Steady-states homeostasis

Bonventre JV, Leaf A. Sodium homeostasis Steady states without a setpoint. Kidney Int 1982 21 880-883. 

This maintenance of relatively constant or steady-state internal conditions is referred to as homeostasis. It is important because the cells and tissues of the body will survive and function efficiently only when these internal conditions are properly maintained. This is not to say that the internal... 

FRET-based nanosensors have been successfully used to monitor steady state levels of metabolites, nutrients, and ions in mammalian cells [74, 87], Recently FRET-based glucose, sucrose, and amino acid nanosensors have been developed to study the metabolism of glucose, sucrose, and amino acid uptake and metabolism in plant cells [80,89, 91]. The enormous potential of these nanosensors will be crucial for understanding ion (e.g., calcium), metabolite (e.g., sugars), hormone (e.g., auxins, gibberellins etc.), and nutrient (e.g., nitrogen, potassium, phosphorus) requirements and homeostasis in living plant tissues. 

The notion of boundary is, in fact, one central concept in the theory of autopoiesis. Inside the boundary of a cell, many reactions and correspondingly many chemical transformations occur. However, despite all these chemical processes, the cell always maintains its own identity during its homeostasis period. This is because the cell (under steady-state conditions and/or homeostasis) regenerates within its own boundary all those chemicals that are being destroyed or transformed, ATP, glucose, amino acids, proteins, etc. 

Calcium and phosphate enter the body from the intestine. The average American diet provides 600-1000 mg of calcium per day, of which approximately 100-250 mg is absorbed. This figure represents net absorption, because both absorption (principally in the duodenum and upper jejunum) and secretion (principally in the ileum) occur. The amount of phosphorus in the American diet is about the same as that of calcium. However, the efficiency of absorption (principally in the jejunum) is greater, ranging from 70% to 90%, depending on intake. In the steady state, renal excretion of calcium and phosphate balances intestinal absorption. In general, over 98% of filtered calcium and 85% of filtered phosphate is reabsorbed by the kidney. The movement of calcium and phosphate across the intestinal and renal epithelia is closely regulated. Intrinsic disease of the intestine (eg, nontropical sprue) or kidney (eg, chronic renal failure) disrupts bone mineral homeostasis. 

When the steady state is disturbed by some change in external circumstances or energy supply, the temporarily altered fluxes through individual metabolic pathways trigger regulatory mechanisms intrinsic to each pathway. The net effect of all these adjustments is to return the organism to a new steady state—to achieve homeostasis. 

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. 

Physiologically, hormones are responsible for maintaining a steady state— homeostasis—in the human being, and help the organism to cope with environmental changes that affect it. Hormones control enzymatic activities this effect is... 

Calcium homeostasis is also affected by an overload in peroxides. In some instances, this may be due to activation of so-called calcium channels. In the presence of extracellular calcium, exposure of smooth muscle cells to 0.3 mM H2O2 was shown to induce a rapid increase in intracellular calcium concentration, followed by a decrease to a new constant level approximately twice higher than the initial one [140]. Subsequent treatment of the cells with Ca2+-channel blockers, with disulfide-reducing agents or with antioxidants such as trolox, prevented the stabilization of intracellular calcium at the high steady-state concentration. These results suggest that an increased disulfide/thiol ratio activates voltage-dependent calcium channels of the outer cell membrane. 

When self-regulating physiological systems (generally controlled by negative feedback systems, e.g. endocrine, cardiovascular) are subject to interference, their control mechanisms respond to minimise the effects of the interference and to restore the previous steady state or rhythm this is homeostasis. The previous state may be a normal function, e.g. ovulation (a rare example of a positive feedback mechanism), or an abnormal function, e.g. 

Nebert, D. W. Proposed role of drug-metabolizing enzymes regulation of steady state levels of the ligands that effect growth, homeostasis, differentiation, and neuroendocrine functions. Mol. Endocrinol. 1991, 5(9), 1203-1214. 

Abstract CO2 and H are metabolic end-products, which are produced continuously and excreted steadily to maintain steady-state concentrations of CO2/H+ in the body, primarily by the chemoreceptors. To help maintain an adequate speed of reaction compatible with life, these reactions are enhanced by carbonic anhydrase (CA) present in the chemoreceptor cells. The role of chemoreceptors in H homeostasis is the focus of this chapter. The peripheral chemoreceptors very readily sense CO2/H and stimulate ventilation in order to enhance CO2 exhalation. Central chemoreceptors are stimulated similarly but slowly, also resulting in increased ventilation. Altogether, this phase, assisted by respiration alone, can be defined as the acute response. However, the H left behind is excreted by the renal system, rather slowly (chronic phase) without any direct intervention by the chemoreceptors. Thus, respiratory and renal systems are integrated in the long run to maintain ( XT/H homeostasis. 

Table 13.1 Initial metabolite/co-metabolite concentrations and steady-state fluxes of enzymes used as reference values in the homeostasis and total enzymatic flux constraints. Experimentally measured values of Chassagnole et al. (2002) are in brackets. The cometabolite concentrations are assumed to be constant.

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