Citations from 1980 to 1988, obtained from fifty biomedical journals covered by theJournal Citation Reports (JCR) are studied. In purely numerical terms, the evolution of each citation (journal citation), including its impact factor (IF), would depend essentially on three variables for each journal: (i) the yearly rate of increase of items that could be cited (citable items), (ii) the relative yearly increment of the citing journals, (iii) the relative yearly increment of citations. The mechanics of this give rise to the three standard patterns for journal citations, namely: (i) annual impact factors increase each year (ascending evolution), (ii) annual impact factors remain the same each year (constant evolution), (iii) annual impact factors decrease each year (descending evolution). The reason why some journal citation profiles do not fit into the standard patterns is presumably that forces are at work able to alter the numerical mechanics described. The concepts of saturation/unsaturation of the demand for scientific information are introduced, showing how they are reflected in the impact factor figures for the journals cited.