Factor XII is most commonly activated by exposure to which structure?

Factor XII, also known as Hageman factor, is a crucial component of the intrinsic pathway of blood coagulation. Its activation is primarily triggered by contact with negatively charged surfaces. The subendothelium, which refers to the layer of tissue beneath the endothelial cells lining the blood vessels, has such negatively charged structures. When blood comes into contact with the subendothelium, particularly during vascular injury, Factor XII interacts with these surfaces and undergoes a conformational change that activates it.

This process is vital in initiating the blood coagulation cascade, leading to the formation of a fibrin clot. Subendothelial structures, such as collagen and other extracellular matrix components, play a key role in this mechanism by providing the necessary surface for Factor XII to bind and be activated, thus serving as an essential trigger in hemostatic responses.

In contrast, plasma primarily serves as the medium for various proteins and factors involved in coagulation but does not specifically activate Factor XII. Platelets are important in coagulation but do not directly activate Factor XII; they play a more significant role downstream in the coagulation process. Fibrinogen, while an essential part of the clotting mechanism, is a substrate for thrombin and does not activate Factor XII either