Changes in optical properties of human whole blood in vitro due to slow heating

Optical properties of human whole blood were investigated in vitro at 633 nm using a double integrating sphere set-up. The blood flow was maintained at a constant rate through a flow cell while continuously heating the blood at 0.2-1.1°C/min from approximately 25 to 55°C in a heat exchanger. A small, but rather abrupt decrease in the scattering asymmetry factor (g-factor) of 1.7 ± 0.6% and a similar increase in the scattering coefficient of 2.9 ± 0.6% were observed at approximately 45-46°C yielding an increase in the reduced scattering coefficient of 40 ± 10%. Furthermore, a continuous, manifest increase in the absorption coefficient was seen with increasing temperature, on average 80 ± 70% from 25 to 50°C. The effect of the heating on the blood cells was also studied under a white-light transmission microscope. A sudden change in the shape of the red blood cells, from disc-shaped to spherical, was observed at approximately the same temperature at which the distinct changes in g-factor and scattering coefficient were observed, i.e. at 45-46°C. The results indicate that this shape transformation could explain the sudden change in scattering properties.