Types of Circulatory System

Open vs closed circulatory systems

In an open circulatory system, found in insects and most other arthropods, the heart pumps blood (haemolymph) into open body spaces called a haemocoel, where it bathes the tissues directly before draining back towards the heart. Blood flows at low pressure and is not confined to vessels for most of its journey.

In a closed circulatory system, found in earthworms, fish and humans, blood always remains inside blood vessels as it is pumped by the heart around the body. This allows blood to be pumped at higher pressure, so it reaches tissues faster and more directly.

Single vs double circulation

In single circulation, found in fish, blood passes through the heart only once for each complete circuit of the body: it is pumped from the heart to the gills, where it picks up oxygen, then flows directly to the rest of the body before returning to the heart. Because blood loses pressure crossing the narrow gill capillaries, it reaches the body at fairly low pressure.

In double circulation, found in mammals including humans, blood passes through the heart twice for each complete circuit: once through the pulmonary circulation (heart to lungs and back) and once through the systemic circulation (heart to the rest of the body and back). Passing back through the heart between the two circuits re-pressurises the blood, so it reaches body tissues faster and at higher pressure.

Comparing across animals

How it is examined

Exam questions on this standard commonly ask you to compare open and closed systems, or single and double circulation, in terms of blood pressure, speed and efficiency of transport. You may also need to trace the path of blood through a fish or human circulatory diagram and state how many times it passes through the heart in one complete circuit.

Worked exam-style question

Question. The diagram shows a simplified fish heart with two chambers, and a simplified human heart with four chambers. Blood pressure is measured at three points: just after leaving the heart, after passing through the gill or lung capillaries, and just before returning to the heart.

(a) State the number of times blood passes through the heart in one complete circuit in (i) the fish and (ii) the human. (b) Explain why the blood pressure recorded after the gill capillaries in a fish is lower than the blood pressure recorded after the lung capillaries in a human, even though gills and lungs both contain capillary networks.

(c) State one advantage double circulation gives a mammal that single circulation does not give a fish.

Model answer. (a) (i) Once. (ii) Twice.

(b) In the fish, blood flows directly from the gill capillaries to the rest of the body without returning to the heart first, so the pressure lost crossing the narrow gill capillaries is never restored; in the human, blood returns to the heart after the lung capillaries and is re-pressurised by a second pumping action before travelling to the body. (c) Double circulation delivers oxygenated blood to the body at higher pressure and speed, supporting a high, constant body temperature and a more active lifestyle, which single circulation in a fish cannot achieve as effectively.

Practice question

Try this. An insect's open circulatory system pumps haemolymph directly into body cavities rather than through a closed network of vessels. (a) State one way this differs from a closed circulatory system such as a human's.

(b) Suggest one reason why an open circulatory system is adequate for a small, less active animal like an insect. (c) Name the blood vessel type in a closed circulatory system where exchange of substances with tissues occurs.

Exam tip

Key terms

• Double circulation, a circulatory system in which blood passes through the heart twice for each complete circuit of the body.
• Artery, a blood vessel with thick, muscular walls that carries blood away from the heart at high pressure.
• Vein, a blood vessel with thinner walls and valves that carries blood back to the heart at low pressure.
• Capillary, the smallest blood vessel, one cell thick, where exchange of substances between blood and tissues occurs.

Source:SRC-DSKP-EN

Frequently asked questions

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