Lymphatic System

Lymphatic vessels

Lymphatic vessels form an extensive and complex interconnected network of channels, which begin as “porous” blind-ended lymphatic capillaries in tissues of the body and converge to form a number of larger vessels, which ultimately connect with large veins in the root of the neck.

Lymphatic vessels mainly collect fluid lost from vascular capillary beds during nutrient exchange processes and deliver it back to the venous side of the vascular system ( Fig. 5.1 ). Also included in this interstitial fluid that drains into the lymphatic capillaries are pathogens, cells of the lymphocytic system, cell products (such as hormones), and cell debris.

In the small intestine, certain fats absorbed and processed by the intestinal epithelium are packaged into protein-coated lipid droplets (chylomicrons), which are released from the epithelial cells and enter the interstitial compartment. Together with other components of the interstitial fluid, the chylomicrons drain into lymphatic capillaries (known as lacteals in the small intestine) and are ultimately delivered to the venous system in the neck. The lymphatic system is therefore also a major route of transport for fat absorbed by the gut.

The fluid in most lymphatic vessels is clear and colorless and is known as lymph. That carried by lymphatic vessels from the small intestine is opaque and milky because of the presence of chylomicrons and is termed chyle.

There are lymphatic vessels in most areas of the body except bone marrow, and avascular tissues such as epithelia and cartilage.

The movement of lymph through the lymphatic vessels is generated mainly by the indirect action of adjacent structures, particularly by contraction of skeletal muscles and pulses in arteries. Unidirectional flow is maintained by the presence of valves in the vessels.

Lymph nodes

Lymph nodes are small (0.1–2.5 cm long) encapsulated structures that interrupt the course of lymphatic vessels and contain elements of the body’s defense system, such as clusters of lymphocytes and macrophages. They act as elaborate filters that trap and phagocytose particulate matter in the lymph that percolates through them. In addition, they detect and defend against foreign antigens that are also carried in the lymph ( Fig. 5.1 ).

Because lymph nodes are efficient filters and flow through them is slow, cells that metastasize from (migrate away from) primary tumors and enter lymphatic vessels often lodge and grow as secondary tumors in lymph nodes. Lymph nodes that drain regions that are infected or contain other forms of disease can enlarge or undergo certain physical changes, such as becoming “hard” or “tender.” These changes can be used by clinicians to detect pathologic changes or to track spread of disease.

A number of regions in the body are associated with clusters or a particular abundance of lymph nodes ( Fig. 5.2 ). Not surprisingly, nodes in many of these regions drain the body’s surface, the digestive system, or the respiratory system. All three of these areas are high-risk sites for the entry of foreign pathogens.

Lymph nodes are abundant and accessible to palpation in the axilla, the groin and femoral region, and the neck. Deep sites that are not palpable include those associated with the trachea and bronchi in the thorax, and with the aorta and its branches in the abdomen.

Lymphatic trunks and ducts

All lymphatic vessels coalesce to form larger trunks or ducts, which drain into the venous system at sites in the neck where the internal jugular veins join the subclavian veins to form the brachiocephalic veins ( Fig. 5.3 ):

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  Lymph from the right side of the head and neck, the right upper limb, right side of the thorax, and right side of the upper and more superficial region of the abdominal wall is carried by lymphatic vessels that connect with veins on the right side of the neck.

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  Lymph from all other regions of the body is carried by lymphatic vessels that drain into veins on the left side of the neck.

Major lymphatic channels

The thoracic duct is the largest lymphatic channel in the body. It begins in the abdomen, passes superiorly through the thorax and enters the root of the neck to the left of the esophagus ( Fig. 5.4 ). Arching laterally, it passes posterior to the carotid sheath and turns inferiorly in front of the thyrocervical trunk, the phrenic nerve, and the vertebral artery. The thoracic duct terminates in the junction between the left internal jugular and the left subclavian veins ( Fig. 5.4 ).

Near where the thoracic duct drains into the venous system, it is joined by three other lymphatic channels:

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  the left jugular trunk, which drains lymph from the left side of the head and neck,

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  the left subclavian trunk, which drains lymph from the left upper limb, and

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  occasionally, the left bronchomediastinal trunk, which drains lymph from the left half of the thoracic structures ( Fig. 5.5 ).

  

A similar confluence of three lymphatic trunks occurs on the right side of the body. Emptying into the junction between the right internal jugular and right subclavian veins are:

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  the right jugular trunk from the head and neck,

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  the right subclavian trunk from the right upper limb, and

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  occasionally, the right bronchomediastinal trunk carrying lymph from the structures in the right half of the thoracic cavity and the right upper intercostal spaces ( Fig. 5.5 ).

There is variability in how these trunks enter the veins. They may combine into a single right lymphatic duct to enter the venous system or enter as three separate trunks.

Major lymphatic channels that drain different regions of the body as a whole are summarized in Table 5.1 .

Table 5.1

Lymphatic drainage

Lymphatic vessel	Area drained
Right jugular trunk	Right side of head and neck
Left jugular trunk	Left side of head and neck
Right subclavian trunk	Right upper limb, superficial regions of thoracic and upper abdominal walls
Left subclavian trunk	Left upper limb, superficial regions of thoracic and upper abdominal walls
Right bronchomediastinal trunk	Right lung and bronchi, mediastinal structures, thoracic wall
Left bronchomediastinal trunk	Left lung and bronchi, mediastinal structures, thoracic wall
Thoracic duct	Lower limbs, abdominal walls and viscera, pelvic walls and viscera, thoracic wall

Lymph nodes and regions of lymphatic drainage

Components of the lymphatic system include superficial nodes around the head, superficial cervical nodes along the external jugular vein, and deep cervical nodes forming a chain along the internal jugular vein ( Fig. 5.6 ).

The basic pattern of drainage is for superficial lymphatic vessels to drain to the superficial nodes. Some of these drain to the superficial cervical nodes on their way to the deep cervical nodes and others drain directly to the deep cervical nodes.

Larynx

Lymphatics drain regions of the larynx above and below the vocal folds:

Those above the vocal folds follow the superior laryngeal artery and terminate in deep cervical nodes associated with the bifurcation of the common carotid artery.

Those below the vocal folds drain into deep nodes associated with the inferior thyroid artery or with nodes associated with the front of the cricothyroid ligament or upper trachea.

Pharynx and palatine tonsil

Lymphatic vessels from the pharynx drain into the deep cervical nodes and include retropharyngeal (between the nasopharynx and vertebral column), paratracheal, and infrahyoid nodes ( Fig. 5.7 ).

The palatine tonsils drain through the pharyngeal wall into the jugulodigastric nodes in the region where the facial vein drains into the internal jugular vein (and inferior to the posterior belly of the digastric muscle).

Face

Lymphatic drainage from the face primarily moves toward three groups of lymph nodes ( Fig. 5.8 ):

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  submental nodes inferior and posterior to the chin, which drain lymphatics from the medial part of the lower lip and chin bilaterally;

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  submandibular nodes superficial to the submandibular gland and inferior to the body of the mandible, which drain the lymphatics from the medial corner of the orbit, most of the external nose the medial part of the cheek, the upper lip, and the lateral part of the lower lip that follows the course of the facial artery;

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  pre-auricular and parotid nodes anterior to the ear, which drain lymphatics from most of the eyelids, a part of the external nose, and the lateral part of the cheek.