Thirst mechanism

Fluid restriction is generally unnecessary as long as sodium intake is controlled. The thirst mechanism remains intact in CKD to maintain total body water and plasma osmolality near normal levels. Fluid intake should be maintained at the rate of urine output to replace urine losses, usually fixed at approximately 2 L/day as urine concentrating ability is lost. Significant increases in free water intake orally or intravenously can precipitate volume overload and hyponatremia. Patients with stage 5 CKD require renal replacement therapy to maintain normal volume status. Fluid intake is often limited in patients receiving hemodialysis to prevent fluid overload between dialysis sessions. 

Q9 Loss of albumin through the glomerular membrane reduces the concentration of albumin in the blood. Albumin plays a major role in the maintenance of ECF volume, and when there is a deficiency additional fluid passes from plasma into the tissues to form oedema. Passage of extra fluid from the circulation into the tissues reduces blood volume, which stimulates the renin-angiotensin system and also triggers the thirst mechanism via osmoreceptors in the hypothalamus. 

Isotonicity of the extracellular space is regulated by (i.) thirst mechanism, (2.) ADH, and (S.) dilution and concentration potential of the kidneys. Maintenance of extracellular isovoiaemia is effected by a change in renal sodium excretion. For this reason, disturbances in the sodium supply primarily result in changes in the extracellular fluid volume. Isohydria is also continually regulated within the normal range. 

Fluid restriction is generally unnecessary provided sodium intake is controlled, although fluid intake between dialysis sessions is generally limited for hemodialysis patients. An intact thirst mechanism maintains total body water and effective plasma osmolality near normal. Since urine volume is relatively fixed at approximately 2 L/day, fluid restriction below this amount should be avoided. Large amounts of free water administered orally or as IV fluid may induce hyponatremia and volume overload. When the patient develops... 

A. Damage to the thirst mechanism from snrgical trauma, leading to excessive consnmption of liquids... 

Personnel required to wear toxic-agent protective clothing are also at high risk for dehydration, which is a contributing factor for developing heat injury. The thirst mechanism is not adequate to... 

Water intake is regulated in part by the thirst mechanism (see I Figure 15.9). When the body loses large amounts of water, salivary secretions decrease, and a dry feeling develops in the mouth. This and other sensations are recognized as thirst, and water is drunk to relieve the condition. The fluid intake compensates for the fluid lost, and balance is reestablished. 

Trace the events of the regulation of fluid balance by the thirst mechanism. 

Humans normally will seek out fluids because of a thirst mechanism that induces a craving for liquids when the body needs fluid. People who cannot access the desired fluids are at risk for inadequate intake. For example, people who are stranded in areas without clean water sources are at risk for hypovolemia. Fluid deficits are also found in people who are unable to obtain food and fluids without assistance (e.g., infants, unconscious individuals, and immobile or mobility challenged individuals) and are not given adequate food and fluids. 

The thirst mechanism is stimulated to drive the individual to seek and ingest fluids to increase fluid volume in the body. 

Problems arise when there is a failure or a decrease in function in any of the fluid regulation mechanisms. For example, in aging, the thirst mechanism is suppressed. The elderly patient may not drink adequate fluids owing to a lack of the thirst drive, and hypovolemia could result. Problems also arise if the regulating mechanisms fail to excrete excess water from the body, such as might occur with renal failure, and fluid overload results. 

A common cause of true hypovolemia is dehydration owing to inadequate intake of fluids, excessive loss of fluids, or a combination of the two. Dehydration is a true danger in the elderly because the thirst mechanism, which stimulates one to drink fluids, is diminished with age. Additionally, children, whose total-body fluid content is high and have high fluid needs, can dehydrate more quickly than adults if they are ill and nausea or gastrointestinal upset causes them to refuse fluid intake. 2... 

Decreased intake of fluids—the elderly are at risk for hypernatremia owing to a decreased thirst mechanism that results in decreased fluid intake and dehydration. 

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