Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

IV. Mature RBC
199
(a)
FIGURE 7-6 The hemoglobin-oxygen dissociation curve and factors
influencing the position of the curve.
1977 ). There is an inverse relationship between metabolic
rate (oxygen consumption per gram of tissue) and body
weight ( Kleiber, 1961 ). Consequently, the higher P 50 in
smaller animals should be beneficial in meeting tissue oxygen
requirements associated with their higher metabolic
rates. The RBCs of most mammal species adapted to living
at high altitude exhibit greater Hb oxygen affinity than
those living at low altitudes, and RBCs from animals of the
same species adapted to living at high altitude may have
greater Hb oxygen affinity than their counterparts living at
low altitude ( León-Velarde et al. , 1996 ).
A number of factors, including innate Hb oxygen-binding
characteristics, H concentration, pCO 2 , organic phosphate
concentrations, Cl concentrations, and temperature, influence
the oxygen affinity of Hb within RBCs. In addition to
their innate Hb oxygen-binding characteristics, different Hb
types respond to differing degrees to these various effectors
( Bårdgard et al. , 1997 ; Haskins and Rezende, 2006 ).
Fetal Hbs in ruminants have higher oxygen affinities than
their respective adult Hbs ( Battaglia et al. , 1970 ; Blunt
et al. , 1971 ). HbC-containing RBCs from neonatal or anemic
adult goats exhibit lower oxygen affinity, with a greater
Bohr effect, than RBCs containing only adult Hb ( Huisman
et al. , 1969 ). Both factors should potentiate the delivery of
oxygen to the tissue in goats with HbC. The presence of
MetHb (iron in the ferric state) in one or more of the Hb
monomers not only results in decreased oxygen carrying
capacity but also in increased oxygen affinity of the remaining
monomers ( Hrinczenko et al. , 2000 ). The binding of Hb
to the cytoplasmic domain of band 3 decreases the oxygen
affinity of bound Hb, but a human RBC membrane has
about 1 million copies of band 3 compared to 471 million
Hb tetramers ( Jensen, 2004 ).
(b)
FIGURE 7-7 Coupled O 2 , and CO 2 transport and H buffering in
RBCs. Reactions taking place in the tissues (a) and lungs (b) are shown.
Hb-CO 2 denotes carbamino Hb, and Hb-H denotes Hb binding H .
Abbreviation: CA, carbonic anhydrase.
2 . Effects of H , CO 2 , and Temperature
The addition of H to a suspension of RBCs results in a
decrease in oxygen affinity of Hb (Bohr effect), an increase
in P 50 , and a shift of the oxygen dissociation curve to the
right ( Fig. 7-6 ). The magnitude of the Bohr effect is defined
numerically as Δ log P 50 /-Δ pH. There is an inverse relationship
between the magnitude of the Bohr effect and average
body size of various animal species ( Riggs, 1960 ).
In capillaries of metabolizing tissues, CO 2 enters RBCs
and about 80% is rapidly converted to H 2 CO 3 by carbonic
anhydrase ( Hsia, 1998 ). This carbonic acid spontaneously
ionizes to H and HCO 3
. The increased H concentration
decreases the oxygen affinity of Hb and facilitates oxygen
delivery to the tissues. DeoxyHb is a weaker acid than
OxyHb; therefore, DeoxyHb binds the excess H and limits
the decrease in pH. The increased HCO 3
diffuses out of the
cell down a concentration gradient and C1 moves in (chloride
shift) to maintain electrical neutrality ( Fig. 7-7a ).
The direct binding of CO 2 to Hb in carbamino groups
lowers oxygen affinity, but this effect is generally considered
to be of minor importance. An exception appears to be in