The bulk viscosity of blood obtained by in vitro viscom-etry (Figure 1) does not fully address the viscous properties that determine microvessel perfusion for a given pressure gradient. In microvessesls larger than 20 mm in diameter, it has been demonstrated that at low physiological hematocrits, and in the normal flow state, the apparent viscosity (h) varies linearly with HMICRO (see Zweifach and Lipowsky ). In vitro studies of blood behavior in small-bore tubes have demonstrated that reductions of HMICRO attendant to diameter reductions result in a decrease in effective viscosity as well, which has been referred to as the Fahraeus-Linqvist effect. . In vivo, increased viscosity occurs with reductions in g, suggesting that the heterogenous perfusion of the microvasculature in the low flow state may arise in part due to substantial elevations in h. However, in vitro studies with small tubes do not reflect these trends and suggest that additional mechanisms may contribute to shear-dependent resistance increases in small blood vessels .
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.