The big idea: The heart is a muscular pump that pushes blood around the body. It has four chambers: two thin-walled atria on top that receive blood, and two thicker-walled ventricles below that pump it out.
The heart is really two pumps side by side. The right side handles deoxygenated blood (it sends it to the lungs); the left side handles oxygenated blood (it sends it to the body).
A wall called the septum separates the two sides so the two kinds of blood never mix.
The four chambers and four great vessels. Blue = deoxygenated blood (right side → lungs); red = oxygenated blood (left side → body). The left ventricle has the thickest wall because it pumps to the whole body.
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- Atrium (plural: atria)
- An upper, thin-walled chamber of the heart that receives blood arriving in the veins and passes it down into a ventricle.
- Ventricle
- A lower, thick-walled chamber of the heart that pumps blood out into an artery. The left ventricle has the thickest wall.
- Septum
- The muscular wall that separates the right side of the heart from the left, keeping deoxygenated and oxygenated blood apart.
- Atrioventricular (AV) valve
- A valve between an atrium and a ventricle that stops blood flowing backwards into the atrium when the ventricle contracts.
- Semilunar valve
- A valve at the exit of each ventricle (into the pulmonary artery and the aorta) that stops blood flowing back into the ventricle.
- Deoxygenated / oxygenated blood
- Deoxygenated blood is low in oxygen (returning from the body); oxygenated blood is high in oxygen (returning from the lungs).
Why the left ventricle has the thickest wall: The left ventricle pumps blood to the whole body, so it must generate a high pressure.
That needs a lot of muscle, so its wall is the thickest of the four chambers.
The right ventricle pumps only to the nearby lungs at lower pressure, so its wall is thinner; the atria barely pump at all (just down into the ventricles), so their walls are thinnest.
| Chamber | Blood it holds | Receives from / pumps to | Wall |
|---|---|---|---|
| Right atrium | deoxygenated | receives from the vena cava (body) | thin |
| Right ventricle | deoxygenated | pumps to the pulmonary artery (lungs) | thicker than atrium |
| Left atrium | oxygenated | receives from the pulmonary vein (lungs) | thin |
| Left ventricle | oxygenated | pumps to the aorta (whole body) | THICKEST — most muscle |
Each heartbeat is a repeating sequence called the cardiac cycle.
It is driven by muscle contracting and relaxing, and by valves that snap shut at the right moments so blood only ever flows one way.
- Cardiac cycle
- One complete heartbeat — the repeating sequence of contraction (systole) and relaxation (diastole) of the heart chambers.
- Systole
- The contraction phase, when a chamber squeezes to push blood out.
- Diastole
- The relaxation phase, when the chambers refill with blood.
- Double circulation
- A system in which blood passes through the heart twice for each full circuit of the body — once for the lungs (pulmonary circuit) and once for the body (systemic circuit).
The cardiac cycle — three stages: 1. Atria contract (atrial systole). The AV valves are open, so blood is pushed down from the atria into the ventricles.
2. Ventricles contract (ventricular systole). The AV valves shut (this makes the first heart sound, 'lub') and the semilunar valves open, so blood is forced out into the pulmonary artery and the aorta.
3. Everything relaxes (diastole). The semilunar valves shut (the second heart sound, 'dub') and the chambers refill, ready for the next beat.
| Stage | What contracts / relaxes | Valves | Effect on blood |
|---|---|---|---|
| Atrial systole | atria contract | AV valves open | blood pushed from atria into ventricles |
| Ventricular systole | ventricles contract | AV valves shut, semilunar valves open | blood forced out into the pulmonary artery and aorta |
| Diastole | atria and ventricles relax | semilunar valves shut, AV valves open | chambers refill with blood returning to the heart |
Pressure, valves and heart sounds — the data link: Valves are pushed open and shut by pressure differences, and this is exactly what a cardiac-cycle pressure graph shows.
A valve opens when the pressure behind it becomes higher than the pressure in front of it; it shuts when the pressure in front becomes higher (which would push blood backwards).
The heart sounds are the valves closing: 'lub' is the AV valves shutting at the start of ventricular systole, and 'dub' is the semilunar valves shutting at the start of diastole.
Double circulation — twice through the heart: Mammals have double circulation: blood passes through the heart twice on each full trip round the body.
Pulmonary circuit: right side of the heart → lungs → back to the heart (picks up oxygen).
Systemic circuit: left side of the heart → body → back to the heart (drops off oxygen).
The advantage: after the blood loses pressure in the lungs, the left ventricle re-pressurises it, so the body gets blood at high pressure and oxygenated and deoxygenated blood are kept completely separate.
Right side (deoxygenated)
- Receives blood from the body (via the vena cava)
- Right atrium → right ventricle
- Pumps out through the pulmonary artery
- Sends blood to the lungs to pick up oxygen
Left side (oxygenated)
- Receives blood from the lungs (via the pulmonary vein)
- Left atrium → left ventricle
- Pumps out through the aorta
- Sends blood to the whole body at high pressure
A memory hook: Atria are Above and Accept blood; ventricles are below and vent it out.
And the odd one out for blood colour: the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood — the only vessels where 'artery = oxygenated' does not hold.
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How this is tested: On Paper 2 a 2-mark Describe question often asks you to trace the circulation of blood from the heart to the lungs — name the chambers and vessels in order, and use the word deoxygenated.
On Paper 1 a 1-mark data question is a favourite: read a cardiac-cycle pressure graph and match the curves to the atrium, ventricle and aorta, or deduce the cause of a heart sound from where the lines cross.
A 2-mark Explain question may ask why blood must be pumped twice to get from the lungs to the body — the double-circulation reasoning.
IB-style question — trace blood from the heart to the lungs
Describe the circulation of deoxygenated blood from the heart to the lungs. [2]
How to score both marks
- Start where the deoxygenated blood arrives. Deoxygenated blood returns from the body in the vena cava and enters the right atrium, then passes into the right ventricle.
- Follow it out to the lungs. The right ventricle contracts and pumps the blood into the pulmonary artery, which carries it to the lungs to be oxygenated. (Mark 1: right atrium → right ventricle. Mark 2: pulmonary artery → lungs.)
Final answer
Deoxygenated blood enters the right atrium, passes into the right ventricle, and is pumped through the pulmonary artery to the lungs.
Follow the blue side for the pathway to the lungs: vena cava → right atrium → right ventricle → pulmonary artery → lungs. Then the red side for the pathway to the body: pulmonary vein → left atrium → left ventricle → aorta → body.
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Explore the labelled diagram, charts and maps for this topic in full study mode.
✓ Why this scores full marks: The structures are named in the correct order and the route ends at the lungs.
A common slip is forgetting the pulmonary artery (the vessel that actually carries the blood out), or writing 'vein' instead — the vessel leaving the heart is always an artery.
| Feature | Single circulation (e.g. fish) | Double circulation (mammals) |
|---|---|---|
| Times blood passes the heart per body circuit | once | twice |
| Two separate circuits? | no — one loop | yes — pulmonary (lungs) and systemic (body) |
| Pressure delivered to the body | lower (drops after the gills) | high (re-pressurised by the left ventricle) |
| Oxygenated and deoxygenated blood kept apart? | less so | yes — right side deoxygenated, left side oxygenated |