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 caffeine content of an average 5 ounce cup of brewed coffee can vary from 50 to 120mg. (an 8 ounce cup has between 80mg and 135mg). Espresso contains about 100mg of caffeine per 2 ounce cup. An average cup of espresso has about 2 to 3 times as much caffeine as an average cup of instant coffee. An average 5 ounce cup of black tea contains about 30-60mg. Instant tea contains about 12 - 28mg caffeine.