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Acid-Base Balance

Acid-Base Balance. Outlines. Introduction. Body acidity has to be kept at a fairly constant level. Normal pH range within body fluids 7.35 - 7.45. Normal pH is constantly being attacked by metabolic processes within the body that produces acidic metabolites. Acid-Base Balance.

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Acid-Base Balance

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  1. Acid-Base Balance

  2. Outlines

  3. Introduction Body acidity has to be kept at a fairly constant level. Normal pH range within body fluids 7.35 - 7.45. Normal pH is constantly being attacked by metabolic processes within the body that produces acidic metabolites.

  4. Acid-Base Balance • An acid is a proton donor and a base is a proton acceptor. • Physiologically, there are two groups of important acids: • Carbonic acid (H2CO2) • Non carbonic acid

  5. Carbonic acid (H2CO2) • Carbonic acid comes from CHO and fat metabolism and results in 15,000 mmol of CO2/day. Carbonic acid metabolism is mostly handled by respiration. Recall: • CO2 + H20   H2CO3

  6. Factors involved in gas exchange • Many facotrs affects gas exchange, this process occurs between the alveoli & pulmonary capillaries & between capillaries & tissue. Its involvied : • Partial pressure • Diffusion • Ventilation-perfusion matching • Oxyhemoglobin dissociation

  7. 1. Partial pressure • When assessing a patient's oxygenation, one of the first steps is to look at PaO2 is the partial pressure of oxygen dissolved in blood. • Normal PaO2 is 80 to 100 mmHg; however this is normal for a healthy person, less than 60 years old, breathing room air.

  8. Normal acid-base balance • Normal plasma pH = 7.4 (Range: 7.35 - 7.45) • CO2 + H20 H2CO3 HCO3- +H+

  9. Blood Gas Norms

  10. Acid-Base Regulation • Three mechanisms to maintain pH • Respiratory (CO2) • Buffer (in the blood: carbonic acid/bicarbonate, phosphate buffers, Hgb) • Renal (HCO3-)

  11. Buffers • A buffer is a substance that can give or accept protons • i.e. H+, in a manner that tends to minimise changes in the pH of the solution. • Usually buffers are composed of a weak acid (proton donor) and a weak base (proton acceptor) as shown in the following equation.

  12. Regulation • The process of acid-base regulation involves: • Chemical buffering by intracellular and extracellular buffers  • Control of pCO2 by normal respiratory function  • Control of HCO3- concentration and acid excretion by the kidney 

  13. The Renal function • Reclaim filtered HCO3- (therefore, avoid HCO3- loss) • Regenerate HCO3- in an amount equal to that used as buffer • In contrast to the respiratory system, can fully compensate without any changes in the bicarbonate pool.

  14. Respiratory Function • the respiratory system is able to compensate for changes in the acid/base balance by increasing or decreasing ventilatory rate. • This would result in an increase or decrease of pCO2 in the blood. Thus changes are compensated at cost, i.e. changes in the bicarbonate pool.

  15. Abnormal acid-base balance • Acid-base imbalances can be defined as acidosis or alkalosis. • Acidosis is a state of excess H+ • Acidemia results when the blood pH is less than 7.35     • Alkalosis is a state of excess HCO3- • Alkalemia results when the blood pH is greater than 7.45

  16. Acid-base disturbanceDisorder type • Primary change in blood HCO3-→Metabolic disorder • Primary change in blood pCO2→Respiratory disorder

  17. Steps for ABG interpretation • Step1: Look at Pao2 (hypoxemia) • Step2: look at pH (acid or alkaline) • Step 3: look at Paco2 (resp. acidosis, alkalosis or normal) • Step4: look at Hco3 (metabolic acidosis, alkalosis, or normal) • Step5: look back at pH (compensated or uncompensated)

  18. ABG interpretation ROME Use the acronym ROME (Respiratory opposite, Metabolic equal) to help you remember

  19. Examples • PaO2: 90 mmHg • pH: 7.25 • P aCO2: 50 mmHg • HCO3-: 22mEq\L (uncompensated respiratory acidosis)

  20. Examples • PaO2: 90 mmHg • pH: 7.25 • P aCO2: 40 mmHg • HCO3-: 17mEq\L (uncompensated metabolic acidosis)

  21. Examples • PaO2: 90 mmHg • pH: 7.37 • P aCO2: 60 mmHg • HCO3-: 38mEq\L (compensated respiratory acidosis with metabolic alkalosis\ main disordered is acidosis and alkalosis is the compensated, because the pH is on the acid side of 7.4))

  22. Examples • PaO2: 90 mmHg • pH: 7.42 • P aCO2: 48 mmHg • HCO3-: 35 mEq\L (compensated metabolic alkalosis with respiratory acidosis \ main disordered is alkalosis and acidosis is the compensated, because the pH is on the alkaline side of 7.40)

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