Free Transport Systems in Animals revision notes for OCR A Level Biology – covering specification points 3.1.2(a) and 3.1.2(b).
The Need for Transport Systems
As an animal (and plant) increases in size and complexity, their surface area to volume ratio (SA:V) decreases; diffusion alone cannot meet its metabolic requirements.
The table below outlines the limitations on diffusion in multicellular animals:
| Limitation | Reason |
|---|---|
| Low SA:V ratio | A lower SA:V provides less space for substances to enter or leave the organism via the skin compared to their volume. |
| High metabolism | Metabolically active tissues (e.g. muscles, brain) need a high rate of oxygen and glucose delivery for making ATP. |
| Long diffusion distance | Internal cells are far from the external surface, and those that aren’t may be covered by an impermeable barrier; it takes too long for substances to get to where they are needed. |
To overcome these limitations, animals have evolved specialised transport systems which:
- Deliver substances to cells (e.g. oxygen, glucose, amino acids, hormones)
- Remove toxic waste products (e.g. carbon dioxide, urea)
- Deliver hormones (hormonal signalling)
Types of Circulatory Systems
There are different types of circulatory systems, open and closed.
The table below compares closed and open circulatory systems:
| Feature | Open Circulatory System | Closed Circulatory System |
|---|---|---|
| Definition | Blood (or haemolymph) is not enclosed in vessels. | Blood is enclosed in vessels. |
| Examples | Insects (e.g. grasshoppers), molluscs (e.g. snails). | Vertebrates (e.g. mammals, fish), some invertebrates (e.g. annelid worms). |
| Transport Medium | Haemolymph: blood and tissue fluid (does not transport oxygen in insects). | Blood: remains separate from tissue fluid. |
| Pressure | Low-pressure. | High pressure (overall, varies). |
| Pumping Mechanism |
Varies: • Body movement • Peristalsis • Heart (simple and open) |
The heart pumps blood through vessels; capillaries allow exchange with tissues. |
Types of Closed Circulatory Systems: Single and double
In OCR A level Biology, you need to know about the structure of the double circulatory system in mammals and the single circulatory system in fish (as well as the open circulatory system in insects).
The table below outlines the features of these three types of circulatory systems for comparison:
| Feature | Double (Mammals) | Single (Fish) | Open (Insects) |
|---|---|---|---|
| No. of circuits | 2 | 1 | 0 |
| Pathway | Heart → pulmonary → heart → systemic | Heart → gills → body | Heart → body |
| Heart passes per cycle | 2 | 1 | 1 |
| Blood pressure to body | High | Low | Low |
| Oxygen flow to body |
• High flow • High pressure |
• Lower flow • Lower pressure |
None: • No oxygen in haemolymph. • Via tracheae. |
| Blood vessels | Closed | Closed | Mostly absent* |
*Some insects have open-ended tubular extensions on their heart that deliver blood to more metabolically active regions.
