Can someone explain me why does the Blood pressure falls down slowly followed by the development of anemia?

I can only guess that your question relates to the situation following a loss of blood. If the volume of blood falls, the blood pressure must drop, rather like the pressure in a deflated balloon. The body has mechanisms to compensate for this, including constricting the veins and arterioles to reduce the volume through which the blood circulates. The kidneys also produce less urine and the retained water stays in the circulation, to improve the circulating volume. This has the effect of diluting the concentration of red cells in the blood, with the development of the measured anaemia. The body also responds by generating new red cels and more haemoglobin, but this is a slower process, so it takes time for the anaemia to resolve. The movement of fluid in and out of the bloodstream has nothing to do with the blood viscosity, but is determined by the concentrations of salts and protein in the plasma.

The RBCs add thickness to the blood, which increases viscosity, therefore keeps fluid in the vessels. If haemoglobin is low, viscosity is reduced, and fluid is more likely to leak outside the vessels, thus reducing volume, thus reducing blood pressure.