Every cell in a large animal needs oxygen and glucose delivered and its waste carried away, faster than diffusion alone could manage. A circulatory system does the delivering: a pump pushes blood round a closed loop of vessels, and valves keep it flowing one way. Comparing a fish with a mammal shows why we have two circuits, not one (Extended).
The circulatory system is a system of blood vessels with a pump (the heart) and valves that ensure a one-way flow of blood. Extended: a fish has a single circulation, in which blood passes through the heart once in each complete circuit of the body. A mammal has a double circulation: blood passes through the heart twice in each circuit, once on the way to the lungs and once on the way to the rest of the body.
Fig. 1: in a fish, blood goes heart, gills, body, heart; in a mammal the right side of the heart pumps blood to the lungs and the left side pumps it to the body (red = oxygenated, blue = deoxygenated)
Section 01
Pump, vessels and valves.
Switch the sim between the fish and the mammal and watch the blood cells change colour as they travel. Red cells are carrying oxygen; blue ones have given most of it up. Whichever animal you pick, the blood always moves round the loop in the same direction.
The pump: the heart, made of muscle, contracts to push blood into the vessels under pressure.
The vessels: arteries carry blood away from the heart, capillaries run through every tissue, and veins bring blood back to the heart.
The valves: found in the heart and in veins, they close if blood starts to flow backwards, so the flow is one way only.
The readouts: "times through the heart per circuit" shows once for the fish and twice for the mammal; "pressure to the body" shows low for the fish and high for the mammal.
Examiner trap
A circulatory system is not just "the heart and blood". The syllabus description has three parts: blood vessels, a pump, and valves to ensure one-way flow. Name all three.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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In the sim, how many times does the blood pass through the fish heart in one complete circuit?
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Which organ does fish blood flow through straight after leaving the heart?
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In the mammal view, what colour are blood cells on the way from the lungs back to the heart?
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What does the sim show for the pressure of blood reaching the body in a fish?
Section 02
Single and double circulation.
Extended. In a fish, blood leaves the two-chambered heart, passes through the gill capillaries, then goes straight on to the body organs before returning to the heart. In a mammal, blood returns to the heart after the lungs and is pumped a second time.
Fish (single)
Mammal (double)
Passes through the heart
once per circuit
twice per circuit
Route
heart → gills → body → heart
right side → lungs → left side → body → right side
Circuits
one
pulmonary (lungs) and systemic (body)
Pressure of blood reaching the body
low: pressure is lost in the gill capillaries
high: blood is pumped again after the lungs
Advantage 1, pressure: blood returning from the lungs is pumped again, so it reaches the body at high pressure and flows quickly. Oxygen and glucose are delivered fast enough for a high metabolic rate, needed by an active animal that keeps its body warm.
Advantage 2, gentle on the lungs: the right side can pump to the lungs at a lower pressure, which does not damage the delicate lung capillaries.
Advantage 3, no mixing: the two sides of the heart are separated, so oxygenated and deoxygenated blood do not mix and the body receives fully oxygenated blood.
Examiner trap
"Twice through the heart" is a description, not an advantage. For an explain the advantages question, say what the second pumping achieves: higher pressure, faster delivery of oxygen and glucose to respiring cells.
Stage 2 · Exam
Exam-style questions
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Which description of the circulatory system is correct?
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Extended: what is meant by a double circulation?
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Extended: which is an advantage of a double circulation over a single circulation?
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Extended: in a fish, blood reaches the body organs at low pressure mainly because:
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Circulatory systems, mastered.
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