Transport in animals: (b) the different types of circulatory systems

The idea

Circulatory systems can be classified by whether blood stays within vessels and by how many circuits exist. In a closed system blood is confined to vessels, whereas in an open system blood leaves vessels and bathes the tissues in a haemocoel, as in many insects. Systems also differ in the number of circuits: a single circulation has blood passing through the heart once per complete circuit, as in fish, while a double circulation passes through the heart twice, as in mammals, with a pulmonary circuit to the lungs and a systemic circuit to the body. Learners should compare these types, describe the advantages of a double circulation in maintaining pressure and rapid flow to the body, and recognise examples of each type.

What earns the marks

• A closed circulatory system keeps blood within vessels, allowing higher pressure and faster flow, whereas an open system lets blood leave vessels into a haemocoel at lower pressure.
• A single circulation, as in fish, has blood passing through the heart once per circuit, so blood loses pressure as it passes through the gills before reaching the body.
• A double circulation, as in mammals, has blood passing through the heart twice per circuit, with separate pulmonary and systemic circuits.
• A double circulation maintains higher pressure and faster flow to body tissues because blood returns to the heart after the lungs before being pumped to the body.
• Systems can be compared by vessel arrangement, number of heart chambers and number of circuits through the heart.

Common mistakes and how to put them right

• Confusing open and closed systems with single and double circulation.

  Put it right: These are two separate classifications, so describe each independently.

• Thinking fish have a double circulation because blood passes through gills and body.

  Put it right: Blood passes through the heart only once, so it is a single circulation.

• Believing open systems have no vessels at all.

  Put it right: Blood still leaves the heart through vessels before entering the haemocoel, so correct this by describing the open-ended arrangement.

Exam tips

• State the classification explicitly, for example 'closed and double', before giving the comparison.
• When comparing single and double circulation, focus on pressure and flow to the body as the key advantage.
• Name a named example organism for each type to secure the comparison mark.