Long-term, high levels of glucose in the blood can damage both the blood vessels and the nerves anywhere in the body.
Damage to the blood vessels may mean that parts of the body are not getting an adequate supply of blood and therefore the oxygen it carries, so areas can become damaged. Damaged areas on the feet can then become ulcerated and infected.
Damage to the nerves (neuropathy) may result in loss of feeling, typically in the feet and hands. Any injury or ulcer may go unnoticed, because pain perception is impaired, and can get much worse. Healing may be slow because of the poor blood supply.
And of course the blood vessels in the eyes may be damaged too, affecting vision, so you don't notice all this happening!
So it is important to look after your feet and check them (or have them checked) regularly to spot any problems at an early stage. And it is important to control blood glucose levels then these problems may never arise.
Best wishes
MaryChristine