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Acid Base Balance

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BIOL 1840U
Peter Cheung

Acid Base Balance I—Lung I. Body Buffer Mechanisms a. Physiological pH = 7.4 tightly controlled! b. Buffered Solutions—resist changes in pH following addition of H+ or OH- i. Function best w/in 1 pH unit of pKa c. Henderson-Hasselbach Equati-n i. pH = pK + aog[A/HA] d. Isohydric principal—all non-bicarbonate buffers in ECF are in equilibrium w/ the [H+] => if [H+] changes, acid/base ratios of all buffers are affected e. Body buffering—can be divided into extracellular and intracellular buffering; i. Whole Blood: Plasma proteins, bicarbonate system, & red cell hemoglobin all grouped in one buffer compartment b/c equilibrate quickly (~50%) ii. Intracellular Buffering: reacts more slowly than blood, but very large buffering capacity (~50%) II. Bicarbonate Buffer System a. CO2 produced during metabolism is hydrated to cabonic acid which rapidly dissociates CO2 + H2O ↔ H + HCO3 - b. pKa = 6.1, so pH = 6.1 + log[HCO3/ dissolved CO2] = 6.1 + log[HCO3-]/ 0.03*pCO2 c. ratio of HCO3 to pCO2 is key for determining pH!!! d. Bicarbonate buffer system is unique b/c it’s an OPEN SYSTEM (CO2 is volatilized by ventilation => able to resist change better than a closed one III. CO2 Transfer a. From Tissues to Erythrocytes: i. CO2 produced & transferred into blood as dissolved gas ii. Carbonic anhydrase converts CO2 → HCO3 + H+ iii. H+ buffered by hemoglobin & HCO3- leaves erythrocytes via Cl/HCO3 exchanger (anion shift) b. From Erythrocytes to Alveolus: i. HCO3- shifts back into erythrocytes via exchanger ii. HCO3- → CO2 via carbonic anhydrase iii. Dissolved CO2 diffuses out of cells into plasma → alveolus iv. Excess CO2 ventilated off! c. Only a small amount of produced CO2 is actually carried in blood, the rest is converted to bicarbonate d. Normal plasma values: pH 7.4, pCO 40mmHg, CO 1.2mM, HCO 24mM - 2 2 3 (Raymond has replaced a table image with equivalent text) IV. Physiological Effects a. ↑ ventilation => ↓ pCO2 b. ↓ pH => ↑ ventilation V. Disturbances in Acid/Base Status a. Respiratory Acidosis—decreased ventilation => ↑ pCO2, ↑ [HCO3], & ↑ [H+] i. Causes ↓ pH
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