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.
Letâs use 80kg as an average. 2. A single cup of coffee on average contains about 250ml of water, and about 135mg of caffeine (link). 3. Lethal dose (oral intake for a rat, which has similar metabolism â although we should note, not identical metabolism) is about 192 mg/kg for caffeine and 90 mL/kg for the water. 4. However, the other part of the equation is we need to evaluate involves rates of elimination.