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.
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| DISTRIBUTION OF LYMPHATICS IN LUNGS AND PLEURA |
