arterial blood is insignificant only the venous blood is pulsatile at the active pulser rate hence the ratio lines 980 are only a function of the venous oxygen saturation accordingly sp a o 2 can be derived from the magnitude of the pulse rate related ratio lines 980 in the same manner as sp a o 2 is derived from the magnitude of the pulse rate related ratio lines propagation of optical radiation through tissue is affected by absorption and scattering processes the operation of pulse oximeters was described qualitatively above using an analysis based on the beer lambert law of absorption equation 3 this approach however fails to account for the secondary effects of light scattering at pulse oximeter wavelengths the primary light scatterer in blood is erythrocytes i e red blood cells a qualitative understanding of the effects of scattering on pulse oximetry is aided by a description of red blood cell properties within flowing blood human blood is a suspension of cells in an aqueous solution the cellular contents are essentially all red blood cells with white cells making up less the 1 600 th of the total cellular volume and platelets less than 1 800 th of the total cellular volume normally the hematocrit which is the percentage of the total volume of blood occupied by cells is about 50 in large vessels and 25 in small arterioles or venules red blood cells are extremely deformable taking on various shapes in response to the hydrodynamic stresses created by flowing blood for example assuming a laminar blood flow within a vessel.