Why are some patients of diabetes treated by giving injections of insulin?

A person suffers from diabetes when his/her body fails to produce adequate amount of Insulin.

Insulin is a hormone secreted by pancreas that helps in regulating the blood glucose level. If insulin is not produced in sufficient amount in blood, the level of glucose in the blood increases which leads to various serious conditions like retinopathy, blindness, fatigue, excessive urination and other serious complications too. So in order to maintain the insulin level in the blood of a diabetic person, insulin injections are administered artificially into them.