Effects of Aging on Water, Electrolyte, and Acid-Base Balance

Effects of Aging on Water,Electrolyte, and Acid-Base Balance

The human body’s percentage of water reduces greatly over time, with embryos and early fetuses being comprised of more than 90% water. As solids accumulate during fetal development, an infant at birth is made up of 70–80% water. Infants have much more extra-cellular fluid than adults, and therefore, a much higher sodium chloride content compared to potassium, magnesium, and phosphate salts.

Body water distribution begins to change approx-imately two months after birth, nearing the adult pattern by the age of two years. The concentrations of plasma electrolytes are similar in infants and adults. However, potassium and calcium values are higher, while magnesium, bicarbonate, and total pro-tein levels are lower, during the first few days of life than at any other part of the life cycle. Upon reach-ing puberty, a child’s body water content is based on differences in gender, with males developing greater relative amounts of skeletal muscle. During infancy, problems with fluid, electrolyte, and especially acid-base balances are most common. In infants, they may be related to the following conditions:

■■ Very low residual lung volume – about half that of adults, relative to body weight. With alterations in respiration, the partial pressure of carbon dioxide can change quickly and significantly.

■■ High rates of fluid intake and output – about seven times more than in adults. An infant may exchange half of the extracellular fluid every day. Though having more proportional body water than adults, excessive fluid shifts can still be harmful. Even small changes in fluid balance can become serious. Another point is that infants can survive for only three to four days without water, while adults can survive for about 10 days.

■■ Relatively high metabolic rate – about twice as much as in adults. This yields many more metabolic wastes and acids that must be excreted by the kidneys. Along with buffer systems that are still inefficient, this results in more likelihood of acidosis developing.

■■ High rate of insensible water loss due to larger surface area relative to body volume – about three times as much as in adults. Infants lose large amounts of water through their skin.

■■ Inefficiency of kidneys – since the kidneys are immature at birth, only functioning about half as well as adult kidneys in the concentration of urine, they are poor at removing acids from the body.

Total body water decreases gradually as we age, predominantly from the intracellular compartment. Adults have approximately 50% to 60% of their bodies made up by water. These decreases reduce the dilution of waste products, toxins, and administered medi-cations. The glomerular filtration rate declines, and the body cannot regulate pH via the urinary system as efficiently. More water begins to be lost because of an increased inability to concentrate the urine. As the skin becomes thinner, the rate of insensible perspira-tion increases. Older adults should increase their daily water intake to combat this condition. They are less able to conserve body water than younger people and are also less responsive to thirst cues. When homeostasis is interrupted, the body takes longer to return to normal.

Body mineral content is lower as muscles and bones decrease in mass. Body fat increases, however. Exercise and increased ingestion of dietary minerals can help in this regard. Because respiratory compensa-tion decreases with age, the risk of respiratory ­acidosis increases, compounded by arthritic ­conditions and other conditions such as emphysema. As the other systems in the body decline in function, they affect fluid, electrolyte, and acid-base balances as well. ­Conditions that make the elderly more prone to acid-base imbalances include congestive heart failure with edema and diabetes mellitus. Nearly all disorders of the body systems, with increased aging, partially affect the balances of fluids, electrolytes, acids, and bases.