Low stomach acid can cause digestive problems, as we age our stomach acid production declines. Herbs such as bitters, can increase the production of stomach acid, and therefore help with digestive issues. Increasing the stomach acid will not effect the ph of the blood or lymph system.
As I have said before, the ph of the stomach and skin can be manipulated, if you want to do this. I didn't say they SHOULD be, just that they can be.
In a perfect world we would all have a good diet and good lifestyle, but this is not a perfect world.
Alkalising proponents DO suggest trying to alkalise the stomach, skin and lymph system. And once again, why would you want to alkalise areas of the body that should be acidic, skin, stomach etc. And once again, alkalising these areas will not effect the ph of the lymph or blood systems, but will effect digestion and pathogen resistance.