Blood sugar, used in a physiological context, is a misnomer and misleading. Physiologically, the term means only glucose in the blood. Other sugars are present, sometimes in more than trace amounts, but only glucose serves as a controlling signal for metabolic regulation. Other sugars are, to some extent, inert. Glucose, transported via the bloodstream from the intestines or liver to body cells, is the primary source of energy for the body's cells.
Blood sugar concentration, or glucose level, is tightly regulated in the human body. Normally, the blood glucose level is maintained between about 4 and 6 mM. The normal blood glucose level is about 90mg/100ml, which works out to 5mM/L, since the molecular weight of glucose, C6H12O6, is about 180 g/mol daltons. The total amount of glucose in circulating blood is therefore about 3.3 to 7g (assuming an ordinary adult blood volume of 5 liters, plausible for an average adult male). Glucose levels rise after meals for an hour or two by a few grams and are usually lowest in the morning, before the first meal of the day.
Failure to maintain blood glucose in the normal range leads to conditions of persistently high (hyperglycemia) or low (hypoglycemia) blood sugar. Diabetes mellitus, characterized by persistent hyperglycemia from any of several causes, is the most prominent disease related to failure of blood sugar regulation.
Although it is called"blood sugar," other simple sugars aside from glucose are found in the blood, such as fructose and galactose. But only glucose levels are used as metabolic regulation signals (via insulin and glucagon). |