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The absorption spectra of oxyhemoglobin and deoxyhemoglobin differ. The oxyhemoglobin has significantly lower absorption of the 660 nm wavelength than deoxyhemoglobin, while at 940 nm its absorption is slightly higher. This difference is used for the measurement of the amount of oxygen in a patient's blood by an instrument called a pulse oximeter.
A CO-oximeter measures the absorption of light passing through blood from few as two or three wavelengths of light to several dozens of wavelengths, in order to distinguish oxyhemoglobin, and deoxyhemoglobin (formerly called 'reduced' hemoglobin), and thus determine the oxyhemoglobin saturation (the percentage of oxygenated hemoglobin compared to the total amount of available hemoglobin (Hb)).
Experiments shown that when the content of oxygen in the breathing gas changed gradually, the contrast of these images changed gradually. Ogawa proposed and proved that the oxyhemoglobin and deoxyhemoglobin is the major contribution of this difference. [7]
The average red blood cell contains 250 million hemoglobin molecules. [7] Hemoglobin contains a globin protein unit with four prosthetic heme groups (hence the name heme-o-globin); each heme is capable of reversibly binding with one gaseous molecule (oxygen, carbon monoxide, cyanide, etc.), [8] therefore a typical red blood cell may carry up to one billion gas molecules.
Histidine residues in hemoglobin can accept protons and act as buffers.Deoxygenated hemoglobin is a better proton acceptor than the oxygenated form. [1]In red blood cells, the enzyme carbonic anhydrase catalyzes the conversion of dissolved carbon dioxide to carbonic acid, which rapidly dissociates to bicarbonate and a free proton:
Then oxyhemoglobin and deoxyhemoglobin will have similar contributions to the total absorption (black) and the effective attenuation (magenta) coefficient spectra, as shown in Figure 6 (b). Figure 7: : Effective penetration depth in breast tissue (StO2 ≈ 70%). Effective attenuation coefficient: λ min = 730 nm; NIR window = (626–1316) nm.
Binding of carbon dioxide to hemoglobin to form carbaminohemoglobin. Carbaminohemoglobin (carbaminohaemoglobin BrE) (CO 2 Hb, also known as carbhemoglobin and carbohemoglobin) is a compound of hemoglobin and carbon dioxide, and is one of the forms in which carbon dioxide exists in the blood. [1]
Specifically, the working of the oximeter is based on the difference in the light absorption spectrum of oxyhemoglobin and deoxyhemoglobin. Oxyhemoglobin absorbs more infrared light (wavelength of 940 nm) while deoxyhemoglobin absorbs more red light (wavelength of 660 nm).