The multistep process of blood clot formation to stop bleeding is called coagulation. Blood coagulation is a process that changes circulating substances within the blood into an insoluble gel. The gel plugs leaks in blood vessels and stops the loss of blood. The process requires coagulation factors, calcium and phospholipids.

How a blood clot is made

The coagulation cascade is a complex chemical process that uses as many as 10 different proteins (called blood clotting factors or coagulation factors) found in plasma in the blood. Put simply, the clotting process changes blood from a liquid to a solid at the site of an injury.

Injury

A small tear in a blood vessel wall (for example, from a cut on the skin or an internal injury) causes bleeding.

Vessel constriction

To control blood loss the blood vessel narrows (called constriction), thus limiting blood flow through the vessel.

Platelet plug

In response to the injury, tiny cells in the blood called platelets are activated. The platelets stick to one another and to the wound site to form a plug. The protein von Willebrand factor (VWF) helps the platelets stick to each other and to the blood vessel wall.

Fibrin clot

Next, clotting factor proteins trigger production of fibrin, a strong, strand-like substance that forms a fibrin clot, a mesh-like net that keeps the plug firm and stable. Over the next several days to weeks, the clot strengthens and then dissolves as the wounded blood vessel wall heals.

Blood Coagulation Factors

The coagulation factors (proteins) are manufactured by the liver.
Ionized calcium (Ca++) is available in the blood and from intracellular sources.
Phospholipids are prominent components of cellular and platelet membranes. They provide a surface upon which the chemical reactions of coagulation can take place.

Coagulation can be initiated by either of two distinct pathways

The Intrinsic pathway can be initiated by events that take place within the lumen of blood vessels. The Intrinsic pathway requires only elements (clotting factors, Ca++, platelet surface etc.) found within, or intrinsic to the vascular system.
The Extrinsic pathway is the other route to coagulation. It requires Tissue Factor (tissue thromboplastin), a substance which is “extrinsic to”, or not normally circulating in the vessel. Tissue Factor is released when the vessel wall is ruptured.

Regardless of whether the Extrinsic or Intrinsic pathway starts coagulation, completion of the process follows a common pathway. The common pathway involves the activation of factors: X, V, II, XIII and I.

The coagulation factors are numbered in the order of their discovery. There are 13 numerals but only 12 factors. Factor VI was subsequently found to be part of another factor. The following are coagulation factors and their common names:

Factor I – Fibrinogen
Factor II – Prothrombin
Factor III – Tissue thromboplastin (tissue factor)
Factor IV – Ionized calcium (Ca++)
Factor V – Labile factor or proaccelerin
Factor VI – Unassigned
Factor VII – Stable factor or proconvertin
Factor VIII – Antihemophilic factor
Factor IX – Plasma thromboplastin component, Christmas factor
Factor X – Stuart-Prower factor
Factor XI – Plasma thromboplastin antecedent
Factor XII – Hageman factor
Factor XIII – Fibrin-stabilizing factor

The liver must be able to use Vitamin K to produce Factors II, VII, IX, and X.

Previous Topic

Back to Lecture

Next Topic