The correct functions of your lungs contribute to the normal ph level of between 7.35 and 7.45. if your lungs do not exchange and remove carbon dioxide from your blood, the blood ph will change. a ph 6.4 reading of your blood indicates

The reading of the blood pH value of 6.4 indicates a health issue because of the pH value. It is because the concentrations of H+ ions in the body becomes 10 times more than the usual level in the body.  

The pH of the blood becomes less than 7.35 due to enhanced production of hydrogen ions by the body or the incapability of the body to produce bicarbonate ions in the kidney, the condition is known as metabolic acidosis that eventually results in acidemia.