Physiology

1. Environment surrounding individual cells is vastly different from external environment

2. Internal environment remains relatively constant under conditions of health

The percent of water in various tissues

• - skin, muslce, heart, brain = 70-80%
• - bone = 25% 
• - fat = 10% 

If water content is computed as lean body mass, the difference between individuals becomes insignificant

• 1. Reduced intake
• 2. Excessive loss from gut
• 3. Excessive sweating
• 4. Excessive loss in expired air
• 5. Excessive loss in urine