Redistribution of Blood During Exercise

During exercise, the body must redirect blood flow so that working muscles receive more oxygen and nutrients while less active organs receive less. This process — known as the redistribution of blood flow or vascular shunting — is a vital mechanism that enables sustained physical performance. This lesson covers the mechanisms of vasoconstriction and vasodilation, the concept of vascular shunting, and how the body prioritises blood supply during activity. This is a frequently examined area of the AQA GCSE PE specification (3.1.1.2).

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Why Blood Must Be Redistributed

At rest, blood is distributed throughout the body to serve all organs and tissues. However, during exercise, the oxygen demand of the working muscles increases dramatically — in some cases by ten to twenty times the resting level. The body cannot increase total blood volume (it remains approximately 5 litres), so it must redistribute the existing blood supply.

The key principle is: blood is diverted away from non-essential (inactive) organs towards the working (active) muscles.

At Rest	During Exercise
Muscles receive approximately 15–20% of cardiac output	Muscles receive up to 80–85% of cardiac output
Digestive system receives approximately 20–25%	Digestive system receives as little as 3–5%
Kidneys receive approximately 20%	Kidneys receive approximately 3–4%
Skin receives approximately 5%	Skin receives approximately 2% (initially; may increase later for cooling)
Brain receives approximately 15%	Brain maintains approximately 15% (always protected)

Exam Tip: AQA often presents these values as percentages or in a table. The key pattern to remember is that the percentage going to muscles increases enormously, while the percentages to the digestive system, kidneys and liver decrease. The brain always receives the same proportion because it is a vital organ.

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The Mechanisms: Vasoconstriction and Vasodilation

The redistribution of blood is achieved through two complementary mechanisms involving the arterioles (small blood vessels that branch off from arteries):

Vasodilation

Vasodilation occurs when the smooth muscle in the walls of arterioles relaxes, causing the arterioles to widen (increase in diameter). This reduces resistance to blood flow and allows more blood to pass through to the tissues supplied by those arterioles.

During exercise, vasodilation occurs in the arterioles supplying the working muscles. This ensures that a greater volume of oxygenated blood reaches the muscles, providing the oxygen and glucose needed for aerobic respiration and the removal of waste products such as carbon dioxide and lactic acid.

Vasoconstriction

Vasoconstriction occurs when the smooth muscle in the walls of arterioles contracts, causing the arterioles to narrow (decrease in diameter). This increases resistance to blood flow and reduces the volume of blood reaching the tissues supplied by those arterioles.

During exercise, vasoconstriction occurs in the arterioles supplying non-essential organs — those that are not directly involved in the exercise. These include the digestive system, kidneys and liver.

Mechanism	What Happens	Where (During Exercise)	Effect
Vasodilation	Arterioles widen	Arterioles to working muscles	More blood flows to muscles
Vasoconstriction	Arterioles narrow	Arterioles to non-essential organs (gut, kidneys, liver)	Less blood flows to these organs

graph TD
    A[Heart pumps blood into aorta] --> B[Arterioles to working muscles]
    A --> C[Arterioles to non-essential organs]
    B -->|VASODILATION - arterioles widen| D[Increased blood flow to muscles]
    C -->|VASOCONSTRICTION - arterioles narrow| E[Decreased blood flow to organs]
    D --> F[More O₂ and nutrients delivered]
    E --> G[Blood redirected to muscles instead]

    style A fill:#e74c3c,color:#fff
    style B fill:#27ae60,color:#fff
    style C fill:#f39c12,color:#fff
    style D fill:#2ecc71,color:#fff
    style E fill:#e67e22,color:#fff
    style F fill:#27ae60,color:#fff
    style G fill:#e67e22,color:#fff

Exam Tip: You must use the correct terminology. Do not say "blood vessels get wider" — say "arterioles undergo vasodilation." AQA will award more marks for precise scientific language.

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Vascular Shunting

The combined effect of vasodilation in some arterioles and vasoconstriction in others is called vascular shunting (also known as the shunting mechanism or blood shunting). It is the process by which blood is redirected from inactive areas to active areas during exercise.

How Vascular Shunting Works

1. The body detects an increase in exercise intensity (through chemoreceptors detecting rising CO₂ levels and falling O₂ levels, and through proprioceptors detecting muscle movement).
2. The brain sends signals via the sympathetic nervous system.
3. Arterioles supplying working muscles undergo vasodilation, increasing blood flow to those muscles.
4. Arterioles supplying non-essential organs undergo vasoconstriction, reducing blood flow to those organs.
5. The overall effect is that a much larger proportion of cardiac output is directed to the working muscles.

The Role of Pre-capillary Sphincters

Pre-capillary sphincters are small rings of smooth muscle located at the entrance to capillary beds. They act as gatekeepers: