In a medical study, patients are classified in 8 ways according to whether they have blood type $A B^{+}$, $A B^{-}, A^{+}, A^{-}, B^{+}, B^{-}, O^{+},$ or $O^{-},$ and also accord ing to whether their blood pressure is low, normal, or high. Find the number of ways in which a patient can be classified.