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.
Instant coffee, also called soluble coffee because it dissolves in water, contains an average of 93 milligrams of caffeine in a prepared 8-ounce serving, according to the Center for Science in the Public Interest. The range of caffeine content in an 8-ounce cup of instant coffee is 27 to 173 milligrams. The same amount of regular brewed coffee contains an average of 133 milligrams of caffeine, with a range of 102 to 200 milligrams. The type of coffee beans and brewing methods affect the caffeine content in coffee.