Laminar Blood Flow

Since laminar flow itself occurs at low values of Re( = Dupl/x), the most likely situations are those characterized by low velocity (u) or high viscosity (p,), such as those involving the slow flow of polymers in extrusion reactors, or of blood in certain organs in animals. Even if not a close approximation in some cases, the predictable performance of an LFR may serve as a limiting model for actual performance. 

Clinically, PC-MRA is often used for laminar flow with few pulsations as for example in the cerebral venous sinuses. Many users apply it as a thick slab 2D technique with short acquisition times and primary projective vessel depiction. 3D PC-MRA demands relatively long measurement times for data acquisition and is somewhat sensitive to patient movements. Principally, phase contrast methods additionally enable a quantification of blood flow velocity and the assessment of flow directions. 

Instead of using a furrowed or dimpled membrane support plate, Sobey [41] observed that a single flow deflector in a flat membrane channel could produce many vortices under oscillatory flow conditions, an effect named the vortex wave. An important feature of the vortex wave is that it could occur under low crossflow velocity conditions or with laminar flow so that it can be used for shear-sensitive fluids. Millward et al. [42] tested the effect of vortex waves on plasma filtration with waves produced by flow deflectors with cross-sectional area of 1 x 1 mm in a 2.25 mm high channel as shown in Figure 8.22. The aim was to improve membrane applications for the separation of plasma from cellular blood components for both donor... 

As outlined earlier, hemodialysis and hemofiltration require the removal of solutes smaller than albumin from blood. Solute mass transfer rates across hemodialysis membranes cannot exceed the diffusivity of the solute In water. Solute diffusivity decreases with Increasing molecular diameter (Stokes-Einstein relationship) consequently, solute mass transfer rates for hemodlalyzers intrinsically decrease with increasing molecular size. In addition to limitations Imposed by diffusion In solution, mass transfer is further limited by diffusion resistance in the membrane as well as boundary layer effects resulting from laminar flow both of these effects are also functions of molecular size. The quantitation of mass transfer In hemodlalyzers has been reviewed extensively (22). 

Blood flow Slow rate of flow Turbulent flow Fast rate of flow Laminar flow... 

Tubular Pinch Effect. The lateral movement of particles across the streamlines in laminar flow was first observed and recorded in 1836 by Poiseville. He noted that the region immediately adjacent to the walls of blood capillaries tends to be free of blood cells. 

The PNF-coated shafts were assembled into the test chambers. The test chambers were flushed with isotonic saline to displace the air interface. The blood flow through each of the chambers was adjusted to 200 mL/min using a Ward s doppler flow ultrasound cuff. The rotation of the shafts was maintained at 200 rpm. Under these conditions, a laminar flow regime was maintained with a shear rate of 150 s-1. Experiments were carried out for time periods of 60 min. After completion of each of the experiments, the chamber was flushed with isotonic saline. 

What important dimensionless number(s) appear in the dimensionless partial differential mass transfer equation for laminar flow through a blood capillary when the important rate processes are axial convection and radial diffusion ... 

In addition, microfiuidics makes use of phenomena not available in the macroscopic world. Well-predictable and controllable laminar flow patterns can be achieved by appropriate design of microchannels. Substance transport by diffusion becomes faster as dimensions shrink. Sample volumes are on the order of microliters to picoliters, and flow on the order of pl/min to nl/min can be realized, comparable to the sima-tion in capillary blood vessels. 

This immunoassay is based on the laminar flow in the microfluidic channel. Generally, the large particles such as blood cells are hard to diffuse significantly with time in the microfluidic laminar flow channel, while the fine particles such as ions and small molecules can rapidly diffuse between steams. Yager s group [3] developed a novel microfluidic whole blood immunoassay based on the diffusion, which is called the competitive diffusion immunoassay (DIA). As shown in Fig. 1, a simple Y-shape microstructure was... 

Inasmuch as the concentration of unbound hormone in plasma is too low to account for the amount of hormone that crosses the capillary bed, it has been postulated that a special mechanism must exist to promote release of hormone from the bound state, in particular from plasma albumin. On the other hand, it can be clearly shown that the dissociation rate from each of the transport proteins is fast enough to replenish the pool of unbound hormone during capillary transit. Furthermore, blood cells in the capillary lumen result in mixing of the contentsthus preventing laminar flow at the capillary wall and local depletion of the free hormone pool- Since the rate of dissociation from albumin is faster than from PA and, especially from TBG, it can be expected that much, if not most, of the and T3 that leaves the capillary is derived from the albumin-bound pool even though albumin binds only a small fraction of the total hormone. 

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