Page 141 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Binding and Transport of O 2 in Blood the right signifies an affinity decrease, and a
shift to the left signifies an affinity increase, re-
Hemoglobin (Hb) is the O 2-carrying protein of sulting in flattening and steepening, respec-
red blood cells (RBCs) (mol. mass: 64 500 Da). tively, of the initial part of the curve. Shifts to
Hb is also involved in CO 2 transport and is an the left are caused by increases in pH (with or
important blood pH buffer (! pp. 124 and without a P CO 2 decrease) and/or decreases in
138ff.). Hb is a tetramer with 4 subunits P CO 2 , temperature and 2,3-bisphosphoglyc-
(adults: 98%: 2α + 2" = HbA; 2% 2α + 2δ = erate (BPG; normally 1 mol/mol Hb tetramer).
HbA 2), each with its own heme group. Heme Shifts to the right occur due to decreases in pH
consists of porphyrin and Fe(II). Each of the and/or increases in P CO 2 , temperature and 2,3-
four Fe(II) atoms (each linked with one his- BPG (! B). The half-saturation pressure (P 0.5 or
tidine residue of Hb) binds reversibly with an P 50) of O 2 (! B, dotted lines) is the PO 2 at which
O 2 molecule. This is referred to as oxygenation S O 2 is 0.5 or 50%. The P 0.5, which is normally
(not oxidation) of Hb to oxyhemoglobin (Oxy- 3.6 kPa or 27 mmHg, is a measure of shifting to
Hb). The amount of O 2 which combines with the right (P 0.5") or left (P 0.5#). Displacement of
Hb depends on the partial pressure of O 2 (P O 2 ): the O 2 dissociation curve due to changes in pH
oxygen dissociation curve (! A, red line). The and P CO 2 is called the Bohr effect. A shift to the
Respiration of the Hb tetramer (positive cooperativity) and P CO 2 "), larger quantities of O 2 can be absorbed
right means that, in the periphery (pH#,
curve has a sigmoid shape, because initially
bound O 2 molecules change the conformation
from the blood without decreasing the P O 2 ,
which is the driving force for O 2 diffusion (! B,
thereby increase hemoglobin-O 2 affinity.
When fully saturated with O 2, 1 mol of tet-
5 rameric Hb combines with 4 mol O 2, i.e., broken lines). A higher affinity for O 2 is then
re-established in the pulmonary capillaries
64 500 g of Hb combine with 4 ! 22.4 L of O 2. (pH", P CO 2 #). A shift to the left is useful when
Thus, 1 g Hb can theoretically transport the PA O 2 is decreased (e.g., in altitude hypoxia),
1.39 mL O 2, or 1.35 mL in vivo (Hüfner num- a situation where arterial S O 2 lies to the left of
ber). The total Hb concentration of the blood the S O 2 plateau.
([Hb] total) is a mean 150 g/L (! p. 88), corre- Myoglobin is an Fe(II)-containing muscle
sponding to a maximum O 2 concentration of protein that serves as a short-term storage
9.1 mmol/L or an O 2 fraction of 0.203 L O 2/L molecule for O 2 (! p. 72). As it is monomeric
blood. This oxygen-carrying capacity is a func- (no positive cooperativity), its O 2 dissociation
tion of [Hb] total (! A, yellow and purple curves curve at low P O 2 is much steeper than that of
as compared to the red curve). HbA (! C). Since the O 2 dissociation curve of
The O 2 content of blood is virtually equivalent to fetal Hb (2α + 2γ = HbF) is also steeper, S O 2
values of 45 to 70% can be reached in the fetal
the amount of O 2 bound by Hb since only 1.4% of O 2
in blood is dissolved at a P O 2 of 13.3 kPa (! A, orange umbilical vein despite the low PO 2 (3–4 kPa or
line). The solubility coefficient (α O 2 ), which is 22–30 mmHg) of maternal placental blood.
10µmol ! [L of plasma] – 1 ! kPa , is 22 times smaller
– 1
than α CO 2 (! p. 126). This is sufficient, because the fetal [Hb] total is
Oxygen saturation (S O 2 ) is the fraction of 180 g/L. The carbon monoxide (CO) dissocia-
Oxy-Hb relative to [Hb] total, or the ratio of ac- tion curve is extremely steep. Therefore, even
tual O 2 concentration/ O 2-carrying capacity. At tiny amounts of CO in the respiratory air will
normal P O 2 in arterial blood (e.g., Pa O 2 = dissociate O 2 from Hb. This can result in carbon
12.6 kPa or 95 mmHg), S O 2 will reach a satura- monoxide poisoning (! C). Methemoglobin,
tion plateau at approx. 0.97, while S O 2 will still Met-Hb (normally 1% of Hb), is formed from
amount to 0.73 in mixed venous blood (PV O 2 = Hb by oxidation of Fe(II) to Fe(III) either spon-
5.33 kPa or 40 mmHg). The venous S O 2 values in taneously or via exogenous oxidants. Met-Hb
different organs can vary greatly (! p. 130). cannot combine with O 2 (! C). Methemoglobin
O 2 dissociation is independent of total Hb if reductase reduces Fe(III) of Met-Hb back to
Fe(II); deficiencies of this enzyme can cause
plotted as a function of S O 2 (! B). Changes in O 2 methemoglobinemia, resulting in neonatal
128 affinity to Hb can then be easily identified as anoxia.
shifting of the O 2 dissociation curve. A shift to
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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