Principles of blood rheology.

Blood rheology, that embraces the study of the de formation and flow of the blood, is an outbursting science. There are reasons to think that the develop ment of blood rheology (or Hemorheology) will imply deep changes in medical thought concerning the diag nosis, prognosis or therapy of several clinical disor ders. Hemorheology is not a specific field of medicine or of any of the classic specialties. Likewise, it is not also foreseeable that it will assume the form of an in dependent speciality in the future. In fact, several scholars pertaining to different academic sciences, particularly biochemists, biophysicists, physiologists, hematologists and cardiologists, are involved in the investigations concerning hemorheology. Most of the works that have beem so far published on this subject sprang out from a multidisciplinar cooperation and it is plausible that this characteristic be maintained in the near future. Hemorheology studies circulatory phenomena and their implications from an original point of view; whi le the investigations so far performed were almost ex clusively confined to the study of bloodvessels’wall, rheology is mostly concerned with the study of blood vessels’content. In fact, the circulation may not be considered apart from the vascular content since the rheologic behaviour of the blood, depending on the properties of its components (cells and plasma), will ultimately influence the blood viscosity, peripheral re sistance and, finally the relationships between blood pressure and volume. The utmost importance of erythrocyte deformabi lity in blood viscosity is due to two main reasons: a) the pressure of the intravascular bloodflow induces alterations of globular shape (usually discoid), com promising the fluency of bloodstream; b) In the mi crovessels and capillaries with a diameter inferior to the size of the erythrocyte, the ability of globular de formation assures nutrient exchange and tissue oxige nation to be carried out under conditions of maxi mum efficacy. The erythrocyte deformability in any territory of the circulation, though mainly in the mi crocirculation, may be viewed as an undoubtedly vital phenomenon influencing the rheologic properties of the blood and also as being the main determinant of globular half-life. Erythrocyte deformability is affected by factors intrinsic to the globule and by exogenous forces ap plied to its surface. In the latter case, the change of shape is mostly dependent from the distribution of the exogenous forces into the cell surface and from geo metric factors such as the vascular morphology. The intrinsic deformability of the erythrocyte is ruled by three main factors (viscoelastic properties of the membrane, viscosity of the globular contents, erythrocyte geometry), which are dependent on seve ral influences namely the concentration of ATP and Ca2 ~ on the spectrin-action cellular frame that coats the internal face of the erythrocytes. Erythrocyte deformability does not connote by it self that the circulation be perfect or oxygen delivery and tissue perfusion be adequate. In a normal bloodstream the erythrocytes easily cross narrowest arteries and capillaries without mo difying their disc-like shape; in crotched points the erythrocytes take peculiar shapes e. g. drop-like, of circumflex accent-like, etc.; in venules, though the erythrocytes keep their discoid shape, they tend to clump together forming temporary rouleaux that may persist under conditions of retarded bloodflow; when normal drainage is hampered erythrocytes frequently adhere to the endothelium clogging perhaps perma