Constancy in a system, such as the human body, maintained by sensing, feedback, and control mechanisms. A familiar example of a system in homeostasis is a house with a thermostat. When the temperature in the house dips too far below the desired temperature, the thermostat senses this and sends a signal to the furnace to turn on. When the target temperature level is reached, the thermostat senses this, too, and signals the furnace to shut off. The human body has a number of functions that are controlled by homeostatic mechanisms, including heartbeat, blood pressure, body temperature, electrolyte balance, respiration, and blood glucose regulation.
In a person who doesn’t have diabetes, the body has a number of mechanisms in place to keep blood glucose in a fairly limited range. The pancreas reacts to low blood glucose levels by decreasing its insulin secretion. If the blood glucose level drops lower, the alpha cells of the pancreas release more of a hormone called glucagon, which stimulates the liver to manufacture more glucose and release it into the bloodstream. At the same time, the adrenal glands secrete more of a hormone called epinephrine. In addition to stimulating the manufacture and secretion of glucose by the liver, epinephrine keeps the body’s tissues from using as much glucose. Epinephrine is thought to cause some of the physical symptoms of hypoglycemia—such as sweating, trembling, and heart palpitations. Other counterregulatory hormones, including growth hormone and cortisol, also help raise blood glucose levels by increasing glucose production and limiting glucose utilization.
A person with Type 1 diabetes has lost one or more of these defense mechanisms. Since his pancreas no longer makes insulin and he must inject it, his pancreas cannot control the level of insulin in his bloodstream through decreased secretion. What’s more, after a person has had Type 1 diabetes for about five years, the alpha cells of his pancreas have a diminished glucagon response to hypoglycemia. People who maintain their blood glucose levels close to normal (often called “tight” control), may also have a reduced epinephrine response, meaning that their blood glucose levels must fall much farther than the normal threshold for hypoglycemia before the epinephrine response occurs. This condition, known as defective glucose counterregulation, puts people at risk for severe hypoglycemia. These individuals also tend to have hypoglycemia unawareness, in which they do not experience or do not notice any of the classic physical early warning signs of hypoglycemia. Consequently, they can suddenly develop severe hypoglycemia.