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PubMed1991

Development of lower body negative pressure as a countermeasure for orthostatic intolerance.

Exposure to prolonged (1-4 hr) lower body negative pressure (LBNP) is a countermeasure against postflight orthostatic intolerance which is used in the Soviet space program and planned for use in the American space program. LBNP in combination with fluid-loading is believed to act by promoting a transient positive fluid balance resulting in an increase in vascular, as well as extravascular fluid. Inflight LBNP also may provide beneficial orthostatic effects by restoring baroreceptor reflex functions and/or lower body venous compliance. Current research efforts at the Johnson Space Center are directed towards increasing the effectiveness and efficiency of the LBNP and saline countermeasure. A promising avenue may involve combining pharmacologic agents, such as inhaled anti-diuretic hormone, or mineralocorticoids, with mechanical stimuli such as LBNP.

Humans↗
PubMed1995

Sodium homeostasis in primary and secondary forms of hypertension.

The role of extracellular fluid status and sodium balance in various forms of hypertension often has been difficult to define precisely because of the limitations of direct measurements of these factors. Isotopic measurements of various fluid volume compartments in the body have suffered from being insensitive and imprecise. Estimates of sodium content have also been difficult because of the ready diffusion of this ion among many body compartments. Studies of sodium balance utilizing metabolic ward techniques have been too time-consuming and arduous for large-scale investigations. Thus, inferences on the role of sodium and volume in blood pressure have been derived largely from indirect observations based on the response of blood pressure to manipulation of sodium or volume status, or both, and by changes in dietary sodium, or administration of intravenous sodium loads or diuretics. Such studies, while indirect and often crude in precision, have yielded important new insights into the etiology and pathogenesis of hypertension. The roles of agents that influence renal handling of sodium and water, and vascular biology are also being investigated for their influence on sodium homeostasis.

Animals↗
PubMed1986

Interpretation of the urine electrolytes and osmolality in the regulation of body fluid tonicity.

To evaluate urinary solutes in terms of their effect on body fluid tonicity, it is necessary to consider: whether that solute serves as an effective osmole in terms of ECF-ICF fluid shifts, its ability to accumulate in the body even if it is an osmotically effective particle, and the original number of body particles from which it derived. Thus, urea can be excluded since it is an ineffective osmole and only urine cations and anions need be considered. With respect to the former, one must separate the proportion of dietary versus endogenous potassium in this analysis as their effects differ. With respect to urine anions, urine chloride need not contribute to the loss of 'particles that count' when its excretion is accompanied by ammonium (i.e. equivalent to a bicarbonate gain). Thus, in the example cited at the beginning of this article, the excretion of hyperosmolar urine may not change body fluid tonicity if all the urinary potassium was of dietary origin and all the bicarbonate generated was retained as such. Finally, it is necessary to integrate defence of ECF volume (sodium balance), potassium balance, acid-base balance and intercompartmental fluid shifts to understand the overall renal response to defend tonicity. While there is utility in measuring urinary osmolality (assessment of medullary physiology, ADH action, water abstraction, concentrating power), in situations where body fluid tonicity is deranged, it is necessary to evaluate the urine sodium concentration along with the concentration of other solutes which may serve as effective osmoles in the urine.

Ammonia↗
PubMed1990

Body composition, normal electrolyte concentrations, and the maintenance of normal volume, tonicity, and acid-base metabolism.

Because the internal environment of the body is largely a fluid medium, the preservation of the volume and composition of the body fluids is absolutely vital to circulatory status and the extraordinarily complicated functions of the human body. The fluid compartments do not exist as fixed spaces with identical compositions but rather are in constant interchange with each other and have strikingly different compositions. Methods of movement of solutes and water include diffusion along electrochemical gradients, by hydrostatic pressure, osmotic forces, bulk flow, primary and secondary active transport, capillary blood flow, and oncotic pressure. Complex feedback control mechanisms exist to ensure homeostasis or equilibrium and include participation by the kidneys, lungs, gastrointestinal tract, the circulatory system, the endocrine system, and the CNS. The maintenance of extracellular volume is centered around the control of balance of the sodium salts. Multiple afferent (or sensing) and efferent (or effector) mechanisms exist to accomplish this homeostasis. The most important determinants of the osmolality or tonicity of the body fluids is the excretion or retention of water by the kidney, thirst mechanisms, and the intake of water. The serum sodium concentration is the laboratory test most often used clinically to assess tonicity. The pH of the body fluids and the major acid-base buffer systems are also carefully regulated. The lungs are responsible for the elimination of the carbon dioxide produced by cellular metabolism, and the kidneys excrete hydrogen ions and regulate the concentration of bicarbonate in the body fluids. Urinary net acid excretion, the hydrogen ions excreted as titratable acid and ammonium ions minus any bicarbonate, equals the acid added to the ECF from the diet and metabolism plus any fecal losses of alkali.

Acid-Base Equilibrium↗
PubMed1989

Effect of high frequency oscillatory ventilation on fluid filtration rate in isolated perfused rabbit lungs.

We have studied the effect of changing ventilatory frequency (VF) of high frequency oscillatory ventilation (HFOV) on fluid filtration rate (FFR) in twelve isolated rabbit lungs perfused at constant blood flow. Mean pulmonary artery pressure (Ppa), mean left atrial pressure (Pla), airway pressure (Paw), pulmonary vascular resistance (PVR), pH, O2 and CO2 partial arterial pressures (PaO2 and PaCO2) and plasma colloid osmotic pressure (COP), were measured. We ventilated the lungs with a modified Bird Mark 7 ventilator which could achieve HFOV (range 5-30 Hertz). In each experiment VF was randomly varied on ten different occasions, maintaining each variation for ten minutes. The first group of six rabbits was ventilated under normal haemodynamic conditions. The other six rabbits were ventilated after the production of hydrostatic lung oedema. Blood gas exchange in both groups of rabbits was satisfactory. There was no statistically significant correlation between VF and FFR. We conclude that variations in VF using HFOV does not alter lung fluid balance in normal and in hydrostatic oedema rabbit lungs.

Animals↗
PubMed2003

Regulation of urine reprocessing in the maintenance of sodium and water balance in the terrestrial Christmas Island red crab Gecarcoidea natalis investigated under field conditions.

Land crabs produce isosmotic urine but reduce salt loss by reabsorbing salt via the gills to produce a dilute excretory fluid (P). This branchial salt reclamation is regulated in response to changes in dietary salt availability. The regulation of branchial Na reabsorption and osmotic status was investigated in the terrestrial crab Gecarcoidea natalis on Christmas Island. Confinement within field enclosures had no general effect on salt and water balance compared with crabs free in the rainforest but there were seasonal effects. Extracellular fluid volume was 27.9% body mass during the wet season but only 22.7% in the dry season. Urine production was 53 ml kg(-1) day(-1) in the dry season but 111 ml kg(-1) day(-1) in the wet season, while water flux rates were 140 ml kg(-1) day(-1) and 280 ml kg(-1) day(-1), respectively. Serotonin but not dopamine increased urine production by at least 16% but only during the dry season when rates were seasonally lowered. Crabs acclimated to drinking 50% seawater increased haemolymph osmotic pressure and downregulated branchial reabsorption of salt. Net Na flux (J(net)) and unidirectional Na influx (J(in)) were investigated in branchial perfusion experiments. In red crabs acclimated to drinking freshwater, J(in), J(net) and the activity of the Na(+)/K(+)-ATPase were increased by serotonin, indicating that the increase of sodium absorption was due to a stimulation of the ATPase. Red crabs drinking 50% seawater reduced J(net) primarily due to increased passive loss (J(out)), since both J(in) and Na(+)/K(+)-ATPase were unchanged. Serotonin and dopamine abolished the increased diffusive loss and re-established J(net) with no change in J(in). G. natalis exhibits different regulatory systems. Branchial salt uptake can be adjusted via the leak component when adequate salt is available but also by stimulated active uptake under diluting conditions. The gills are important sites of ion pumping in euryhaline aquatic crabs, and the upregulation of J(net) in red crabs is reminiscent of that in marine crabs. Serotonergic stimulation of branchial uptake, independent of cAMP, and hormonally modulated ion leakage are presently unique to terrestrial species.

Animals↗