Photoplethysmography uses infrared light to quantify which component of blood?

• A. Red blood cells
• B. White blood cells
• C. Platelets
• D. Plasma proteins

Answer: (A)

Photoplethysmography hinges on how blood absorbs light. Near-infrared light penetrates tissue and is absorbed mainly by hemoglobin inside red blood cells. As arterial blood volume in a region rises and falls with each heartbeat, the amount of light absorbed changes in a pulsatile way, producing the signal detected by the sensor. Thus, infrared light in this context quantifies red blood cells (via their hemoglobin content) and the associated blood volume changes. The other blood components—white blood cells, platelets, and plasma proteins—don’t provide the same distinct near-infrared absorption signal needed for this technique.