Glucose is a sugar and the main energy source used by the body. Carbohydrates that you eat are broken down, converted to glucose and then absorbed by the bloodstream. Circulating glucose is one of several blood sugars, which also include fructose and galactose, but when discussing "blood sugar" most people mean glucose. Blood glucose is usually maintained by the human body as 70-130 mg/dL, and the levels of glucose are influenced by many hormones, including those involved in blood pressure regulation.
Insulin and energy usage
Blood sugar levels usually increase after eating, with levels reaching 180 mg/dL according to the American Diabetes Association. When receptors in the pancreas sense increases in blood glucose levels, the pancreas releases insulin. Insulin is a hormone that aids in the removal of glucose from the blood in a variety of ways: it promotes the entry of glucose into cells, enhances the storage of glycogen or fatty acids, and prevents the usage of fats and protein as energy. Fats and protein somewhat compete with glucose as sources of energy in the body.
Glucagon and hypoglycemia
Glycogen is formed by the liver and sometimes the muscles or other tissues in a process called glycogenesis. The process involves the conversion of glucose through structural manipulations of the sugar ring and collecting molecules as a chain and attaching them to a glycogen primer. This is the form in which glucose is stored in the body for later use in animal cells. Its plant counterpart is starch, so glycogen is often referred to as animal starch. The granules take up less storage space than triglycerides (i.e. fat).
When blood sugar levels decrease too much, a condition called hypoglycemia, the pancreas releases glucagon. Glucagon is a hormone that promotes the release of glycogen from the liver. Glucose units are removed from the glycogen chains and released into the circulation to increase the blood levels of glucose to a normal range.
When the body's ability to regulate blood glucose is impaired, such is the case in diabetes mellitus, blood sugar levels are consistently increased and carbohydrate intake has to be monitored. This is referred to as hyperglycemia and can cause damage to the circulatory system; increase pressure in the eyes, leading to blindness; impair healing, particularly in the extremities; and cause blood pressure complications.
Fructose is a simple sugar, a monosaccharide, found in fruits and honey and is twice as sweet as cane sugar, known as sucrose. The breakdown of fructose involves a long pathway and its intake is recommended for hypoglycemia. Galactose, also called brain sugar, is found in dairy products as a component of lactose. Hydrolysis of lactose by the body produces both galactose and glucose, so it induces insulin production as well as the breakdown of the galactose into a glucose unit that can be attached to glycogen chains.
Regulating blood sugar levels
So as you can see, the body has several different hormones involved in the regulation of blood glucose. Even if you aren't eating glucose per se, the body breaks down carbohydrates to obtain that energy source. Insulin is probably the more well known hormone in this process, but the pancreas and liver have other pathways they can use to maintain the necessary levels and prevent damage.