Page 151 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

6 Acid–Base Homeostasis

pH, pH Buffers, Acid–Base Balance bound forms (! p. 124). This, in turn, increases
the pulmonary elimination of CO 2.
The pH indicates the hydrogen ion activity or
+
the “effective” H concentration of a solution Other non-bicarbonate buffers of the blood in-
+
+
(H activity = f H · [H ], where square brackets clude plasma proteins and inorganic phosphate
–
2–
+
(H 2PO 4 H + HPO 4 ) as well as organic
mean concentration; ! p. 378), where phosphates (in red blood cells). Intracellular organic
pH = – log (f H ! [H ]) [6.1] and inorganic substances in various tissues also func-
+
In healthy individuals, the pH of the blood is tion as buffers.
usually a mean pH 7.4 (see p. 142 for normal The buffer capacity is a measure of the buff-
range), corresponding to H activity of about ering power of a buffer system (mol · L –1 ·
+
40 nmol/L. The maintenance of a constant pH [∆pH] ). It corresponds to the number of
–1
is important for human survival. Large devia- added H or OH ions per unit volume that
+
–
tions from the norm can have detrimental ef- change the pH by one unit. The buffer capacity
fects on metabolism, membrane permeability, therefore corresponds to the slope of the titra-
and electrolyte distribution. Blood pH values tion curve for a given buffer (! p. 380, B). The
below 7.0 and above 7.8 are not compatible buffer capacity is dependent on (a) the buffer
with life. concentration and (b) the pH. The farther the
Various pH buffers are responsible for main- pH is from the pK a of a buffer system, the
taining the body at a constant pH (! p. 379). smaller the buffer capacity (! p. 380). The
One important buffer for blood and other body buffer capacity of the blood is about
fluids is the bicarbonate/carbon dioxide 75 mmol · L · (∆pH) – 1 at pH 7.4 and constant
–1
(HCO 3 /CO 2) buffer system: P CO 2 . Since the buffer capacity is dependent on
–
–
CO 2 + H 2O HCO 3 + H . + [6.2] the prevailing P CO 2 , the buffer base concentra-
The pK a value (! p. 378f.) determines the pre- tion of the blood (normally about 48 mEq/L) is
vailing concentration ratio of the buffer base normally used as the measure of buffering
and buffer acid ([HCO 3 ] and [CO 2], respectively power of the blood in clinical medicine
–
in Eq. 6.2) at a given pH (Henderson–Has- (! pp. 142 and 146). The buffer base concen-
selbalch equation; ! A). tration is the sum of the concentrations of all
–
The primary function of the CO 2/HCO 3 buffer components that accept hydrogen ions,
+
buffer system in blood is to buffer H ions. i.e., HCO 3 , Hb , Oxy-Hb , diphosphoglycerate
–
–
–
However, this system is especially important anions, plasma protein anions, HPO 4 , etc.
2 –
because the concentrations of the two buffer Changes in the pH of the blood are chiefly
components can be modified largely indepen- due to changes in the following factors (! A
dent of each other: [CO 2] by respiration and and p. 142ff.):
[HCO 3 ] by the liver and kidney (! A; see also ! H ions: Direct uptake in foodstuffs (e.g.,
–
+
p. 174). It is therefore classified as an open vinegar) or by metabolism, or removal from
buffer system (! p. 140). the blood (e.g., by the kidney; ! p. 174ff.).
Hemoglobin in red blood cells (320 g Hb/L ! OH ions: Uptake in foodstuffs containing
–
erythrocytes! ! MCHC, p. 89 C), the second (basic) salts of weak acids, especially in pri-
most important buffer in blood, is a non-bicar- marily vegetarian diet.
bonate buffer. ! CO 2: Its concentration, [CO 2], can change
–
HbH Hb + H + [6.3] due to alterations in metabolic production or
–
Oxy-HbH Oxy-Hb + H + [6.4] pulmonary elimination of CO 2. A drop in [CO 2]
The relatively acidic oxyhemoglobin anion leads to a rise in pH and vice versa (! A: [CO 2]
–
+
(Oxy-Hb ) combines with fewer H ions than is the denominator in the equation).
–
deoxygenated Hb , which is less acidic (see ! HCO 3 : It can be eliminated directly from
–
also p. 124). H ions are therefore liberated the blood by the kidney or gut (in diarrhea)
+
upon oxygenation of Hb to Oxy-Hb in the lung. (! pp. 176, 142). A rise or fall in [HCO 3 ] will
–
Reaction 6.2 therefore proceeds to the left,
138 thereby promoting the release of CO 2 from its lead to a corresponding rise or fall in pH (! A:
[HCO 3 ] is the numerator in the equation).
–
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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