Overview of the Vascular System

The innermost wall of all blood and lymph vessels comprises a single layer of endothelial cells that regulate the passage of molecules in and out of the fluid.

These cells are polarized in the direction of blood flow and separated from the outer layers by the basal lamina.

Endothelial cells and basal lamina form primary blood vessels that mature into arteries, capillaries, and veins.

Endothelial cells have distinct basal and apical surfaces. While the basal surface connects to the basal lamina, the apical surface is coated with glycocalyx. The Glycocalyx forms a selective, mesh-like barrier for the solutes and fluids passing across the endothelial lining.

Properties of endothelial cells, such as their shape, glycocalyx composition, and  presence of intercellular junctions vary depending on their function.

For example, junction proteins connect adjacent blood endothelial cells without gaps to form zipper-like junctions and prevent interstitial fluid leakage.

In contrast, some lymphatic vessels may have a discontinuous endothelium, where the interjunctional gaps between the cells allow draining of lymph, immune cells, and macromolecules.