The fluctuation of blood sugar (red) and the sugar-lowering hormone insulin (blue) in humans during the course of a day with three meals. One of the effects of a sugar-rich vs a starch-rich meal is highlighted.
Blood sugar concentration, or glucose level, refers to the amount of glucose present in a mammal's blood. In birds and reptiles the processing of sugars is done differently, the pancreas is slightly more well developed in birds than in mammals, perhaps as a partial compensation for the lack of saliva and chewing. It produces carbohydrate, fat and protein digesting enzymes which are secreted into the small intestine. The liver has two distinct lobes each with its own duct leading into the small intestine. The liver, as in mammals, houses the bile, which in birds however is acidic and not alkaline as it is in mammals. Many birds do not have a gall bladder to hold the bile, and it is secreted directly into the pancreatic ducts.
Normally, in mammals the blood glucose level is maintained at a reference range between about 3.6 and 5.8 mM (mmol/l). It is tightly regulated in the human body as a part of metabolic homeostasis. Other sugars (eg, fructose) do not participate in the control mechanisms and are, thus, largely irrelevant to metabolic control.
Normal blood glucose levels are about 90 mg/100ml, equivalent to 5mM (mmol/l) (since the molecular weight of glucose, C6H12O6, is about 180 g/mol). The total amount of glucose normally in circulating human blood is therefore about 3.3 to 7g (assuming an ordinary adult blood volume of 5 litres, 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. Transported via the bloodstream from the intestines or liver to body cells, Glucose is the primary source of energy for body's cells, fats and oils (ie, lipids) being primarily a compact energy store.
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.