IB Biology HLTransport in Animals & PlantsPaper 1 & 2~14 min read
The Cardiac Cycle
One heartbeat is a sequence of squeezes in a fixed order, timed by the heart itself. Every valve that opens or shuts during it does so for one reason only: the pressure on one side became bigger than the pressure on the other. Read the pressure graph with that in mind and it stops being intimidating.
📚 What you need to know
The heartbeat is myogenic: it starts in the heart itself, with no signal from the nervous system.
The sinoatrial node (SAN) in the wall of the right atrium sends a wave of depolarisation across the atria, making them contract.
A region of non-conducting tissue stops the wave passing straight to the ventricles, delaying it long enough for the atria to finish emptying.
The wave passes instead to the atrioventricular node (AVN), then down the bundle of His in the septum, then out through the Purkyne fibres.
The ventricles therefore depolarise from the base upwards, so they contract from the bottom up.
Systole is contraction, diastole is relaxation. Contraction decreases volume and so increases pressure in that chamber.
Valves open when pressure behind them is greater, and close when pressure in front is greater.
Events happen simultaneously on the left and right sides of the heart.
What sets the rhythm
Contracting from the base upwards matters: it squeezes blood towards the arteries at the top, the way you would empty a toothpaste tube.
Stage
Event
1
Sinoatrial node sends out a wave of excitation
2
Atria contract
3
Atrioventricular node sends out a wave of excitation
4
Purkyne tissue conducts the wave to the base of the ventricles
5
Ventricles contract
The three phases
Atrial systole
The walls of the atria contract, so atrial volume decreases and atrial pressure increases.
Atrial pressure rises above ventricular pressure, so the AV valves are pushed open and blood is forced into the ventricles.
Ventricular pressure and volume rise slightly as they fill. The ventricles are relaxed – ventricular diastole coincides with atrial systole.
Ventricular systole
The walls of the ventricles contract, so ventricular volume decreases and ventricular pressure increases sharply.
Ventricular pressure rises above atrial pressure, which forces the AV valves shut and prevents backflow.
Ventricular pressure then rises above the pressure in the aorta and pulmonary artery, which forces the semilunar valves open, and blood is pushed out of the heart.
The atria are relaxing at this time – atrial diastole coincides with ventricular systole – and begin to fill with blood again.
Diastole
Atria and ventricles are both relaxed.
Ventricular pressure drops below the pressure in the arteries, so the semilunar valves close.
Blood returns to the heart in the vena cava and pulmonary vein, and the atria fill.
Atrial pressure rises above ventricular pressure, so the AV valves open and blood flows passively into the ventricles – no atrial contraction is needed for this.
The cycle then begins again with atrial systole.
Stage of the cycle
Atrioventricular valves
Semilunar valves
Atrial systole
Open
Closed
Ventricular systole
Closed
Open
Diastole
Open
Closed
🧠
Squeeze, shrink, push
Muscle contracts → chamber volume decreases → pressure increases → blood is pushed to wherever the pressure is lower, and a valve opens or closes to allow it. Every line of a cardiac cycle answer is one of those four steps.
Reading the pressure graph
The whole cycle here lasts 0.8 seconds. Everything shown for the left side happens on the right side at the same time, at lower pressures.
Point
What is happening
Valve event
A
End of diastole. The atrium has filled and atrial pressure is above ventricular pressure
AV valve open
A to B
Atrial systole. The atrium contracts, atrial pressure rises and blood is forced into the ventricle, which fills a little more
AV valve still open
B
Start of ventricular systole. The ventricle contracts and its pressure rises above atrial pressure
AV valve shuts
C
The ventricle keeps contracting until its pressure exceeds the pressure in the aorta
Aortic valve opens and blood is forced into the aorta
D
Start of diastole. The ventricle has emptied and its muscle relaxes, so pressure falls below that in the newly filled aorta
Aortic valve closes
D to E
Early diastole. The ventricle stays relaxed and its pressure keeps falling, while blood flowing in from the pulmonary vein raises atrial pressure
All valves closed
E
Atrial pressure now exceeds the pressure in the emptied ventricle
AV valve opens, and the ventricle fills passively
After point E there is a small dip in both traces. The ventricle is still expanding as its muscle relaxes, which increases its volume and lowers its pressure a little further, while blood draining out of the atrium briefly lowers atrial pressure too. Then both rise slowly as filling continues. It is a small detail, but examiners like it.
Getting heart rate from the graph
heart rate (beats per minute) = 60 ÷ length of one cardiac cycle in seconds
Worked examples
WE 1
Heart rate from a trace
One complete cardiac cycle on the graph above takes 0.8 seconds. Calculate the heart rate in beats per minute. (2 marks)
Step 1: beats in one second
1 ÷ 0.8 = 1.25 beats per secondStep 2: beats in one minute
1.25 × 60 = 75
75 beats per minutethe short cut is 60 ÷ cycle length, which gives the same answer in one step
WE 2
A faster heart
On a second trace, one cardiac cycle lasts 0.65 seconds. Calculate the heart rate and state one reason it might be higher than at rest. (3 marks)
Step 1: the calculation
60 ÷ 0.65 = 92.3 beats per minuteStep 2: round sensibly
About 92 beats per minute.
Step 3: a reason
The person may have been exercising, so their muscles need oxygen and glucose delivered faster and carbon dioxide removed faster.
92 beats per minutea shorter cycle always means a faster rate – check your answer makes sense before writing it down
WE 3
Explaining a valve event
Using the graph, explain what happens at point C. (3 marks)
Point 1: the chamber
The ventricle is contracting, so its volume is decreasing and its pressure is rising steeply.
Point 2: the crossing
At C the ventricular pressure rises above the pressure in the aorta.
Point 3: the consequence
Because the pressure behind the valve is now greater than the pressure in front of it, the aortic valve is forced open and blood is ejected into the aorta.
Ventricular pressure exceeds aortic pressure, so the aortic valve opensat every crossing point say which pressure has become greater than which – that is the mark
💡 Exam tips
Learn the order: SAN → atria contract → AVN → bundle of His → Purkyne fibres → ventricles contract.
Always explain the delay at the AVN: it lets the atria empty before the ventricles squeeze.
Describe every valve event as a comparison of two pressures.
Use systole for contraction and diastole for relaxation, and remember one chamber can be in systole while another is in diastole.
To find heart rate, measure one full cycle from the graph, then use 60 ÷ cycle length.
State that events occur simultaneously in the left and right sides of the heart.
⚠ Common mistakes
Saying the brain starts each heartbeat. The heartbeat is myogenic and starts at the SAN.
Saying pressure increases because the chamber gets bigger. Contraction decreases volume, which increases pressure.
Measuring half a cycle when calculating heart rate.
Saying valves open and close on their own. They respond to pressure differences.
Thinking the ventricles fill only during atrial systole. Most filling is passive, during diastole.
Reading the aorta line as the ventricle line. Check the key before you answer.
Up next: Roots & Water Uptake – back to plants, and the one situation where water really is pushed rather than pulled.
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