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.
Coffee contains between 75 and 150 mg. of caffeine per 8 oz. cup, depending on the roasting strength and other factors. It is difficult to measure the nutritional content of coffee, because it varies depending on many things. Generally, the darker the coffee is roasted, the less caffeine it contains.