Understanding Body Water Balance: Functions, Maintenance, and Practical Tips

1. WATER IN THE BODY

2. WATER IN THE BODY The body contains two fluid “compartments.” • Intracellular compartment, refers to fluid inside cells • Extracellular compartment includes • (1) blood plasma (_20% of total extracellular fluid) • (2) interstitial fluids, which primarily comprise fluid flowing in microscopic spaces between cells.

3. Sources of interstitial fluid include: • 1. Lymph • 2. Saliva • 3. Fluids in the eyes • 4. Fluids secreted by glands and the digestive tract • 5. Fluids that bathe the nerves of the spinal cord • 6. Fluids excreted from the skin and kidneys • Much of the fluid lost through sweating comes from extracellular fluid, predominantly blood plasma

4. Functions of Body Water • 1. Provides the body’s transport and reactive medium. • 2. Diffusion of gases occurs across moist body surfaces. • 3. Waste products leave the body through the water in urine and feces. • 4. Absorbs considerable heat with only minimal changes in temperature from its heat stabilizing qualities. • 5. Watery fluids lubricate joints, keeping bon surfaces from grinding against each other. • 6. Being non compressible, water provides structure and form through the turgor it imparts to the body’s tissues.

5. Water Balance: Intake Versus Output The water content of the body remains relatively stable over time. Appropriate fluid intake rapidly restores an imbalance

6. Water Intake • In a normal environment, a sedentary adult requires about 2.5 L of water daily. • For an active person in a warm environment, the water requirement often increases to between 5 and 10 L daily.

7. The sources of water intake: • 1. Liquids • 2. Foods • 3. Metabolic processes

8. Metabolizing food molecules for energy forms carbon dioxide and water. • For a sedentary person, metabolic water provides about 25% of the daily water requirement. • Additionally, each gram of glycogen joins with 2.7 g of water as the glucose units link together thus making glycogen a heavy energy fuel. • Glycogen subsequently releases this water during its catabolism for energy.

9. Water Output • The body loses water in four ways: • 1. In urine • 2. Through the skin • 3. As water vapor in expired air • 4. In feces

10. WATER REQUIREMENT DURING EXERCISE • The factors determine water loss through sweating: 1. Severity of physical activity : The evaporative loss of 1 L of sweat releases about 600 kCal of heat energy from the body to the environment. 2. Environmental temperature

11. 3. Humidity: • Relative humidity, which refers to the water content of the ambient air, impacts the efficiency of the sweating mechanism in temperature regulation. • At 100% relative humidity, the ambient air is completely saturated with water vapor. • This blocks evaporation of fluid from the skin surface to the air, thus minimizing this important avenue for body cooling. • When this happens, sweat beads on the skin and eventually rolls off without generating a cooling effect.

12. The major physiologic defense against overheating comes from evaporation of sweat from the skin’s surface. • sweat loss equal to 2% to 3% of body mass decreases plasma volume. This amount of fluid loss strains circulatory functions and ultimately impairs exercise capacity and diminishes thermoregulatory control.

13. Practical Recommendations for Fluid Replacement in Exercise: Pre-exercise hyperhydration • (1) delays dehydration. • (2) increases sweating during exercise. • (3) blunts the increase in body temperature compared with exercising without prior fluids. • As a practical step, a person should consume 400 to 600 mL (13–20 oz) of cold water 10 to 20 minutes before exercising.

14. How to Distinguish Among Heat Cramps, Heat Exhaustion, and Heat Stroke