Page 418 - Atlas of Histology with Functional Correlations

smooth muscle fibers in smaller muscular arteries or arterioles regulate

blood flow into the capillary beds.

Terminal arterioles give rise to the smallest blood vessels, the
capillaries. Because of their very thin walls, capillaries are sites for the
exchange of gases, metabolites, nutrients, and waste products between blood
and interstitial tissues.

Lymphatic Vessels

The main function of the lymphatic vascular system is to collect excess
tissue fluid and proteins, called lymph, from the intercellular spaces of the
connective tissue and return it into the venous blood vascular system. Lymph
is a clear fluid and an ultrafiltrate of the blood plasma. Numerous lymph

nodes are located along the route of the lymph vessels. In the maze of lymph
node channels, the collected lymph is filtered of cells and particulate matter.
Lymph that flows through the lymph nodes is also exposed to the numerous
macrophages that reside here. These engulf foreign microorganisms and

other suspended matter. The lymph vessels also bring to the systemic
bloodstream lymphocytes, fatty acids absorbed through the capillary lymph
vessels called lacteals in the small intestine, and immunoglobulins
(antibodies) produced in the lymph nodes. Thus, the lymphatic vessels serve

as an important component of the immune system of the body.

Endothelium

The endothelium lining the lumina of blood vessels performs numerous
physiologic, metabolic, and secretory functions. The endothelial cells form a
semipermeable barrier between blood and the interstitial tissue. The cells

are anchored to the basal lamina and attached to each other by adhesion
junctions. The presence of pinocytotic vesicles in the endothelial cells
indicates a bidirectional movement of molecules between blood and tissues.
The smooth lining of the endothelium in the blood vessels and the secretion
of anticoagulants by the endothelial cells prevent blood clotting. The

endothelium surface is also lined by glycocalyx protein. In addition,
endothelial cells secrete prostacyclin, an antithrombotic substance that
prevents platelet adhesion in the blood vessels and blood clot formation.

Endothelial cells also produce vasoactive chemicals nitrous oxide and its

related compounds to induce vasodilation and increase blood flow.

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