Eye redness is due to swollen or dilated blood vessels, which cause the surface of the eye to look red, or bloodshot.
Considerations
There are many possible causes of a red eye or eyes. Some are cause for concern; some are medical emergencies. Others are of no consequence or concern at all. The degree of redness or appearance of blood usually does not correlate to how serious the situation is. It is generally more important whether you also have eye pain or impaired vision.
just the top lines from a the medical site.