Pump, vessels, valves — the three components
Pump — the heart, a muscular organ whose contractions generate the pressure that pushes blood round the circuit. Blood vessels — a closed network of arteries, veins and capillaries that directs the flow. Valves — structures in the heart and veins that open when pressure is higher upstream and close when it reverses, ensuring one-way flow. Valves never generate pressure; only the heart does. All Core content, required of both tiers.
(Extended) Single vs double circulation
Single circulation (fish): heart → gills → body → heart. Blood passes through the heart once per circuit, and pressure has already dropped across the gill capillaries before it reaches the body. Double circulation (mammal): two loops, one heart — pulmonary (right side → lungs → left side) and systemic (left side → body → right side). Blood passes through the heart twice per circuit. The septum divides the two sides; the left ventricle has the thicker wall.
(Extended) Two advantages of double circulation
Advantage 1 — pressure. Blood is re-pressurised at the heart between the two circuits, so the left ventricle delivers it to body tissues at high pressure; a fish cannot do this, because its blood goes straight on to the body after the gills. Advantage 2 — separation. Oxygenated and deoxygenated blood are kept completely separate, so tissues always receive fully oxygenated blood and the lungs receive fully deoxygenated blood — maximising both diffusion gradients.
Drawn from real examiner reports.
The heart pumps — it does not transport
The heart pumps; the blood and the vessels transport. "The heart transports blood around the body" earns nothing — examiners want "pumps" or "pushes". Keep the three jobs distinct: the heart generates the pressure by muscular contraction, the vessels direct the flow, and the blood itself carries oxygen, nutrients and wastes.
Flagged s23 P31 Q7a
Left ventricle sits on the right of a diagram
A heart diagram is drawn as if you are facing the patient, so the left ventricle appears on the right of the page. The left side handles oxygenated blood (lungs → left atrium → left ventricle → aorta → body); the right side handles deoxygenated blood. The thicker, more muscular wall is always the left ventricle — never the right.
Flagged s23 P12/13 Q19/Q20 · s22 P11/13/33 Q19
Septum separates; valves stop backflow
The septum is the muscular wall between the left and right sides of the heart — it keeps oxygenated and deoxygenated blood separate. Valves are flaps that prevent backflow: atrioventricular valves between atrium and ventricle, semi-lunar valves at the exits of the ventricles. Name the specific valve — "a valve" on its own is not enough.
Flagged w22 P42 Q6a ii · s22 P41 Q2a iii
Pulmonary artery carries deoxygenated blood
Vessels are named by direction, not by oxygen content. The pulmonary artery carries deoxygenated blood — it is an artery because it leaves the heart (right ventricle → lungs). The pulmonary vein carries oxygenated blood back to the left atrium. Arteries always carry blood away from the heart; veins always carry it toward the heart.
Flagged s23 P11 Q19 · s23 P23 Q18
Double counts heart passes, not laps
The blood completes one lap of the body per cycle either way. "Double" counts how many times it passes through the heart: once per circuit in a fish (heart → gills → body → heart), twice in a mammal (heart → lungs → heart → body → heart). "In a double circulation the blood travels round the body twice" is the classic wrong option.
One advantage is half the marks
"State the advantages of a double circulation" is worth two marks and expects two different ideas: blood is re-pressurised at the heart between the circuits, so it reaches the body at high pressure; and oxygenated and deoxygenated blood stay completely separate. Writing "the blood pressure is higher" twice in different words scores one mark only.
List the components separately
For "state the features of a circulatory system" [3], write three separate lines: a pump (the heart); a network of blood vessels; valves that ensure one-way flow. One mark each — merging them into a single sentence often loses a mark.
Describe a circulation as a pathway
Fish: heart → gills (oxygenation) → body (delivery) → heart, then state that blood passes through the heart once. Mammal: name the pulmonary loop (right side → lungs → left side) and the systemic loop (left side → body → right side), then state twice.
Give both advantages, every time
Two marks, two ideas: (1) blood is re-pressurised at the heart between the circuits, so tissues receive it at high pressure; (2) oxygenated and deoxygenated blood are kept separate, so tissues receive fully oxygenated blood. Never spend both marks on pressure.
Cardiac output — and the unit
Cardiac output = heart rate × stroke volume, so 72 beats/min × 75 cm³/beat = 5400 cm³/min. State the unit; a missing unit costs the mark. For any percentage change, divide by the starting value, never the final one.
Circulatory system: a closed system consisting of a pump (heart), a network of blood vessels, and valves that ensure blood flows in one direction only throughout the body.
Each component has a distinct role:
| Component | Role |
|---|---|
| Heart (pump) | Contracts rhythmically to generate blood pressure and push blood around the circuit |
| Blood vessels | A closed network of tubes (arteries, veins, capillaries) that directs blood flow |
| Valves | Structures that open when blood pressure is higher on the upstream side and close when pressure falls, preventing backflow |
Valve function: valves do NOT create pressure — the heart creates pressure. Valves only maintain one-way flow by closing against the direction of pressure.
State the three components of a circulatory system.
State three features of a circulatory system that are common to all animals that possess one. [3]
