1.1.1Topic 1 · Lifestyle, Health and Risk
The Need for a Circulatory System — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions
5 min read 8 parts Lesson 3 of 23 Unit quiz
1.1.1 The Need for a Circulatory System — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain why large multicellular animals need a transport system, using surface area to volume ratio, and distinguish open from closed and single from double circulations.Every living cell needs…
- surface area to volume ratio (SA:V)
- Metabolic rate
- Specialisation
- Waste removal
- transport medium
- single circulation
Section 1 of 8Why diffusion is not enough
Every living cell needs oxygen and nutrients and must remove carbon dioxide and other wastes.
As an organism increases in size, its volume grows faster than its surface area, so the surface area to volume ratio (SA:V) falls. Consider cubes: a cube of side 1 cm has surface area 6 cm2, volume 1 cm3, so SA:V = 6:1. A cube of side 4 cm has surface area 96 cm2, volume 64 cm3, so SA:V = 1.5:1. The larger organism has proportionally less surface through which to exchange substances, yet more cells to supply.
Section 2 of 8Factors that create the need
- Active, warm-blooded animals have high demands for oxygen and glucose, so need rapid delivery.
- A mouse has a higher mass-specific metabolic rate than an elephant and also a much higher heart rate.
- In multicellular animals exchange surfaces such as lungs and the gut are in one place,
- while the cells needing the supplies are elsewhere,
- so substances must be moved between them.
A transport system must have a transport medium (blood), vessels, a pump (heart) to create pressure, and a mechanism to keep the flow in one direction, such as valves.
Section 3 of 8Types of circulatory system
| Feature | Open | Closed |
|---|---|---|
| Blood location | Pumped into body cavity (haemocoel), bathes organs directly | Stays inside vessels |
| Pressure | Low | High, can be adjusted |
| Speed and control | Slow, little control over distribution | Fast, can direct blood to active organs |
| Examples | Insects, many molluscs | Vertebrates, earthworms |
Closed systems can be single or double.
- and a systemic circuit carries it from heart to the body and back.
Section 4 of 8Transport in other animals and plants
Comparing organisms reinforces the principles.
- so their haemolymph carries nutrients but not much oxygen.
Plants also need mass transport (xylem and phloem), because the roots and shoots are far apart. The general idea is the same: a small SA:V and the high demand of active tissues require a specialised system that moves materials in bulk.
Section 5 of 8Mass transport in mammals
In humans the transport system relies on blood plasma, red blood cells that carry oxygen, and a network of arteries, arterioles, capillaries, venules and veins.
You should link this to lessons on the structure of the heart and blood vessels and to later topics on gas exchange.
Section 6 of 8Exam-style questions
Explain why a mouse needs a circulatory system but a flatworm does not. [3]3 marks
The flatworm is thin and flat so has a large SA:V ratio and short diffusion distance; oxygen reaches all cells by diffusion. [1] The mouse is larger with a smaller SA:V ratio and cells deep inside the body. [1] The mouse also has a high metabolic rate and so needs rapid supply of oxygen and nutrients that diffusion alone cannot supply. [1]
A cube-shaped organism has sides of 2 cm. Calculate its surface area to volume ratio and state what happens to the ratio if the side doubles. [3]3 marks
Surface area = 6 x 2 x 2 = 24 cm2; volume = 2 x 2 x 2 = 8 cm3. [1] Ratio = 24:8 = 3:1. [1] If the side doubles to 4 cm the ratio falls to 1.5:1 (halves). [1]
Describe the advantages of a double circulation compared with a single circulation. [3]3 marks
Blood is returned to the heart after passing through the lungs so can be pumped again at high pressure. [1] This gives faster flow of blood to the body tissues, so more rapid delivery of oxygen and glucose. [1] Lungs can operate at a lower pressure, which avoids damage to the delicate capillaries. [1]
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Surface area to volume ratio | Surface area divided by volume; falls as organisms get larger. |
| Mass transport | Bulk movement of substances in a fluid, driven by pressure differences. |
| Open circulation | A system in which blood is not confined to vessels and bathes tissues directly. |
| Closed circulation | A system in which blood is contained in vessels. |
| Single circulation | Blood passes through the heart once in each complete circuit. |
| Double circulation | Blood passes through the heart twice in each complete circuit. |
| Metabolic rate | The rate at which an organism uses energy. |
| Diffusion | Net movement of particles from high to low concentration. |
Section 8 of 8Exam technique
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