Limiting permeability

In nearly all oil or gas reservoirs there are layers which do not contain, or will not produce reservoir fluids. These layers may have no porosity or limited permeability and are generally defined as non reservoir intervals. The thickness of productive (net) reservoir rock within the total (gross) reservoir thickness is termed the net-to-gross or N/G ratio. 

BCS Class IV Low-solubility, low-permeability drugs. These compounds have very poor oral bioavailability. They are not only difficult to dissolve but often exhibit limited permeability across the GI mucosa. These drugs tend to be very difficult to formulate and can exhibit very large intersubject and intrasubject variability. 

Amide 39 is a potent and selective inhibitor of GlyT-1 both in vitro (Ki = 1.79 nM) and in vivo (CSF-glycine ED2oo 3.9 mg/kg, rat, p.o.) but with limited permeability across Madin-Darby canine kidney (MDCK) cell membranes [78]. Optimization led to fused [3.1.0] and [3.3.0] azabi-cyclic analogs 40 (PF-3463275) [78] and 41 [79], respectively. Analogs of both systems demonstrated excellent potency (fC = 1.7-95 nM), and improved permeability, PK, and in vivo efficacy. Spatial working memory... 

TIMESCALES AND LIMITING PERMEABILITIES RELATED TO THE VARIOUS PROCESSES... 

As a result, these membranes suffer two deficiencies low flux rate due to the low porosity (i.e., limited permeability) and high fouling rate due to the asymmetric pore-size distribution having small pores on the surface [113]. 

Target accessibility to probe may be restricted because of limited permeability of cell membrane and crosslinking of cellular proteins during fixation. 

The use of yeast cells as a eukaryotic complement to the Ames test led to the development of several protocols for the detection of mutation, gene conversion and recombination. The formal introduction of methods [23] followed by much development work from Zimmermarm s laboratory led to large systematic studies [24, 25] and OECD guidelines for the test battery (OECD 480, 481). However the assays are now rarely used, at least in part because of concerns over low sensitivity, thought to reflect limited permeability of the cell wall. 

The guarded hot-plate method can be modified to perform dry and wet heat transfer testing (sweating skin model). Some plates contain simulated sweat glands and use a pumping mechanism to deliver water to the plate surface. Thermal comfort properties that can be determined from this test are do, permeability index (T), and comfort limits. Permeability index indicates moisture—heat permeability through the fabric on a scale of 0 (completely impermeable) to 1 (completdy permeable). This parameter indicates the effect of skin moisture on heat loss. Comfort limits are the predicted metabolic activity levds that may be sustained while maintaining body thermal comfort in the test environment. 

Bozic and Svetina [36] analysed a different situation, where addition of membrane constituents happens from the external milieu, and there is no metabolism inside, but there is limited permeability. They supposed that the membrane assumes spontaneous membrane curvature. This is non-zero if the properties of the inside and outside solutions differ, or if the two layers of a bilayer membrane differ in composition, or if some membrane-embedded constituents are asymmetrically shaped. They were able to show that under these assumptions membrane division is possible provided TLkC4 > 1.85, where T is the time taken to double the membrane area, L is the hydraulic permeability of the membrane, k is the bending modulus, and C is the spontaneous membrane curvature. In this model growing vesicles first retain spherical shape, then are distorted to a dumbbell, then to a pair of asymmetric vesicles coupled by a narrow neck, and finally to a pair of spherical vesicles linked by a narrow neck. Separation of the two daughter vesicles occurs as a result of mechanical agitation in the solution. 

Despite their apparent efficacy, the usefulness of sodium chloride solutions in the treatment of edematous corneas with a traumatized epithelium appears to be limited. The intact corneal epithelium exhibits limited permeability to inorganic ions. In the absence of an intact epithelium the cornea imbibes salt solutions, which reduces the osmotic effect. In the management of corneal edema associated with traumatized epithelium, hypertonic saline solutions may be of limited value due to their increased ability to penetrate the epithelial barrier. 

It was soon recognized that gastric HVK -ATPase is the site of action for omeprazole [27,43] and that enzyme inhibition parallels inhibition of gastric acid secretion in laboratory animals [38]. Physico-chemically, omeprazole represents a lipid-permeable weak base with a pK of 4 [42]. At physiological pH, it is predominantly unionized and this neutral form passes freely across biological membranes. However, in an acidic environment with a pH below 4, it is predominantly protonated. This results in a limited permeability of the drug [28]. Due to the unique structure of the gastric... 

The predominant circulating form of vitamin Bg is pyridoxal phosphate. Absorbed pyridoxine is oxidized and phosphorylated in intestinal mucosal cells, liver, and erythrocytes. Pyridoxine enters hepatocytes and erythrocytes by passive diffusion and is mostly retained by phosphorylation. Pyridoxal phosphate is transported in the blood bound to albumin. The blood-brain barrier has limited permeability to pyridoxal. 

Any assessment of the central effects of noradrenaline following its parenteral administration has to be made with care since the blood-brain barrier has a limited permeability to the catecholamines. The feelings of anxiety and apprehension reported by subjects who have received adrenaline... 

Capillaries in the Brain (the Blood-Brain Barrier). Capillaries in the brain are less permeable than capillaries in other tissues. This limited permeability, which is frequently called the blood-brain barrier, is essential for brain function. Reduced permeation provides a buffer that maintains a constant brain extracellular environment, even at times when blood chemistry is changing. The basis for this lower permeability is the relative paucity of pores in the brain endothelium. Therefore, molecules that move from blood to brain must diffuse through the endothelial cell membranes. As expected from this observation, the permeability of brain capillaries depends on the size and lipid solubility of the solute. In general, molecules that are larger than several hundred in molecular weight do not permeate into the brain. Empirical relationships between cerebrovascular permeability and the oil / water partition coefficient have been developed [26] (see Figure 5.27) ... 

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