The kidneys are the main organs of the urinary system. They function chiefly to filter blood in order to remove wastes and excess water. The waste and water are excreted as urine. The kidneys also reabsorb and return to the blood needed substances, including amino acids, sugar, sodium, potassium, and other nutrients.
But many popular brands actually contain considerably less than an old-fashioned cup of black coffee. An 8.4-ounce serving of Red Bull, for instance, has a relatively modest 76 to 80 mg of caffeine, compared to the 95 to 200 mg in a typical cup of coffee, the Mayo Clinic reports.
The active ingredient in coffee is caffeine, the most commonly consumed psychoactive substance in the world . The caffeine content in a cup of coffee is highly variable, ranging from 50 to 400+ mg per cup. A small home-brewed cup of coffee could contain 50 mg, while a big 16 oz Starbucks grande can contain over 300 mg.