IB Biology HLTransport in Animals & PlantsPaper 1 & 2~12 min read
The Mammalian Heart
Four chambers, four valves, four big vessels. Learn which is which and the rest of this topic – the cardiac cycle, the pressure graph, heart disease – becomes much easier, because every one of those depends on knowing where the blood is at any moment.
📚 What you need to know
The heart is made of cardiac muscle, which contracts repeatedly and continuously without fatiguing.
Four chambers: two atria on top, two ventricles below. The septum separates the left and right sides so the two bloods do not mix.
Atria receive blood from veins and have thin muscular walls, because they only pump it a short distance into the ventricles.
Ventricles have thick muscular walls. The left ventricle is thickest, because it pumps blood all the way around the body.
Blood arrives in the vena cava and pulmonary vein; it leaves in the pulmonary artery and aorta.
Atrioventricular valves (tricuspid on the right, bicuspid on the left) stop blood flowing back into the atria. Semilunar valves (pulmonary and aortic) stop it flowing back from the arteries.
Valves open when the pressure behind them is greater than the pressure in front, and close when it is not.
The pacemaker, or sinoatrial node (SAN), sits in the wall of the right atrium and starts each heartbeat.
The plan of the heart
Remember the diagram is drawn as if the heart were inside a person facing you – so the right side of the heart appears on the left of the page.
Why the walls differ
Chamber
Wall
Why
Atria
Thin layer of muscle
They only push blood a few centimetres, down into the ventricles below
Right ventricle
Thick, but thinner than the left
Pumps blood only as far as the lungs, and at a lower pressure to protect the alveolar capillaries
Left ventricle
Thickest of all
Generates enough pressure to push blood all the way around the body and back
Every question about wall thickness has the same answer shape: thicker muscle → more force → higher pressure → blood pushed further. Write that chain and you will pick up all the marks whichever chamber they ask about.
The valves
Valves are flaps of tissue that can only bend one way. They are not muscles and they do not decide anything – they are pushed open or slammed shut by pressure differences.
A valve opens when the pressure of blood behind it is greater than the pressure in front of it.
A valve closes when the pressure in front of it is greater than the pressure behind it.
Valve
Where it is
What it prevents
Tricuspid (atrioventricular)
Between right atrium and right ventricle
Backflow from the right ventricle into the right atrium
Bicuspid (atrioventricular)
Between left atrium and left ventricle
Backflow from the left ventricle into the left atrium
Pulmonary (semilunar)
Between right ventricle and pulmonary artery
Backflow from the pulmonary artery into the right ventricle
Aortic (semilunar)
Between left ventricle and aorta
Backflow from the aorta into the left ventricle
The atrioventricular valves are held in place by tough cords, sometimes called valve tendons. Ventricular pressure gets very high, and without the cords the valve flaps would be blown inside out into the atria.
Following the blood
Two vessels bring blood in – vena cava and pulmonary vein. Two take it away – pulmonary artery and aorta.
The heart’s own blood supply
The heart is a muscle, so it needs its own delivery of oxygen and glucose for aerobic respiration. It cannot use the blood inside its chambers – that blood is just passing through. Instead, coronary arteries branch off the aorta and run across the surface of the heart, supplying the cardiac muscle itself.
Myogenic muscle. Cardiac muscle contracts without any nerve signal from the brain, because the sinoatrial node in the right atrium wall generates its own rhythm. That is why a heart keeps beating in a transplant box, and it is the starting point of the next page.
Worked examples
WE 1
Thickness of the left ventricle
Explain why the wall of the left ventricle is thicker than the wall of the left atrium. (3 marks)
Point 1: what the atrium does
The left atrium only pushes blood a short distance down into the ventricle, so little force is needed.
Point 2: what the ventricle does
The left ventricle pumps blood into the aorta and all the way around the body.
Point 3: the link
More muscle allows a stronger contraction, generating the high pressure needed to push blood that far and overcome the resistance of the vessels.
More muscle, more force, higher pressure, further travelthis chain works for any wall-thickness question – learn it once and reuse it
WE 2
How valves work
Explain how the atrioventricular valves prevent the backflow of blood. (3 marks)
Point 1: opening
When atrial pressure is higher than ventricular pressure, the valve is pushed open and blood flows into the ventricle.
Point 2: closing
When the ventricle contracts, ventricular pressure rises above atrial pressure and the valve is forced shut.
Point 3: the cords
Cords attached to the valve flaps stop them turning inside out under the high ventricular pressure, so no blood returns to the atrium.
Pressure opens them and pressure closes themnever say a valve “decides” or “chooses” – it only responds to pressure
WE 3
The route through the heart
Describe the path taken by a red blood cell from the vena cava to the aorta. (4 marks)
Step 1: into the right side
From the vena cava into the right atrium, then through the tricuspid valve into the right ventricle.
Step 2: to the lungs
Out through the pulmonary valve into the pulmonary artery, which carries it to the lungs.
Step 3: back to the heart
After gas exchange it returns in the pulmonary vein to the left atrium, then through the bicuspid valve into the left ventricle.
Step 4: out to the body
The left ventricle contracts, the aortic valve opens and the cell leaves in the aorta.
Right side → lungs → left side → bodynaming the valves as you pass through them is an easy way to pick up the extra mark
💡 Exam tips
Remember the diagram is mirrored: the right side of the heart is drawn on the left.
Atria receive, ventricles pump. Atria are above, ventricles below.
Learn the valve names and positions – tricuspid right, bicuspid left, then the two semilunar valves.
Always explain valve behaviour in terms of pressure differences.
Say the heart muscle is supplied by the coronary arteries, not by the blood inside the chambers.
Use myogenic and sinoatrial node when asked what starts the heartbeat.
⚠ Common mistakes
Labelling left and right the wrong way round on a diagram.
Saying the left ventricle is thicker because it pumps more blood. Both ventricles pump the same volume; the left one pumps it further.
Saying valves contract. They are flaps moved by pressure.
Forgetting the cords that anchor the atrioventricular valves.
Claiming the heart is supplied by blood in its chambers. It needs coronary arteries.
Saying the brain tells the heart to beat. The beat is myogenic; nerves only adjust the rate.
Up next: The Cardiac Cycle – the same heart in motion, and the pressure graph that shows exactly when each valve opens and closes.
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