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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:
| Location | Sphincter Action During Exercise | Effect |
|---|---|---|
| Working muscles | Open (relax) | Increased blood flow to muscles |
| Digestive organs | Close (contract) | Reduced blood flow to gut |
| Kidneys | Close (contract) | Reduced blood flow to kidneys |
| Skin | Open (relax) | Increased blood flow for heat loss |
Without vascular shunting, the working muscles would not receive enough oxygen and glucose to sustain exercise. The total cardiac output, even at its maximum (around 20–35 litres per minute in a trained athlete), is not enough to fully supply every organ at exercise intensity. The body must make choices — it prioritises the organs that need blood most urgently during physical activity.
| Scenario | Implication |
|---|---|
| Eating a large meal before exercise | Blood is directed to the digestive system for digestion. During exercise, blood is redirected away from the gut, which can cause stomach cramps, nausea, or a "stitch." This is why performers are advised to eat 2–3 hours before exercise. |
| High-intensity exercise in hot conditions | The body must send blood to both the working muscles (for O₂) and the skin (for cooling). This creates competing demands and can lead to earlier fatigue or heat-related illness. |
| Warm-up before exercise | A gradual warm-up allows the body to begin redirecting blood to the muscles progressively, preparing the cardiovascular system for the demands of the main activity. |
Exam Tip: A common exam question links vascular shunting to the recommendation of not eating immediately before exercise. Blood is needed by both the digestive system and the working muscles, creating a conflict. Explaining this shows excellent application of knowledge.
graph LR
subgraph "AT REST"
A1[Heart] --> B1[Muscles ~20%]
A1 --> C1[Digestive System ~25%]
A1 --> D1[Kidneys ~20%]
A1 --> E1[Brain ~15%]
A1 --> F1[Other ~20%]
end
subgraph "DURING EXERCISE"
A2[Heart] --> B2[Muscles ~85%]
A2 --> C2[Digestive System ~3%]
A2 --> D2[Kidneys ~3%]
A2 --> E2[Brain ~4%]
A2 --> F2[Other ~5%]
end
| Factor | At Rest | During Exercise |
|---|---|---|
| Blood flow to muscles | ~15–20% | ~80–85% |
| Blood flow to digestive system | ~20–25% | ~3–5% |
| Blood flow to kidneys | ~20% | ~2–4% |
| Arterioles to muscles | Normal diameter | Vasodilated (widened) |
| Arterioles to inactive organs | Normal diameter | Vasoconstricted (narrowed) |
| Pre-capillary sphincters at muscles | Partially open | Fully open |
| Pre-capillary sphincters at gut | Partially open | Mostly closed |
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