You have asked a complex question. I will try to explain this as clearly as I can. People who have diabetes have a lack of insulin in their blood. Insulin is made in an organ called the pancreas. Insulin is important to allow glucose (blood sugar) to get into the cells of the body. Put another way, insulin opens the door to let blood sugar to enter most cells in the body. Blood sugar is a food for the bodies cells. If insulin is low or absent in the blood then the cells don't get fed the blood sugar they need. If the blood sugar can not get into the bodies cells then it builds up in the blood stream and the sugar count increases on the blood tests that we do. Also, as the blood sugar increases and can not get into the bodies cells it has the effect of drawing water out of the cells and shrinks them up making them even less healthy.
The nerves in the body are affected a bit differently. Nerve cells will allow blood sugar in with out insulin, however without insulin present the sugar is not used by the nerve cell properly and the sugar accumulates in the cell. Over time this will damage the nerve cell and cause the nerve to die. This causes numbness and tingling in the feet and sometimes in the hands.Blood vessels are also made up of cells. As the sugar builds up in these cells it swells them up and this causes a narrowing of the blood vessel. This causes a decrease in the circulation to the feet, the kidneys and the eyes. This is why people with diabetes often loose their legs their eye sight and kidney function.
It is very important that people with diabetes learn about their condition, control their blood sugar, and exercise.