Blood sugar refers to the concentration of glucose in the blood stream. Blood sugar, also referred to as blood glucose, is the sugar in the blood that the blood stream carries to supply energy to cells that carry out the functions of the body. The human body normally regulates blood sugar to keep the levels in check.
The word "sugar" in "blood sugar" is a bit of a misnomer and actually refers to the sugar required by the body's cells, glucose. The human body breaks down carbohydrates in the digestive system to form a variety of sugars, including glucose. This means that after a meal, blood sugar levels increase as the glucose concentration in the blood stream rises. The body then releases insulin, which helps reduce blood sugar levels.
Excess glucose is stored in the liver and muscles. Food, activity and stress all have an effect on glucose levels in the blood stream. When blood sugar levels are outside of the normal range, it can be an indication of a medical condition. Hyperglycemia, or high blood sugar, is an indicator of diabetes. With diabetes, the body doesn't produce adequate insulin to help cells breakdown glucose, elevating blood sugar to unsafe levels.