微循环血液流变学（英语）

微循环血液流变学（英语） 华东理工大学2010—2011学年第_2_学期 《生物流体力学基础(双语)》课程论文 2011.04 班级 过程083 学号10082348 姓名 吴琦玲 开课学院 动力工程及过程机械系 任课教师 张洪波 成绩______ 论文题目:FLOW IN MICROVASCULAR NETWORKS 论文要求:Write in English and at least 1000 words; anything about hemorheology. 教师评语: FLOW IN MICROVASCULAR NETWORKS Key Words: microcirculation, microvessels, microvascular networks, hemorheology Abstract: We review major experimental and theoretical studies on microcirculation. We discuss the substantial heterogeneity in the distributions of RBC velocity and concentration between vascular segments of similar size in the microcirculation and theoretical modeling of blood flowin a microvascular network. 1.INTRODUCTION The microcirculation represents the smallest blood vessels in the body and it consists of the capillary network ,the smallest vessels of 4–8μm inner diameter ;the arterioles, vessels up to ?100μm in the arterial system; and the venules ,vessels somewhat larger in the venous system. The microcirculation is responsible for regulating blood flow in individual organs and for exchange between blood and tissue. Approximately 80% of the total pressure drop between the aorta and the vena cava occurs in these vessels. These features differentiate the microcirculation from the larger vessels of the macrocirculation , which serve as conduits to and from the heart and peripheral organs and as high and low pressure reservoirs essential to cardiac function. Another distinction is that microcirculatory vessels are embedded within an organ whereas most macrocirculatory vessels are not. This enables communication between the parenchymal tissue and thesevessels. The deleterious consequences of diseases such as hypertension, sickle cell anemia, and diabetes exclusively, or to a great extent, afflict the microcirculation. 2. THE HISTORY OF EXPERIMENTAL STUDIES ON MICROVESSELS The history of experimental studies on microvessels goes back to the seventeenth century with the advent of the microscope. This led to Malpighi’s discovery of the capillary system while van Leeuvenhoek described the complex branching network of microcirculatory vessels in the tail of the eel and measured the velocity of red cells in precapillary vessels to be on the order of 2 mm/s. To better understand the factors that determine flow in the blood vessels, in 1830 a French physician, Poiseuille, performed his now-classic experiments on the hydrodynamics of tube flow (Sutera&Skalak 1993). The principles revealed in those studies form the basis of much of our current understanding of blood flow in the larger vessels and in the microcirculation. In the 1930s, a Swedish physiologist, F?ahræus, investigated the unique properties of blood flow in small glass tubes and set the foundation for subsequent research on microvascular flow and hemorheology (Goldsmith et al. 1989). A renewed interest in the field since the 1960s has led to significant advances in the mechanics of the microcirculation (Chien 1987, Cokelet 1987, Fung & Zweifach 1971, Skalak et al. 1989). Recent experimental description of the microcirculation has progressed greatly, in large part because of developments in intravital microscopy and image analysis, the development of fluorescent probes for in vivo measurements, and new techniques for measuring molecular concentrations with high spatial and temporal resolution. As methodologies for direct studies in the microcirculation progress, comparisons betweenin vitro and in vivo rheological findings are providing new insights to the unique features of the microcirculation. Recent reviews give an account of experimental and theoretical developments in the field of microvascular hemorheology (Baskurt &Meiselman 2003, Mchedlishvili&Maeda 2001, Pries et al. 1996, Secomb 2003), flow mechanics (Schmid-Schonbein 1999), and mathematical models (Secomb 2003). 3. FLOW IN MICROVASCULAR NETWORKS There are certain anatomic and topographic features of microvascular networks that appear to be common to a variety of organs and tissues. A functional feature commonly present in vascular beds is the substantial heterogeneity in the distributions of RBC velocity and concentration between vascular segments of similar size in the microcirculation. In skeletal muscle, which has been studied most extensively, the coefficient of variation (Standard Deviation/mean) of RBC velocity in capillaries, arterioles, and venules typically varies between 50% and 100% (Duling & Damon 1987). These heterogeneities are the hallmark of the microcirculation and significantly impact its exchange function. They can be interpreted as fractal properties of the microcirculation (Beard & Bassingthwaighte 2000). The heterogeneity of RBC velocities is determined by several factors, among them the topology and geometry of the microvascular pathways resulting in different effective hydrodynamic resistances along these pathways. The source of hematocrit heterogeneity is uneven separation of RBCs and plasma at microvascular bifurcations. Daughter branches with higher bulk flow draw a disproportionately larger number of RBCs. When flow in a daughter branch is sufficiently small, the branch might receive few or no RBCs, which in the extreme results in a plasmatic vessel. The relationship between the ratio of RBC flux into a daughter branch and the total RBC flux in the parent vessel of a bifurcation versus the ratio of the corresponding bulk flows is called “the bifurcation law.” Support for the bifurcation law was obtained empirically in vitro and in vivo (Pries et al. 1996) and, in some cases, can be established on theoretical grounds (Enden & Popel 1994). At the network level, the tendency of vascular segments with lower blood flow to have lower vessel hematocrit results in a lower mean hematocrit calculated over segments of similar dimensions. By analogy with the F?ahræus effect for a single segment, this effect was termed the “network F?ahræus effect” (Pries et al. 1996). In essence, the low-flow vessels in a network can be likened to the cell-depleted layer in an individual microvessel. The apparent viscosity of blood in a vascular segment is a function of segment diameter, the local hematocrit and, in some situations, wall shear rate (e.g., if RBC aggregation is significant). Theoretical modeling of blood flowin a microvascular network involves several steps. First, the geometry of the network is specified, i.e., segment interconnections and their lengths and diameters (and cross-sectional shapes, if not cylindrical) are measured. Pressure-flow relationships are expressed in terms of Equation 1 with the apparent viscosity μ specified. In addition, the bifurcation law describes bulk a flowversus theRBCflux relationship at every bifurcation. Therefore, for a network containing N segments, the problem consists of 2N nonlinear algebraic equations with respect to the unknown flow rate Q and discharge hematocrit H for each segment. Alternatively, the flow can be expressed in terms of pressures at the ends of the segment according to Equation 1. At the inlet and outlet nodes of the network, either flow or pressure can be specified. Solving the system of algebraic equations results in a prediction of the distribution of flow, pressure, and hematocrit throughout the network. This scheme was applied to predict the distribution of hemodynamic parameters in a rat mesentery network consisting of several hundred segments, for which detailed morphologic and hemodynamic measurements are available (Pries et al. 1996, 1994). Using the values of μ based on in vitro measurements resulted in significant discrepancies a between the predicted and measured values; minimizing these discrepancies yielded the values of in vivo apparent viscosity that were higher than the corresponding in vitro values; the difference was attributed to several sources including one due to the ESL, which decreases the effective luminal diameter available to blood flow in the microcirculation by retarding plasma flow near the endothelial-cell surface. These calculations suggest that for a given discharge hematocrit, the total network hydrodynamic resistance is considerably higher than the resistance based on in vitro values of μ (Pries et al. 1996). This disparity persisted in both a arterioles and venules as large as 30 to 40μm i.d. Thus far, the rat mesentery is the only tissue for which theoretical predictions have been compared with experimental measurements on a segmentby- segment basis; it remains to be seen whether the formulated in vivo apparent viscosity and bifurcation laws are quantitatively the same for other tissues. The emerging understanding of the dynamic nature of the ESL and its dependence on such factors as plasma composition suggests that this structure may be an important determinant of local blood-flow regulation in a network. Below we consider other regulatory factors such as wall shear stress, oxygen level, and arteriolar wall hoop stress. REFERENCE: [1]Baskurt OK, Meiselman HJ. 2003. Blood rheology and hemodynamics. Semin. 29:435–50 Thromb.Hemost. [2] Bishop JJ, Nance PR, Popel AS, Intaglietta M, Johnson PC. 2001a. Effect of erythrocyte aggregation on velocity profiles in venules. Am. J. Physiol. Heart Circ. 280: H222–36 Physiol. [3] Bishop JJ, Nance PR, Popel AS, Intaglietta M, Johnson PC. 2004. Relationship between erythrocyte aggregate size and flow rate in skeletal muscle venules. Am. 286:H113–20 J. Physiol. Heart Circ. Physiol. [4] Cornelissen AJ, Dankelman J, VanBavel E, Spaan JA. 2002. Balance between myogenic, flow-dependent, and metabolic flow control in coronary arterial tree: a model study. Am. J. Physiol. Heart Circ. Physiol. 282:H2224–37 [5] Das B, Johnson PC, Popel AS. 2000. Computational fluid dynamic studies of leukocyte adhesion effects on non-Newtonian blood flow through microvessels. Biorheology 37:239–58