Measurement of capilar area in hearts

I am having problems in measuring the area of capilars in heart slide samples.
I am setting the scala, including the number of pixels that have the lengh of the scala included in my pictures taken with a microphone Leica MD5000, and including the units (um). When I check the length in pixels and microns of the picture’s scale, the measurement is correct. But when I measure the total size of the tissue included in the picture, the results don’t make any sense. So sombosy could help me?

Did you check if you images are properly scaled when you open them in Fiji (under properties)?