Redistribution of Blood During Exercise

This lesson covers the redistribution of blood during exercise as required by the OCR GCSE PE specification (J587). You need to understand how the body redirects blood flow during physical activity to ensure that working muscles receive the oxygen they need, and how this is achieved through vasodilation and vasoconstriction.

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What Is Blood Redistribution?

At rest, blood is distributed fairly evenly throughout the body, supplying all organs and tissues. However, during exercise, the working muscles have a dramatically increased demand for oxygen and nutrients. The body cannot increase the total volume of blood, so instead it redirects blood from areas where it is less needed to areas where it is needed most.

This process is called blood redistribution (or blood shunting).

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Blood Distribution at Rest vs During Exercise

Organ/Area	At Rest (% of cardiac output)	During Exercise (% of cardiac output)
Skeletal muscles	~15-20%	~80-85%
Digestive system	~20-25%	~3-5%
Brain	~15%	~3-4%
Kidneys	~20%	~2-4%
Skin	~5%	~2% (increases during prolonged exercise for cooling)
Heart	~5%	~4-5%

Exam Tip: The most important figures to remember are: skeletal muscles receive approximately 15-20% at rest but 80-85% during exercise. The digestive system receives much less blood during exercise, which is why eating a large meal before sport can cause stomach cramps.

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How Blood Is Redistributed

Blood redistribution is controlled by the autonomic nervous system (involuntary) and involves two key mechanisms:

1. Vasodilation

Vasodilation is the widening of blood vessels (specifically arterioles) to increase blood flow to an area.

• During exercise, the arterioles supplying the working muscles undergo vasodilation.
• This increases the amount of blood flowing to the muscles, delivering more oxygen and nutrients.
• Vasodilation also occurs in the arterioles supplying the skin during prolonged exercise, to help with thermoregulation (cooling the body by bringing warm blood closer to the surface).

2. Vasoconstriction

Vasoconstriction is the narrowing of blood vessels (specifically arterioles) to decrease blood flow to an area.

• During exercise, the arterioles supplying non-essential organs (such as the digestive system and kidneys) undergo vasoconstriction.
• This reduces the amount of blood flowing to these organs, allowing more blood to be redirected to the working muscles.
• The organs are not harmed because they can function with less blood for the duration of the exercise.

graph TD
    A["Exercise Begins"] --> B["Working muscles<br>need more O₂"]
    B --> C["Vasodilation<br>in arterioles to muscles"]
    B --> D["Vasoconstriction<br>in arterioles to<br>non-essential organs"]
    C --> E["More blood flows<br>to working muscles"]
    D --> E

    style A fill:#4a90d9,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#e67e22,color:#fff

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Why Is Blood Redistribution Important for Sport?