All the components necessary for the clotting process to proceed are found in the blood. As such, the proteins required for such clotting to take place are part of the intrinsic pathway of blood coagulation. This pathway involves a series of proteins, protein cofactors, and enzymes, which interact in reactions that take place on membrane surfaces. These reactions are initiated by tissue injury and result in the formation of a fibrin clot (Figure 1).

The intrinsic pathway is initiated by the activation of factor XII by certain negatively charged surfaces, including glass. High-molecular-weight kininogen and prekallikrein are two proteins that facilitate this activation. The enzyme form of factor XII (factor XIIa) catalyzes the conversion of factor XI to its enzyme form (factor XIa). Factor XIa catalyzes the conversion of factor IX to the activated form, factor IXa, in a reaction that requires calcium ions. Factor IXa assembles on the surface of membranes in complex with factor VIII; the factor IXa–factor VIII complex requires calcium to stabilize certain structures on these proteins associated with their membrane-binding properties. Factor X binds to the factor IXa–factor VIII complex and is activated to factor Xa. Factor Xa forms a complex with factor V on membrane surfaces in a reaction that also requires calcium ions. Prothrombin binds to the factor Xa–factor V complex and is converted to thrombin, a potent enzyme that cleaves fibrinogen to fibrin, a monomer. The monomer fibrin molecules then link together (polymerize) to form long fibres. Later, additional bonding between the units of the polymer is promoted by an enzyme known as factor XIIIa, which stabilizes the newly formed clot by cross-linkages. Although the detailed mechanisms are not known, this cascade, or waterfall, effect offers the possibility for amplification of a small signal associated with tissue injury into a major biologic event—the formation of a fibrin clot. Furthermore, careful regulation of this system is possible with the participation of two protein cofactors, factor VIII and factor V.

Certain negatively charged surfaces, including glass, kaolin, some synthetic plastics, and fabrics, activate factor XII to its enzyme form, factor XIIa. In contrast, certain materials have little tendency to activate factor XII. Inactive surfaces include some oils, waxes, resins, silicones, a few plastics, and endothelial cells, the most inert surface of all. The physicochemical properties that determine activity are not known. The problem is important, for modern surgery requires a perfectly inactive material to make substitutes (prostheses) for heart valves and sections of blood vessels. The formation of clots (thrombi) on these surfaces can lead to serious or even fatal complications. Open-heart surgery requires pumping of blood through equipment that does not activate the blood-clotting process significantly. Similarly, blood filtration of waste products during kidney dialysis must not lead to the generation of fibrin clots. To minimize the activation of blood coagulation when blood flows over foreign surfaces, special drugs (anticoagulants) such as heparin are employed.

The activity of the intrinsic pathway may be assessed in a simple laboratory test called the partial thromboplastin time (PTT), or, more accurately, the activated partial thromboplastin time. Plasma is collected and anticoagulated with citrate buffer; the citrate binds and effectively removes functional calcium ions from the plasma. Under these conditions, a fibrin clot cannot be generated. A negatively charged material, such as the diatomaceous material kaolin, is added to the plasma. Kaolin activates factor XII to its enzyme form, factor XIIa, which then activates factor XI. The process is blocked from further activation because of the lack of calcium ions, which are required for the next reaction, the activation of factor IX. Upon the addition of calcium ions and a phospholipid preparation (which serves as an artificial membrane for the assembly of the blood-clotting protein complexes), the duration of time is recorded until a visible clot is formed. This reaction takes place in a matter of 25 to 50 seconds, depending upon the formulation of chemicals used. In practice, the clotting time of a test plasma is compared to the clotting time of normal plasma. Delayed clotting, measured as a prolonged partial thromboplastin time, may be due to a deficiency in the activity of one or more of the blood-clotting factors or to a chemical inhibitor of blood coagulation.