How does caffine?!

How does caffine?

how do you get energy from caffine? what body part does it react with?

Caffeine is a xanthine alkaloid compound that acts as a stimulant in humans. The word comes from the Italian term for coffee, caffè.[1] Caffeine is also called guaranine when found in guarana, mateine when found in mate, and theine when found in tea, although these terms are not considered valid chemical names. It is found in the leaves and beans of the coffee plant, in tea, yerba mate, and guarana berries, the kola nut, the Yaupon Holly, and in small quantities in cocoa. Overall, caffeine is found in the beans, leaves, and fruit of over 60 plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants.
In humans, caffeine is a central nervous system (CNS) stimulant, having the effect of temporarily warding off drowsiness and restoring alertness. Beverages containing caffeine, such as coffee, tea, soft drinks and energy drinks enjoy great popularity; caffeine is the world's most widely consumed psychoactive substance, but unlike most other psychoactive substances, it is legal and unregulated in nearly all jurisdictions. In North America, 90% of adults consume caffeine daily.[2] The U.S. Food and Drug Administration lists caffeine as a "Multiple Purpose Generally Recognized as Safe Food Substance".[3]
Many natural sources of caffeine also contain widely varying mixtures of other xanthine alkaloids, including the cardiac stimulants theophylline and theobromine and other substances such as polyphenols which can form insoluble complexes with caffeine.
Caffeine is a central nervous system and metabolic stimulant,[23] and is used both recreationally and medically to reduce physical fatigue and restore mental alertness when unusual weakness or drowsiness occurs. Caffeine stimulates the central nervous system first at the higher levels, resulting in increased alertness and wakefulness, faster and clearer flow of thought, increased focus, and better general body coordination, and later at the spinal cord level at higher doses.[24] Once inside the body, it has a complex chemistry, and acts through several mechanisms as described below.
Metabolism
Caffeine is metabolized in the liver into three primary metabolites: paraxanthine (84%), theobromine (12%), and theophylline (4%)Caffeine is completely absorbed by the stomach and small intestine within 45 minutes of ingestion. After ingestion it is distributed throughout all tissues of the body and is eliminated by first-order kinetics.[25]
The half-life of caffeine—the time required for the body to eliminate one-half of the total amount of caffeine consumed at a given time—varies widely among individuals according to such factors as age, liver function, pregnancy, some concurrent medications, and the level of enzymes in the liver needed for caffeine metabolism. In healthy adults, caffeine's half-life is approximately 3–4 hours. In women taking oral contraceptives this is increased to 5–10 hours,[26] and in pregnant women the half-life is roughly 9–11 hours.[27] Caffeine can accumulate in individuals with severe liver disease when its half-life can increase to 96 hours.[28] In infants and young children, the half-life may be longer than in adults; half-life in a newborn baby may be as long as 30 hours. Other factors such as smoking can shorten caffeine's half-life.[29]
Caffeine is metabolized in the liver by the cytochrome P450 oxidase enzyme system (specifically, the 1A2 isozyme) into three metabolic dimethylxanthines,[30] which each have their own effects on the body:
Paraxanthine (84%): Has the effect of increasing lipolysis, leading to elevated glycerol and free fatty acid levels in the blood plasma.
Theobromine (12%): Dilates blood vessels and increases urine volume. Theobromine is also the principal alkaloid in cocoa, and therefore chocolate.
Theophylline (4%): Relaxes smooth muscles of the bronchi, and is used to treat asthma. The therapeutic dose of theophylline, however, is many times greater than the levels attained from caffeine metabolism.
Each of these metabolites is further metabolized and then excreted in the urine.