Bulk flow of fluid

Bulk flow is expressed as the material or energy carried by the bulk flow of fluid into or out of the control region. 

Explain how hydrostatic forces and osmotic forces regulate the bulk flow of fluid across the capillary wall... 

Figure 15.7 Starling principle a summary of forces determining the bulk flow of fluid across the wall of a capillary. Hydrostatic forces include capillary pressure (Pc) and interstitial fluid pressure (PJ. Capillary pressure pushes fluid out of the capillary. Interstitial fluid pressure is negative and acts as a suction pulling fluid out of the capillary. Osmotic forces include plasma colloid osmotic pressure (np) and interstitial fluid colloid osmotic pressure (n,). These forces are caused by proteins that pull fluid toward them. The sum of these four forces results in net filtration of fluid at the arteriolar end of the capillary (where Pc is high) and net reabsorption of fluid at the venular end of the capillary (where Pc is low).

When chemisorption is involved, or when some additional surface chemical reaction occurs, the process is more complicated. The most common combinations of surface mechanisms have been expressed in the Langmuir-Hinshelwood relationships 36). Since the adsorption process results in the net transfer of molecules from the gas to the adsorbed phase, it is accompanied by a bulk flow of fluid which keeps the total pressure constant. The effect is small and usually neglected. As adsorption proceeds, diffusing molecules may be denied access to parts of the internal surface because the pore system becomes blocked at critical points with condensate. Complex as the situation may be in theory,... 

Cations striking a cathode liberate electrons. A series of dynodes multiplies the number of electrons by 105 before they reach the anode, electroosmosis Bulk flow of fluid in a capillary tube induced by an electric field. Mobile ions in the diffuse part of the double layer at the wall of the capillary serve as the pump. Also called electroendosmosis. electroosmotic flow Uniform, pluglike flow of fluid in a capillary tube under the influence of an electric field. The greater the charge on the wall of the capillary, the greater the number of counterions in the double layer and the stronger the electroosmotic flow. 

Under aqueous conditions, the inner wall of the capillary becomes negatively charged as a result of the presence of acidic silanol groups. In an electric field, this negative charge results in a bulk flow of fluid in the capillary toward... 

The bulk flow of fluid that results whenever a sound wave is present in the medium is given the term microstreaming. Microstreaming, enhances mass transfer and as such aids interparticle collisions, again resulting in the particles achieving a non-Dysthe equilibrium orientation. 

This is beyond the resolution of the NMR imaging techniques that are discussed in Section 5. In this section, we shall consider the theory behind the application of PFG methods to measure diffusion coefficients, and then go on to consider how diffusion coefficients in porous media reveal structural information. Some PFG pulse sequences that overcome particular problems experienced for porous media are discussed. PFG methods on samples in which there is bulk flow of fluid will be considered in Section 6. 

The movement of solute molecules across the capillary wall occurs primarily by two mechanisms diffusion and solvent drag. Diffusion is the passive mechanism of transport that rapidly and efficiently transports small solutes over the small distances (tens of microns) between the blood supply (capillaries) and tissue cells. Solvent drag refers to the movement of solute that is entrained in the bulk flow of fluid across the capillary wall and is generally neghgible, except in cases of large molecules with small diffusivities and high transcapiUary fluid flow. 

The method of inducing bulk flow of fluid within the device can take two general forms pressure driven flow and electroosmotically induced flow. Both methods have advantages and disadvantages and are somewhat dependent on the construction material. Although there are many methods of inducing pressure driven flow, the ultimate flow characteristics are similar. 

Closed-cell foamed plastic (unicellular foam) n. A cellular plastic in, which interconnecting cells are too few to permit the bulk flow of fluids through the mass. 

Bulk flow plays only a minor role in the exchange of specific solutes between blood and tissue cells. A far more important function of bulk flow is to regulate distribution of extracellular fluid between the vascular compartment (plasma) and the interstitial space. Maintenance of an appropriate circulating volume of blood is an important factor in the maintenance of blood pressure. For example, dehydration and hemorrhage will cause a decrease in blood pressure leading to a decrease in capillary hydrostatic pressure. As a result, net filtration decreases and net reabsorption increases, causing movement, or bulk flow, of extracellular fluid from interstitial space into the vascular compartment. This fluid shift expands the plasma volume and compensates for the fall in blood pressure. 

The flow of cerebrospinal fluid is essentially unidirectional that is, it flows from its site of formation in the choroid plexus through the ventricles to its site of exit at the arachnoid villi. Drugs in this fluid can either enter the brain tissue or be returned to the venous circulation in the bulk flow of cerebrospinal fluid carried through the arachnoid villi. Some drugs, such as penicillin, wUl not leave the cerebrospinal fluid compartment by bulk flow but will be actively transported by the choroid plexus out of the fluid and back into the blood. Finally, drugs may diffuse from brain tissue directly into blood capUlaries. 

Advection is the transport of dissolved contaminant mass due to the bulk flow of groundwater, and is by far the most dominant mass transport process [2]. Thus, if one understands the groundwater flow system, one can predict how advection will transport dissolved contaminant mass. The speed and direction of groundwater flow may be characterized by the average linear velocity vector (v). The average linear velocity of a fluid flowing in a porous medium is determined using Darcy s Law [2] ... 

Abbott NJ (2004) Evidence for bulk flow of brain interstitial fluid significance for physiology and pathology. Neurochem Int 45 545-552... 

Cserr HF, Ostrach LH (1974) Bulk flow of interstitial fluid after intracranial injection of blue dextran 2000. Exp Neurol 45 50-60... 

Rosenberg GA, Kyner WT, Estrada E (1980) Bulk flow of brain interstitial fluid under normal and hyperosmolar conditions. Am J Physiol 238 F42-F49 Rosenberg GA, NavratU M (1997) MetaUoproteinase inhibition blocks edema in intracerebral hemorrhage in the rat. Neurology 48 921-926... 

D. Properties of Bulk Flow Inside Fluid Beds. 297... 

Whether an increase in vascular permeability results in mucosal edema depends on the balance between the amount of leakage into the mucosa and the rate of clearance from the mucosa, either through the lymphatics or across the epithelium into the airway lumen. The increase of vascular permeability produced by inflammatory stimuli can result in the bulk flow of plasma into the airway mucosa (Renkin, 1992). The amount of plasma leakage depends upon the number of gaps that form in the endothelium of the leaky vessels, the duration of the gaps and the intravascular pressure that drives the extravasation (Clough, 1991 Taylor and Ballard, 1992). The movement of plasma proteins and other osmotically active solutes into the mucosa can increase the interstitial oncotic pressure, which favors the net movement of fluid out of vessels and further increases the amount of leakage (Taylor and Ballard, 1992). 

The bulk motion of fluid is common throughout the environment this advective motion is described mathematically by the direction and the magnitude of its velocity. If a chemical is introduced into flowing air or water, the chemical is transported at the same velocity as the fluid. While spreading due to Fickian transport may occur at the same time, as described in the next section, the center of mass of the chemical moves by advection at the average fluid velocity. 

Figure 9.1. General representation of the surface renewal model. An eddy arrives at the interface and resides there for randomly varying periods of time. During this period, there is plug flow of fluid elements. The bulk fluid is considered to be located at an infinite distance from the interface. Pictorial representation adapted from Scriven (1968, 1969).

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