The body maintains a cell and tissue equilibrium, and a loss is in most cases repaired.

The blood fluid lost in a haemorrhage is quickly restored by a withdrawal of the fluid from the tissues into the blood, but the cells lost are restored by new formation of cells in the blood-forming organs.

The blood cells are all formed in bone marrow and in the lymph nodes, and not from the cells which circulate in the blood, and the stimulus to new cell formation which the loss of blood brings about affects this remote tissue.
In general, repair takes place most easily in tissues of a simple character, and where there is the least differentiation of cell structure for the purposes of function.

A high degree of function in which the cell produces material of a complex character necessitates a complex chemical apparatus to carry this out, and a complicated mechanism is formed less easily than a simple one.

In certain tissues the cells have become so highly differentiated that all formative activity is lost.