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

Due to the presence of buffers in the human blood that neutralizes the blood pH.

The blood pH stays neutral when the same amount of lactic acid is added to the blood because of the presence of buffer in the human blood. This buffer neutralizes the pH and maintain it in the normal range.

Human blood contains a buffer of carbonic acid (H2CO3) and bicarbonate anion (HCO3-) that maintain blood pH in the range of 7.35 and 7.45. If the blood pH exceeds from 7.8 and lowers from 6.8 then this will leads to death so the buffers maintains the pH of the blood in the normal range.

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