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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.
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 |
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.
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 |
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
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.
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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