H The Lymphatic System and Lymph Nodes
The lymphatic system comprises an extensive vascular network that drains fluid, called lymph, from bodily tissues and returns it to the venous circulation. The system starts peripherally as blind lymphatic capillaries, and continues centrally as thin vascular vessels and then collecting ducts that finally empty into major veins at the root of the neck. The lymph transported in these channels is filtered through lymph nodes that are interposed along the way.
Lymph nodes are round, oval, or bean-shaped structures that vary in size according to their location. Some lymph nodes, such as the preauriculars, if palpable at all, are typically very small. The inguinal nodes, in contrast, are relatively larger—often 1 cm in diameter and occasionally even 2 cm in an adult.
In addition to its vascular functions, the lymphatic system plays an important role in the body's immune system. Cells within the lymph nodes engulf cellular debris and bacteria and produce antibodies.
Only the superficial lymph nodes are accessible to physical examination. These include the cervical nodes (p. 133), the axillary nodes (p. 300), and nodes in the arms and legs.
Lateral axillary nodes
Recall that the axillary lymph nodes drain most of the arm. Lymphatics from the ulnar surface of the forearm and hand, the little and ring fingers, and the adjacent surface of the middle finger, however, drain first into the epitrochlear nodes. These are located on the medial surface of the arm about 3 cm above the elbow. Lymphatics from the rest of the arm drain mostly into the axillary nodes. A few may go directly to the infraclaviculars.
The lymphatics of the lower limb, following the venous supply, consist of both deep and superficial systems. Only the superficial nodes are palpable. The superficial inguinal nodes include two groups. The horizontal group lies in a chain high in the anterior thigh below the inguinal ligament. It drains the superficial portions of the lower abdomen and buttock, the external genitalia (but not the testes), the anal canal and perianal area, and the lower vagina.
The vertical group clusters near the upper part of the saphenous vein and drains a corresponding region of the leg. In contrast, lymphatics from the portion of leg drained by the small saphenous vein (the heel and outer aspect of the foot) join the deep system at the level of the popliteal space. Lesions in this area, therefore, are not usually associated with palpable inguinal lymph nodes.
H Fluid Exchange and the Capillary Bed_
Blood circulates from arteries to veins through the capillary bed. Here fluids diffuse across the capillary membrane, maintaining a dynamic equilibrium between the vascular and interstitial spaces. Blood pressure (hydrostatic pressure) within the capillary bed, especially near the arteriolar end, forces fluid out into the tissue spaces. In effecting this movement, it is aided by the relatively weak osmotic attraction of proteins within the tissues (interstitial colloid oncotic pressure) and is opposed by the hydrostatic pressure of the tissues.
As blood continues through the capillary bed toward the venous end its hydrostatic pressure falls, and another force gains dominance. This is the colloid oncotic pressure of plasma proteins, which pulls fluid back into the vascular tree. Net flow of fluid, which was directed outward on the arteriolar side of the capillary bed, reverses itself and turns inward on the venous side. Lymphatic capillaries, which also play an important role in this equilibrium, remove excessive fluid, including protein, from the interstitial space.
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