General Chemistry

Spectroscopy and the Color of Blood

The changes that occur in blood upon oxygenation and deoxygenation are visible not only at the microscopic level, as detailed above, but also at the macroscopic level. Clinicians have long noted that blood in the systemic arteries (traveling from the heart to the oxygen-using cells of the body) is red-colored, while blood in the systemic veins (traveling from the oxygen-using cells back to the heart) is blue-colored (see Figure 7). The blood in the systemic arteries is oxygen-rich; this blood has just traveled from the lungs (where it picked up oxygen inhaled from the air) to the heart, and then is pumped throughout the body to deliver its oxygen to the body's cells. The blood in the systemic veins, on the other hand, is oxygen-poor; it has unloaded its oxygen to the body's cells (exchanging the O2 for CO2, as described below), and must now return to the lungs to replenish the supply of oxygen. Hence, a simple macroscopic observation, i.e., noting the color of the blood, can tell us whether the blood is oxygenated or deoxygenated.

What causes this color change in the blood? We know that the shape of the heme group and the hemoglobin protein change, depending on whether hemoglobin is oxygenated or deoxygenated. The two conformations must have different light-absorbing properties. The oxygenated conformation of hemoglobin must absorb light in the blue-green range, and reflect red light, to account for the red appearance of oxygenated blood. The deoxygenated conformation of hemoglobin must absorb light in the orange range, and reflect blue light, to account for the bluish appearance of deoxygenated blood. We could use a spectrophotometer to examine a dilute solution of blood and determine the wavelength of light absorbed by each conformation. For an approximate prediction of the wavelength of light absorbed and for the colors of light absorbed for a given complementary color, a table such as Table 1 in the introduction to the Experiment ("Relations Between Electronic Transition Energy and Color") could be used.

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Revised: 2004-08-08