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In the rabbit, the arterial and venous vasa vasorum of the pulmonary artery from its origin to the entrance of the right and left pulmonary arteries into the lung were studied and described through the use of a specially prepared silicone rubber injection mass. The physical-chemical characteristics of this material allowed detailed filling of the vascular bed under physiological conditions. Special features of the rich arterial and venous vasa vasorum of the pulmonary artery include: (1) spiral palisading of small vessels and (2)arterial and venous vasal confluences that result from the junction of branches from multiple supply and drainage sites. Both of these may be special adaptations for the normal rapid stretch of the pulmonary artery.

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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

Pressure-volume determinations were made in a reproducible proximal segment of the pulmonary arterial system of swine using a known reciprocating volume at physiologic rates of cycling. The distensible response of vessels separated from normal supporting structures depended on extraneous mechanical influences. Antecedent freezing decreased distensibility and altered the pattern of response. At low inputs fresh specimens in continuity with supporting structures displayed pressure-volume curves of increased distensibility with increasing volume. At high inputs the curves were of sigmoid shape with a lower region of increased instantaneous distensibility and a higher region of decreased instantaneous distensibility. The findings differ from distensibility studies of large arteries by other methods.

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