The human body has several closed loop and feedback control systems that enable it to maintain homeostasis for various health-related elements, i.e. body temperature, blood pressure, blood glucose, blood pH.
Homeostasis is the ability of the human body (or a cell) to maintain a constant internal environment of stability and balance in response to environmental changes. Homeostasis is an important characteristic of living things and a unifying principle of biology.
Keeping a stable internal environment requires constant adjustments as conditions change inside and outside the cell. The adjusting of systems within a cell is called homeostatic regulation. Because the internal and external environments of a cell are constantly changing, adjustments must be made continuously to stay at or near the set point (the normal level or range). Homeostasis can be thought of as a dynamic equilibrium rather than a constant, unchanging state.
The nervous and endocrine systems control homeostasis in the body through feedback mechanisms involving various organs and organ systems. Examples of homeostatic processes in the body include body temperature control, blood pH balance, blood glucose regulation, water and electrolyte balance, blood pressure, and respiration.
The endocrine system plays an important role in homeostasis because it uses hormones to regulate the blood and the activity of body cells to stay within a tight range. The release of hormones into the blood is controlled by a stimulus. The stimulus either causes an increase or a decrease in the amount of hormone secreted. Then, the response to a stimulus changes the internal conditions and may itself become a new stimulus. This self-adjusting mechanism is called feedback regulation.
The specific system we're talking about here is the Blood Glucose Regulation System. The hormones are insulin and glucagon. The stimulus is raised or lowered blood glucose.
Blood Glucose Regulation System
The primary goal of the body’s Blood Glucose Regulation System is to keep blood glucose in a tight range between 70 and 110 mg/dL (or 3.89 to 6.11 mmol/L).
As depicted in the following diagram, when blood glucose rises (for example, after eating), the hormone insulin is secreted from the beta cells of the pancreas, triggering muscle and fat cells to absorb glucose from the bloodstream causing blood glucose to decrease.
When blood glucose falls (after heavy exercise or lack of food for extended periods), glucagon is secreted from the alpha cells of the pancreas, causing the liver to release stored glycogen as glucose into the bloodstream, causing blood glucose to rise.
However, in a diabetic's body, when blood glucose rises, the muscle and fat cells are unable to absorb glucose from the bloodstream (because they are insulin-resistant). This causes blood glucose to continue to rise beyond the upper target level (this is called hyperglycemia).
As depicted in the following diagram, the Blood Glucose Regulation System isn't working as designed. To try to correct the high glucose problem, the pancreas continues excreting more and more insulin to try to "push" the excess glucose into the cells. If the problem is not corrected, this can lead to hyperinsulinemia.
And, if insulin resistance, hyperglycemia and hyperinsulinemia continue for an extended number of years, this can lead to Type 2 diabetes.
Note: This topic is discussed in more detail (at the cellular level) in the DTD Science of Diabetes ebook.