Pulmonary blood flow
- Dual circulation of the lung
- Bronchial circulation – arterial supply of tracheobronchial tree (as far as the terminal bronchioles), and other thoracic structures. About 2% of cardiac output.
- Pulmonary circulation – site of gas exchange between alveolar air and capillary blood. Mixed venous blood in pulmonary artery.
Main pulmonary artery is much thinner-walled than the aorta. Much less smooth muscle in the walls of the vessels of the pulmonary arterial tree. No highly muscular arteriole.
As a result low resistance to pulmonary blood flow. Pulmonary vascular resistance (PVR) is about 1/10 of systemic vascular resistance.
Pulmonary vessels are more distensible than systemic vessels. Blood at a time seen about 30% arteries, 40% capillaries; 30% veins. This can change with lung inflation or during hypoxia.
Pulmonary capillaries in the adult:280 billion capillaries supply 300 million alveoli with 50-100 m2 surface area. Diffusion distance normally less than 0.5 micron.