Acid-Base Balance

Acid-Base Balance Physical therapy Level I Prof. Fathia El Sharkawi Body fluids • The distribution of fluid throughout the body can be broken down into two general categories: intracellular fluid ( ICF) and extracellular fluid ( ECF). Intracellular fluid is approximately 40% of the total body weight. • ICF is mostly water and is rich in K + anions ,SO 4 2, and protein. • ECF contains more Na + , Cl, HCO 3, Mg ++ , HPO 4 , and Ca ++ . • The extracellular fluid comprises approximately 20% of total body weight and further subcategorizes as plasma at approximately 5% of body weight and interstitial space which is approximately 12% of body weight (which includes lymph, synovial fluid, cerebrospinal fluid, GI tract fluids, and fluids in the eyes and ears) Body fluids • Fluid balance refers to the proper levels of water and electrolytes being in the various body compartments • Electrolytes are chemical substances that release cations (positively charged ions) and anions (negatively charged ions) when they are dissolved in water. • Electrolyte balance • Electrolytes help maintain acid -base balance • The most important electrolytes include HPO 4 ,Na + , K + , Cl, Ca ++ • Na + is the most abundant extracellular cation; involved in nerve impulse transmission, muscle contraction, and creation of osmotic pressure. Electrolyte balance • Cl− is a major extracellular anion; involved in regulating osmotic pressure between body compartments, forming HCI in stomach,, and involved in the “chloride shift” process in blood. • K + is the most abundant cation in ICF; involved in maintaining fluid volume, nerve impulse transmission, muscle contraction, and regulating pH. Acid base balance • Acids are electrolytes that release hydrogen ions (H + ) when they are dissolved in water. • Bases are electrolytes are release hydroxide ions (OH when they are dissolved in water. • Acid base balance is primarily regulated by the concentration of H + (or the pH level) in body fluids, especially ECF. Acid base balance • Normal pH range of ECF is from 7.35 to 7.45. • Most H + comes from metabolism-glycolysis, oxidation of fatty acids and amino acids, and hydrolysis of proteins. • pH in body fluids is regulated by acid base buffer systems (primary control), respiratory centers in brain stem, and by kidney tubule secretion of H + + . • Acid base buffer systems are chemical reactions that consist of a weak acid and a weak base, to prevent rapid, drastic changes in body fluid pH. one of the most carefully regulated conc. in the body is that of H + ion. Acid base balance • Source of H • a. Aerobic respiration of glucose produces CO 2 , which reacts with water to from carbonic acid. carbonic acid dissociates to release H + and bicarbonate ions. • b. Anaerobic respiration of glucose produce lactic acid. • c. Incomplete oxidation of fatty acids releases acidic ketone bodies. • d. Oxidation of sulfur containing amino acids produce sulfuric acid • e. Hydrolysis of phosphoproteins and nucleic acids gives rise to phosphoric acid. There are 3 major mechanisms to maintain the pH of the blood: Buffering systems Exhalation of CO2 Kidney secretion of H+ and regeneration of HCO3- The major output for H+ is through the exhalation of CO2 through the lung and renal secretion Mechanisms to maintain normal pH of the blood Regeneration of Bicarbonate from the kidneys 1. HCO3- ions are freely filtered by glomeruli. - 2. HCO3 ions combine with H+ ions forming carbonic acid that dissociates into CO2 and water 3. CO2 is reabsorbed into the tubules. 4. CO2 reacts with water forming carbonic acid. 5. Carbonic acid dissociates into bicarbonate and protons (H+ will be secreted into the tubular lumen). 6. HCO3- is back again into the interstitial fluids. • If HCO3- ions were not reabsorbed, the buffering capacity of the blood would be rapidly depleted. Disorders of Hydrogen ion Homeostasis 1. Metabolic Acidosis : Causes: 1. Excess H+ Production ❑ Increased production of organic acids (usually lactic acid) as a result of anaerobic metabolism. This may result from tissue hypoxia. ❑ Increased ketone bodies production in DKA, or lactic acidosis in case of sepsis. ❑ Ingestion of Acids such as salicylate overdose or methanol intake results in the formation of formic acid. pH ↓ / HCO3- ↓ 2. Excessive Loss of HCO-3 ❑ e.g. Gastrointestinal secretions are high in sodium HCO3-. The loss of small bowel contents or excessive diarrhea results in the loss of large amounts of bicarbonate resulting in metabolic acidosis. This may be seen in such conditions as Cholera. ❑ Renal Failure. ❑ Chloride loading may also cause loss of HCO3- 3. Inadequate Excretion of H+ ❑ This results from renal tubular dysfunction and usually occurs in conjunction with inadequate reabsorption of HCO3-. Any form of renal failure may result in metabolic acidosis due to retention of H+ in the ECF. Disorders of Hydrogen ion Homeostasis 2. Metabolic Alkalosis : pH ↑ / HCO3- ↑ 2. Excessive reabsorption of HCO-3 1. Excess H+ Loss ❑ Gastric secretions contain large quantities of H+ ions. Loss of gastric secretions results in metabolic alkalosis. ❑ This occurs in prolonged vomiting, for example, pyloric stenosis or gastrointestinal disorders. ❑ HCO3- and Chloride concentrations are linked. If chloride losses are excessive, then HCO3- will be reabsorbed to maintain electrical neutrality. So, loss of Chloride ions either due to diuretics use or prolonged vomiting will increase reabsorption of HCO3-(Saline-responsive Metabolic Alkalosis). ❑ Mineralocorticoid excess. 3. Excessive ingestion of Alkali ❑ Although this is not common. But Metabolic alkalosis may occur in case of an overdose of alkalis such as an infusion of HCO3- or citrate-containing blood products or antacids.