Which law describes the relationship used by the pulse oximeter to determine saturation?

Which law describes the relationship used by the pulse oximeter to determine saturation?

• A. Ohm's Law
• B. Beer's Law
• C. Darcy's Law
• D. Faraday's Law

Answer: (B)

The main concept here is how light absorption relates to the amount of a specific absorber in a medium. Pulse oximetry relies on Beer-Lambert law, which connects how much light is absorbed to the concentration of absorbing molecules and the distance the light travels through the tissue. In practice, a pulse oximeter sends two wavelengths of light through a finger (or another site) and measures how much light makes it to the sensor. Oxygenated and deoxygenated hemoglobin absorb those two wavelengths differently, so by applying Beer-Lambert principles, the device uses the ratio of absorptions at the two wavelengths to estimate the fractions of HbO2 and Hb that determine arterial oxygen saturation (SpO2). The law’s core idea—absorbance increases with concentration and path length—underpins how the device converts light measurements into a saturation value. Other laws describe different phenomena that don’t govern light attenuation in tissue. Ohm’s Law is about electrical current and resistance, Darcy’s Law concerns fluid flow through porous media, and Faraday’s Law deals with induction from changing magnetic fields. None of these describe how light absorption by hemoglobin relates to its concentration, which is why Beer-Lambert law is the appropriate description here.

The main concept here is how light absorption relates to the amount of a specific absorber in a medium. Pulse oximetry relies on Beer-Lambert law, which connects how much light is absorbed to the concentration of absorbing molecules and the distance the light travels through the tissue. In practice, a pulse oximeter sends two wavelengths of light through a finger (or another site) and measures how much light makes it to the sensor. Oxygenated and deoxygenated hemoglobin absorb those two wavelengths differently, so by applying Beer-Lambert principles, the device uses the ratio of absorptions at the two wavelengths to estimate the fractions of HbO2 and Hb that determine arterial oxygen saturation (SpO2). The law’s core idea—absorbance increases with concentration and path length—underpins how the device converts light measurements into a saturation value.

Other laws describe different phenomena that don’t govern light attenuation in tissue. Ohm’s Law is about electrical current and resistance, Darcy’s Law concerns fluid flow through porous media, and Faraday’s Law deals with induction from changing magnetic fields. None of these describe how light absorption by hemoglobin relates to its concentration, which is why Beer-Lambert law is the appropriate description here.