The Structure of the Pulmonary Arteries Varies Among Species
The main pulmonary arteries that accompany the bronchi are elastic, but the smaller arteries adjacent to the bronchioles and the alveolar ducts are muscular. The adult pig and the cow have a thick medial muscle layer in the smaller pulmonary arteries; the horse has less muscle; and the sheep and dog have only a thin muscle layer.
The amount of smooth muscle in the media of pulmonary arteries determines the reactivity of the vasculature to alveolar hypoxia and other neural and humoral stimuli (see later discussion).The small pulmonary arteries lead into pulmonary capillaries, which form an extensive branching network of vessels within the alveolar septum, almost covering the alveolar surface. Not all capillaries are perfused in the resting animal. As a result, vessels that are unperfused in the resting animal can be recruited when pulmonary blood flow increases (e.g., during exercise). Pulmonary veins with thin walls conduct blood from capillaries to the left atrium and also form a reservoir of blood for the left ventricle. The reservoir of blood in the pulmonary veins can be used to initiate a change in cardiac output (e.g., at the start of a sudden burst of exercise).