Dehydration — Consequences and Prevention

Dehydration is one of the most significant threats to sporting performance and athlete health. This lesson builds on the hydration content from Lesson 9 and focuses specifically on the five consequences of dehydration that AQA GCSE PE requires you to know, how to prevent dehydration, and how its effects vary across different sports and conditions.

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Recap: What Is Dehydration?

Dehydration is a state in which the body loses more water than it takes in, resulting in insufficient water for normal bodily functions. During exercise, the primary cause of dehydration is sweating — the body's mechanism for cooling itself down.

Key Fact	Detail
Average person at rest	Loses about 2.5 litres of water per day through urine, sweat, breathing and faeces
During moderate exercise	Can lose 0.5–1 litre of sweat per hour
During intense exercise in hot conditions	Can lose 2–3 litres of sweat per hour
Just 2% loss of body weight through water	Causes a measurable decline in physical and cognitive performance

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The Five Consequences of Dehydration

1. Blood Thickening (Increased Blood Viscosity)

When the body is dehydrated, there is less water available for blood plasma (the liquid component of blood). This causes the blood to become thicker and more viscous.

Effect	Detail
Blood becomes thicker	Less water in blood plasma means higher concentration of red blood cells
Harder for the heart to pump	Thicker blood creates more resistance in blood vessels
Reduced oxygen delivery	Thicker blood flows more slowly, delivering less oxygen to working muscles
Reduced nutrient delivery	Glucose and other nutrients are transported less efficiently
Reduced waste removal	Carbon dioxide and lactic acid are removed from muscles more slowly

This is one of the most significant consequences of dehydration because it triggers a chain reaction of further problems.

Exam Tip: Blood thickening is the root cause of many other dehydration effects. If you explain this first in an exam answer, you can then logically connect the other consequences to it. This chain of reasoning scores high marks.

2. Increased Heart Rate

Because the blood is thicker and harder to pump, the heart must beat faster to maintain adequate blood flow to the muscles and organs.

Effect	Detail
Heart rate increases	The heart compensates for reduced blood volume by beating more frequently
Cardiac drift	Heart rate gradually increases over time during exercise, even if intensity stays the same
Increased perceived effort	The same exercise feels harder because the cardiovascular system is under greater strain
Earlier fatigue	The heart reaches its maximum rate sooner, limiting the duration and intensity of exercise
Increased risk of cardiac events	In extreme cases, severe dehydration can contribute to cardiac arrhythmias

3. Increased Body Temperature

Water is essential for thermoregulation — the body's ability to maintain a safe core temperature. When dehydrated, this system is compromised.

Effect	Detail
Less sweat produced	Dehydrated body conserves water by reducing sweat output
Reduced cooling	Less sweat means less evaporative cooling from the skin
Core temperature rises	Without effective cooling, body temperature increases progressively
Risk of heat exhaustion	Symptoms include headache, dizziness, nausea, excessive sweating
Risk of heatstroke	A life-threatening condition where core temperature exceeds 40°C; the body's cooling system fails completely

graph TD
    A[Dehydration] --> B[Less Water Available for Sweat]
    B --> C[Reduced Sweat Production]
    C --> D[Less Evaporative Cooling]
    D --> E[Core Body Temperature Rises]
    E --> F[Heat Exhaustion]
    F --> G[Heatstroke - Medical Emergency]
    style A fill:#ffcdd2,stroke:#c62828
    style G fill:#ff8a80,stroke:#c62828

4. Slower Reaction Time and Reduced Concentration

Dehydration affects the brain as well as the body:

Effect	Detail
Reduced blood flow to the brain	Thicker blood delivers less oxygen and glucose to the brain
Slower neural processing	The brain works less efficiently without adequate hydration
Slower reaction time	Athletes respond more slowly to stimuli (e.g. a ball, an opponent's move)
Poor decision-making	Tactical awareness and game intelligence decline
Reduced concentration	Difficulty focusing on the task, leading to errors and poor performance
Impaired coordination	Brain-muscle communication is less effective

This consequence is particularly damaging in sports that require quick reactions and tactical thinking — such as tennis, cricket, football and basketball.

5. Muscle Fatigue and Cramps

Dehydration directly affects muscle function:

Effect	Detail
Reduced blood flow to muscles	Less oxygen and glucose reach working muscles, limiting energy production
Build-up of waste products	Lactic acid and CO2 are removed more slowly, causing discomfort and fatigue
Electrolyte imbalance	Sweat contains sodium, potassium and other electrolytes; losing them disrupts muscle function
Muscle cramps	Involuntary, painful contractions caused by dehydration and electrolyte loss
Earlier onset of fatigue	Muscles tire more quickly when dehydrated, reducing performance and endurance
Increased risk of muscle injury	Fatigued, dehydrated muscles are more prone to strains and tears

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Summary Table: Five Consequences

Consequence	Key Mechanism
1. Blood thickening	Less water in plasma → thicker blood → harder for heart to pump
2. Increased heart rate	Heart beats faster to compensate for reduced blood volume and thicker blood
3. Increased body temperature	Less sweat produced → reduced cooling → core temperature rises
4. Slower reactions	Reduced blood flow to brain → slower processing, poor decisions
5. Muscle fatigue/cramps	Reduced blood flow to muscles → less oxygen → electrolyte imbalance → cramps

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Evaluating Dehydration Effects Across Different Sports

The impact of dehydration varies depending on the type of sport:

Sport Type	Why Dehydration Is Particularly Damaging
Endurance sports (marathon, cycling, triathlon)	Long duration means more sweat loss; blood thickening reduces oxygen delivery; increased body temperature; muscle fatigue over hours
Team sports (football, rugby, hockey)	Combination of endurance and high-intensity bursts; need for concentration and decision-making; cramps during match
Racquet sports (tennis, badminton)	Long matches in hot conditions; need for quick reactions and concentration; risk of cramps
Combat sports (boxing, MMA, judo)	Athletes may deliberately dehydrate to make weight categories — extremely dangerous; reaction time and power both affected
Precision sports (archery, shooting, golf)	Reduced concentration and slower reactions directly affect accuracy; less physical but still demanding mentally
Swimming	Despite being in water, swimmers still sweat and can become dehydrated; less obvious because sweat is not visible

Exam Tip: AQA often asks you to evaluate the effects of dehydration on a specific sport. When answering, think about which consequences matter most for that sport. For a marathon runner, blood thickening and increased body temperature are most critical. For a tennis player, slower reactions and muscle cramps may be more relevant. Tailoring your answer to the specific sport demonstrates application of knowledge and scores higher marks.

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Preventing Dehydration

Strategy	Detail
Drink before exercise	Consume 400–600ml of water 2–3 hours before exercise to start well-hydrated
Drink during exercise	Take small, regular sips (150–200ml every 15–20 minutes) during exercise
Drink after exercise	Replace fluids lost during exercise; aim to drink 150% of fluid lost
Monitor urine colour	Pale straw colour = well hydrated; dark yellow = dehydrated
Use sports drinks when appropriate	Isotonic sports drinks replace electrolytes (sodium, potassium) as well as water
Acclimatise to heat	Train in hot conditions gradually to improve the body's cooling efficiency
Wear appropriate clothing	Light, breathable fabrics help the body cool itself through evaporation

When to Use Water vs Sports Drinks

Situation	Recommended Fluid
Exercise lasting <60 minutes	Water is sufficient
Exercise lasting >60 minutes	Isotonic sports drink (replaces water, electrolytes and provides carbohydrates)
Very hot conditions	Isotonic sports drink recommended even for shorter sessions
Everyday hydration	Water is the best option

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Hydration and Weigh-In Sports