A dog may have blood shot eyes when there is blood present in the eyelids or blood in the eyes vessels, which can be caused by conditions such as hyperemia and ocular vasculature. These health issues cause large amounts of blood to build in in the eyes, resulting in inflammation and visible redness.
Other health issues that can cause a dog to have bloodshot eyes are, orbital disease, hemorrhages in the front of the eyes, glaucoma and inflammation of the iris, conjunctiva, eyelid, sclera or cornea. In some cases, red eyes can mean that a dog is suffering from a life-threatening systemic disease. If a dog is displaying signs of inflammation in one or both eyes, it should be brought to a veterinarian immediately.