Clinical signs attributed to hyperviscosity may result from the regional effects of hyperviscosity, including tissue hypoxia, acidosis, and hypoglycemia, and from the formation of microthrombi within the microcirculation. An important caveat, however, is that the same clinical signs may result from coexisting perinatal circumstances in the presence or absence of hyperviscosity. Potentially affected organs include the central nervous system, the kidneys and adrenal glands, the cardiopulmonary system, and the gastrointestinal tract. Blood viscosity depends on the interaction of frictional forces in whole blood. These forces are defined as shear stress (refers to frictional forces within a fluid) and shear rate (a measure of blood flow velocity). The shear rate in the aorta is 230/s and only 11.5/s in the small arterioles and venules. As the viscosity increases, such as in the microcirculation, blood with a high hematocrit may virtually cease flowing. The frictional forces identified within whole blood and their relative contributions to hyperviscosity in the newborn include the following: