Water as a basic constituent of the organism

Water serves as an indispensable structural and functional element of the organism, forming the basis of its existence. It is vital for the proper functioning of cells, tissues, and the entire system. It plays a central role in maintaining homeostasis, providing the environment where metabolic processes occur. Due to its ability to dissolve chemical substances, it facilitates the transport of nutrients and the elimination of metabolic byproducts. It impacts digestion, detoxification, and body temperature regulation. Additionally, it performs transport functions and participates in thermoregulation.

Water is present in all body structures, but its distribution is not uniform across different tissues. Water in the human body can be categorized into intracellular water, which accounts for 34–40% of body mass, and extracellular water, making up approximately 20%. The space where water exists outside of cells can be divided into: intravascular space (blood plasma, lymph) – about 5% of body mass, extravascular space – about 15% of body mass, and transcellular space (e.g., water in the digestive tract, joints, eye and inner ear fluids) – around 2–4% of body mass.

The fluid content in the body (engl. Total Body Water – TBW) varies depending on age, sex, and body composition, particularly due to fat tissue percentage. The highest water concentration is found in fat-free mass (Free Fat Mass – FFM) – about 70–80%, while fat tissue contains around 30%. In infants, water constitutes approximately 75% of body weight, but this proportion gradually decreases with age. In adults, it ranges from 50–60%, and in the elderly, it drops to 45–50%. These differences stem from age, body structure, and gender. For instance, in men over 60, water accounts for an average of 54% of body weight, whereas in women, it is only 46%. Several methods exist to assess total fluid content, including laboratory tests, non-invasive measurements, and symptom observation. Water is vital for bodily functions, so fluid intake should be tailored to individual needs based on age and sex.