Water Properties and Homeostasis in the Human Body
Water: Essential for Life and Homeostasis
Water constitutes approximately 60% of the human body. It is distributed as follows: one-third is located outside the cells (extracellular fluid), and the remaining two-thirds are inside the cells.
Cells depend on the internal environment (extracellular fluid) to perform their functions properly. The internal environmental conditions, such as pH, nutrients, and ion concentrations, are crucial for cell life. Maintaining these conditions in a constant state of balance is called homeostasis. The mechanisms that regulate these parameters constitute the homeostatic processes.
Body Systems Maintaining Homeostasis
- Transport of fluid (renewal): This is a function of the circulatory system. Every minute, a molecule completes a full circuit around the system. This constant renewal is facilitated by the proximity of all cells to capillaries (the distance between them is approximately 50 microns), allowing continuous contact between the circulatory system and the extracellular fluid.
- Nutrient supply to the extracellular fluid: Achieved through the circulatory, respiratory, gastrointestinal, and musculoskeletal systems.
- Elimination of metabolic end products: Waste products, if not removed, can block enzyme activity. The respiratory, urinary, and digestive systems are primarily responsible for this.
- Regulation of body function: Primarily through the neuroendocrine system.
Fundamental Properties of Water
- High boiling point: 100°C. This allows for significant cohesion at room temperature due to hydrogen bonds.
- High density at 4°C: This allows many aquatic species to survive. If the bottom of the seas froze, many species would die. Because ice is less dense than liquid water, it floats, and the water at the bottom of the seas never drops below 4°C.
- High specific heat (heat required to raise 1 gram of water by 1°C): This allows for significant heat changes while the temperature remains relatively stable.
- High heat of vaporization (amount of heat needed to change from liquid to gas): 536 calories. This means that for every drop of sweat, 536 calories (as heat) are eliminated from the body. The more adapted we are to heat, the more we sweat and the better we cool down. This is a trainable ability.
- High conductivity: Water is an excellent conductor of temperature. This allows the entire organism to maintain a consistent temperature, as enzymes have an optimal temperature range for constant function.
- High dielectric constant: Being a dipole, water easily dissolves ionic substances (cations and anions) and crystalline structures.
- Large capacity for ion hydration (atoms that have gained or lost electrons):
- Anion: An atom that has gained electrons and has a negative charge.
- Cation: An atom that has lost electrons and has a positive charge.
Different water molecules surround an ion. For example, a sodium ion is always surrounded by water. Therefore, it is easier for potassium to enter cells (which is a larger atom but is not accompanied by water molecules) than for sodium (which, despite being a smaller atom, is always surrounded by several water molecules).
- Amphipathic solvent molecules (molecules with polar and nonpolar parts) form micelles: Amphipathic molecules spontaneously arrange themselves, leaving the polar part in contact with water and the nonpolar or hydrophobic area on the inside. These structures are called micelles. They are formed due to the ordering of water. Phospholipids are a type of amphipathic molecule that arranges to form, among other structures, cell membranes.
- Polar solvent for ionic compounds: For example, glucose.
- High surface tension: At the surface of the water, there is greater cohesion between molecules than in the rest of the liquid. This causes the formation of menisci (adherence to the walls of the container).
- Transparency: If water were opaque, there would be no life in the sea, given the absence of algae that produce oxygen.
- Weak electrolyte: Water can behave as both an acid and a base.