TAILIEUCHUNG - Chapter 048. Acidosis and Alkalosis

Normal Acid-Base Homeostasis Systemic arterial pH is maintained between and by extracellular and intracellular chemical buffering together with respiratory and renal regulatory mechanisms. The control of arterial CO2 tension (PaCO2) by the central nervous system and respiratory systems and the control of the plasma bicarbonate by the kidneys stabilize the arterial pH by excretion or retention of acid or alkali. The metabolic and respiratory components that regulate systemic pH are described by the Henderson-Hasselbalch equation: . | Chapter 048. Acidosis and Alkalosis Normal Acid-Base Homeostasis Systemic arterial pH is maintained between and by extracellular and intracellular chemical buffering together with respiratory and renal regulatory mechanisms. The control of arterial CO2 tension PaC02 by the central nervous system and respiratory systems and the control of the plasma bicarbonate by the kidneys stabilize the arterial pH by excretion or retention of acid or alkali. The metabolic and respiratory components that regulate systemic pH are described by the Henderson-Hasselbalch equation HCOj pH ứSPi 0jB01 Under most circumstances CO2 production and excretion are matched and the usual steady-state PaCO2 is maintained at 40 mmHg. Underexcretion of CO2 produces hypercapnia and overexcretion causes hypocapnia. Nevertheless production and excretion are again matched at a new steady-state PaCO2. Therefore the PaCO2 is regulated primarily by neural respiratory factors Chap. 258 and is not subject to regulation by the rate of CO2 production. Hypercapnia is usually the result of hypoventilation rather than of increased CO2 production. Increases or decreases in PaCO2 represent derangements of neural respiratory control or are due to compensatory changes in response to a primary alteration in the plasma HCO3- . The kidneys regulate plasma HCO3- through three main processes 1 quot reabsorption quot of filtered HCO3- 2 formation of titratable acid and 3 excretion of NH4 in the urine. The kidney filters 4000 mmol of HCO3- per day. To reabsorb the filtered load of HCO3- the renal tubules must therefore secrete 4000 mmol of hydrogen ions. Between 80 and 90 of HCO3- is reabsorbed in the proximal tubule. The distal nephron reabsorbs the remainder and secretes protons as generated from metabolism to defend systemic pH. While this quantity of protons 40-60 mmol d is small it must be secreted to prevent chronic positive H balance and metabolic acidosis. This quantity of secreted protons is represented

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