THE BLOOD VESSELS STRUCTURE



The lung receives its blood supply from two vascular systemsthe bronchial and pulmonary circulations. The nutritive blood flow to. all but the alveolar structures comes from the bronchial circulation. About one third of the venous effluent drains into the systemic veins and back to the right ventricle. The remainder drains into the pul­monary veins and, along with the Thebesian veins in the heart, represents a component of the 1 to 2 per cent right-to-left shunt found in normal sub­jects.

The pulmonary arterial system runs alongside the airways from the hila to the periphery. The arteries down to the level of the subsegmental air­ways (2 mm diameter) are thinwalled, predom­inantly elastic vessels. Beyond this, the arteries become muscularized until they reach diameters of 30 |xm, where the muscular coat disappears. Most of the arterial pressure drop takes place in these small muscular arteries, which are respon­sible for the active control of blood flow distri­bution in the lung. The pulmonary arterioles empty into an extensive capillary network and drain into thin-walled pulmonary veins, which eventually join with the arteries and bronchi at the hilum and exit the lung to enter the left atrium.