Hemoglobin, a protein found within red blood cells, transports oxygen throughout the body, carrying it from the lungs to various tissues and organs, and then returning carbon dioxide to the lungs to be exhaled. Hemoglobin’s interaction with light, known as its “absorption spectrum,” allows us to understand and measure its function in the bloodstream.
Understanding Hemoglobin’s Light Interaction
An absorption spectrum describes how a substance absorbs different wavelengths of light. When light shines on a material, some wavelengths are absorbed while others are reflected or transmitted. Hemoglobin’s structure, particularly its iron-containing heme groups, enables it to absorb light across a broad range, from ultraviolet to near-infrared.
The presence or absence of oxygen significantly alters hemoglobin’s light absorption. Oxygenated hemoglobin (oxyhemoglobin) has a distinct absorption pattern compared to deoxygenated hemoglobin (deoxyhemoglobin). Oxyhemoglobin absorbs more infrared light and less red light, appearing bright red. Deoxyhemoglobin absorbs more red light and less infrared light, appearing a darker, reddish-blue. This difference is fundamental to many medical applications.
The Role of Light in Oxygen Transport
The distinct light absorption patterns of oxyhemoglobin and deoxyhemoglobin are directly related to their biological function in oxygen transport. As hemoglobin binds to oxygen in the lungs, it transforms into oxyhemoglobin, which has a bright red appearance. This allows oxygen to be efficiently loaded onto the hemoglobin molecules.
When oxyhemoglobin travels to tissues that need oxygen, it releases the oxygen, converting back into deoxyhemoglobin. This deoxygenated form is the darker, bluish-red color often observed in venous blood. The change in color reflects the shift in hemoglobin’s structure as it picks up and releases oxygen, making its absorption characteristics a natural indicator of its oxygen-carrying status.
Measuring Oxygen Levels with Light
The differing absorption spectra of oxygenated and deoxygenated hemoglobin form the basis of the pulse oximeter. This non-invasive device clips onto a finger or earlobe and uses two light-emitting diodes (LEDs): one emitting red light (around 660 nanometers) and the other infrared light (around 940 nanometers).
These wavelengths are chosen because oxyhemoglobin and deoxyhemoglobin absorb them differently. A photodetector measures the light passing through the tissue. By analyzing the ratio of absorbed red and infrared light, the pulse oximeter calculates the percentage of oxygen saturation (SpO2) in arterial blood, representing the ratio of oxygenated hemoglobin to total hemoglobin. The device also accounts for pulsatile blood flow to isolate the arterial signal.
Beyond Oxygen: Other Clinical Insights
Hemoglobin’s absorption spectrum provides insights beyond oxygen saturation, especially when other substances bind to hemoglobin. For instance, in carbon monoxide poisoning, carbon monoxide binds to hemoglobin, forming carboxyhemoglobin (COHb). COHb has an absorption spectrum similar to oxyhemoglobin, which can mislead conventional pulse oximeters into reporting falsely high oxygen saturation. Specialized co-oximeters are required to accurately measure COHb levels by analyzing light absorption at multiple wavelengths.
Methemoglobinemia is another condition where hemoglobin’s iron is oxidized, making it unable to bind oxygen. Methemoglobin also has a distinct absorption spectrum, and its presence can cause pulse oximeters to display a saturation reading that plateaus around 82-86%, regardless of actual oxygen saturation. This “refractory hypoxemia” where oxygen saturation appears low despite supplemental oxygen, along with a bluish-brown skin discoloration, signals methemoglobinemia. Co-oximetry is necessary for an accurate diagnosis and measurement of methemoglobin levels.