FIGURE 12.

Schematic overview of the lymphatic system in peripheral tissues. The lymphatic vascular tree is composed of several distinct segments. I: blind lymphatic capillaries, for which the primary function is the uptake of excess extracellular fluid. The capillaries have no smooth muscle cells, and their fluid uptake is believed to rely on the unique valve-like microstructural organization of endothelial cells’ flaps and button. A magnified illustration of flaps extending from the borders of the lymphatic endothelial cell. The flaps lack cell adhesions at their tips and are only anchored on the sides by buttons (red), which are cell-cell junctions created by cadherin and tight junction proteins. Uptake of fluid and solutes is unidirectional, and efflux cannot occur because any increase in the intraluminal back-pressure will close the flaps. Also, the basal lamina surrounding lymphatic capillaries is incomplete, facilitating uptake of larger structures, such as immune cells. II: lymphatic collectors are surrounded by smooth muscle cells that are in part oriented along the lymphatic vessel wall rather than encircling the vessel as in blood vessels. They are also endowed with valves that open in response to an increase in pressure created by contraction of the smooth muscle cells. The segment between two valves is called a lymphangion. Intrinsic properties of the muscle cells, the fluid load, and the autonomic nervous system regulate lymph vessel contractility. III: lymph nodes receive lymph from large prenodal collecting lymph vessels. Lymph within the node is transported from the afferent to the efferent lymphatics via a peripheral path (blue arrows pointing to flow from the subcapsular to the medullary sinus) or a central path (flow from the subcapsular sinus to the centrally placed B-cell follicles and T-cell cortex, not shown). IV: postnodal collecting lymphatic channel is shown. Approximately 50% of lymph fluid is transferred to the venous compartment within the node, whereas little exchange of proteins take place at this site. Thus most proteins entering the node, and ∼50% of the total lymph production, exit via the postnodal collecting lymphatic channels and are returned to the venous circulation by the thoracic or right lymphatic ducts (not shown) (49).