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.
An 8-ounce cup of brewed coffee contains between 95 and 200 milligrams of caffeine, while an 8-ounce cup of instant coffee ranges from 27 to 173 milligrams. Single-serve varieties of brewed coffee contain 75 to 150 milligrams, while specialty drinks such as lattes contain between 63 and 175 milligrams.