Re: How can an eyes lens be clear and recieve red blood?

As you point out, it is inconsistent to imagine a lens that is both clear and colorless and that has red blood flowing through it. So, it must be that one of these assumptions is wrong! Either the lens is not clear, or it doesn’t have blood flowing through it. In fact, it is the latter assumption that is wrong. The lens of the eye does NOT have blood flowing through it.

But how can this be, you ask? The lens is alive, isn’t it? And living things need blood to bring them oxygen, don’t they? I mean, your brain can’t live without blood, so how does the lens?

Indeed, the lens is a living tissue, and there are living cells within it. But these are very special cells with special properties. Most importantly for us, they are adapted to survive and grow with very little oxygen. This is in large part because the grow VERY slowly over the lifetime of the person.

Still, they do need some oxygen, and they do need to get rid of other waste products. The lens cells must rely entirely on diffusion (i.e. passive exchange) of these materials into and out of the lens. To facilitate this, the lens is bathed in a flowing bath of fluid called the "aqueous humor", located just in front of the lens and behind the "cornea" that covers the surface of the eye. This fluid has no blood cells, so it is clear. But it can slowly exchange gases and other materials into and out of the lens. The fluid is constantly recycled so that these gases and things don’t build up.

Note that this process of diffusional exchange is fundamentally the same as what any cell does in an organ with blood flow. In other words, ALL cells rely on diffusion of these gases and waste products, it’s just that the lens doesn’t get the added benefit of nearby, fast-flowing red blood cells to facilitate the diffusion. This means that the lens cells can only grow and metabolize at a much reduced rate. It also means that the size of the lens is limited. If it got too big, the gases and things could never diffuse to the middle of the lens.

The Web site from the National Eye Institute provides more information on this, as well as a diagram of the eye that will help you understand the anatomical relationship of the lens and the aqueous humor.

Finally, a few other interesting facts about the aqueous humor are:

1) Glaucoma is increased pressure in the eye that is caused by a block in the flow of the aqueous humor.

2) The aqueous humor is actually more important for focusing light on the retina (and therefore for seeing) than is the lens itself! The cornea and the aqueous humor beneath it are highly refractile all by themselves. The lens adds some power, but this is more important for "refining" the image.