Formulas relating to diffraction of the focused Gaussian beam from a narrow blood microvessel have been derived. A native vessel has been considered as a set of moving random screens. The correlation function of intensity fluctuations of statistically inhomogeneous speckled speckles has been studied with applications to flow measurement. Dependencies of statistical characteristics of biospeckles with a small number of scatterers on the number of scattering screens have been analyzed. It has been shown that the value of Doppler shift in the scattered light essentially depends on the spatial velocity distribution in the blood flow.