Blood Supply

It is important to realize that there are two blood supplies to each lung. The most obvious vessels are the pulmonary arteries which are branches of the pulmonary trunk carrying deoxygenated blood from the right ventricle of the heart and pulmonary veins returning oxygenated blood into the left atrium. The alveoli receive oxygen and nutrients as gaseous exchange between blood and air occurs through their walls.

The bronchi and bronchioles, connective tissue, and visceral pleura receive oxygenated blood from the much smaller bronchial arteries which arise from the descending thoracic aorta. Anastomoses between these systems become important when either is occluded by disease.

Nerve Supply

The lungs receive an autonomic nerve supply through the pulmonary plexus, comprising branches from the sympathetic trunk and parasympathetic branches from the vagus nerves. Sympathetic activity produces bronchodilation and parasympathetic bronchoconstriction. Sensory nerves from stretch receptors in the bronchi and alveolar walls are important in the reflex regulation of the degree of lung inflation. Over 50% of these receptors are in the bronchial tree within the lungs and the rest are located in the trachea and main bronchi outside the lungs.

Despite their name of stretch receptors, they respond to pressure difference across the walls rather than physical stretching of the walls of the lower respiratory tract. As the pressure changes during ventilation, stretch receptors are stimulated and inhibit contraction of the muscles of inspiration in the respiratory centres in the medulla, allowing expiration through passive elastic recoil.