We present here a bird-eye view of time-dependent optical transmission of blood in red-near infrared spectral range. This issue is of the key importance both for fundamental understanding and for various applications connected with non-invasive optical blood analysis. A number of experiments measuring kinetics of blood transmission in the case of natural heart pulsations and of artificial kinetics following over-systolic occlusion is reviewed. The comprehensive theoretical approach has to consider scattering- associated mechanism rather than the widely accepted absorptionassociated one. Light scattering occurs on RBC aggregates. The size of aggregates and their shape change in time due to blood flow variations. It results in the corresponding changes of optical transmission.