9.1 Circulatory systems
- Syllabus
- 0610–2026–2027
- Topic
- 9.1
- Level
- —
Learning objectives
9.1.1Circulatory system as a system• Describe the circulatory system as a system of blood vessels with a pump and valves to ensure one-way flow of blood9.1.2Single circulation of a fish• Describe the single circulation of a fish9.1.3Double circulation of a mammal• Describe the double circulation of a mammal9.1.4Advantages of a double circulation• Explain the advantages of a double circulation
Define the circulatory system
The circulatory system is a system of blood vessels with a pump and valves that ensures one-way flow of blood.
| Component | Function |
|---|---|
| pump: the heart | generates pressure that moves blood |
| blood vessels | provide routes for blood around the body |
| valves | prevent backflow, maintaining one-way flow |
These components work together: the heart moves blood into vessels, the vessels connect the circulation, and valves stop blood reversing direction.
Valves do not pump blood. The heart supplies the force; valves prevent backflow.
Describe single circulation in a fish
In a single circulation, blood passes through the heart once during one complete circuit of the body.
| Stage | What happens |
|---|---|
| heart → gills | the heart pumps deoxygenated blood to the gills |
| gills → body | blood gains oxygen at the gills and travels to body tissues |
| body → heart | blood delivers oxygen, becomes deoxygenated and returns to the heart |
The complete fish pathway is: heart → gills → body → heart.
Single circulation means one passage through the heart per complete circuit, not that blood travels in only one vessel.
Describe double circulation in a mammal
In a double circulation, blood passes through the heart twice during one complete journey through the pulmonary and systemic circuits.
| Circuit | Route | Main job |
|---|---|---|
| pulmonary circulation | heart → lungs → heart | blood gains oxygen and loses carbon dioxide |
| systemic circulation | heart → body → heart | blood delivers oxygen to tissues and returns deoxygenated |
Together the two loops give the route heart → lungs → heart → body → heart.
Double circulation means two passages through the heart per complete journey. It does not mean that blood circulates around the body twice.
Explain the advantages of double circulation
Double circulation lets the heart restore pressure between the lung circuit and the body circuit, while keeping oxygenated and deoxygenated blood separate.
| Feature | Advantage |
|---|---|
| blood returns to the heart after the lungs | it can be pumped to the body at high pressure for rapid flow |
| a separate pulmonary circuit | lower pressure can protect delicate lung capillaries |
| separate sides and circuits | oxygenated blood does not mix with deoxygenated blood |
| rapid, oxygen-rich systemic flow | tissues receive oxygen efficiently for high rates of respiration |
The key causal chain is: two circuits → pressure can be adjusted and blood kept separate → efficient oxygen delivery to body tissues.
The advantage is not merely that blood passes through the heart twice; it is the resulting pressure control, separation and more efficient oxygen supply.