I = imread('blood1.tif');
figure; imshow(I);
figure, imhist(I) %look at the hist to get a threshold, e.g., 140
BW=roicolor(I, 140, 255); % makes a binary image
figure, imshow(BW) % all pixels in (140, 255) will be 1 and white the rest is 0 which is black
border=bwperim(BW); % finds all the border pixels
figure, imshow(border) %shows all the border pixels (1's) as white 