In medicine, blood sugar is glucose in the blood. Blood sugar concentration is an important factor in diabetes.
Blood sugar levels are regulated by negative feedback in order to keep the body in homeostasis. The levels of glucose in the blood are monitored by the pancreas, when the concentration of glucose falls, perhaps due to exercise, the pancreas releases more glucagon. Glucagon is a hormone which targets cells in the liver, causing them to convert glycogen into glucose, increasing blood sugar levels.
If the pancreas detects an increase in blood sugar, which could be a result of eating, then it secretes more insulin. This hormone causes the liver to convert more glucose into glycogen and so decreases blood sugar levels. Diabetes mellitus type 1 is caused by insufficient or non-existent production of insulin, while type 2 is due to inadequate response to endogenous insulin (a state termed "insulin resistance", both of which result in too much glucose in the blood. Type 1 diabetes is treated by replacing the absent insulin by injection of exogenous insulin. Type 2 diabetes is treated with sulfonylureas and metiglinides, which increase pancreatic secretion of insulin; metformin, which reduces hepatic synthesis of glucose (gluconeogenesis); and thiazolidinediones (aka "glitazones"), which increase cellular sensitivity to circulating insulin. In late stage type 2 diabetes, exogenous insulin may also be required.