Blood Flow and the Double Circulatory System
This lesson covers the double circulatory system as required by the OCR GCSE PE specification (J587). You need to understand that humans have a double circulatory system consisting of the pulmonary circuit and the systemic circuit, and be able to describe the pathway of blood through both circuits.
What Is a Double Circulatory System?
Humans have a double circulatory system, which means blood passes through the heart twice on each complete circuit around the body. The two circuits are:
- Pulmonary circuit — carries blood between the heart and the lungs.
- Systemic circuit — carries blood between the heart and the rest of the body.
graph LR
A["Heart<br>(right side)"] -->|"Pulmonary artery<br>(deoxygenated)"| B["LUNGS"]
B -->|"Pulmonary veins<br>(oxygenated)"| C["Heart<br>(left side)"]
C -->|"Aorta<br>(oxygenated)"| D["BODY<br>(muscles, organs)"]
D -->|"Vena cava<br>(deoxygenated)"| A
style A fill:#4a90d9,color:#fff
style B fill:#27ae60,color:#fff
style C fill:#e67e22,color:#fff
style D fill:#9b59b6,color:#fff
Why Is a Double Circulatory System Important?
The double circulatory system is more efficient than a single circulatory system because:
- Blood passes through the heart twice, meaning it is pumped at high pressure in both circuits.
- In the pulmonary circuit, the heart pumps blood to the lungs at lower pressure (to avoid damaging the delicate alveoli).
- In the systemic circuit, the heart pumps blood to the body at higher pressure (to ensure blood reaches every organ and muscle, including those far from the heart such as the toes).
- This means oxygenated blood is delivered to the muscles at high pressure and high speed, which is essential during exercise.
The Pulmonary Circuit
The pulmonary circuit is the loop between the heart and the lungs. Its purpose is to exchange gases — picking up oxygen and releasing carbon dioxide.
Pathway
- Deoxygenated blood enters the right atrium from the body via the vena cava.
- Blood passes through the tricuspid valve into the right ventricle.
- The right ventricle pumps blood through the pulmonary valve into the pulmonary artery.
- The pulmonary artery carries deoxygenated blood to the lungs.
- In the lungs, blood flows through tiny capillaries surrounding the alveoli (air sacs).
- Gaseous exchange occurs: oxygen diffuses into the blood, and carbon dioxide diffuses out of the blood into the alveoli.
- Oxygenated blood returns to the heart via the pulmonary veins, entering the left atrium.
The Systemic Circuit
The systemic circuit is the loop between the heart and the rest of the body. Its purpose is to deliver oxygen and nutrients to the body's cells and collect carbon dioxide and waste products.
Pathway
- Oxygenated blood enters the left atrium from the lungs via the pulmonary veins.
- Blood passes through the bicuspid valve into the left ventricle.
- The left ventricle pumps blood through the aortic valve into the aorta.
- The aorta branches into smaller arteries, then arterioles, then capillaries that reach every tissue in the body.
- At the capillaries, oxygen and nutrients diffuse from the blood into the body's cells, and carbon dioxide and waste products diffuse from the cells into the blood.
- Deoxygenated blood is collected in venules, then veins, and returns to the heart via the vena cava, entering the right atrium.
Complete Pathway of Blood
Here is the full pathway of blood through both circuits, starting from the vena cava:
flowchart TD
A["Vena cava<br>(deoxygenated blood<br>from the body)"] --> B["Right atrium"]
B --> C["Tricuspid valve"]
C --> D["Right ventricle"]
D --> E["Pulmonary valve"]
E --> F["Pulmonary artery"]
F --> G["LUNGS<br>(gaseous exchange:<br>CO₂ out, O₂ in)"]
G --> H["Pulmonary veins"]
H --> I["Left atrium"]
I --> J["Bicuspid valve"]
J --> K["Left ventricle"]
K --> L["Aortic valve"]
L --> M["Aorta"]
M --> N["BODY<br>(O₂ and nutrients delivered,<br>CO₂ and waste collected)"]
N --> A
style A fill:#4a90d9,color:#fff
style G fill:#27ae60,color:#fff
style N fill:#e67e22,color:#fff
Exam Tip: Practise writing the pathway from memory. A common 4-6 mark question asks you to describe the pathway of blood from the vena cava back to the vena cava, naming all chambers, valves, and vessels.
The Double Circulatory System During Exercise
During exercise, the demand for oxygen increases dramatically. The double circulatory system responds by:
- Increasing heart rate — the heart beats faster, pumping more blood per minute.
- Increasing stroke volume — the heart pumps a larger volume of blood with each beat.
- Increasing cardiac output — the total volume of blood pumped per minute increases (cardiac output = stroke volume x heart rate).
- Vasodilation — arterioles supplying working muscles widen, allowing more blood to flow to the muscles.
- Vasoconstriction — arterioles supplying non-essential organs (such as the digestive system) narrow, redirecting blood to where it is needed most.
This ensures that working muscles receive a much greater supply of oxygenated blood during exercise than at rest.
Comparison of the Two Circuits
| Feature | Pulmonary Circuit | Systemic Circuit |
|---|
| Route | Heart → lungs → heart | Heart → body → heart |
| Purpose | Gas exchange (O₂ in, CO₂ out) | Deliver O₂ and nutrients; remove CO₂ and waste |
| Pressure | Lower (to protect delicate alveoli) | Higher (to push blood around the entire body) |
| Side of heart | Right side | Left side |
| Key artery | Pulmonary artery | Aorta |
| Key vein | Pulmonary veins | Vena cava |
Common Exam Mistakes
- Confusing the two circuits — the pulmonary circuit goes to the lungs; the systemic circuit goes to the body. Students sometimes mix these up.
- Not naming the valves — when describing the pathway, always include the valves (tricuspid, bicuspid/mitral, pulmonary, aortic).
- Forgetting that blood passes through the heart twice — the key feature of a double circulatory system is that blood passes through the heart twice on each complete loop.
- Not explaining WHY a double system is beneficial — the answer is about maintaining high pressure in both circuits for efficient delivery.
- Confusing arteries and veins in the pulmonary circuit — remember that the pulmonary artery carries deoxygenated blood and the pulmonary veins carry oxygenated blood.
Summary