Blood Cells - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 72

Hemoglobin is the iron-containing, oxygen-transport metalloprotein in solution
in the RBC cytoplasm ( ∼ 97% of RBC dry content; ∼ 35% of total content). This

tetramer contains 2 copies of 2 distinct globin types ( α 2 β 2 )

and 4 binding sites, one per globin. Hemoglobin tetramer is a ellipsoidal protein
(6.4

α and β ; hemoglobin-A;

5.0 nm).
Hemoglobin carries 97 to 98% of oxygen to cells for metabolism. Hemoglobin
oligomerization enables cooperativity between multiple binding sites ( allostery ).
Binding of a first O 2 molecule to any heme induces a shift in the corresponding
F-helix. This structural change decreases the binding energy for subsequent O 2
binding in the other 3 subunits (cooperative binding).
Oxyhemoglobin is formed when oxygen binds to Hb heme in RBCs of the
pulmonary capillaries running along pulmonary alveolus wall. Deoxyhemoglobin
corresponds to hemoglobin that has released its oxygen. Oxyhemoglobin and
deoxyhemoglobin differ by their absorption spectra. 37

× 5.5 ×

3.5.7.1 Hemoglobin-Oxygen Association and Dissociation

Oxygen saturation is a relative measure of the amount of oxygen that is dissolved or
carried in a given medium. Oxygen saturation in blood ( S O 2 ) yields the percentage
of hemoglobin binding sites occupied by oxygen.
Functional oxygen saturation refers to the ratio of oxyhemoglobin to all types of
functional hemoglobin:

oxyhemoglobin/(oxyhemoglobin + deoxyhemoglobin).

Fractional oxygen saturation is the ratio of oxyhemoglobin to the total amount of
hemoglobin that is able to carry oxygen, including dyshemoglobins:

oxyhemoglobin/(oxyhemoglobin + deoxyhemoglobin + carboxyhemoglobin

+ methemoglobin).

Hemoglobin-oxygen association-dissociation curve has a sigmoidal shape that
results from cooperative binding of oxygen to hemoglobin. At low partial pressures
of oxygen ( p O 2 ), most hemoglobin is deoxygenated. When oxygen partial pressure
reaches about 10 kPa, blood oxygen saturation approaches 100%. 38

37 Pulse oximeter measures functional saturation by transmitting two light wavelengths that are
differentially absorbed by oxyhemoglobin and deoxyhemoglobin. Oxyhemoglobin has signif-
icantly lower absorption of 660 nm wavelength than deoxyhemoglobin, while at 940 nm its
absorption is slightly higher. Functional saturation can also be invasively assessed from arterial
blood gas samples. Co-oximeter uses 4 light wavelengths to differentiate between oxyhemoglobin,
deoxyhemglobin, carboxyhemoglobin, and methemoglobin and to measure fractional saturation.
38 Hypoxemia leads to an oxygen arterial saturation ( S aO 2 ) below 90%. Oxygen venous saturation
( S vO 2 ) measures the amount of oxygen consumed by the body. S vO 2 below 60% indicates lack of
oxygen.