Biochemical Foundations of Human Body Vital Activity - Volkov N.I., Nesen E.N. 2000

Biochemical Foundations of Human Body Vital Activity
Water and Mineral Metabolism
Water and Its Role in the Body

Water (H₂O) is one of the most vital compounds in The Human Body. Vital metabolic processes cannot take place without water, and life itself is impossible without it. Losing as little as 10–20% of the body's water leads to death.

An athlete's physical performance, recovery rate, ability to cope with various stressors, and overall health status all depend on the body's water content.

Water content in the body varies depending on age, sex, and current functional state. In an adult, water accounts for approximately 2/3 of body weight, or about 42 kg: roughly 60% in men and 50% in women. In children, the water content per 1 kg of body weight is 2 to 4 times higher than in adults.

Water is distributed unevenly among different Tissues, with its concentration ranging from 0.3% in dental enamel to 99% in biological fluids. Half of the body's total water is found in Muscles, about 1/8 in the Skeleton, and 1/20 in the Blood (Table 4).

The body's water content changes throughout a person's life: the highest amount is found in the embryo (up to 97%), and the lowest in the Aging Organism (down to 50%). About 63% of the body's water is located inside Cells, known as intracellular water. The remaining water comprises the body's biological fluids, such as plasma, intercellular fluid, Lymph, etc., which is referred to as extracellular water (Fig. 22). Within the body, water exists in various states, thereby exerting distinct influences on biochemical processes.

States of water in the body. Depending on the degree of binding, water exists in three main states: free, Hydration, and immobilized.

Class="center">TABLE 4 Distribution of water among Organs and tissues

Fig. 22 Distribution of water in body tissues

Free water forms The basis of many biological fluids, including blood, lymph, saliva, and urine. It participates in METABOLISM between body cells and the external environment, delivers nutrients, removes intracellular metabolic waste, helps regulate body Temperature, and performs a mechanical role by facilitating the gliding of articulating joint surfaces. Additionally, it acts as a unique solvent for various substances. When retained in the body, free water accumulates beneath the skin, causing edema. Its loss leads to a reduced Blood Plasma volume and impaired tissue blood supply, which consequently diminishes the delivery of oxygen and nutrients, thereby affecting the function of the brain, Cardiovascular system, and skeletal muscles.

Hydration water is part of the hydration shells surrounding inorganic ions, Proteins, Polysaccharides, and Nucleic Acids. It plays a role in forming the spatial structures of most Biopolymers. Hydration water does not freeze at temperatures below 0 °C and exhibits no solvent properties. Its quantity remains nearly constant throughout life, decreasing only during aging. The loss of hydration water leads to tissue "drying out," particularly skin wrinkling.

Immobilized Water is localized within the enclosed structures of various molecules or membranes, though it is not part of their hydration shells. This water resides in the pores permeating Introduction/36.html">Biological Membranes and Ribosomes, as well as in nuclei, Cell/35.html">Mitochondria, and other structures, with which it is tightly bound. Unlike hydration water, immobilized water freezes at temperatures below 0 °C, dissolves substances, and participates in metabolic reactions.

A dynamic equilibrium exists among the Different types of water, allowing them to transform into one another. For instance, the content of hydration water can increase at the expense of its immobilized and free fractions.

Properties of water. Water performs diverse Functions in the body due to the specific Structure of its molecules. A water molecule is a dipole with a negative charge on the oxygen atom and a positive charge on the hydrogen atoms (Fig. 23; a, b). Due to their polarity, water molecules can form weak Hydrogen Bonds among themselves or with other charged substances. The electrostatic attraction of water molecules to charged particles is known as hydration. During hydration, complex crystal-like structures—hydrates and micelles—are formed (Fig. 23, c, e). This helps stabilize The structure of proteins, nucleic acids, and other substances, playing a crucial role in the expression of their functions.

Fig. 23 Structure and properties of the water molecule: a — molecular model of water; b — dipole; c — hydration via hydrogen bonding between water molecules and Other Compounds (alcohol); d — Hydrophobicity and hydrophilicity of substances; e — micelle

The high polarity of water molecules ensures the solubility of numerous crystalline substances and their dissociation into ions:

Many organic molecules, such as Fatty acids, nucleic acids, and proteins, contain non-polar regions (Fig. 23, d) that are incapable of interacting with water (hydrophobic), alongside polar regions that tend to form aqueous shells (hydrophilic). In aqueous solutions, hydrophilic molecules form structures in which non-polar hydrophobic regions are sequestered inside, while hydrophilic regions are located On the surface, interacting with water molecules. These resulting structures are called micelles (Fig. 23, e). Micelle formation plays a major role in the assembly of supramolecular structures and cell membranes. Such structures exhibit high ion mobility, which underlies the electrical excitability of membranes.

Biological Role of water. Water participates in the following processes:

✵ dissolving numerous substances, thereby accelerating Chemical Reactions;

✵ transporting substances during food Digestion IN THE gastrointestinal tract, delivering nutrients to body cells, and excreting Metabolic waste products via urine and sweat;

✵ maintaining the structures and functions of cellular Organelles; this property ensures the precise Organization of chemical processes within the organism; for example, an increase in cellular water content can induce mitochondrial Swelling, altering energy production (ATP) within them;

✵ participating in biochemical reactions involving CARBOHYDRATES, Lipids, proteins, and ATP metabolism (Hydrolysis, hydration, and dehydrogenation); for instance, The breakdown of ATP proceeds with the involvement of water and is termed ATP hydrolysis;

✵ in maintaining the Acid-Base Balance of the Body's internal environment, as water partially dissociates into H⁺ and OH^- ions (see Chapter 5);

✵ in generating osmotic pressure, which depends on the concentration of organic and inorganic substances dissolved in it, as well as on protein hydration;

✵ in providing mechanical protection for rubbing surfaces (acting as a lubricant), such as joints, ligaments, and muscles;

✵ in the body's thermoregulation processes, since about 50% of the heat released is dissipated from the body through water evaporation.

Thus, water maintains the dynamic constancy of The chemical composition, osmotic pressure, metabolic reactions, and body temperature, thereby ensuring the Stability of the internal environment (Homeostasis) and the acid-base balance of the body.