How does age affect body fluid volumes?

How does age affect body fluid volumes?

• A. Fluid volume increases with age as muscle tissue expands.
• B. Fluid volume is the same for all ages.
• C. A newborn has about 80% fluid volume of total body weight; declines with age as fat replaces muscle.
• D. Fluid volume decreases with age due to higher bone mineral content.

Answer: (C)

Total body water as a percentage of body weight changes with age because body composition shifts from mostly lean tissue at birth to more fat tissue later. A newborn has a high water content, about 75–80% of body weight, because lean tissues and extracellular fluids are abundant. As we age, fat replaces some lean muscle, and fat contains much less water, so the overall fraction of body weight that is water declines. This makes the statement that a newborn has about 80% fluid volume and that it decreases with age as fat replaces muscle the best description of how age affects body fluid volumes. The other ideas don’t fit because fluid content is not the same across ages, aging is typically associated with loss of lean mass rather than expansion of muscle, and bone mineral content isn’t the main factor driving total body water change.

Total body water as a percentage of body weight changes with age because body composition shifts from mostly lean tissue at birth to more fat tissue later. A newborn has a high water content, about 75–80% of body weight, because lean tissues and extracellular fluids are abundant. As we age, fat replaces some lean muscle, and fat contains much less water, so the overall fraction of body weight that is water declines. This makes the statement that a newborn has about 80% fluid volume and that it decreases with age as fat replaces muscle the best description of how age affects body fluid volumes. The other ideas don’t fit because fluid content is not the same across ages, aging is typically associated with loss of lean mass rather than expansion of muscle, and bone mineral content isn’t the main factor driving total body water change.