Page 153 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

+
–
Bicarbonate/Carbon Dioxide Buffer venous blood (H + HCO 3 ! CO 2 + H 2O) (! B1).
The lungs eliminate the additional CO 2 so
The pH of any buffer system is determined by quickly that the arterial P CO 2 remains practi-
the concentration ratio of the buffer pairs and cally unchanged despite the addition of H +
the pK a of the system (! p. 378). The pH of a (open system!).
bicarbonate solution is the concentration ratio The following example demonstrates the quantita-
of bicarbonate and dissolved carbon dioxide
([HCO 3 ]/[CO 2]), as defined in the Henderson– tively small impact of increased pulmonary CO 2 +
–
elimination. A two-fold increase in the amount of H
–
Hasselbalch equation (! A1). Given [HCO 3 ] = ions produced within the body on a given day (nor-
24 mmol/L and [CO 2] = 1.2 mmol/l, [HCO 3 ]/ mally 60 mmol/day) will result in the added produc-
–
[CO 2] = 24/1.2 = 20. Given log20 = 1.3 and pK a = tion of 60 mmol more of CO 2 per day (disregarding
6.1, a pH of 7.4 is derived when these values are non-bicarbonate buffers). This corresponds to only
about 0.3% of the normal daily CO 2 elimination rate.
–
set into the equation (! A2). If [HCO 3 ] drops
Acid–Base Homeostasis ratio of the two variables will not change, and ery has basically similar effects. Since OH +
to 10 and [CO 2] decreases to 0.5 mmol/L, the
–
An increased supply of OH ions in the periph-
–
the pH will remain constant.
CO 2 ! HCO 3 , [HCO 3 ] increases and the
–
–
+
When added to a buffered solution, H ions
venous P CO 2 becomes smaller than normal. Be-
–
combine with the buffer base (HCO 3 in this
cause the rate of CO 2 elimination is also re-
case), resulting in the formation of buffer acid
duced, the arterial P CO 2 also does not change in
–
+
(HCO 3 + H ! CO 2 + H 2O). In a closed system
At a pH of 7.4, the open HCO 3 /CO 2 buffer
system makes up about two-thirds of the
amount of buffer acid formed (CO 2) equals the
6 from which CO 2 cannot escape (! A3), the the illustrated example (! B2). –
amount of buffer base consumed (HCO 3 ). The
–
buffer capacity of the blood when the P CO 2 re-
inverse holds true for the addition of hy- mains constant at 5.33 kPa (! p. 138). Mainly
–
–
droxide ions (OH + CO 2 ! HCO 3 ). After addi- intracellular non-bicarbonate buffers provide
+
tion of 2 mmol/L of H , the aforementioned the remaining buffer capacity.
–
baseline ratio [HCO 3 ]/[CO 2] of 24/1.2 (! A2) Since non-bicarbonate buffers (NBBs) func-
changes to 22/3.2, making the pH fall to 6.93 tion in closed systems, their total concentration
–
(! A3). Thus, the buffer capacity of the HCO 3 / ([NBB base] + [NBB acid]) remains constant,
CO 2 buffer at pH 7.4 is very low in a closed sys- even after buffering. The total concentration
tem, for which the pK a of 6.1 is too far from the changes in response to changes in the
target pH of 7.4 (! pp. 138, 378ff). hemoglobin concentration, however, since
hemoglobin is the main constituent of NBBs
If, however, the additionally produced CO 2
–
is eliminated from the system (open system; (! pp. 138, 146). NBBs supplement the HCO 3 /
! A4), only the [HCO 3 ] will change when the CO 2 buffer in non-respiratory (metabolic)
–
same amount of H is added (2 mmol/L). The acid–base disturbances (! p. 142), but are the
+
–
corresponding decrease in the [HCO 3 ]/[CO 2] only effective buffers in respiratory acid–base
ratio (22/1.2) and pH (7.36) is much less than in disturbances (! p. 144).
a closed system. In the body, bicarbonate buff-
ering occurs in an open system in which the
partial pressure (P CO 2 ) and hence the concen-
tration of carbon dioxide in plasma ([CO 2] =
α · P CO 2 ; ! p. 126) are regulated by respiration
(! B). The lungs normally eliminate as much
CO 2 as produced by metabolism (15 000–
20 000 mmol/day), while the alveolar P CO 2 re-
mains constant (! p. 120ff.). Since the plasma
P CO 2 adapts to the alveolar P CO 2 during each res-
piratory cycle, the arterial P CO 2 (Pa CO 2 ) also re-
mains constant. An increased supply of H in
+
140 the periphery leads to an increase in the P CO 2 of
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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