Final answer:
Body water levels are similar to those of adults by the age of 20 years. This similarity is due to the body's completion of physical development and the distribution of water in intracellular and extracellular fluids. Daily water intake and loss are balanced to maintain physiological functions.
Step-by-step explanation:
By the age of 20 years, body water levels are similar to those of adults. Human beings are composed mainly of water, which represents a significant portion of our body mass. Infants start with about 75% of their body mass as water, but this percentage decreases to about 50-60% in adults. This percentage further decreases in old age to as low as 45%. The body water percentage varies due to changes in the proportions of organs, muscle, fat, bone, and other tissues.
The adult body's water is distributed evenly throughout, as a component of both the intracellular and extracellular fluids. Intracellular fluid accounts for approximately 50% of the weight in a lean person, while extracellular fluid makes up about 20% of the body weight. This emphasizes the importance of water as a critical component of our physiological functions and the body's ability to regulate its water balance through mechanisms such as the kidneys, which can conserve or expel water as needed.
On a daily basis, an adult typically consumes around 2500 mL of aqueous fluids, with metabolic processes generating additional water as well. Adults also lose around 2500 mL of water daily through various routes like urine and insensible water loss—which involves evaporation from the skin and water vapor in expelled air. Body water balance is crucial for maintaining proper physiological function and overall health.