Pulmonary aspects of acid-base balance, Henderson-Hasselbach equation, Davenport diagram, respiratory acidosis/alkalosis

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Biomedical Science
BMS 420
Charles Miller

30 November Final 75% old questions 25% since last exam Quiz #4 – next Friday Pulmonary Aspects of Acid-Base Balance Why is pH regulation important? Normal between 7.35 – 7.45 pH 6.6 – 7.8 compatible with life H reacts with negatively charged molecules such as proteins to alter function. For example, Hb combines with less oxygen when H combines with Hb Enzymes become altered if pH is out of balance Drug effectiveness is altered with changes in pH + Myocardial contractility is altered with decreased pH (↑ H ) Lots of examples of altered pH inducing arrhythmias, seizures, and vascular collapse Compared with other ions, H is highly regulated. Na for example can vary 1,000,000 + + times more than H . Thus H is precisely controlled. The basic defenses against changes in H involve the respiratory system (can react within a few minutes), kidney (hours/days and the most powerful of the three) and buffers (keep them tied up until balance restored). Acidemia: blood pH below 7.35 Alkalemia: blood pH above 7.45 Acidosis: abnormal process leading to acidemia Alkalosis: abnormal process leading to alkalemia Lung diseases will affect the pH, PaO , PaCO a2d therefo2e these are important parameters used in the strategies for the treatment of lung diseases. Cyanosis, which is the bluish color of the mucus membrane, occurs when the % saturation of oxygen and hemoglobin drops to below 80% or PaO of 50 – 60 mm Hg. 2 By altering the alveolar ventilation, the body has great control over acid-base balance. Increasing the alveolar ventilation will increase the pH Hyperventilation decreases the PaCO and hypove2tilation increases the PaCO and 2 decreases the pH Since the alveolar ventilation can change drastically, the pH can be changed substantially. Changes in H in the arterial blood can have substantial impacts on the rates of ventilation. + + Increases in H have a greater impact on increasing ventilation than decreases in H have on decreasing ventilation. The pH resulting from CO dissol2ing in blood and the dissociation of carbonic acid is given by the Henderson-Hasselbach Equation - pH = pKA + log (HCO )/(CO 3 2 Since CO obe2s Henry’s Law, CO can be repl2ced by (.03 x PCO ) 2 pKA = 6.1 and is a constant related to the dissociation of carbonic acid. HCO in3arterial blood = 24 mmol/liter Dissolved CO = .02 mmol/mm Hg/liter pH = 6.1 + log 24/(0.03 x 40 mm Hg) = 6.1 + log 24/1.2 pH = 6.1 + log 20 = 6.1 + 1.3 = 7.4 - **As long as the ratio of (HCO ) to (3CO + .03) =220, the pH is 7.4. HCO is regulated by kidneys and pCO by the lungs. Both can be lowered or increased, hence 3 2 any of the four disturbances will cause a characteristic acid-base change. The disturbances are: Respiratory acidosis Respiratory alkal
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