Overview
This lecture covers the distribution, balance, and regulation of water and electrolytes in the human body, and explains their roles in cellular function and health.
Body Water Composition
- The human body is made up mostly of water, with adults having about 37-42 liters.
- Newborns are about 70% water; adult males are about 60% and adult females 55%.
- Body water percentage decreases with increased body fat and with age, reaching around 45% in the elderly.
- Proper hydration can reduce headaches, muscle aches, kidney stones, and may lower colon cancer risk.
Fluid Compartments and Electrolyte Basics
- Body water is divided into intracellular (inside cells) and extracellular (outside cells) compartments.
- Extracellular fluids include interstitial fluid, blood plasma, and cerebrospinal fluid.
- Water in the body contains solutes (dissolved substances) such as proteins, glucose, and electrolytes.
- Electrolytes dissociate into ions in water; cations have a positive charge, anions have a negative charge.
- Composition of solutes varies between compartments; e.g., more proteins inside cells, more chloride anions outside.
Osmoregulation and Water Movement
- Osmoregulation controls fluid balance and solute composition in the body.
- Fluid compartments are separated by selectively permeable membranes allowing water movement by osmosis.
- Osmosis is the passive movement of water from areas of high to low concentration.
- Cells control electrolyte movement to indirectly regulate water distribution and keep osmolality equal inside and outside.
- Sodium is much more concentrated in extracellular fluid than inside cells.
Effects of Solution Concentrations on Cells
- Hypertonic solutions cause cells to shrink as water leaves the cell.
- Hypotonic solutions cause cells to swell as water enters the cell.
- Isotonic solutions have equal concentrations of dissolved particles inside and outside, so cell volume remains constant.
- Cells actively pump electrolytes to maintain water volume and prevent cell damage.
Key Terms & Definitions
- Electrolyte — substances that dissociate into charged ions when dissolved in water (e.g., sodium, potassium, chloride).
- Cation — a positively charged ion.
- Anion — a negatively charged ion.
- Solute — dissolved substances in a solution.
- Osmosis — the movement of water across a semi-permeable membrane toward higher solute concentration.
- Extracellular Fluid — fluid outside cells, including interstitial and intravascular fluid.
- Hypertonic — having higher osmotic pressure than another fluid, causing water to move out of cells.
- Isotonic — having equal osmotic pressure inside and outside, resulting in no net water movement.
- Hypotonic — having lower osmotic pressure than another fluid, causing water to move into cells.
Action Items / Next Steps
- Review the next section: Water’s Importance to Vitality.
- Complete any assigned interactive activities related to fluid and electrolyte balance.