Final answer:
The extracellular fluid (ECF) remains constant as a percentage of body weight throughout life, representing about 20% of the body weight, divided between the interstitial fluid and blood plasma.
Step-by-step explanation:
The body fluid that stays constant as a percentage of body weight throughout life is the extracellular fluid (ECF). While the total body water content can vary significantly with age and body composition, the ECF, which represents about 20% of the body weight, remains relatively consistent.
Extracellular fluid broadly includes interstitial fluid, which is the fluid that bathes and surrounds the tissue cells, and the blood plasma, the liquid component of blood. About 15% of body weight is accounted for by interstitial fluid and 5% by blood plasma. Other types of ECF like cerebrospinal fluid (CSF), lymph, and synovial fluids are relatively small in volume.
It is important to note that while the percentage of ECF remains fairly constant, the total volume of ECF and the balance of solutes within it are vital to physiological functions and are tightly regulated by various mechanisms in the body to maintain homeostasis.