Why is glucose known as blood sugar?

Glucose is the kind of sugar that circulates in the blood.

For example, lactose (sugar found in milk), is a disaccharide (ie two linked sugar molecules) made up of one glucose molecule and one galactose molecule. Galactose is almost like glucose, but it will be changed into glucose in the liver and then released into the blood. All carbohydrates, including starches and grains, eventually get broken down and converted into glucose molecules, which are then released into the blood.