Von Willebrand's factor acts as a plasma carrier of factor VIII and circulates in the blood as factor VIII-VWF complex. Its deficiency therefore impairs the intrinsic pathway of coagulation and prolongs the GPTT as the intrinsic pathway of coagulation. Ref:Kligman, Behrman, Jenson, Stanton (2008), Chapter 477, "VON WILLEBRAND DISEASE", In the book, "Nelson's Textbook of Pediatrics", Volume 2, 18th Edition, New Delhi, Page 2073