Lesson 8 of 9 / Transport in humans
The cardiac cycle
What makes each valve open or close during a heartbeat?
I can explain filling and ejection through systole, diastole and pressure changes.
I can explain heart structure, muscular action and valves.Syllabus
Syllabus K325, Topic 5(f). Explain heart structure, muscular action and valves.
I can explain systole and diastole during a heartbeat.Syllabus
Syllabus K325, Topic 5(g). Explain systole and diastole during a heartbeat.
About 5 min
Make a guess. You are not marked.
Heart valves open and close during every beat. What makes them move?
Show the answer
Differences in blood pressure on each side.
A valve opens when pressure behind it is higher and shuts when pressure ahead is higher. Valves have no muscle.
The key idea
Contraction raises chamber pressure; relaxation lowers it. Pressure differences determine flow and valve position.
Left-heart pressure over one heartbeat (illustrative values)
Thick red line: left ventricle. Purple line: aorta. Dashed blue line: left atrium. 1: bicuspid valve closes. 2: semilunar valve opens. 3: semilunar valve closes. 4: bicuspid valve opens. Between 1 and 2, and between 3 and 4, both valves are closed. One cycle lasts 0.8 s, so heart rate = 60 / 0.8 = 75 beats per minute. Shading shows the phase chosen above.
Both ventricles work at the same time. The heart diagram leaves out the brief moments when both valve sets are closed; the pressure graph below marks them. Pressure determines when each valve changes position.
Explanation
A cardiac cycle is one sequence of contraction and relaxation. Systole refers to contraction and diastole to relaxation, so specify whether you mean the atria or ventricles.
During filling, relaxed ventricles have lower pressure than the atria. The atrioventricular valves open and blood flows into the ventricles. Atrial systole then pushes in the remaining blood while the ventricles are still relaxed.
The ventricles contract and their pressure rises above atrial pressure, closing the atrioventricular valves. When ventricular pressure exceeds pressure in the outgoing arteries, the semilunar valves open and blood is ejected. Both ventricles contract together.
As the ventricles relax, their pressure falls below arterial pressure and the semilunar valves close. Once ventricular pressure falls below atrial pressure, the atrioventricular valves reopen and filling resumes.
There are brief transitions when both valve sets are closed while pressure changes. The diagram focuses on filling, atrial contraction and ejection, rather than implying that semilunar valves remain open throughout every moment of systole.
Exams test the cycle with a pressure-time graph of the left atrium, left ventricle and aorta. Read it by finding where two lines cross, because a valve changes position whenever the pressures on its two sides swap. (1) Ventricle line rises above the atrium line: the bicuspid valve closes. (2) Ventricle line rises above the aorta line: the semilunar valve opens and ejection starts. (3) Ventricle line falls below the aorta line: the semilunar valve closes. (4) Ventricle line falls below the atrium line: the bicuspid valve opens and filling starts. Both valves are closed between points 1 and 2 and between points 3 and 4.
Heart rate can be read from the same graph. One cycle in the illustrative graph lasts 0.8 s, so the heart rate is 60 / 0.8 = 75 beats per minute. During exercise the heart rate rises, for example from 70 to 150 beats per minute, so more blood reaches the muscles each minute. That blood brings more oxygen and glucose for aerobic respiration and removes carbon dioxide and heat faster.
Step by step
- 1
Name the contracting chamber
Atrial and ventricular systole are different phases.
- 2
Compare pressures
Blood tends to move from higher pressure to lower pressure.
- 3
Set the valves
Explain which opening permits forward flow and which closure prevents backward flow.
Worked example
The start of ventricular contraction
Why does ventricular contraction not normally force blood back into the atria?
One way to explain it
Contraction raises ventricular pressure above atrial pressure. This pressure difference closes the atrioventricular valves, preventing backflow into the atria. Blood can be ejected into the arteries once ventricular pressure exceeds arterial pressure.
Why this answer works
- Start with muscle contraction.
- Use the pressure comparison to explain valve closure.
- Do not say both valve sets open at the same time.
Watch out for this
A student says: "All four chambers contract at the same time." What is wrong with this?
Show the answer
The atria contract together first; the ventricles then contract together. This sequence supports filling followed by ejection.
Check your understanding
During ventricular ejection, which valve arrangement allows one-way flow?
Show the answer
Atrioventricular valves closed; semilunar valves open
Blood leaves for the arteries while backward flow into the atria is blocked.
Exam-style questions
Check your understanding
One cardiac cycle lasts 0.75 s. What is the heart rate?
Show the answer
80 beats per minute
Heart rate = 60 / time for one cycle = 60 / 0.75 = 80 beats per minute.
Quick recall
What makes each valve open or close during a heartbeat?