration of the morphology of the pulmonary microcirculation. The silicone elastomer microvascular casting method developed in this laboratory appeared to be admirably suited to such investigation. 6, 7 METHODS Detailed descriptions of the silicone rubber injection method have been published and will not be here given in detail.6> The injection material, General Electric Silicone Rubber RTV-201, § is specially prepared for intravascular injection. It is immiscible with water, has a low surface tension, catalyzes without heat production, and after appropriate adjustment of viscosity can be infused under physiological pressures. Prior wash out of blood is unnecessary and in light of the ability of these silicones to mirror, at least partially, the physiologic state of the blood vessels at the time of perfusion, such wash out is undesirable 8 Procedure. Studies were carried out in cats, dogs and rabbits. Anesthesia was accomplished with urethane (rabbits and cats, 2.5 gm per kg, intramuscularly) and sodium pentobarbital (dogs, 30 mg per kg, intravenously). The trachea was exposed and cannulated, and through a laparotomy incision a loose ligature was secured around the abdominal aorta below the superior

Vasodilator effects of adrenomedullin on small pulmonary arteries and veins in anaesthetized cats

“…changes of the pulmonary vessels directly. In addition, attention was focused on the small muscular pulmonary arteries and veins on account of their thicker smooth muscle layers compared with pulmonary vessels of other sizes (Sobin et al , 1966). They are also highly responsive to many neurohumoral stimuli (Shirai et al , 1993; 1994a,b; 1996).…”

“…of the pulmonary arterioles and venules were not observed in this study, but we believe that these vessels are probably far less responsive to AM. The reasons behind this are, (1) that the arterioles and venules have few smooth muscle layers (Sobin et al , 1966), and (2) that feline arterioles of less than 100 μm i.d. display only very small i.d.…”

Morphometric Basis of the Sheet-Flow Concept of the Pulmonary Alveolar Microcirculation in the Cat

“…It was measured in thick histological sections in alveolar walls that could be positioned by the tilting stage exactly parallel to the optical axis of the microscope. Under these conditions the "sheet" on edge was not displaced from the 7 The statistical analysis of randomly cut thin sections of septa (1, 14) was found to be an unsatisfactory alternative to the optical sectioning technique for thickness determination for this study. With paraffin block sections l/i thick and attendant processing artifact, measurement would still be on the basis of an optical cross section in the light microscope.…”

“…In recent times the living microcirculation of the mammalian lung surface has been extensively studied by visual observation (4) and photography (5) through the intact thorax. A serious limitation of this method is that the microcirculation of the pleural surface is not representative of the respiratory alveoli of the bulk of the lung which are inaccessible to the examining microscope (6,7).…”

“…All subsequent preparations confirmed these initial observations. We believe that this tight mesh may be described as a sheet in which the two vascular endothelial surfaces are essentially parallel and held together by cells and connective tissue which act as posts (7). We shall call this a sheet model.…”