Water is the main fluid in the human body and is essential for survival, performance, and health. It helps move nutrients, remove wastes, control body temperature, protect organs, and keep blood volume high enough for circulation. Dehydration happens when the body loses more fluid than it takes in, often through sweating, breathing, urination, diarrhea, or vomiting.
Even mild dehydration can reduce concentration, physical endurance, and normal body function.
Key Facts
- Total body water is about 60% of body mass in many adults, but the percentage varies with age, sex, and body composition.
- Water balance can be estimated as fluid intake minus fluid loss over time.
- Percent dehydration = fluid mass lost ÷ original body mass × 100%.
- Losing 1 L of water is approximately equal to losing 1 kg of body mass.
- Blood plasma needs water to help transport glucose, amino acids, electrolytes, hormones, and wastes.
- Electrolytes such as Na+, K+, and Cl- help control fluid movement, nerve signals, and muscle contraction.
Vocabulary
- Dehydration
- Dehydration is a condition in which the body has less water than it needs for normal function.
- Electrolyte
- An electrolyte is a charged mineral, such as sodium or potassium, that helps regulate fluid balance and nerve and muscle activity.
- Plasma
- Plasma is the liquid part of blood that carries blood cells, nutrients, hormones, and waste products.
- Osmosis
- Osmosis is the movement of water across a membrane from an area of lower solute concentration to an area of higher solute concentration.
- Thermoregulation
- Thermoregulation is the process by which the body keeps its internal temperature within a safe range.
Common Mistakes to Avoid
- Waiting until you feel very thirsty to drink is a mistake because thirst can lag behind actual fluid loss, especially during exercise or hot weather.
- Assuming clear urine always means perfect hydration is a mistake because urine color can be affected by vitamins, medicines, and how recently a person drank fluids.
- Replacing heavy sweat loss with plain water only for a long time is a mistake because large sodium losses may also need to be replaced to support fluid balance.
- Thinking dehydration only affects athletes is a mistake because illness, heat, high altitude, low fluid intake, and certain medications can cause dehydration in many people.
Practice Questions
- 1 A student weighs 60.0 kg before soccer practice and 58.8 kg after practice. Estimate the liters of water lost and calculate the percent dehydration.
- 2 During a hot day, a person drinks 1.5 L of fluid but loses 2.3 L through sweat and urine. What is the net fluid balance for the day?
- 3 Explain why dehydration can make the heart work harder and reduce the body's ability to cool itself during exercise.