The clotting mechanism is broken into 2 stages:[4][5][6]

1. Primary hemostasis: Formation of a weak platelet plug
2. Secondary hemostasis: Stabilizing the weak platelet plug into a clot by the fibrin network

Primary Hemostasis 

Primary hemostasis is the formation of a weak platelet plug which is achieved in four phases: vasoconstriction, platelet adhesion, platelet activation, and platelet aggregation.

Vasoconstriction is the initial response whenever there is vessel injury. Vasospasm of the blood vessels occurs first in response to injury of the vasculature. This vasospasm, in turn, stimulates vasoconstriction. Vasoconstriction is primarily mediated by endothelin-1, a potent vasoconstrictor, which is synthesized by the damaged endothelium. Damaged endothelium exposes sub-endothelial collagen, von Willebrand factor (vWF), releases ATP, and inflammatory mediators. vWF is synthesized by megakaryocytes which later gets stored in a-granules of platelets. Weibel-Palade bodies of the endothelium also synthesize vWF. It is the combination of exposure of vWF, subendothelial collagen, ATP, and inflammatory mediators which provide the gateway into the second phase of primary hemostasis, platelet adhesion.

Platelet adhesion is the process by which platelets attach to the exposed subendothelial vWF. Post vascular damage, platelets begin to roll along vessel walls and adhere to areas of exposed subendothelial collagen and vWF. Platelet membranes are rich in G protein (Gp) receptors located within the phospholipid bilayer. Specifically, it is Gp Ib-IX receptor on platelets that bind to vWF within the endothelium that creates the initial connection between the two. Once bound, a variety of events can occur in the third phase of primary hemostasis to activate the platelet.

Platelet activation consists of platelets undergoing two specific events once they have adhered to the exposed vWF (i.e. the damaged vessel site). First, platelets will undergo an irreversible change in shape from smooth discs to multi-pseudopodal plugs, which greatly increases their surface area. Second, platelets secrete their cytoplasmic granules.

Platelet activation is mediated via thrombin by two mechanisms. Thrombin directly activates platelets via proteolytic cleavage by binding the protease-activated receptor. Thrombin also stimulates platelet granule release which includes serotonin, platelet activating factor, and Adenosine Diphosphate (ADP). ADP is an important physiological agonist which is stored specifically in the dense granules of platelets. When ADP is released, it binds to P2Y1 and P2Y12 receptors on platelet membranes. P2Y1 induces the pseudopod shape change and aids in platelet aggregation. P2Y12 plays a major role in inducing the clotting cascade. When ADP binds to its receptors, it induces Gp IIb/IIIa complex expression at the platelet membrane surface. The Gp IIb/IIIa complex is a calcium-dependent collagen receptor which is necessary for platelet-to-endothelial adherence and platelet-to-platelet aggregation. Simultaneously, platelets synthesize Thromboxane A2 (TXA2). TXA2 further intensifies vasoconstriction and platelet aggregation (next step in the primary hemostasis process). The process of platelet activation readies the local environment for platelet aggregation.

Platelet aggregation begins once platelets have been activated. Once activated, the Gp IIb/IIIa receptors adhere to vWF and fibrinogen. Fibrinogen is found in the circulation and forms a connection between the Gp IIb/IIIa receptors of platelets to interconnect them with each other. This ultimately forms the weak platelet plug.

Ultimately, primary hemostasis allows the culmination of a weak platelet plug to temporarily protect from hemorrhage until further stabilization of fibrinogen to fibrin via thrombin occurs in secondary hemostasis.

Secondary Hemostasis

Secondary hemostasis involves the clotting factors acting in a cascade to ultimately stabilize the weak platelet plug. This is accomplished by completing three tasks: (1) triggering activation of clotting factors, (2) conversion of prothrombin to thrombin, and (3) conversion of fibrinogen to fibrin.  These tasks are accomplished initially by 1 of 2 pathways; the extrinsic and intrinsic pathway, which converge at the activation of factor X and then complete their tasks via the common pathway. Please note that calcium ions are required for the entire process of secondary hemostasis. 

The extrinsic pathway includes tissue factor (TF) and factor VII (FVII). It is initiated when TF binds to FVII, activating FVII to factor VIIa (FVIIa), forming a TF-FVIIa complex. This complex, in turn, activates factor X (FX). Note, the TF-FVIIa complex can also activate factor IX of the intrinsic pathway, which is called the alternate pathway. Once Factor X is activated to FXa by TF-FVIIa complex, the cascade continues down the common pathway (see below).

The intrinsic pathway includes Hageman factor (FXII), factor I (FXI), factor IX (FIX), and factor VIII (FVIII). The process is initiated when FXII comes into contact with exposed subendothelial collagen and becomes activated to FXIIa. Subsequently, FXIIa activates FXI to FXIa, and FXIa activates FIX to FIXa. FIXa works in combination with activated factor VIII (FVIIIa) to activate factor X. Once Factor X is activated by FIXa-FVIIIa complex, the cascade continues down the common pathway (see below).

The common pathway is initiated via the activation of Factor Xa. Factor Xa combines with Factor Va and calcium on phospholipid surfaces to create a prothrombinase complex ultimately activating prothrombin (aka Factor II) into thrombin. This activation of thrombin occurs via serine protease cleaving of prothrombin. Now, thrombin activates factor XIIIa (FXIIIa). FXIIIa crosslinks with fibrin forming the stabilized clot.

Thrombosis is the process of blood clot (thrombus) formation in a blood vessel. Virchow triad is an important concept that highlights the primary abnormalities in pathology that can lead to the clotting mechanism proceeding to thrombosis. The triad is composed of stasis or turbulent blood flow, endothelial injury, and hypercoagulability of the blood. [7][8][9][10]

1. Abnormal (stasis) or turbulent blood flow can lead to thrombosis. Normal blood flow is laminar. Turbulent blood flow leads to endothelial injury thus promoting the formation of a thrombus. An example of turbulent blood flow is in the aneurysm of weakened vessels. Another aspect of abnormal blood flow, venous stasis, such as in post-operative bed rest, long distance traveling in a car or plane, or immobility due to obesity can lead to endothelial injury thus promoting thrombosis.
2. Endothelial Injury leads to platelet activation and the formation of a thrombus. This may be a result of inflammation of the endothelial surface of the vasculature. Hypercholesterolemia is an example of a chronic inflammatory condition which progresses into endothelial injury.
3. Hypercoagulability (thrombophilia) is any disorder of the blood that predisposes a person to thrombosis. This may be a result of inherited clotting disorders such as a Factor V Leiden mutation or an acquired clotting disorder such as disseminated intravascular coagulation.

Hemorrhage occurs when blood escapes from its vessel walls.

Platelet dysfunction, or clotting factor dysfunction, can be further broken down into which part of the clotting mechanism physiology is affected.

Disorders of Primary Hemostasis: vWF, Platelet defects, or Receptor Interference

• Von Willebrand Factor disease
• Bernard-Soulier disease
• Glanzmann thrombasthenia
• Medication-induced

Disorders of Secondary Hemostasis: Clotting Factor Defects

• Factor V Leiden
• Vitamin K deficiency
• Hemophilia
• Anti-phospholipid antibody syndrome
• Disseminated intravascular coagulation
• Liver disease
• Medication-induced

Defects in Small Vessels

• Trauma
• Aneurysm rupture
• Vasculitides

In addition to the pathophysiology, a few ideas to keep in mind when you have a patient with clotting mechanism disorders:

Patients with:

• Primary hemostasis defects typically present with small bleeds in the skin or mucosal membranes. This includes petechiae and/or purpura.
• Secondary hemostasis defects typically present with bleeds into soft tissue (muscle) or joints (hemarthrosis).
• Direct defects in small blood vessels typically present with palpable purpura and ecchymosis. These may collect and become larger to develop a hematoma. 

Also, laboratory testing involving PTT or PT/INR can be divided by the physiological mechanisms:

• Disorders exclusively effecting primary hemostasis do not affect the PT/INR or PTT, they only increase bleeding time
• Disorders that affect the extrinsic pathway of secondary hemostasis affect the PT/INR
• Disorders that affect the intrinsic pathway of secondary hemostasis affect the PTT