Gluconeogenesis refers to a group of metabolic reactions, some of them highly exergonic and irreversible, which are regulated both locally and globally (by insulin, glucagon, and cortisol). The purpose of this system, localized in both the cytosol and mitochondria, is to maintain blood glucose level constant throughout fasting state. The balance between stimulatory and inhibitory hormones regulates the rate of gluconeogenesis. Liver and secondarily the kidney are the organs that supply circulating blood and consequently, various tissues with glucose. Many tissues depend primarily on glucose to maintain adequate energy levels for their proper function during fasting.