why do you think the blood ph stays neutral while the same amount of lactic acid in water was very acidic

This happens because blood has its own buffers which maintain the pH. Alkaline (high pH) buffers are released into the blood stream when the blood becomes too acidic, and conversely, acidic buffers (low pH) are released if the blood pH becomes too alkaline.

The blood pH is maintained within a pH range of 7.35 - 7.45. This is because our body's immune system needs to operate in optimal conditions and thus, be able to fight off illness and disease.

The answer is blood has an acid-base buffer in it hence is cautioned from fluctuation in pH even after adding small amounts of acid or base. Usually, in blood, the presence of carbonic acid (which dissociated into H + and CO3-) acts as a weak acid pH buffer. The lack of a buffer in the water, on the other hand, makes it prone to changes in pH on the input of acid or base.