Page 139 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

CO 2 Binding in Blood, CO 2 in CSF dissociation is determined by connecting
these two points by a line called “physiologic
The total carbon dioxide concentration CO 2 dissociation curve.”
(= chemically bound “CO 2” + dissolved CO 2) of The concentration ratio of HCO 3 to dis-
–
mixed venous blood is about 24–25 mmol/L; solved CO 2 in plasma and red blood cells differs
that of arterial blood is roughly 22–23 mmol/L. (about 20 : 1 and 12 : 1, respectively). This re-
Nearly 90% of this is present as HCO 3 (! A, flects the difference in the pH of plasma (7.4)
–
right panel, and table on p. 124). The partial and erythrocytes (ca. 7.2) (! p. 138ff.).
pressure of CO 2 (P CO 2 ) is the chief factor that de-
termines the CO 2 content of blood. The CO 2 dis-
CO 2 in Cerebrospinal Fluid
sociation curve illustrates how the total CO 2
concentration depends on P CO 2 (! A). Unlike HCO 3 and H , CO 2 can cross the blood-
+
–
The concentration of dissolved CO 2, [CO 2], in cerebrospinal fluid (CSF) barrier with relative
plasma is directly proportional to the PCO 2 in ease (! B1 and p. 310). The P CO 2 in CSF there-
plasma and can be calculated as follows: fore adapts quickly to acute changes in the P CO 2
[CO 2] = α CO 2 ! P CO 2 (mmol/L plasma in blood. CO 2-related (respiratory) pH changes
or mL/L plasma), [5.6] in the body can be buffered by non-bicarbonate
where α CO 2 is the (Bunsen) solubility coefficient
buffers (NBBs) only (! p. 144). Since the con-
Respiration for CO 2. At 37 !C, –1 ! kPa , centration of non-bicarbonate buffers in CSF is
α CO 2 = 0.225 mmol ! L
very low, an acute rise in P CO 2 (respiratory acid-
–1
osis; ! p. 144) leads to a relatively sharp
After converting the amount of CO 2 into
volume CO 2 (mL = mmol ! 22.26), this yields
5 α CO 2 = 5 mL ! L –1 ! kPa . decrease in the pH of CSF (! B1, pH""). This
decrease is registered by central chemosen-
–1
The curve for dissolved CO 2 is therefore linear sors (or chemoreceptors) that adjust respira-
(! A, green line). tory activity accordingly (! p. 132). (In this
Since the buffering and carbamate forma- book, sensory receptors are called sensors in
tion capacities of hemoglobin are limited, the order to distinguish them from hormone and
relation between bound “CO 2” and P CO 2 is cur- transmitter receptors.)
vilinear. The dissociation curve for total CO 2 is The concentration of non-bicarbonate
calculated from the sum of dissolved and buffers in blood (hemoglobin, plasma pro-
bound CO 2 (! A, red and violet lines). teins) is high. When the CO 2 concentration in-
CO 2 binding with hemoglobin depends on creases, the liberated H ions are therefore ef-
+
the degree of oxygen saturation (S O 2 ) of fectively buffered in the blood. The actual
–
hemoglobin. Blood completely saturated with HCO 3 concentration in blood then rises rela-
O 2 is not able to bind as much CO 2 as O 2-free tively slowly, to ultimately become higher
blood at equal P CO 2 levels (! A, red and violet than in the CSF. As a result, HCO 3 diffuses
–
lines). When venous blood in the lungs is (relatively slowly) into the CSF (! B2), result-
loaded with O 2, the buffer capacity of ing in a renewed increase in the pH of the
hemoglobin and, consequently, the levels of CSF because the HCO 3 /CO 2 ratio increases
–
chemical CO 2 binding decrease due to the Hal- (! p. 140). This, in turn, leads to a reduction in
dane effect (! p. 124). Venous blood is never respiratory activity (via central chemosen-
completely void of O 2, but is always O 2-satu- sors), a process enhanced by renal compensa-
–
rated to a certain degree, depending on the tion, i.e., a pH increase through HCO 3 reten-
degree of O 2 extraction (! p. 130) of the organ tion (! p. 144). By this mechanism, the body
in question. The S O 2 of mixed venous blood is ultimately adapts to chronic elevation in P CO 2 —
about 0.75. The CO 2 dissociation curve for S O 2 = i.e., a chronically elevated P CO 2 will no longer
0.75 therefore lies between those for S O 2 = 0.00 represent a respiratory drive (cf. p. 132).
and 1.00 (! A, dotted line). In arterial blood,
P CO 2 ! 5.33 kPa and S O 2 ! 0.97 (! A, point a). In
126 mixed venous blood, P CO 2 ! 6.27 kPa and S O 2
! 0.75 (! A, point V). The normal range of CO 2
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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