Glucose, the fuel of all cellular processes, is distributed to all cells dissolved in the blood. The concentration of glucose in the blood must be high enough to ensure that the cells have enough fuel.
However, extra glucose beyond the immediate needs of the cells can harm many important organs and tissues, especially where the vessels are tiny, as in the retina of the eye, the extremities (hands and, especially, feet), and the kidneys. Diabetes is a disease in which there is a chronic overconcentration of glucose in the blood.
The amount of glucose in the blood is controlled mainly by the pancreas. Absorption by the small intestine puts the glucose from ingested food into the blood. Insulin is a hormone released into the blood from the pancreas in response to increased blood glucose levels. Most cells have receptors that bind the insulin, which allows glucose into the cells for cellular respiration. The cells of the liver, muscles, and adipose tissue (fat) take up the glucose and store it as glycogen. At times when your intestines aren't absorbing much glucose, like hours after a meal, the production of insulin is suppressed and the stored glucose is released into the blood again.