A thick red vein in your dog’s eye is known as the central retinal vein. This vein runs through the middle of the retina, which is the innermost part of the eye. This vein is responsible for carrying oxygenated blood back to the heart. The retina is very delicate tissue and if this vein becomes blocked up, then this can seriously affect how well your dog sees. If the vein gets blocked up, it cannot carry blood back to the heart, so instead it will start to fill up with blood. This blood will spill into the vitreous humour (the clear jelly-like substance) around the retina. This blood build-up is known as oedema and it causes swelling around the retina, which can make it difficult for your pet to see. The thick red veins in your dog’s eye are most common in older dogs.