Functional anatomy of the lung of the snake Pituophis melanoleucus

The respiratory system in Pituophis melanoleucus is composed of a trachea, a single bronchial lung containing the pulmonary vasculature, and a large membranous caudal air sac. Total air volume (VL) in a 1-kg snake is 97.5 ml and varies with size (M) according to the equation VL = 0.388 M0.80 where VL is in milliliters and M is in grams. These air volumes are similar to those measured in many lizards and turtles. Relative volumes of the trachea, vascular lung, and air sac in Pituophis are about 4, 20, and 76%, respectively. The respiratory tissue is composed of a honeycomb network of capillary-bearing partitions, which collectively forms the gas exchange surface. This respiratory tissue is confined to the proximal 20% of the lung. Total respiratory surface area (SA) in a 1-kg snake is 2.00 x 10(3) cm2 and varies with size according to the equation SA = 3.03 M0.94 where SA is in square centimeters. Total SA/VL within the bronchial lung is large, being approximately 101 cm2/ml. Gradients in respiratory surface area and lung air volume are reversed so that SA/VL is highest near the cranial tip of the lung and decreases towards the air sac. Harmonic mean thickness of the air-blood barrier was found to be 0.46 +/- 10(-4) cm with the result that the anatomic diffusion factor, or ratio of respiratory surface area to mean diffusion distance, for a 1-kg snake is 0.44 x 10(8) cm. This index of diffusion capacity is very close to that reported for two lungs of a turtle.