Overview
This lecture covers the importance, functions, distribution, balance, sources, and regulation of water in the human body, including dietary factors and hydration recommendations.
Water's Importance and Functions
- Water is one of six essential nutrients and is critical for survival and normal body function.
- Without water, humans survive only 5–7 days depending on environmental conditions.
- Water is a major structural component of cells, giving them shape and volume.
- Acts as a solvent for most chemical reactions in the body; sometimes participates directly in reactions.
- Maintains blood volume, aids waste removal, and regulates internal temperature via sweating.
Distribution of Water in the Body
- About 60% of total body weight is water.
- Two-thirds of body water is intracellular (inside cells); one-third is extracellular (outside cells).
- Most water lost from the body comes from extracellular fluid, especially during sweating.
- Water balance is maintained by keeping intake and loss roughly equal.
Water Intake and Loss
- Intake comes from fluids (water, beverages), foods (especially fruits and vegetables), and limited amounts from metabolic water.
- Major water loss occurs via urination; other losses include sweat, feces, and breath.
- Dehydration results from intake less than loss; hyperhydration occurs with excess intake but is usually temporary.
- Water toxicity is rare but can be deadly if massive amounts are consumed rapidly.
Dietary Factors Affecting Water Balance
- Diuretics like alcohol and caffeine increase water loss through urine.
- High sodium (salt) intake causes water retention in tissue, requiring extra water to restore balance.
Body Responses to Water Imbalance
- Dehydration triggers thirst and reduces saliva production.
- Hormonal responses include release of antidiuretic hormone (ADH), aldosterone, and the renin-angiotensin system, which conserve water and sodium and maintain blood pressure.
- Prolonged dehydration can impair cardiovascular, nervous, respiratory, and renal function and may be fatal in extreme cases.
Hydration Recommendations
- Adults should generally consume 2–3 liters of water per day, but needs vary by metabolism, activity, body size, and environment.
- Best self-check: urine should be clear and urination frequent; dark urine and infrequent urination signal dehydration.
Key Terms & Definitions
- Intracellular fluid — water inside cells (about two-thirds of total body water).
- Extracellular fluid — water outside cells (one-third of total body water).
- Dehydration — state where water loss exceeds intake.
- Hyperhydration — temporary state of excess body water.
- Diuretic — substance that increases urine production and water loss (e.g., caffeine, alcohol).
- ADH (Antidiuretic Hormone) — hormone that signals kidneys to conserve water.
- Metabolic water — water produced as a byproduct of chemical reactions in the body.
Action Items / Next Steps
- Monitor urine frequency and clarity to assess hydration status.
- Adjust water intake based on activity, environment, and diet (especially when consuming caffeine, alcohol, or high-sodium foods).
- Read related textbook sections on water balance and hormonal regulation.