- There are significant differences between adults and infants, especially premature infants, with respect to fluid distribution, regulation of fluid and electrolyte balance, and acid–base homeostasis.
- Accordingly, infants experience more problems than adults in these areas.
1. Proportion and distribution of water
- A newborn’s total body mass is about 75% water
- (and can be as high as 90% in a premature infant);
- an adult’s total body mass is about 55–60% water.
- (The “adult” percentage is achieved at about 2 years of age.)
- Adults have twice as much water in ICF as ECF,
- but the opposite is true in premature infants.
Because ECF is subject to more changes than ICF,
- rapid losses or gains of body water are much more critical in infants.
- Given that the rate of fluid intake and output is about seven times higher in infants than in adults, the slightest changes in fluid balance can result in severe abnormalities.
- The metabolic rate of infants is about double that of adults.
- This results in the production of more metabolic wastes and acids,
- which can lead to the development of acidosis in infants.
- Infant kidneys are only about half as efficient in concentrating urine as those of adults.
- (Functional development is not complete until close to the end of the first month after birth.)
- As a result, the kidneys of newborns can neither concentrate urine nor rid the body of excess acids as effectively as those of adults.
- The ratio of body surface area to body volume of infants is about three times greater than that of adults.
- Water loss through the skin is significantly higher in infants than in adults.
5. Breathing rate.
- The higher breathing rate of infants (about 30 to 80 times a minute) causes greater water loss from the lungs.
- Respiratory alkalosis may occur because greater ventilation eliminates more CO2 and lowers the PCO2.
6. Ion concentrations.
- Newborns have higher K + and Cl − concentrations than adults.
- This creates a tendency toward metabolic acidosis.
By comparison with children and younger adults,
- older adults often have an impaired ability to maintain fluid, electrolyte, and acid–base balance.
- a decreased volume of intracellular fluid and
- decreased total body K + due to declining skeletal muscle mass and increasing mass of adipose tissue (which contains very little water).
Age-related decreases in respiratory and renal functioning may compromise acid–base balance
- by slowing the exhalation of CO2
- and the excretion of excess acids in urine.
- decreased blood flow,
- decreased glomerular filtration rate, and
- reduced sensitivity to antidiuretic hormone,
- Due to a decrease in the number and efficiency of sweat glands, water loss from the skin declines with age.
- Because of these age related changes, older adults are susceptible to several fluid and electrolyte disorders:
- often occur due to inadequate fluid intake
- or loss of more water than Na + in vomit, feces, or urine.
8. Hyponatremia
may occur due to
9. Hypokalemia
10. Acidosis
Another cause
- inadequate intake of Na+ ;
- elevated loss of Na + in urine, vomit, or diarrhea;
- or impaired ability of the kidneys to produce dilute urine.
- often occurs in older adults who chronically use laxatives to relieve constipation
- or who take K + depleting diuretic drugs for treatment of hypertension or heart disease.
- may occur due to impaired ability of the lungs and kidneys to compensate for acid–base imbalances.
- decreased production of ammonia (NH3) by renal tubule cells,
- which then is not available to combine with H+
- and be excreted in urine as NH4+ ;
- is reduced exhalation of CO2.
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