BIOL 1104 Lecture Notes - Carbonic Anhydrase, Buffer Solution, Pulmonary Alveolus

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11 Feb 2013
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 Equation
i. pH = pKa + log[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
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, pCO2 40mmHg, CO2 1.2mM, HCO3- 24mM
(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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