How can you mask images that are a different shape than the ones you need?

I am searching an image of an eye for small red blobs, each smaller than 10 pixels in radius. After masking out all non-red pixels, I am left with a sizable number of pixels, some of which are not in the dots I need. Instead, they are in veins, which I do not want to appear in the final image. How do I create a mask for these veins?

Image attached is a sample image I am trying to search.