Fluid balance normal intake output

Fluid balance in the body is maintained or restored primarily by variations in urine output. Normally, this amounts to about 1400 mL/day, but the total excretion fluctuates according to water intake. The primary factor that controls urine production is the rate of water reabsorption from the renal tubules in the kidneys. This rate is regulated chiefly by the pituitary hormone vasopressin and by the adrenal cortex hormone aldosterone. 

Large deficits or excesses in the body water are reason for concern. In healthy individuals, the total amount of body water remains reasonably constant. Therefore, an increase or decrease in water intake brings about an appropriate increase or decrease in water output to maintain the balance. Fig. W-4 illustrates the intake of water, the routes of water output, and the movement of fluid between the compartments of the body. Water enters the body as a liquid, and as a component of the food-including metabolic water derived from the breakdown of food. Water is lost from the body by (1) the skin as perspiration, (2) the lungs as water vapor in expired air, (3) the kidneys as urine, and (4) the intestines in the feces. As Fig. W-4 shows, under normal conditions total water intake is approximately equal to total water output by the various routes. 

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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