One of the main functions of the skeleton of humans (and other land vertebrates) is the buffering of metabolic ions (H+) within the blood and interstitial fluid to maintain the pH balance within narrow boundaries. This is possible because the skeleton is a large store of alkaline minerals called hydroxyapatite. The pH of the blood and extracellular fluid can be lowered during the course of normal metabolism, because many reactions result in the production of H+. The kidneys and lungs raise pH by eliminating large amounts of H+ and CO2 respectively, but the alkali minerals of the bone are used if these pathways are not effective at normalising pH. The pH of arterial blood is close to 7.40, but this drops to around 7.36 in venous blood because of mixing with the metabolic waste from the interstitial fluid.
Interestingly, diet can also have acidifying or alkalising effects on the blood. Very high protein diets, soft drinks and foods containing high quantities of sugar can acidify the blood as a by product of metabolism. Under acidic conditions, osteoclast resorptive activity is increased and the deposition of alkaline minerals in bone by osteoblasts is reduced. This releases hydroxyl ions (OH–) into solution to buffer the acidic conditions. The result of chronic acidosis caused by poor food choice is therefore a demineralisation of the skeleton and the development of osteoporosis. Shifting the acid base balance can be achieved with changes to the diet. Diets high in fruits and vegetables and calcium are able to shift the balance of pH back to alkalising because the by products of metabolism produce fewer H+ and therefore do not require so much buffering.
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