RED CELL & FIBRIN. RED CELL & FIBRIN This scanning electron micrograph (SEM) depicted a closer view of number of red blood cells found enmeshed in a fibrinous matrix on the luminal surface of an indwelling vascular catheter, Magnified 5698x. In this instance, the indwelling catheter was a tube that was left in place creating a patent portal directly into a blood vessel. Note the biconcave cytomorphologic shape of each erythrocyte, which increases the surface area of these hemoglobin-filled c