Acid_Base_Phys_overview - Overview of Acid-Base Physiology...

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Overview of Acid-Base Physiology Sources of Acid Our normal metabolism of carbohydrates, lipids, and proteins for energy produces acid as a byproduct. On a mixed diet of plant and animal foods we tend to produce a net amount of acid equal to 1 mEq/Kg body wt/day. Most of the acid is in the form of carbonic acid (H 2 CO 3 ) which is derived from the complete oxidation of carbohydrate and lipid to CO 2 . Carbonic acid is categorized as a volatile acid since it is in equilibrium with carbon dioxide, and therefore its concentration in the blood is regulated by the respiratory system. The other source of acid is classified as non-volatile (fixed) acid. It is derived from the incomplete oxidation of carbohydrate, fat and protein. For example, under anaerobic conditions carbohydrate is incompletely oxidized to lactic acid. During conditions of deficient dietary calorie intake or insulin deficiency, fatty acids are incompletely oxidized to ketones such as acetoacetic acid and B-hydroxybutyric acid. As their name implies, ketones are acids. Proteins are metabolized to strong acids. For example, sulfur containing amino acids such as methionine and cysteine are metabolized to sulfuric acid (H 2 SO 4 ). Arginine and lysine are metabolized to hydrochloric acid (HCl). Phospholipids are metabolized to phosphoric acid. Exogenous fixed acids derived from drugs or toxic substances include salicylic acid (from aspirin), formic acid (from methanol), and glycolic acid (from ethylene glycol). The non-volatile (fixed) acids are buffered by chemical buffer systems such as the bicarbonate system in the blood, and regulated by renal excretion. Regulation of Acid/Base Since acid is being produced continuously, it must be regulated to prevent the development of acidosis. There are 3 regulatory systems within the body that control acid/base balance in order to maintain a normal range of pH between 7.35-7.45. Your text defines the normal range as 7.37-7.42. They are: 1) the chemical buffer system in the plasma and cells; 2) the respiratory system; and 3) the renal system. The chemical buffer system responds the fastest to changes in pH. The respiratory system responds within minutes, and the renal system responds the slowest over hours and days. The chemical buffer system includes the extracellular bicarbonate system, and the intracellular organic phosphate system, proteins (especially hemoglobin in RBC), and bone carbonate. The respiratory system regulates acid/base balance via alterations in breathing depth and rate (hyper/hypoventilation). It is easiest to equate CO 2 with acid (H + ). With hyperventilation, more CO 2 is expired and there is a net loss of acid from the body. With hypoventilation, less CO 2 is expired and there is a net gain of acid in the body. The carbonic acid – bicarbonate buffer system equation highlights this relationship.
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