You are viewing a free preview of this lesson.
Subscribe to unlock all 8 lessons in this course and every other course on LearningBro.
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).
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 |
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.
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.
The redirection is achieved through two simultaneous processes:
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
| 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 |
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:
Subscribe to continue reading
Get full access to this lesson and all 8 lessons in this course.