If you drink alkaline water - unless it’s heavily buffered, it’s no longer alkaline as soon as it hits the stomach - special cells in the stomach acidify the content of the stomach by pumping out protons in exchange for sodium ions. Then originally neutral, acidic and alkaline water all get alkaline in the duodenum as cells lining the gut secrete bicarbonate ions in exchange for chloride ions. The main regulator of pH is carbon dioxide produced as we metabolize food. CO2 + water produces carbonic acid which dissociates into bicarbonate and protons. As long as our lungs work reasonable well, we regulate the acidity of the blood by breathing - if we exercise, more CO2 is produced, the blood pH drops and prompts us to breath more and get rid of the excess CO2. If our blood is too acidic for other reasons, it still causes us to increase respiration to get rid of more CO2 and bring the acidity to normal levels.