|MadSci Network: Anatomy|
Our eyes get bloodshot because something is irritating them. It's as simple as that.
Now, lots of different things can irritate your eyes. You can get soap in them, stare at a computer all day and let them dry out, or get punched in one. All of these things will make your eyes bloodshot.
Your eyes are full of little blood vessels. Look at the whites of your eyes really closely in the mirror and you'll see them all over the place. What happens when your eyes get irritated is that the blood vessels swell up and start to look big and red, and the whole eye looks bloodshot.
Anytime something irritates a part of your body, you get a very similar response. Go ahead and scratch the back of your hand (lightly!) with your fingernail. It'll turn white for a few seconds, but then it will turn red. The same things happen in your eye. They get irritated, they turn red. Usually a few other things happen to irritated body parts along with getting red. Depending on how bad they're irritated, your body parts can swell up, get hot, and hurt. Swelling, heat, redness, and pain are all part of a process called "inflammation" that your body uses to keep damage to a minimum when something bad happens to it.
Hope that helps,
Try the links in the MadSci Library for more information on Anatomy.