Circulatory system = vessels + pump + valves
A circulatory system has three co-operating parts. Blood vessels are the tubes that carry blood around the body. The pump — the heart — contracts to generate the pressure that pushes blood through those vessels. Valves, in the heart and in veins, ensure blood flows in one direction only by preventing backflow. A full description names all three; "the heart pumps blood" alone is incomplete, dropping both the vessel network and the one-way-flow role of valves.
(Extended) Single circulation — heart once per circuit
In a fish, blood is pumped from the heart to the gills, where it gains oxygen and loses carbon dioxide; it then travels straight on to the body, delivering oxygen to the tissues, before returning to the heart. Blood therefore passes through the heart once per complete body circuit. Because pressure falls sharply as blood crosses the narrow gill capillaries, the blood that goes on to the body tissues arrives at a much lower pressure than when it left the heart.
(Extended) Double circulation — heart twice per circuit
In a mammal, two separate loops both pass through the heart. The pulmonary circulation runs heart → lungs → heart, where blood gains oxygen and loses carbon dioxide. The systemic circulation runs heart → body → heart, delivering oxygen to the tissues. Because blood returns to the heart and is re-pumped between lungs and body, it passes through the heart twice per circuit. That re-pressurising step is the defining difference from single circulation.
Drawn from real examiner reports.
Valves dropped from the description
A complete description needs vessels, a pump and valves that ensure one-way flow, but candidates give only the vessel-and-pump part, or write "valves" without saying what they do. Without valves the pump's pressure would push blood backwards as well as forwards, so flow direction could not be controlled — the valve point is not decoration.
Flagged Jun 2022 P41 Q1ci; Nov 2023 P43 Q1bi
Valves ≠ "they just stop backflow"
A valve opens when the pressure behind it exceeds the pressure in front of it, and closes when the pressure in front would otherwise drive blood backwards. "Valves stop backflow" gives the effect, not the mechanism. The same reasoning explains why blood leaves the heart: the ventricle must reach a pressure higher than that in the vessel beyond it.
Flagged Jun 2022 P22 Q7
Circulation pathway ≠ heart structure
"Describe the double circulation of a mammal" wants the pathway — the two loops, and the fact that blood passes through the heart twice per circuit. It does not want the heart's internal chambers, septum or valve names, which belong to the separate heart-structure leaf. Such facts can be entirely true and still score nothing, because they answer a different question.
(Extended) One loop is single, not double
Writing heart → lungs → body → heart describes one continuous loop that visits the heart once — that is single circulation, not double. A correct double-circulation answer must bring blood back to the heart between the lungs and the body: heart → lungs → heart (pulmonary), then heart → body → heart (systemic). The return to the heart in the middle is the whole point.
(Extended) Counting vessels, not heart passes
What separates single from double circulation is the number of times blood passes through the heart per complete body circuit — once versus twice — not the number of blood vessels, the number of organs visited, or the size of the animal. Counting loops on a diagram only works if you check that each loop both starts and ends at the heart.
(Extended) "More efficient" explains nothing
Asked to explain the advantage of double circulation, candidates write "it is more efficient" or "the pressure is higher" and stop. The chain must be explicit: blood is re-pumped between lungs and body → it reaches the body tissues at higher pressure → so oxygen is delivered to those tissues faster. The re-pumping step is the link most often missing.
"Veins have valves" is only half of it
Asked what keeps blood flowing one way, candidates answer "veins have valves". True, but incomplete: valves are also present in the heart, at its exits. On a schematic, marking valves only in veins — or worse, only in arteries — loses the mark. Answer at whole-system level: valves in the heart and in veins together ensure one-way flow.
Three-part checklist for "describe"
For "describe the circulatory system", run the same checklist every time: vessels carry blood around the body; the heart is the pump generating the pressure; valves in the heart and veins ensure one-way flow. Three marks, three points — match the number of points to the tariff.
(Extended) Trace the loop, don't draw a heart
Give the circuit as an ordered pathway and state the number of heart passes. Single: heart → gills → body → heart, once per circuit. Double: heart → lungs → heart, then heart → body → heart, twice. Never substitute chamber detail for the pathway itself.
Diagram: one pump box, valves, arrows
Draw the heart as a single pump box with vessels entering and leaving it, mark valves at the heart exits and within veins, and draw arrows in one direction only. A fish gets one loop that includes the gills; a mammal gets two loops that each start and end at the heart.
Pressure percentages: check the divide
For "percentage of pressure remaining", divide the smaller leaving value by the larger entering value: 12/30 × 100 = 40%, never 30/12. For the change from one heart pass to two, give the percentage increase — (2−1)/1 × 100 = 100% — not "twice" or "doubled".
Define a valve and state what it ensures in the circulatory system.
Describe the circulatory system of an animal. (3 marks)
