1.2.1Unit 1: Lifestyle, Transport, Genes and Health
The Need for a Circulatory System — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 13 of 29 Unit quiz
Part 1 of 4Why Diffusion Is Not Enough
Small organisms exchange substances directly, but large ones cannot meet their needs this way.
Surface area to volume
- As an organism grows, its volume increases faster than its surface area, so its surface area to volume ratio falls.
Diffusion distance
- In a large body the distance from the surface to the inner cells is far too great for diffusion alone to supply them in time.
High metabolic rate
- Active animals such as mammals use oxygen and glucose rapidly, so supply by diffusion alone would not meet demand.
Single-celled organisms
- These have a high ratio and short distances, so simple diffusion across the cell surface is enough.
Part 2 of 4Features of Mass Transport Systems
Features of Mass Transport Systems
Fluid medium
- Blood carries dissolved and suspended substances such as oxygen, nutrients, hormones and wastes around the body.
Pump
- The heart creates the pressure that moves blood quickly around the system, so transport is by mass flow, not by diffusion.
Vessels
- A closed network of tubes keeps blood flowing in the right direction and allows the flow to be directed to tissues that need it.
Gradients
- Constant flow removes products from exchange surfaces, which maintains steep concentration gradients for diffusion.
Part 3 of 4The Double Circulation
1
Deoxygenated blood from the body enters the right atrium, passes to the right ventricle, and is pumped to the lungs in the pulmonary artery.
2
In the lungs the blood picks up oxygen and loses carbon dioxide, and returns to the left atrium in the pulmonary vein.
3
Oxygenated blood passes to the left ventricle, which pumps it at high pressure into the aorta and around the body.
4
The blood returns to the right side of the heart in the vena cava, completing one circuit of the body.
5
Blood passes through the heart twice in every full circuit, which is why the system is described as a double circulation.
| Circuit | From | To |
|---|---|---|
| Pulmonary | Right ventricle | Lungs and back to left atrium |
| Systemic | Left ventricle | Body and back to right atrium |
Part 4 of 4Exam Checklist & Tips
1.2.1 – Key Points for the Exam
- Mass transport Bulk movement of substances in a fluid over long distances, usually driven by pressure.
- Closed circulation A system in which blood stays within vessels all the time.
- Double circulation A system in which blood passes through the heart twice in each circuit.
- Surface area to volume ratio The surface area of an organism divided by its volume.
Common Mistakes
- Stating that large organisms have a larger surface area to volume ratio;
- the ratio becomes smaller as size increases.
- Saying that all arteries carry oxygenated blood; the pulmonary artery carries deoxygenated blood.
- Describing a double circulation as two hearts; it is one heart with two pumps and two circuits.
- Forgetting that the circulation also removes waste, as well as delivering oxygen and nutrients.
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