To prevent the backflow of blood during systole and diastole, Artries from the heart dont need valves since their blood pressure is already high enough to not allow backflow of blood, but since veins' blood pressure is lower, they need valves just so the blood doesn't seep back in into the heart again!
If a major change in pH occurred, the protease enzymes would stop functioning or function very slowly, assuming the original pH was the most optimal pH for them.
