is that our bodies are normally very efficient at keeping the pH of our insides, a major aspect of our "milleu interieur", pretty constant. Our body tissues, including blood, are equipped with buffering substances and processes that effectively neutralize either acidic or alkaline substances both inside and outside of our cells. Most extracellular fluids are thus maintained at a constant pH of 7.4, very slightly alkaline on the pH scale. Except for certain normal variations I will describe below, deviations of even 0.5 pH unit from this in these fluids are uncommon and can be life-threatening.

Under a number of pathological circumstances, the most common of which is severe, untreated insulin-dependent diabetes, acids produced by the abnormal metabolism of fats can quickly build up, resulting in life-threatening keto-acidosis, with blood pH dropping as low as 6.7.

Other fairly common causes of acidosis are disruption of respriation by COPD, chronic obstructive pulmonary disease such as severe emphasema or asthma. CO2 builds up in the blood because it can't be efficiently excreted through the lungs. Even then, the pH decreases only slightly because of the buffering efficiency of the blood.

Urine pH can vary more widely than that of blood, and is affected by thinigs such as diet and degree of hydration, but is generally in the slightly acidic range, pH 5.5 to 7.2.

pH in the digestive tract may normally be as low as 1.5-2.0 in the stomach as a result of secretion of the hydrochloric acid needed for gastric digestion, and as high as 7.7 in the small intestine due to neutralization by the "alkaline tide" provided by pancreatic secretions. This is required for the pancreatic and intestinal enzymes to become activated and complete the breakdown of dietary carbohydrates, fats and proteins into the sugars, fatty acids and amino acids our bodies use as fuels and "building blocks".

Rare diseases, some of which may be familial, or genetically transmitted, can result in the accumulation abnormal metabolic products or abnormally high quantites of normal products, some of which may be acids. These can cause harmful pH changes in the body that overwhelm its otherwise normalizing "homeostatic" processes.