Why can normal people maintain a certain blood sugar level?

Under the regulation of liver, hormones and nervous system, the blood sugar of normal people fluctuates within a certain range, and the fasting blood sugar is 3.9 ~ 6.

1mmol/L, and the blood sugar 2 hours after meal is not more than 7.8 mmol/L, where the blood sugar refers to the venous plasma glucose concentration. -It won't be as high as 7.8 two hours after a meal.