contain many proteins and organic phosphates that can bind H + . For example, hemoglobin (Hb) in red blood cells combines with H+ from H2CO3, minimizing the increase in free H + . Recall from Chapter 21 the buffering reaction:

H2CO3 + HbO2

This reaction raises the plasma [HCO3_]. In acute respiratory acidosis, such chemical buffering processes in the body lead to an increase in plasma [HCO3~] of about 1 mEq/L for each 10 mm Hg increase in Pco2 (see Table 25.4). Bicarbonate is not a buffer for H2CO3 because the reaction

is simply an exchange reaction and does not affect the pH.

An example illustrates how chemical buffering reduces a fall in pH during respiratory acidosis. Suppose Pco2 increased from a normal value of 40 mm Hg to 70 mm Hg ([CO2(d)] = 2.l mmol/L). If there were no body buffer bases that could accept H+ from H2CO3 (i.e., if there was no measurable increase in [HCO3_]), the resulting pH would be 7.16:

Renal Compensation. The kidneys compensate for respiratory acidosis by adding more H+ to the urine and adding new HCO3~ to the blood. The increased Pco2 stimulates renal H+ secretion, which allows the reabsorption of all filtered HCO3~. Excess H+ is excreted as titratable acid and NH4+; these processes add new HCO3~ to the blood, causing plasma [HCO3~] to rise. This compensation takes several days to fully develop.

With chronic respiratory acidosis, plasma [HCO3~] increases, on average, by 4 mEq/L for each 10 mm Hg rise in Pco2 (see Table 25.4). This rise exceeds that seen with acute respiratory acidosis because of the renal addition of HCO3~ to the blood. One would expect a person with chronic respiratory acidosis and a Pco2 of 70 mm Hg to have an increase in plasma HCO3~ of 12 mEq/L. The blood pH would be 7.33:

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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