This paper addresses the mechanisms behind selective endothelial permeability and their regulations. The singular properties of each of the seven blood-tissues barriers. Then, it further revisits the physical, quantitative meaning of permeability, and the way it should be measured based on sound physical chemistry reasoning and methodologies. Despite the relevance of permeability studies one often comes across inaccurate determinations, mostly from oversimplified data analyses. To worsen matters, the exact meaning of permeability is being lost along with this loss of accuracy. The importance of proper permeability calculation is illustrated with a family of derivatives of kyotorphin, an analgesic dipeptide.