Thrombin effects on molecular structure of fibrin fibers in plasma. Clots were made with normal pooled human plasma (0.3 mg/mL), thrombin (0.1 and 1.0 U/mL), and CaCl₂ (10 mM) and analyzed by turbidimetry. (A) Fibrin fiber radius. (B) Number of protofibrils within the fibrin fiber. (C) Protein density of fibrin fibers. (D) Distance between protofibrils inside the fibrin fibers. The results represent the mean values ± SD; n = 3. Statistical significance, using an unpaired t test, is denoted with ***P < .005 and ****P < .001 for comparison.