Which coagulation factor is not in circulating form in blood?
**Core Concept**
The coagulation cascade involves a series of enzymatic reactions that ultimately lead to the formation of a fibrin clot. These reactions are catalyzed by various coagulation factors, which can be present in either the intrinsic or extrinsic pathway. Some of these factors are synthesized in the liver and secreted into the bloodstream, while others are synthesized in the endothelial cells or platelets and remain bound to these cells.
**Why the Correct Answer is Right**
Factor VII is synthesized in the liver and secreted into the bloodstream in a circulating form. However, factors V and VIII are synthesized in the endothelial cells and platelets, respectively, and are not found in free, circulating forms in the blood. Instead, they are bound to von Willebrand factor (VWF) and released from these cells as a complex. Factor V is also synthesized in the liver, but it is bound to lipoproteins and other proteins in the bloodstream.
**Why Each Wrong Option is Incorrect**
**Option A:** Factor VII is indeed a circulating coagulation factor, synthesized in the liver and secreted into the bloodstream.
**Option B:** Factor V is actually found in circulating form in the blood, bound to lipoproteins and other proteins.
**Option C:** Factor VIII is not found in free, circulating form in the blood, but is instead bound to VWF and released from endothelial cells.
**Clinical Pearl / High-Yield Fact**
Von Willebrand factor (VWF) plays a critical role in the regulation of coagulation by binding to and stabilizing factor VIII, as well as serving as a bridge for platelet adhesion to the site of injury.
**Correct Answer:** C. Factor VIII.