The Role of Blood Flow and Blood Vessels in Haemostasis

The normal flow of blood is smooth and without turbulence. This is referred to as laminar flow. If blood flow becomes turbulent, this can trigger clotting. Under physiological circumstances turbulence will occur if a blood vessel is punctured or severed. Slow blood flow can also activate coagulation.

The endothelial cells which line blood vessels are not passive participants in the passage of blood along the vessel. As well as having a role in regulating the tone of the underlying smooth muscle and therefore the diamter of the blood vessel and the blood flow they are crucial components of the blood clotting system, as well as having a role in the process of inflammation.

Normal endothelium provides a smooth lining to the wall of the blood vessel and thus facilitate laminar flow. Furthermore, under normal circumstances the endothlelial cells of an intact blood vessel secrete prostacyclin (PGI2). Prostacyclin is a vasodilator and an anticoagulant. Its secretion under normal circumstances makes sense: the vessel's diameter is maintained and the clotting cascade is damped down to prevent overenthusiastic activation.

If a blood vessel is breached the endothelial cells will be damaged in the vicinity of the injury. These endothelial cells decrease the synthesis of prostacyclin and instead release endothelin-1, which is a vasoconstrictor. Vasoconstriction adopts a fundamental approach to limiting blood loss by curtailing blood flow to the breach.

The damaged endothelial cells also express von Willebrand factor (vWF). Platelets bind to vn Willebrand factor and the endothelial cells thus help to anchor the platelets exactly where they are needed.

The endothelial cells separate the contents of the blood from the underlying tissue. The underlying tissue expresses the imaginatively named tissue factor. Tissue factor can activate platelets and the coagulation cascade. If a blood vessel is damaged the endothelial cell barrier is lost and the blood is exposed to tissue factor, as well as collagen. Collagen can also initiate clotting.