The cornea needs to be transparent so that light can pass through so there can be no blood vessels there, otherwise the light would be obscured.
Without blood vessels the cornea must get it’s oxygen directly from the air. The oxygen first dissolves in the tears and then diffuses throughout the cornea to keep it healthy. Carbon dioxide is released via the same process back out to the atmosphere. Some oxygen can diffuse through the aqueous humour within the eye but this is slow and limited. The main pathway is through the front of the eye so if this is restricted by your eyes being closed for extreme periods of time or via the use of the wrong type of contact lenses, then the cornea can become oxygen deprived.
Without enough Oxygen the cornea will warp, become less transparent, less able to detect pain and can develop scars. Additionally, new blood vessels from the sclera (the white part of the eye) can grow into the cornea and cause further damage and scarring. A blood shot eye happens in response to the cornea looking for another way to get more oxygen. Since the cornea is without blood vessels, the retina pumps up its veins in attempt to absorb more oxygen.
This response can lead to other problems over time like corneal neovascularization and macular degeneration disease. The oxygen supply to the cornea is slightly less absorbed when contact lenses cover the cornea. Contact lenses nowadays are a lot more permeable in order to allow more oxygen to pass through to the eye and some are designed to be in for long periods of time.
Check with your optician to make sure you have the correct type to avoid damaging your eyes. Early symptoms of a lack of oxygen include dryness and irritation as well as blood shot eye. Solutions and moisturisers are available to help keep your eyes healthy and oxygenated.
References: http://www.aclm.org.uk and http://www.avoideyestrain.com