Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Cardiovascular system
Lymphatic system
Lymph drainage pathways

Lymph drainage pathways include lymphatic capillaries, vessels, and ducts.

Lymphatic capillaries begin blindly in Tissues, where they form a network similar to Blood capillaries. The wall of lymphatic capillaries consists of a single layer of endothelial Cells with large pores between them. Their basement membrane is discontinuous or entirely absent, and pericytes are also lacking. Bundles of Collagen fibers extending into the surrounding Connective Tissue attach to the outer surface of the capillary. Their role appears to be keeping the capillary open when there is excess interstitial fluid (during edema). Otherwise, under the pressure of surrounding tissues, they would collapse, preventing the drainage of excess fluid. As edema develops, tissues swell, the fibers "pull apart" the walls of the lymphatic capillaries, the pores between endothelial cells widen, and excess interstitial fluid enters them. Lymphatic capillaries are more permeable than blood capillaries. Lymph removes Proteins that have leaked from the bloodstream, Lipids (in the intestine), Bacteria, malignant tumor cells, and other substances from the intercellular spaces. Thus, lymph Functions to remove excess interstitial fluid, return proteins to the bloodstream, transport nutrients and Hormones, and cleanse tissues of bacteria, their metabolic products, and altered cells. An adult human produces 2—4 liters of lymph per day.

Lymphatic capillaries have a larger diameter than blood capillaries. They permeate almost all tissues, but their density varies across different Organs. They are particularly well-developed in the Lungs, Liver, Kidneys, and serous, mucous, and synovial membranes. Lymphatic capillaries are absent where there are no Blood Vessels—in the Teeth, Cartilage, cornea, and lens of the eye; they are also absent in the Central Nervous system. The latter contains no lymph, and interstitial fluid is replaced by CEREBROSPINAL FLUID. Heart Valves, the Placenta, the umbilical cord, and bones are also devoid of lymphatic capillaries.

Lymphatic capillaries merge into small Lymphatic vessels, which gradually enlarge as they fuse. Lymphatic vessels that collect lymph from capillaries and carry it to larger vessels typically run through tissues alongside a vein and an accompanying artery. However, unlike Veins, lymphatic vessels do not show such a clear tendency to merge. Therefore, several lymphatic vessels often run alongside a single vein and artery. Larger lymphatic vessels accompany neurovascular bundles.

The wall of lymphatic vessels consists of three layers: inner, middle, and outer. In the walls of small vessels, these layers are less distinct. The inner layer is composed of endothelial cells. Elastic fibers are typically found here. The middle layer is formed by smooth Muscle cells arranged circularly or spirally. The myocytes are surrounded by a small amount of connective tissue. The outer layer is usually well-developed and contains longitudinally and obliquely oriented smooth muscle cells. Small blood vessels are found in the outer layer of the walls of large vessels.

Except for the smallest ones, lymphatic vessels are typically equipped with valves. They are more numerous and spaced closer together than venous valves. At the sites of these valves, the vessel narrows sharply, giving them a beaded appearance (Atl. Fig. 83). Lymphatic valves usually have two cusps; they consist of folds of the inner lining with a core of connective tissue in the middle and are lined with endothelium. The valves prevent the backflow of lymph. In the area of valve attachment, Muscle tissue is poorly developed, whereas an accumulation of myocytes is observed between two adjacent valves. Upon contraction, they propel lymph into the next segment of the vessel. The rate of these contractions is 8—10 per minute. Skeletal Muscle contractions, Movements of the digestive tract, pulsation of the aorta and large Arteries, and respiratory movements play a major role in propelling lymph. The latter cause the Thoracic duct to expand during inhalation and compress during exhalation. Thus, the thoracic cavity acts as a pump. The rate of lymph flow varies between fasting and after eating or drinking, and depends on the rate of Lymph formation.

Within the muscular wall of lymphatic vessels, there are plexuses of the Autonomic nervous system, which also encompass nearby blood vessels. Stimulation of sympathetic nerves causes contraction of myocytes in the walls of lymphatic vessels, while parasympathetic stimulation can cause both contraction and relaxation, depending on the initial vascular tone. Large Vessels of the limbs are innervated by sympathetic nerves, whereas the thoracic duct and mesenteric vessels have dual innervation.

All lymphatic vessels eventually drain into the thoracic duct and the Right lymphatic duct. Their walls are similar in Structure to those of veins. The thoracic duct (ductus thoracicus) is larger and typically (in 75% of people) begins with a dilation—the cisterna, located in the transition zone between the thoracic and lumbar spine (Atl. Fig. 82). The left and right lumbar lymphatic trunks, which collect lymph from the pelvic walls and lower limbs, as well as one or more intestinal trunks, empty into the cisterna. The thoracic duct ascends along the aorta, passes with it through the Diaphragm, and runs through the Mediastinum, shifting to the left to reach the left venous angle, where it empties. Along its course, the thoracic duct receives lymphatic vessels from the walls and Organs of the left half of the thoracic cavity, the left upper limb, and the left side of the HEAD and Neck.

The right lymphatic duct (ductus lymphaticus dexter), no more than 1.5 cm in length, forms near the right venous angle, into which it empties. The duct collects lymph from the walls and organs of the right half of the thoracic cavity, the right upper limb, and the right side of the head and neck. Often, this duct is absent, in which case the lymphatic trunks from each of these regions empty directly into the subclavian vein.

Thus, The Lymphatic system returns fluid that entered the tissues from blood capillaries—along with its proteins, salts, and nutrients—as well as tissue waste products, back into the bloodstream.