Hemostasis
(coagulation of the blood)

Coagulation.

     Hemostasis is the process by which the blood stops bleeding. When you cut yourself, you do not want to bleed to death, your body needs to stop the bleeding. Several cellular and molecular mechanisms are involved in that process. I will only try to summarize the main steps involved in this fairly complex phenomenon.

    When there is damage to a blood vessel, the first stage of hemostasis is a vasospasm. It is a local vasoconstriction (contraction of a blood vessel) to decrease blood flow to the site of injury. This reduces the loss of blood and promotes the accumulation of platelets.

    The platelets stick to the wall of the blood vessel and form a thin membrane that clog the vascular damage. This is called the platelet plug or the white thrombus.

    Then there are fibrin strands that aggregates to the damaged site and traps red and white blood cells as well as many more platelets. And, a cascade of reactions, involving different coagulation factors, polymerizes (crystallizes) these fibrin strands which forms the mature blood clot.

    Finally, when the blood vessel is repaired, there are different enzymes that dissolve the clot allowing normal blood flow.

     The following sections explain in more detail the role of various components and reactions involved in this hemostasis phenomenon.

The blood vessel.

     The blood vessel plays an important role in the phenomenon of hemostasis and maintaining normal blood flow. This section does not include a detailed description of the blood vessel, rather only a description of the components involved in hemostasis.

    As you can see in the diagram of the section of a blood vessel, the innermost layer of the vessel is the endothelium. This is a monolayer, it consists of a single layer of cells, endothelial cells. These cells are involved in maintaining an unsticky environment to the blood flow. They secrete substances, prostacyclin (PGI2) and nitric oxide (NO, or nitrous oxide) which prevent clumping of blood platelets.

    The next layer is the subendothelial layer. It is a layer that contains collagen which will promote platelets aggregation when there is damage to the blood vessel.

    The muscle layer of a blood vessel is also participating to the hemostasis processes. By contacting, it reduces the blood flow at that location which limits blood loss and promotes the accumulation of platelets and other molecules involved in thrombus formation.

Blood platelets.

     Platelets are also known as thrombocytes. They contribute to the formation of the thrombus, or blood clots.

    Platelets are cell fragments. They originate from large white blood cells called megakaryocytes. And, megakaryocytes, like all blood cells, are formed in the bone marrow. These megakaryocytes are so large (100-150μm) they do not pass through all blood vessels. Each of them can give rise to thousands of platelets.

    Platelets are smaller than red blood cells (7μm), they only measure 2 to 4μm of diameter. There could be as many as 150-400 billion per liter of blood, and they could live up to 8 to 10 days. Then, they are degraded in the spleen.

     Platelets, those megakaryocyte fragments, have no nucleus but pocess several other organelles. They have a lipid bilayer membrane, adhesion receptors and fibronectin allowing them to adhere to each other. They also pocess mitochondria which provide energy, dense granules containing calcium, ADP (adenosine diphosphate) and serotonin (5-HT), some alpha granules containing fibrinogen, messengers, coagulation factors and growth factors, lysosomes containing some proteolytic enzymes to digest proteins, and actin and myosin filaments allowing them to stick to each others.

Coagulation factors.

    Coagulation factors, as their name suggests, are molecules involved in blood clotting. With the exception of tissue factor (factor III), which will be released upon damage to the blood vessel, most of these factors are soluble molecules present in the blood stream. You will also notice that most of these factors are produced in the liver, and some are vitamin K dependent. Accordingly, liver problems or deficits in vitamin K often influence blood clotting.

List of factors involved in blood coagulation (taken form Wikipédia).

    These factors are not numbered in order of importance, but in order of their discoveries.

The fibrinogen.

    Fibrinogen is clotting factor I. This is the main molecule involved in the formation of a blood clot.

    Fibrinogen is a dimer protein. It is formed of two strands positioned in opposite direction, and joined at their center by negatively charged amino acids. This molecule is generally soluble in the blood.

    But, under the action of thrombin (factor II), the fibrinogen strands will polymerize and become insoluble.

Hemostasis.

    Hemostasis comprise the set of reactions, occuring after a blood vessel injury, to prevent blood loss and maintain proper flow.

    The first step is a vasospasm. It is a local contraction of smooth muscles of the blood at the point where the damage is. This spasm occurs fairly quickly, within 15-60sec, and can last for 20-30min. Several mediators contribute to this vasospasm: the actual dammage of the smooth muscle, the activity of the nociceptors (the pain receptors) and the release of calcium and serotonin (5-hydroxytryptamine (5-HT)) by the vascular endothelium and platelets.

    By contracting, the blood vessel diameter decreases and this reduces the blood flow, hence the blood loss at this location. This also promotes the accumulation of platelets and other clotting factors.

    The second step, which occurs in less than a minute, is the formation of a platelet plug, also called white thrombus. The platelets adhere to the collagen of the blood vessel wall, and to one another thereby forming a temporary plug which stops the bleeding. This activation of the platelets triggers the release several coagulation factors such as the von Willebrand factors, ADP (adenosine diphosphate) and thromboxane A2 (family of prostaglandins involved in inflammation), all of them triggering further aggregation of platelets.

    Then, the third step is the coagulation per see. This is the stage where the final blood clot is formed. This step takes 3 to 6 min and involves a cascade of activation of several coagulation factors which will result in the polymerisation of the fibrin strands. And within this mesh of fibrin filaments, red and white blood cells will get trapped, helping to provide a better seal of the vascular injury.

    The activation cascade of the coagulation factors is illustrated in the left figure. There are two distinct pathways leading to the formation of the prothrombin activator: an intrinsic pathway which involve molecule dissolved in the blood; and an extrinsic pathway requiring factor from the damaged vessel. These two pathways are usually activated simultaneously and are complementary to each other. Though activation of the intrinsic pathway alone can lead to coagulation (which is the case of the blood coagulating in a test tube). Then, both pathways activate a common set of molecule: the prothrombin is activated to thrombin which polymerize the fibrinogen to insoluble fibrin filaments and activate the factor XIII, which in turn stabilize the blood clot.

    After 30 to 60min, the clot starts contracting or retracting. The actin and myosin filaments within the clot are contracting. This helps bringing toghether the walls of the damaged vessel and promote healing. The platelets also release a growth factor (Platelet-Derived Growth Factor (PDGF)) which stimulates cell proliferation and help rebuilding the blood vessel.

    Finally, after a few days or a week, when the vascular damage is repaired, there is fibrinolysis which is the degradation of the clot. The endothelium secretes a tissue plasminogen activator (t-PA), which activate a plasminogen already present in the clot. The plasminogen forms plasmin which is an enzyme that degrades the fibrin within the blood clot. Note that factor XII could also stimulate the activation of plasminogen and participate in fibrinolysis.

    The blood vessel is now fully restored and blood flow returns to normal.

Some pathologies of the blood hemotasis and their treatments.

    First I want to warn my readers that I am not a medical doctor and that I have no experience with human diseases and their treatments.

    Blood coagulation is usually evaluated by measuring time to prothrombin activation. INR (International Normalized Ratio) is used as a measure of comparison, where the coagulation rate in a reference population is set to 1.0. Thus, a patient presenting an INR of 2.0 to 3.0 means that the blood takes 2 to 3 times longer to clot compared to that normal population. These are the values ​​sought by anticoagulotherapies to prevent thrombotic embolism.