Microclimates

An alternative to macroclimate systems is the creation of microclimates. The objects are placed within smaller spaces, such as cases, in which an ideal environment is maintained. One possibiUty is to install equipment to control the climate in individual cases, or groups of cases with similar materials, by mechanical means. 

Certain grades of siUca gel or selected clay minerals are often used. The buffeting material is preconditioned under the selected relative humidity and, after equihbration, installed in the case. This method of microclimate control has proven to be very efficient, not only in exhibition cases and storage spaces, but also in packing crates used for the transportation of sensitive objects. 

Hydrogen sulfide has traditionally been a problem in the tarnishing of silver and the discoloration of bronze patinas. This gas can be dealt with in the filters of the climate-control system as well as through the use of proper absorbing agents. For example, a paper treated with activated charcoal is fabricated especially for absorbing H2S within a microclimate. 

Air curtains with unheated outdoor air do not provide for the necessary microclimate in the immediate vicinity of the gate, but since no thermal energy is used, they reduce heat losses from the room. 

Microclimate The distinctive pattern of temperature, humidity, air movement, and purity within a relatively small zone either inside or outside a building. 

Microclimate suit A suit worn to protect an operator who is working m adverse conditions of either heat or cold. 

An endothermic animal generates its own body temperature, while an ectothermic animal does not. In general, endothermic animals have constant body temperatures that are typically greater than that of the surrounding environment, while ectothermic animals have variable temperatures. Ectotherms rely on behavioral temperature regulation—a snake will move from sun to shade until it finds a suitable microclimate that is close to its optimal body temperature. When exposed to direct sunlight, an ectotherm can increase its body temperature as much as 1°C (32.8°F) per minute. 

Land use changes in the tropics have resulted in a landscape characterized as a mosaic of logged forests, cleared fields, and successional forests. This results in the transformation from extremely fire resistant rainforest ecosystems to anthropogenic landscapes in which fire is a common event (16, 17), Fires occur in disturbed tropical forests because deforestation has a dramatic effect on microclimate. Deforestation results in lower relative humidities, increased wind speeds, and increased air temperatures. In addition, deforestation results in increased quantities of biomass that are susceptible to fire. This biomass may be in the form of forest slash, leaf litter, grasses, lianas or herbaceous species (16, 18). 

Jones, H.G. (1983). Plants and Microclimate. Cambridge Cambridge University Press. 

Mucus layer (glycoprotein gel, 90% oligosaccharide, implicated in pH microclimate) Unstirred water layer... 

The absorption of short-chain weak acids in the rat intestine, as a function of pH, does not appear to conform to the pH partition hypothesis [44]. Similar anomalies were found with weak bases [77]. The apparent pKa values observed in the absorp-tion-pH curve were shifted to higher values for acids and to lower values for bases, compared with the true pKa values. Such deviations could be explained by the effect of an acid layer on the apical side of cells, the so-called acid pH microclimate [44,70,73,76-84],... 

Shiau et al. [73] directly measured the microclimate pH, pHm, to be 5.2-6.7 in different sections of the intestine (very reproducible values in a given segment) covered with the normal mucus layer, as the luminal (bulk) pH, pH/, was maintained at 7.2. Good controls ruled out pH electrode artifacts. With the mucus layer washed off, pHm rose from 5.4 to 7.2. Values of pHfo as low as 3 and as high as 10 remarkably did not affect values of pHm. Glucose did not affect pHm when the microclimate was established. However, when the mucus layer had been washed off and pHm was allowed to rise to pHfo, the addition of 28 mM glucose caused the original low pHm to be reestablished after 5 min. Shiau et al. [73] hypothesized that the mucus layer was an ampholyte (of considerable pH buffer capacity) that created the pH acid microclimate. 

The tips of villi have the lowest pHm values, whereas the crypt regions have pHm > 8 values [70]. Most remarkable was that an alkaline microclimate (pHm 8) was observed in the human stomach, whose bulk pHfo is generally about 1.7. In the stomach and duodenum, the near-neutral microclimate pH was attributed to the secretion of HCO3 from the epithelium [70]. 

Said et al. [78] directly measured the acid microclimate on the surface of gastrointestinal tract (GIT) epithelial cells (intact with mucus layer) in rats. The pH on the apical (donor) side of the cells varied from 6.0 to 8.0, while the pH on the basolateral (acceptor) side was 7.4. Furthermore, the pH gradient between... 

TABLE 7.2 Microclimate pH on the Apical Side of Epithelial Cells in the GIT in Rats... 

Permeability is a property closely tied to the environment of the epithelial cell surface. There is little point in measuring permeability at pH 1.7, if the microclimate barrier has pH >5 and <8, averaging 6. An in vitro permeability screen based on donor pH 5.0-7.4 and acceptor pH 7.4 seems about right. It will be useful to correct the data for the unstirred water layer effect, using computational methods. 

Precellular solute ionization dictates membrane permeability dependence on mucosal pH. Therefore, lumenal or cellular events that affect mucosal microclimate pH may alter the membrane transport of ionizable solutes. The mucosal microclimate pH is defined by a region in the neighborhood of the mucosal membrane in which pH is lower than in the lumenal fluid. This is the result of proton secretion by the enterocytes, for which outward diffusion is slowed by intestinal mucus. (In fact, mucosal secretion of any ion coupled with mucus-restricted diffusion will provide an ionic microclimate.) Important differences in solute transport between experimental systems may be due to differences in intestinal ions and mucus secretion. It might be anticipated that microclimate pH effects would be less pronounced in epithelial cell culture (devoid of goblet cells) transport studies than in whole intestinal tissue. 

Membrane uptake of nonionized solute is favored over that of ionized solute by the membrane/water partition coefficient (Kp). If Kp = 1 for a nonionized solute, membrane permeability should mirror the solute ionization curve (i.e., membrane permeability should be half the maximum value when mucosal pH equals solute pKa). When the Kp is high, membrane uptake of nonionized solute shifts the ionization equilibrium in the mucosal microclimate to replace nonionized solute removed by the membrane. As a result, solute membrane permeability (absorption rate) versus pH curves are shifted toward the right for weak acids and toward the left for weak bases (Fig. 7). 

Figure 11 Schematic of mucosal membrane sodium-proton exchanger and chloride-bicarbonate exchanger responsible for pH homeostasis in enterocyte cytosol. Microclimate pH is maintained by mucosal slowing of proton diffusion away from the lumenal membrane.

Figure 12 Schematic of generation of mucosal microclimate pH as a transmucosal proton-gradient driving force for di- and tripeptide carrier-mediated translocation across the mucosal membrane into the enterocyte.

YF Shiau, P Fernandez, MJ Jackson, S McMonagle. Mechanisms maintaining alow-pH microclimate in the intestine. Am J Physiol 248 G608-G617, 1985. 

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