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Learn9700Paper 1 · Multiple ChoiceTransport in mammalsThe heart and cardiac cycle
Transport in mammals · Subtopic 2 of 2

The Heart and Cardiac Cycle

Mammals have a double circulation: pulmonary (right ventricle → lungs) and systemic (left ventricle → body). The left ventricle wall is thickest to pump blood at high pressure. Cardiac cycle: atrial systole (atria contract, AV valves open), ventricular systole (ventricles contract; pressure rises; AV valves close when ventricular pressure exceeds atrial; semilunar valves open when it exceeds arterial), diastole (relaxation; semilunar valves close; atria fill). Valves open and close only due to pressure differences. Heartbeat is myogenic: the SAN (pacemaker) sends a wave of excitation over the atria; the AVN delays it, then passes it down the bundle of His and Purkyne fibres so ventricles contract from the apex up. Cardiac output = stroke volume × heart rate.

Cambridge 9700 Paper 1Cambridge 9700 Paper 2Edexcel YBI11 WBI11AQA 7402 Paper 1≈ 45 minRevision 1 · updated 2026-11-02
Section 1 of 14

Learning objectives

  • Describe heart structure and double circulation.
  • Explain pressure and valve changes in the cardiac cycle.
  • Describe how heartbeat is initiated and coordinated.
  • Calculate cardiac output and heart rate from ECGs.
Pause and think

Why is there a short delay at the AVN?

Section 2 of 14

Core idea

Start hereA self-starting pump

A heart removed from the body keeps beating for a while. Its rhythm comes from within.

The big idea

The SAN starts each beat; pressure differences open and close valves so blood flows one way.

Conduction through the heart

The AVN delay lets the atria empty first.

Worked exampleCardiac output

Stroke volume 70 cm³; ECG R–R interval 0.80 s. Cardiac output?

Goal
cm³ min⁻¹.
Strategy
HR = 60/t, then CO = SV × HR.
  1. HR=600.80=75\text{HR}=\frac{60}{0.80}=75
    Beats per minute.
  2. CO=70×75=5250 cm3 min−1\text{CO}=70\times75=5250\ \text{cm}^3\ \text{min}^{-1}
    Multiply.
Answer5250 cm³ min⁻¹.
✓ CheckAbout 5 litres a minute at rest ✓.

Summary

  • Valves follow pressure.
  • SAN, AVN, Purkyne.
  • CO = SV × HR.
Key words
Myogenic
Contraction starts in the muscle itself.
Systole
Contraction phase of the cardiac cycle.
HistoryHarvey, 1628. William Harvey showed that blood circulates in a loop pumped by the heart, overturning ideas that had stood for 1400 years.
Fun factA lifetime of beats. Your heart beats about 100 000 times a day and around 3 billion times in a lifetime.
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