The innermost lining of simple squamous epithelium is called the endothelium and is here represented as a thin line. These cells produce several compounds involved in hemostasis.
Prostaglandin I2, PGI2, also called prostacyclin, is secreted (block arrow) by the endothelium. It repels platelets (dotted ovals) as implied by the dashed arrows. Heparin is membrane bound and reduces (dashed arrow) the level of calcium ions (Ca++) by chelation. It also increases the activity of antithrombin III 1000 times. Antithrombin III inactivates (dashed arrow) thrombin, a key component in clot formation.
Tissue plasminogen activator, TPA is secreted (block arrow) by the endothelium when thrombin is present (solid arrow). TPA is an enzyme that converts plasmininogen, which is inactive, into plasmin, which is active. Plasmin is involved in dissolving (dashed arrow) the fibrin in clots.
Collagen fibers are present in the areolar tissue just beneath the endothelium. They are represented as a layer of curlicues in the map. They play a major role in forming the platelet plug explained in the next tutorial.
Vascular smooth muscle is represented by the zig-zag line. The cells are in a circular arrangement around the vessel so that when they contract the vessel narrows. They are responsible for the 'vascular spasm' that immediately occurs in regions where the vessel is damaged.
Last update: 7/19/2005