Introductory Chapter: Lymphatic System Human Anatomy

1. Introduction

1.1 Lymphatic system human anatomy

The lymphatic system modulates the interstitial fluid volume through a one-way transport system. The residual interstitial fluid is a carriage from the soft tissue interstitial space into the venous circulation through specific lympho-venous connections [1, 2]. Along with the excess interstitial fluid, redundant proteins and “waste” are transported back to the bloodstream by the lymphatic system. In detail, the lymphatic system via a network of lymphatic channels transports through lymph nodes the interstitial fluid, which is defined as lymph when it is inside the lymphatic channels network, and discharges it into the blood circulation [3, 4, 5]. Lymph nodes filter the interstitial flow and break down bacteria, viruses, and other cells and molecules. The lymphatic system is also important in immune surveillance defending the body against microorganisms and foreign particles. The lymphatic system encourages the immune response [2, 6, 7]. The lymphatic system is, therefore, strictly correlated to the circulatory system and immune system, but not only. In fact, the lymphatic system has also an important role in the absorption of fat-soluble vitamins and fatty substances at the gut level, through gastrointestinal tract’s specific lymphatic capillaries called lacteals, so dietary fat is transported into the venous circulation [1]. In addition, lymphatic vessels were recently identified in the brain meninges and the meningeal lymphatic network is fundamental for the removal of toxins and also in draining cerebrospinal fluid and immune cells from the central nervous system to the peripheral lymphatic system [3, 8].

The lymphatic system is a highly complex system with a variable structure and function between anatomical sites and between species. In humans, the lymphatic system includes lymphatic capillaries, lymphatic vessels (afferent and efferent), lymph nodes, and various lymphoid organs (such as the thymus and spleen) [2, 6, 9].

In this introductory chapter, we will focus our attention mainly on the description of the hierarchy lymphatic channels network (lymphatic capillaries and lymphatic vessels).

The organization of lymphatic networks within various organs depends on the functional demands of the organ itself, leading to both common and unique morphological features of the lymph-venous connections and lymphatic channel network [3, 10]. Interstitial fluid comes out of the blood capillary walls due to heart and/or cell osmotic pressure and enters the lymphatic system through small and blind-ended lymphatic capillaries. These capillaries, defined as initial lymphatics, form a mesh-like network and gradually increase their diameter becoming pre-collector vessels, collector vessels, lymphatic trunks, and finally ducts. When soft tissue interstitial pressure increases, the interstitial fluid enters into the lymphatic capillaries through openings in the endothelial layer; whereas, when the pressure inside the lymphatic capillaries rises, the interstitial fluid entering flow is stopped. Lymphatic capillaries are tiny, thin-walled, and blind-end channels that present a larger diameter with respect to blood capillaries. In addition, the lymphatic capillaries are dissipated among blood capillaries to facilitate interstitial fluid collection by the lymphatic capillary network. The lymphatic capillaries endothelial cells overlap but shift to open the capillary wall when interstitial fluid pressure is greater than intra-capillary pressure so permitting interstitial fluid, lymphocytes, bacteria, cellular debris, plasma proteins, and other cells to enter the lymphatic capillaries [3, 6, 11]. The interstitial fluid inside the lymphatic channels network is defined as lymph. Lymph is composed of interstitial fluid with variable amounts of lymphocytes, monocytes, plasma proteins, and other cells. Lymph formation is organ-dependent and it is correlated to the various organs/tissues morphostructural properties [2, 6].

The lymphatic capillaries form large networks of channels called lymphatic plexuses and converge to form larger lymphatic vessels. Collecting vessels are further divided into afferent (pre-nodal) and efferent (post-nodal) vessels depending on their location relative to lymph nodes. Afferent lymphatic vessels transport the unfiltered lymph from tissues to the lymph nodes and efferent lymphatic vessels convey filtered lymph from lymph nodes to subsequent lymph nodes or into the venous system [6]. Lymph flow generally occurs against a pressure gradient and therefore requires both extrinsic forces, such as skeletal muscle movement and arterial pulsations, and intrinsic forces exerted by lymphatic vessels. In fact, the lymph is pumped slowly by the contraction of the lymphatic vessels [2, 3, 12]. To prevent lymph flow backward, collecting lymphatic vessels and larger lymphatic vessels presented a series of one-way valves; notably, the one-way valves are not present in the lymphatic capillaries. These lymphatic valves help the advancement of lymph flow through the lymphatic vessels.

The anatomical structure of each component of the lymphatic vessel network and its surroundings contribute to its function. Figure 1 reported a schematic representation of the lymphatic channel network.

Figure 1.

Schematic representation of the lymphatic channel network showing lymphatic capillaries and vessels. The yellow arrows represent the direction of lymph flow inside the lymphatic channels and the black arrows show the interstitial fluid entering the lymphatic vessels.

The lymphatic channels gradually increased their diameter becoming finally the main lympho-venous connection: the thoracic duct and the right lymphatic duct. The right lymphatic duct is responsible for draining the lymph from the upper right quadrant of the body (the right side of the head, neck, thorax and the right upper limb) into the venous circulation at the junction between the right subclavian vein and the right internal jugular vein. The right lymphatic duct is formed generally by the convergence of the right bronchomediastinal trunk, jugular trunk, and subclavian trunk [6]. However, it is important to underline that its origin and ending presented a changeable anatomy and morphology.

The thoracic duct, also known as the left lymphatic duct or van Hoorne’s canal, drains the lymph of the body except for the territory drained by the right lymphatic duct so it drains lymph from 80% to 90% of the body. The thoracic duct is a thin-walled tubular lymphatic vessel (with 2–6 mm in diameter). The thoracic duct is the largest and longest lymphatic duct in the body. This duct drains lymph at the junction between the left internal jugular vein and the left subclavian vein [1, 2]. The thoracic duct presents a high anatomical variability, but it typically arises in the abdomen as cisterna chyli, which is an expanded lymphatic sac that forms at the convergence of the intestinal lymphatic trunk and lumbar lymphatic trunk. The cisterna chyli is at the level of the 12 thoracic vertebrae (T12) [13]. Notably, the cisterna chyli is present in approximately 40–60% of the population and in people without this cisterna the intestinal and lumbar lymphatic trunks communicate directly with the thoracic duct [14]. From the cisterna, the thoracic duct ascends running to the right of the body midline and posterior to the aorta and it enters the thorax via the aortic hiatus. The thoracic duct then rises in the thoracic cavity anteriorly and to the right of the vertebral column, between the aorta and azygous vein. At about the level of the fifth thoracic vertebra (T5), the thoracic duct crosses to the left of the vertebral column and posterior to the esophagus. Finally, it ascends vertically and then releases the drained lymph in the venous circulation [6, 15, 16].

There is so a continuous and dynamic exchanging circulation of extracellular fluid passing back and forth from the bloodstream to the tissues and lymphatic system. The occlusion of the lymphatic vessels downstream may promote the opening of new lympho-venous connections, resulting in anatomo-morphological changes relevant in both health and disease states [1, 5, 12].

2. Conclusion

The lymphatic system has a fundamental role in extracellular fluid drainage, but it has also important immune functions. The lymphatic system has also a critical role in a clinical context, because the lymphatic and vascular systems have numerous connections, and tumor cell metastasis may pass from one circulatory system to the other.

The lymphatic system is a highly complex and dynamic system and the lymphatic system of human anatomy is fundamental to modulate pathological changes which are relatively unknown, but fundamental in clinical practice.

Acknowledgments

The authors sincerely thank Dr. Marzia Gianò (Anatomy and Physiopathology Division, University of Brescia, Italy) for her assistance in drawing the figure.

Conflict of interest

The authors declare no conflict of interest.