Single and double circulation and the mammalian heart

Organisms (Form and function) · Transport · note 7 of 9

Spec B3.2.14, B3.2.15

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Single and double circulation and the mammalian heartSpec B3.2.14, B3.2.15

In short

Bony fish have a single circulation: blood passes through the heart once, from heart to gills to body. Mammals have a double circulation: blood passes through the heart twice, through separate pulmonary and systemic circuits. The mammalian heart delivers pressurized blood using myogenic cardiac muscle, a pacemaker, thick-walled ventricles, valves, a septum and coronary vessels.

Single circulation of a fish: the heart pumps deoxygenated blood to the capillaries in the gills, then oxygenated blood goes to the capillaries in the body and back to the heart. Double circulation of a mammal: the right side of the heart pumps blood to the capillaries in the lungs and back to the left side, which pumps it to the capillaries in the body and back to the right side. — In a fish blood passes through the heart once per circuit; in a mammal it passes through twice.

Single and double circulation
Feature	Bony fish (single)	Mammal (double)
Sequence of organs	Heart → gills → body → heart	Heart (right side) → lungs → heart (left side) → body → heart
Passes through heart per circuit	Once	Twice
Pressure of blood reaching the body	Low, because pressure is lost in the gill capillaries	High, because blood is pumped again after the lungs
Separation of blood	Only deoxygenated blood in the heart	Septum keeps oxygenated and deoxygenated blood apart

Adaptations of the mammalian heart

Form and function in the heart
Structure	Adaptation
Cardiac muscle	Contracts without nerve stimulation (myogenic); branched, connected cells pass impulses rapidly so the walls contract together; does not fatigue.
Pacemaker (sinoatrial node)	A group of specialised cells in the wall of the right atrium that initiates each heartbeat.
Atria	Thin-walled chambers that collect blood from veins and pump it a short distance into the ventricles.
Ventricles	Thick muscular walls generate high pressure; the left ventricle wall is thicker because it pumps blood around the whole body.
Atrioventricular valves	Close when ventricles contract, preventing backflow into the atria.
Semilunar valves	In the aorta and pulmonary artery; close when ventricles relax, preventing backflow into the ventricles.
Septum	Wall separating left and right sides, keeping oxygenated and deoxygenated blood apart.
Coronary vessels	Supply cardiac muscle with oxygen and glucose and remove CO₂.

A vertical section of the mammalian heart in the frontal plane, labelled vena cava, pulmonary artery, aorta, pulmonary vein, right atrium, left atrium, right ventricle, left ventricle, semilunar valves, atrioventricular valves, septum, muscular wall and coronary arteries, with blue arrows for deoxygenated blood and red arrows for oxygenated blood. — The heart in the frontal plane is drawn as if facing the person, so its left side is on the right of the page. The coronary arteries are drawn schematically (in life they branch from the base of the aorta and run over the outer surface), and the sinoatrial node, in the wall of the right atrium near the vena cava, is not drawn.

Vena cava → right atrium → atrioventricular valve → right ventricle → semilunar valve → pulmonary artery → lungs.
Pulmonary veins → left atrium → atrioventricular valve → left ventricle → semilunar valve → aorta → body.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

What is the difference between arteries and veins?

Arteries carry blood away from the heart at high pressure, so they have thick walls of smooth muscle and elastic fibres and a narrow lumen. Veins return blood to the heart at low pressure, so they have thin flexible walls, a wide lumen and valves that stop blood flowing backwards.

What causes a heart attack?

A heart attack is caused by occlusion of a coronary artery. Fatty plaque builds up in the artery wall and narrows the lumen, and if it ruptures a blood clot can block the artery. Cardiac muscle beyond the blockage is starved of oxygen and part of it dies.

How does water move up a plant?

Water moves up a plant by the cohesion–tension mechanism. Transpiration from leaf cell walls draws water out of the xylem by capillary action, creating tension that pulls the water column up from the roots. Cohesion between water molecules, from hydrogen bonding, keeps the column continuous.

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