Major differences in physiological and pharmacological responsiveness may be observed between blood vessels that differ either in anatomical type or in that they serve different organs or tissues. 2. The intrinsic properties of the vascular smooth muscle appears to differ little between species. No important differences in noradrenergic innervation have been observed, but major differences may exist in neural dilator systems. A few differences have been demonstrated that can be attributed to variations in the receptor population. No clear differences in local humoral mechanisms are known. There is, however, an insufficient number of strictly comparable studies in different species and it is likely that more differences will be found as our knowledge of vascular responsiveness is extended.
