The diagram is depicting a feedback loop for the regulation of blood glucose levels. The number 11 represents insulin, a hormone that signals the body's cells to take up glucose from the blood. Therefore, a decrease in the number of insulin (11) would lead to a decrease in the uptake of glucose by cells and an increase in the concentration of glucose in the blood, a condition known as hyperglycemia. This would not result in an increase in urine production, energy production, or a decreased population, but could lead to decreased immunity over time if left uncontrolled. Therefore, the correct answer is "decreased immunity".