Anatomy and Structural Physiology of the Lymphatic System

Introduction

A better understanding of the anatomy of the lymphatics equips practitioners with the fundamental information required to accurately diagnose lymphedema and select the appropriate surgical option for each patient. Therefore, surgeons working with the lymphatic system need to understand normal lymphatic anatomy, as well as the changes that occur in lymphedema.

The lymphatic system is distinctly different from the vascular system, and our conventional knowledge about the vascular system does not help to understand it. For example, the lymphatic system does not have a pumping organ like a heart. Rather, lymph fluid is propelled extrinsically by muscle movement and intrinsically by peristaltic contraction of the lymph collecting vessels. Blood is circulated through the arterial and venous systems and perfused throughout the body, whereas lymph fluid is produced in the peripheral tissues and transferred via the lymphatic system to return to the blood circulatory system.

Starling’s Law is the basic principle governing the regulation of fluid exchange in the body. The traditional Starling’s Law was recently modified due to new understanding of the function of the endothelial glyocalyx layer. The revised Starling Equation Law suggests that filtered fluid returns to the circulatory system primarily via the lymphatics because there is no reabsorption possible through the blood capillaries and venules. Hence, the main cause of tissue edema is considered to be a result of lymphatic dysfunction.

Normal anatomy of the lymphatics has been investigated over three centuries since the development of the Mercury method in the 17th century. Its use in anatomical preparations and its perception as an anatomical material evolved with the understanding of the circulatory and lymphatic systems. However, mercury was later abandoned for use in anatomical studies due to its toxicity. The author (HS) developed the microinjection technique to demonstrate the lymphatics in cadaveric specimens. This method uses hydrogen peroxide to inflate the lymphatic vessels to identify them, after which a fine needle is cannulated into the vessels and a dye or radiocontrast medium is injected to allow their visualization. In recent times, indocyanine green (ICG) fluorescent lymphography has been used to demonstrate the lymphatics not only in patients with lymphedema but also in cadavers. This new imaging method provides increased imaging data and promotes further understanding about the anatomical changes that occur in lymphedema. Based on the authors’ anatomical findings, ICG lymphography data, and archival diagrams from historical materials, we proposed a new anatomical concept called the “lymphosome,” which demarcates the skin into separate lymphatic territories. Each lymphatic territory is defined by the notion that several superficial lymphatic vessels form a group that connects to first-tier (sentinel) lymph nodes ( Fig. 2.1 ).

An understanding of normal lymphatic anatomy is vitally important for practitioners because it provides the baseline imaging data required to distinguish the altered anatomical structures that occur in lymphedema. However, normal anatomy itself is not enough to interpret imaging data in lymphedema because other factors such as lymphangiogenesis, dermal reflux (dermal backflow), and other structural changes are always associated with lymphedema. The lymphatic anatomy changes after lymph node dissection, and the body attempts to maintain lymph drainage in several different ways. The anatomical changes do not occur only at the surgical site but may also occur in any areas downstream of the lymphatic vessels that connect to the removed nodes. In this chapter, we describe normal lymphatic anatomy in the limbs and introduce the anatomical changes that occur in lymphedema.