High blood sugar levels: In diabetes mellitus, the body either does not produce enough insulin (Type 1 diabetes) or cannot effectively use the insulin it produces (Type 2 diabetes). As a result, glucose accumulates in the bloodstream instead of being absorbed by the cells. This condition is known as hyperglycemia. Elevated blood sugar levels lead to increased osmotic pressure in the blood, pulling fluid from the body tissues into the bloodstream. This excess fluid is then filtered by the kidneys and excreted as urine, resulting in frequent urination.