Vascular System and Innervation of the Lungs
The lungs make up the terminal portion of the respiratory apparatus and the largest portion of respiratory tissue. They are a pair of organs situated within the thoracic cavity and they are supplied with inhaled air that comes from the upper segments of the tract including the nasal cavity, the pharynx, the larynx, the trachea, the bronchi and the bronchioles. Their main function is breathing so that oxygen and carbon dioxide can be exchanged, with oxygen being inhaled and sent to the tissues and carbon dioxide being released via exhalation.
This article will focus upon the blood supply, venous drainage, innervation and lymphatic drainage of the pulmonary tissues.
Blood Supply and Venous Drainage
At the level of the sternal angle, the pulmonary trunk divides into the left and right pulmonary arteries, which carry deoxygenated-low oxygen blood from the right ventricle of the heart.
Each pulmonary artery forms part of the root of their respective lungs, and divides into secondary lobar arteries. Lobar arteries then divide into tertiary segmental arteries. The arteries follow the branching pattern of the bronchi, most often travel anterior to their bronchial counterpart.
Two pulmonary veins (a superior and inferior one) arise from each lung, carrying oxygenated blood from the lungs to the left atrium of the heart.
The pulmonary veins generally run independent of the pulmonary arteries and bronchi.
Systemic (Bronchial) circulation
The bronchial arteries originate from the thoracic aorta and carry oxygenated blood to the supporting tissues of the lungs, and visceral pleura.
The thoracic aorta gives rise to two left bronchial arteries, while the single right bronchial artery stems from either one of the superior posterior intercostal arteries or the left superior bronchial artery.
The bronchial arteries generally travel along the posterior aspect of the main bronchi and their branches, as far as the respiratory bronchioles. Their branches anastomose with the branches of the pulmonary arteries, forming an arterial network throughout the lung tissue.
The parietal pleura receives it blood supply via the arteries of the posterior thoracic wall.
The bronchial veins drain primarily the proximal portions of each lung, which are supplied directly the bronchial arteries.
The pulmonary veins govern the venous drainage of the rest of the lungs, which includes the visceral pleura, peripheral lung tissue and distal parts of the bronchial root.
The right and left bronchial veins empty into into the azygos and hemiazygos veins, respectively.
The innervation of the lungs occurs via the pulmonary plexuses which runs both anteriorly and posteriorly along the lung roots. The nervous structures within the plexuses contain postganglionic sympathetic fibers that originate from the sympathetic trunks which happen to innervate the bronchial tree, the pulmonary vessels and the glands of the bronchial tree.
The characteristics of the sympathetic fibers include bronchodilation, vasoconstriction and inhibition of glandular secretion.
Other fibers that run in the pulmonary plexuses include the preganglionic fibers which are contributed by the vagus nerve (CN X) as well as small parasympathetic ganglia and postganglionic parasympathetic nerves which innervate the smooth muscle of the bronchial tree, the pulmonary vessels and the bronchial glands. The characteristics of parasympathetic fibers include bronchoconstriction, vasodilation and glandular secretomotor stimulation.
Finally, the visceral afferent fibers present function by transmitting information that is needed in the coughing reflex, stretch reception, blood pressure, chemoreception and nociception.
The lungs have a large complex of freely connecting lymphatic vessels. The lymph formed within the lungs drains into the pulmonary lymph nodes which are found along the lobar bronchi. Also, the bronchopulmonary lymph nodes which sit on the left and right major bronchi. Finally the superior and inferior tracheobronchial lymph nodes which conjugate around the superior and inferior aspects of the bifurcation of the trachea.
Coughing is a natural reflex that plays a very important role in the maintenance of healthy lung tissue. The ability to cough allows the body to remove dust, mucus, saliva and other debris from the lungs. If a patient is unable to cough, the likelihood of contracting an infection or an inflammation greatly increases. Another way to keep lung matter healthy and free from particulates is to breathe deeply in a rhythmic manner and there are various techniques and exercises that help on to do so.