Redistribution of Blood During Exercise

At rest, the body distributes blood to all organs and tissues according to their needs. However, when you begin exercising, the body's priorities change dramatically — the working muscles need far more blood (carrying oxygen and glucose) than they do at rest. To meet this demand, the body redirects blood flow away from less active organs and towards the working muscles. This process is called the redistribution of blood (or blood shunting / vascular shunting). Understanding this mechanism is required by the Edexcel GCSE PE specification (1PE0 — Topic 1: Applied Anatomy and Physiology).

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Blood Distribution at Rest

At rest, the body distributes its cardiac output (approximately 5 litres per minute) across all organs and systems. The working muscles receive a relatively small share because they are not under heavy demand.

Organ / System	% of Cardiac Output at Rest	Approximate Volume (ml/min)
Skeletal muscles	15–20%	750–1,000
Digestive system	20–25%	1,000–1,250
Kidneys	20%	1,000
Brain	15%	750
Skin	5–10%	250–500
Heart (coronary circulation)	5%	250
Other organs	Remaining %	Variable

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Blood Distribution During Exercise

During vigorous exercise, the distribution changes dramatically. The working muscles may receive up to 80–85% of the total cardiac output, while organs such as the digestive system and kidneys receive significantly less.

Organ / System	% at Rest	% During Exercise
Skeletal muscles	15–20%	80–85%
Digestive system	20–25%	3–5%
Kidneys	20%	2–4%
Brain	15%	3–4%
Skin	5–10%	Increases (for cooling)
Heart	5%	4–5% (increases slightly)

Exam Tip: Note that blood flow to the skin actually increases during exercise — this is for thermoregulation (cooling the body through radiation and sweating). The brain always receives a minimum blood flow to maintain consciousness and function.

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What Is Vascular Shunting?

Vascular shunting is the mechanism by which the body redirects blood flow towards the working muscles and away from inactive organs during exercise. It is controlled by the vasomotor centre in the brain, which sends nerve signals to the smooth muscle in the walls of blood vessels.

How Vascular Shunting Works

The redirection is achieved through two simultaneous processes:

1. Vasodilation — The blood vessels (arterioles) leading to the working muscles and skin widen, allowing more blood to flow through them.
2. Vasoconstriction — The blood vessels (arterioles) leading to inactive organs (such as the digestive system and kidneys) narrow, restricting blood flow to those areas.

graph TD
    A[Exercise Begins] --> B[Working Muscles<br>Demand More O₂ and Glucose]
    B --> C[Vasomotor Centre<br>in Brain Activated]
    C --> D[Vasodilation of Arterioles<br>to Working Muscles and Skin]
    C --> E[Vasoconstriction of Arterioles<br>to Digestive System, Kidneys, etc.]
    D --> F[Increased Blood Flow<br>to Working Muscles]
    E --> G[Decreased Blood Flow<br>to Inactive Organs]
    F --> H[More O₂ and Glucose<br>Delivered to Muscles]
    G --> H

Key Terminology

Term	Definition
Vasodilation	Widening of blood vessels to increase blood flow
Vasoconstriction	Narrowing of blood vessels to decrease blood flow
Vascular shunting	The overall redistribution of blood from inactive organs to working muscles
Vasomotor centre	The part of the brain (in the medulla oblongata) that controls the diameter of blood vessels
Pre-capillary sphincters	Tiny rings of smooth muscle at the entrance to capillary beds that open or close to control blood flow into the capillaries

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The Role of Pre-Capillary Sphincters

Pre-capillary sphincters are small bands of smooth muscle located at the point where arterioles branch into capillaries. They act like tiny taps or gates:

• During exercise: Sphincters in the muscles open (relax), allowing more blood into the capillary beds serving the working muscles.
• During exercise: Sphincters in inactive organs close (contract), restricting blood flow to those capillary beds.