The normal glucose level in human blood is ranging between 80mg and 120mg/100ml. This level is perfectly maintained by a hormone called insulin secreted by the beta cells of Islets of Langerhans in pancreas. Whenever the glucose level in the blood is increased, sufficient amount of insulin is secreted by beta cells. This insulin is responsible for removing the excess of glucose from the blood and storing it in the liver in the form of glycogen. By doing this, it helps to maintain the glucose level in the blood.
Diabetes mellitus is caused due to the deficiency of insulin. Insufficient quantity of insulin can take only a part of excess of glucose to the liver for storage and the rest of excess of glucose is left in the blood. Thus the glucose level in the blood is always high. This condition is called hyperglycemia. When the blood with excess of glucose enters into the kidneys, the excess of glucose is excreted along with urine. This elimination of excess of glucose along with urine is called diabetes mellitus. When the kidneys receive the blood with lot of glucose, the tissues in the kidneys are destroyed which leads to renal failure. Ultimately this leads to the death of the individual.