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.
Be aware that the amount of caffeine in your cup of coffee will vary depending on the type of coffee and how it's brewed. The coffee at a restaurant or coffee shop, for example, can range from about 100 mg for a small (8-ounce) cup to over 400 mg for a large (16-ounce) cup, depending on the brand and the brew.