Thirst sodium regulation

The sodium-to-water ratio in the body must be properly balanced or unfortunate effects may result. The hypothalamus area of the brain is fairly efficient at regulation of this ratio when sodium is high, thirst results when excess urination causes loss of water, the body also excretes sodium to help maintain the balance. People traveling long distances in desert or other dry climates may ingest salt (NaCl) as a way to retain water in their systems. The kidneys also play a role in sodium regulation. 

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. 

Fig. 4.10. Body fluid homeostasis (constant body water balance). Intake is influenced by availability of fluids and food, thirst, hunger, and the ability to swallow. The rates of breathing and evaporation and urinary volume influence water loss. The body adjusts the volume of urinary excretion to compensate for variations in other types of water loss and for variations in intake. The hormones aldosterone and antidiuretic hormone (ADH) help to monitor blood volume and osmolality through mechanisms regulating thirst and sodium and water balance.

Thirst (in humans) is the conscious desire for water. It is the primary means of regulating water intake, and generally, the thirst sensation ensures that water intake meets or exceeds the body s requirement for water. The sensation of thirst is caused by nerve centers in the hypothalamus of the brain which monitor the concentration of sodium (osmolar-ity) in the blood. When the sodium concentration of the blood increases abiove the normal 310 to 340 mg/100 ml (136 to 145 mEq/1), cells in the thirst center shrink. This shrinking causes more nervous impulses to be generated in the thirst center, thereby creating the sensation of thirst. An... 

There are two components to the regulation of ECE sodium the total amount of sodium retained and its concentration. The former is regulated by mechanisms that directly affect sodium, whereas the latter is essentially regulated via water balance. Thus, whatever sodium is retained in ECF is clothed with the appropriate amount of water to maintain the normal plasma sodium concentration within narrow limits deviations of less than 1% (hard to measure in the laboratory) trigger corrective responses. Thus, a raised plasma sodium concentration (e.g., after water loss) stimulates both thirst and renal water conservation antidiuretic hormone (ADH) from the posterior pituitary reduces urine output through its effect on the renal collecting ducts. Even one of these mechanisms can defend body water thus diabetes insipidus (inadequate production or effect of ADH) does not cause severe dehydration but polydipsia (increased fluid intake thirst is a sensation). 

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