"Every gram of human tissue has one billion cells, and every cell must be within five microns (or five 25,000ths of an inch) of a blood vessel."A gram is puny. About a quarter of a teaspoon of sugar weighs a gram. But that's the weight of a dry item. Wet living cells may be denser and take up less volume. The extent of blood-vessel branching needed to support the billion cells in something the size of a pencil eraser is just incredible.
In people with diabetes, the very tiny blood vessels, the capillaries, in the retina of the eye (and of course elsewhere like the kidney) get clogged and leak. Eventually, when insufficient nutrients reach the retina, new blood vessels grow (which is also incredible). These new vessels are very fragile though and aren't as efficient as the ones they replace.
All those leaking vessels eventually lead to vision loss ... but not at first! In the beginning, when your macula is swelling and your vessels are leaking ... you may notice nothing, feel nothing, see nothing unusual ... maybe a few floaters or some blurry vision. It's why anyone with diabetes really should have someone look deep inside their eyes regularly. (Sadly, millions of people don't even know they have diabetes and their retinopathy is silently progressing.1)
Undiagnosed: 7.0 million
Prediabetes: 79 million