Passive homeostasis

Impermeability of the membrane to protons and other ions plays a major role in preventing severe changes in pH in response to varying environmental changes. Another factor is the high buffering capacity of the cell due to the protein, glutamate, and polyamines (Hill, O Driscoll, and Booth, 1995). 

---

Sodium inflow and acute excitotoxicity. Neurotoxicity of excitatory amino acids is a direct consequence of the excessive excitatory depolarization—may be related to a loss of the ionic homeostasis and/or to a depletion of the energetic stocks of the cell (Olney et al. 1986). Neuronal damage could depend on the extracellular presence of sodium accompanied by a passive flow of chlorine and water (Hablitz and Langmoen 1982). Some considerations suggest that the acute excitatory swelling does not explain completely the damages induced by excitatory amino acids (Pulsinelli et al. 1982). 

Although the stratum corneum acts as a simple physical barrier to outside influences, skin tissue as a whole is very active. It is crucial in maintaining the body s homeostasis, its essential steady-state environment. Skin maintains temperature and balance of electrolytes, the dissolved salts in internal body fluids. It is metabolically active and participates in hormonal and immune regulatory processes. More than serving as a passive barrier, it is proactive in response to xenobiotic insults and can be damaged in the defensive process by developing rashes and other symptoms. 

The stratum comeum is usefully thought of as a brick wall , with the fully differentiated comeocytes comprising the bricks , embedded in the mortar created by the intercellular lipids. A layer of lipid covalently bound to the comified envelope of the comeocyte contributes to this exquisite organization. The intercellular lipids of the stratum comeum include no phosphohpids, comprising an approximately equimolar mixture of ceramides, cholesterol and free fatty acids. These non-polar and somewhat rigid components of the stratum comeum s cement play a critical role in barrier function. On average, there are about 20 cell layers in the stratum comeum, each of which is about 0.5 fim in thickness. Yet, the architecture of the membrane is such that this very thin structure limits, under normal conditions, the passive loss of water across the entire skin surface to only about 250 mL per day, a volume easily replaced in order to maintain homeostasis. 

To sum up the absence of stimulation by antigens and second signals promotes passive cell death. Maintenance, homeostasis, is only guaranteed when the cell continuously... 

Engelhardt JI, Siklos L, Appel SH (1997) Altered calcium homeostasis and ultrastructure in motoneurons of mice caused by passively trans-fened and-motoneuronal IgG. J Neuropath Exp Neurol 56 21—39. 

Disorders of Na" " homeostasis can occur because of excessive loss, gain, or retention of Na or because of excessive loss, gain, or retention of H2O. It is difficult to separate disorders of Na and H2O balance because of their close relationship in establishing normal osmolality in aU body water compartments. As described in detail in Chapter 45, the primary organ for regulating body water and extracellular Na" " is the kidney. However, as a brief introduction to this section, it is important to remind the reader of the functions of healthy kidneys. In the proximal tubules, 60% to 70% of the filtered Na" is actively reabsorbed, with H2O and CT following passively to maintain electrical neutrality and osmotic equivalence. In the descending loop of Henle, H2O, but not... 

Dietary phosphate intake is usually 1.2 to 1.4 g (39 to 45 mmoi)/day, nearly twice the recommended intake, of which approximately 60% to 70% is absorbed, principally in the jejunum. As with calcium, both passive and active transport systems exist 1,25(OH)2D is the prmcipal regulator of the active transport of phosphate. PTH-stimulated synthesis of 1,25(0H)2D thus offsets the phosphaturic effect of PTH. The prevailing serum phosphate concentration also modulates renal 25(0H)D-la-hydroxylase. Phosphate depletion or hypophosphatemia stimulates formation of l,25(OH)2D by the kidneys. In general, at pharmacological concentrations, calcitonin has the opposite effect of PTH. It is unclear, however, if calcitonm has any physiological role in mineral homeostasis in adult humans. 

In bacterial cells homeostasis is achieved by a combination of passive and active mechanisms (Hill, O Driscoll, and Booth, 1995). 

Interestingly, the osmotic concentration of the hemolymph of edematose larvae was nearly normal (28). Apparently sexta larvae have no satisfactory alternative means to eliminate the accumulated ornithine and edema results either from an active attempt to maintain osmotic homeostasis in the hemolymph or as the result of passive water retention. 

The absorption of dietary zinc occurs over the duodenal and jejunal regions of the gastrointestinal tract, and mainly follows via a saturable carrier-mediated transport process (Zapsalis and Beck 1985, Lee et al. 1989). The mechanism and control of zinc absorption from the intestine has not yet been fully elucidated, although absorption of zinc is known to be regulated homeostatically, mainly under the control of pancreatic and intestinal secretion and fecal excretion. Homeostasis may involve metal-binding proteins such as metallothionein and cysteine-rich intestinal protein. Metallothionein plays an essential role in the regulation of zinc metabolism (Richard and Cousins 1975, Petering and Fowler 1986). Other unknown mechanisms may also exist, and the uptake from intestinal mucosa may involve both active and passive transport processes. 

Figure 19.5 Central role of the plasma membrane ATPase in the maintenance of proton homeostasis within the cytoplasm. Protons gain entry into the cytoplasm through passive proton diffusion, which can increase with increasing ethanol from the uptake of protonated acids from symport with amino acids or from metabolism. The cytoplasm and vacuole can buffer proton levels, but the main buffering activity is provided by the action of the ATPase. Saturation of the ATPase can lead to cell death thus metabolic activities are tightly coordinated with ATPase activity.

The most incredible mobility occurs at the cellular level when fluids and electrolytes are exchanged across membranes to maintain homeostasis, the balance in the body needed to sustain life. While some of these exchanges are passive and flow freely with little effort, other exchanges are active, energy-exhausting processes designed to maintain a critical balance of fluid and electrolytes on each side of the cell membrane and an enviromnent that is appropriately charged with acids or bases... 

---