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Regulation of Calcium and Phosphate Filtered phosphate is actively reabsorbed in the proximal tubules In the absence of PTH, phosphate reabsorption is regulated by its transport maximum and excesses are excreted in urine High or normal ECF calcium levels inhibit PTH secretion Release of calcium from bone is inhibited Larger amounts of calcium are lost in feces and urine More phosphate is retained
Influence of Calcitonin Released in response to rising blood calcium levels Calcitonin is a PTH antagonist, but its contribution to calcium and phosphate homeostasis is minor to negligible
Regulation of Anions Chloride is the major anion accompanying sodium in the ECF 99% of chloride is reabsorbed under normal pH conditions When acidosis occurs, fewer chloride ions are reabsorbed Other anions have transport maximums and excesses are excreted in urine
Sources of Hydrogen Ions Most hydrogen ions originate from cellular metabolism Breakdown of phosphorus-containing proteins releases phosphoric acid into the ECF Anaerobic respiration of glucose produces lactic acid Fat metabolism yields organic acids and ketone bodies Transporting carbon dioxide as bicarbonate releases hydrogen ions
Acid Base Balance Normal pH of body fluids Arterial blood is 7.4 Venous blood and interstitial fluid is 7.35 Intracellular fluid is 7.0 Alkalosis or alkalemia – arterial blood pH rises above 7.45 Acidosis or acidemia – arterial pH drops below 7.35 (physiological acidosis)
Hydrogen Ion Regulation Concentration of hydrogen ions is regulated sequentially by: Chemical buffer systems – act within seconds The respiratory center in the brain stem – acts within 1-3 minutes Renal mechanisms – require hours to days to effect pH changes
Chemical Buffer Systems Strong acids – all their H+ is dissociated completely in water Weak acids – dissociate partially in water and are efficient at preventing pH changes Strong bases – dissociate easily in water and quickly tie up H+ Weak bases – accept H+ more slowly (e.g., HCO3 and NH3)
Chemicial Buffer Systems One or two molecules that act to resist pH changes when strong acid or base is added Three major chemical buffer systems Bicarbonate buffer system Phosphate buffer system Protein buffer system Any drifts in pH are resisted by the entire chemical buffering system
Bicarbonate Buffer System A mixture of carbonic acid (H2CO3) and its salt, sodium bicarbonate (NaHCO3) (potassium or magnesium bicarbonates work as well) If strong acid is added: Hydrogen ions released combine with the bicarbonate ions and form carbonic acid (a weak acid) The pH of the solution decreases only slightly
Bicarbonate Buffer System If strong base is added: It reacts with the carbonic acid to form sodium bicarbonate (a weak base) The pH of the solution rises only slightly This system is the only important ECF buffer
Phosphate Buffer System Nearly identical to the bicarbonate system Its components are: Sodium salts of dihydrogen phosphate (H2PO4), a weak acid Monohydrogen phosphate (HPO42), a weak base This system is an effective buffer in urine and intracellular fluid
