LYMPHATIC DRAINAGE OF THE LUNGS AND PLEURA
The lymphatic drainage of the lung plays critical roles in the removal of excess interstitial ﬂuid and particulate matter (free or within macrophages) deposited in the airspaces and in lymphocyte trafﬁcking and immune surveillance. Discrepancies exist between the terminology of the Nomina Anatomica adopted by anatomists for lung lymphatic routes and the terms commonly and conveniently used by clinicians, surgeons, and radiologists. For this reason, in the illustrations, the terms in common usage are included in parentheses after the ofﬁcial Nomina Anatomica designations.
As the lymphatic channels approach the hilum, lymph nodes are present in the following distributions:
1. The pulmonary (intrapulmonary nodes) within the lung, located chieﬂy at bifurcations of the large bronchi
2. The bronchopulmonary (hilar) nodes situated in the pulmonary hilum at the site of entry of the main bronchi and vessels
3. The tracheobronchial nodes, which anatomists subdivide into two groups: a superior group situated in the obtuse angles between the trachea and bronchi and an inferior (carinal) group situated below or at the carina (i.e., at the junction of the two main bronchi)
4. The tracheal (paratracheal) group situated alongside and to some extent in front of the trachea throughout its course; these are sometimes subdivided into lower tracheal (paratracheal) nodes and an upper group in accordance with their relative positions
5. The inferior deep cervical (scalene) nodes situated in relation to the lower part of the internal jugular vein, usually under cover of the scalenus anterior muscle
6. The aortic arch nodes situated under the arch of the aorta
Beginning centrally, the major lymph channels on the right side are (1) the bronchomediastinal lymph trunk, which collects lymph from the mediastinum, and (2) the jugular lymph trunk. The latter commonly unites with (3) the subclavian trunk to form a right lymphatic duct, which in turn joins the origin of the right brachiocephalic vein. In some cases, however, these three major lymphatic channels join the brachiocephalic vein independently. On the left side, the thoracic duct curves behind the internal jugular vein to enter the right brachiocephalic vein at the junction of the subclavian vein and internal jugular veins. There may or may not be a separate right bronchomediastinal lymph trunk; if present, it may join the thoracic duct or enter the brachiocephalic vein independently.
Within the lung, lymphatic plexuses course as two separate arcades, one along the bronchovascular sheath (beginning at the level of the respiratory bronchiole) and the other along the pulmonary veins coursing through the interlobular planes, connective tissue septa, and the pleura. In the bronchi, ﬁne lymph channels in the submucosa communicate with much larger lymphatic vessels in the adventitia. Beyond this point, the lymph is collected by the interlobular lymphatics. The bronchial pathways communicate with the lymph vessels along the accompanying pulmonary arteries. The pulmonary veins that lie at the edge of the respiratory units whether acinus, lobule, or segment are surrounded by connective tissue and have lymphatic plexuses in their walls. They are separated from the bronchi and arteries, but at least centrally, communicating channels connect the various lymphatic systems that form a ﬁne network beneath the pleural surface over the surface of the lungs and the interlobar ﬁssures.
The network was formerly thought to drain it its entirety to the hilar nodes, but it has now been shown to communicate not only with the arterial and venous channels but with the interlobular plexuses as well. Only the portion of the pleural drainage close to the hilum supplies the nodes there. The interlobular vessels pass to the bronchial, arterial, and venous pulmonary plexuses and to the pulmonary and bronchopulmonary nodes.
Almost all the lymph from the lungs eventually reaches the bronchopulmonary (hilar) lymph nodes, with or without passing through pulmonary lymph nodes on its way. Some lymph may bypass the hilum and go directly to the tracheobronchial lymph nodes. From the right lung, drainage from the bronchopulmonary (hilar) group is to the superior and inferior (carinal) tracheobronchial and the right tracheal (paratracheal) nodes. From there, lymph goes either by the way of the bronchomediastinal trunk to the right brachiocephalic vein, via the inferior deep cervical (scalene) lymph nodes to the same vein, or through both of these channels. On the left side, the course is somewhat different. There, either most or all of the drainage from the upper lobe, after passing through the bronchopulmonary (hilar) lymph nodes, moves either by way of the tracheobronchial and tracheal (paratracheal) lymph nodes, bronchomediastinal trunk, scalene nodes, and thoracic duct to the brachiocephalic vein or by way of the aortic arch nodes to the same termination. From the left lower lobe and usually from the lingula, lymph ﬂows to the right after passing through the bronchopulmonary (hilar) nodes and goes mostly to the lower tracheobronchial (carinal) lymph nodes. It then follows the same course as the lymph from the right lung by way of the right tracheal (paratracheal) nodes an important point in disease, especially tumors of the left lower lobe.
A number of factors may cause deviation from these major pathways of lymph drainage. The pulmonary lymphatic vessels contain many valves that normally direct the ﬂow toward the hilum. Obstruction in parts of the system, however, may cause a “backing up” effect with incompetence of the valves, reversal of ﬂow, and opening of collateral channels. It is noteworthy that in pulmonary edema, the pulmonary lymph vessels have been found to be greatly distended (see Plate 4-127).
Some lymph may leave the lungs through vessels that emerge in the pulmonary ligaments and pass to the posterior mediastinal lymph nodes. Nagaishi’s textbook states that some of the pulmonary drainage may even reach intraabdominal lymph nodes, although a speciﬁc transit route is not described. Finally, there are probably cross-connections between the right and left tracheal (paratracheal) nodes, a situation that may further alter the drainage pathways.
Clinically, the nodal positions are described by the regional lymph node classiﬁcation for lung cancer staging as detailed in Plate 4-49. This classiﬁcation is anatomically based and validated, allowing for consistent lymph node mapping used in staging lung cancer.
|DISTRIBUTION OF LYMPHATICS IN LUNGS AND PLEURA|