Endothelium - Role in Hemostasis
The main reason the blood in the vascular system stays fluid is the presence of endothelium. Twenty years ago, it was assumed that endothelium was just a passive barrier between the blood and the interstitium. Now we know that endothelium is actively involved in a large number of physiological and pathological processes.
The endothelium is an active participant in hemostasis. Primarily, it works to decrease thrombosis. But if it is damaged, it promotes thrombosis through exposure of blood to tissue factor.
Here are the factors that the endothelial cell produces and how those factors have a role in anticoagulation:

Prostacyclin, also known as Prostaglandin I2 (PGI2), is constantly produced by endothelial cells. This prostaglandin works to keep vessels open (vasodilation).

It also prevents platelet aggregation by acting on platelets to increase cyclic AMP (cAMP) that effectively reduces the amount of thromboxane A2 (which is made by platelets and which we will cover shortly) which has procoagulant activities.

What happens if the endothelial cell is damaged?

When the platelets contact the collagen, they produce ADP, which further promotes platelet aggregation, as pictured in the diagram at the left.

The endothelium is capable of producing adenosine diphosphatase that degrades ADP.

The endothelium produces nitric oxide in response to ADP from activated platelets. This nitric oxide works to keep the vessel open (vasodilation), which decreases the probability of clotting.

Protein C is released from the endothelium in response to thrombin (a very important molecule in the whole clotting thang and we'll hear about thrombin very soon). This Protein C acts to inactive some of the clotting factors.

A heparin-like substance is produced on the surface of endothelial cells that acts to inactivate thrombin.

Endothelial cells act to help clot dissolution by releasing plasminogen activators. Plasminogen is a key molecule involved in breaking clots down after they have formed.

Okay, for review, here are the products of endothelial cells - How does each work to decrease thrombosis?

  • prostacyclin
  • adenosine diphosphatase
  • nitric oxide
  • Protein C
  • heparin-like substance
  • plasminogen activators

Think then click here.

Self Study Quiz

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