The pulmonary circulation of the chelonian reptile: Morphology, haemodynamics and pharmacology

Abstract

1. Blood pressures measured in lightly anaesthetized turtles (Pseudemys scripta) and tortoises (Testudo graeca) indicate that pressures throughout the ventricle are superimposable, but vascular impedances to blood flow in the pulmonary outflow tract and especially in the large extrinsic pulmonary arteries result in slightly lower pulmonary than systemic arterial systolic pressures in both species. 2. Pulmonary outflow tract impedance is increased by vagal stimulation and acetylcholine and decreased by adrenaline. However, the pulmonary outflow tract apparently contributes little to the overall pulmonary impedance changes which occur during intermittent breathing. 3. An analysis of pulmonary arterial impedance suggests that a large central arterial reservoir actively fills during systole and passively empties through a functionally single peripheral resistance during diastole. Morphological examination as well as in vitro compliance measurements and perfusion with drugs of the extrinsic pulmonary arteries corroborate these data by revealing a highly distensible central arterial reservoir nearly devoid of smooth muscle and vasomotor responses. The more distal pulmonary arteries are much less compliant, contain much smooth muscle, and show marked vasoconstriction in response to acetylcholine and vagal stimulation. 4. Data on pulmonary impedance, morphology and pharmacology are incorporated into a classic ‘Windkessel’ haemodynamic model of the chelonian pulmonary circulation.