Increased blood in the eyes is caused by conjunctival congestion, which can have a variety of causes, such as the following common ones: First, the conjunctiva or cornea of the eye surface can become infected and stimulate the conjunctival tissue to become congested, due to inflammatory factors acting on the conjunctival blood vessels, so that the smooth muscle tone in the walls of the blood vessels is reduced, thus increasing the diameter of the blood vessels and increasing blood flow, triggering red blood in the eyes. Second, due to allergic reactions in the eye, commonly seen in spring catarrhal conjunctivitis, allergic conjunctivitis, etc. When the conjunctival tissue of the eye is exposed to allergens in the external environment, it will reactively trigger vasodilation, resulting in red blood. Third, due to dry eye disease resulting in damage to the conjunctiva and corneal epithelium, triggered by irritative congestion. Fourth, reactive congestion of the ocular surface due to intraocular lesions, such as glaucoma and uveitis.