K224 / K225 / 2027

Lesson 6 of 7 / Transport in humans

How heart contraction and valves move blood

What makes each valve open or close during a heartbeat?

I can explain how contraction, relaxation and valve action produce one-way flow through the heart.

  • I can describe heart structure, contraction and valves.Syllabus

    Syllabus K224 / K225, Topic 5(d). Describe heart structure, contraction and valves.

About 5 min

Make a guess. You are not marked.

Heart valves open and close during every beat. What makes them move?

  1. Nerve signals open each valve in turn.
  2. Small muscles in each valve pull it open.
  3. Differences in blood pressure on each side.
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 model: Late ventricular diastole: ventricular pressure is below atrial pressure. The atrioventricular valve is open; the semilunar valve is closed.Left atriumRelaxedLeft ventricleRelaxedAortaBicuspidopenSemilunarclosedSchematic connections, not organ positions
Late ventricular diastole: ventricular pressure is below atrial pressure. The atrioventricular valve is open; the semilunar valve is closed.

Left-heart pressure over one heartbeat (illustrative values)

048121600.20.40.60.8Time / sPressure / kPa1234

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 heartbeat: one round of contraction and relaxation. Systole means contraction and diastole means relaxation. Always say whether you mean the atria or the ventricles.

While the ventricles are relaxed, their pressure is lower than in the atria. The valves between them open and blood flows into the ventricles. Then the atria contract and push in the rest of the blood.

Next the ventricles contract. Their pressure rises above the pressure in the atria, so the valves between them close. When the pressure in the ventricles is higher than in the arteries, the semilunar valves open and blood is pushed out. Both ventricles contract together.

Then the ventricles relax. Their pressure falls below the pressure in the arteries, so the semilunar valves close. When it falls below the pressure in the atria, the valves between atria and ventricles open again and the ventricles start to fill.

For a short time, both sets of valves are closed while the pressure changes.

Exams test this with a graph of pressure in the left atrium, left ventricle and aorta. Look for where two lines cross: a valve opens or closes whenever the pressure on its two sides swaps. (1) The ventricle line rises above the atrium line: the bicuspid valve closes. (2) The ventricle line rises above the aorta line: the semilunar valve opens and blood leaves. (3) The ventricle line falls below the aorta line: the semilunar valve closes. (4) The ventricle line falls below the atrium line: the bicuspid valve opens and filling starts.

You can work out heart rate from the graph. If one cycle lasts 0.8 s, 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. More blood reaches the muscles each minute, bringing more oxygen and glucose for aerobic respiration, and removing carbon dioxide and heat faster.

Step by step
  1. 1

    Name the chamber that contracts

    Atria contracting and ventricles contracting are different stages.

  2. 2

    Compare the pressures

    Blood moves from where the pressure is higher to where it is lower.

  3. 3

    Set the valves

    Say which valve opens to let blood go forward and which closes to stop it going back.

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?

  1. Atrioventricular valves open; semilunar valves closed
  2. Both sets of valves stay open throughout the heartbeat
  3. Atrioventricular valves closed; semilunar valves open
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?

  1. 75 beats per minute
  2. 80 beats per minute
  3. 133 beats per minute
  4. 45 beats per minute
Show the answer

80 beats per minute

Heart rate = 60 / time for one cycle = 60 / 0.75 = 80 beats per minute.

Quick recall

Card 1 of 3. Answer in your head, then check.

What makes each valve open or close during a heartbeat?

You can return to this lesson any time.