OCR · A-Level · Biology

    Course version: H420 · Version from Sept 2025

    Exchange surfaces: (d) the mechanism of ventilation in mammals

    Ventilation moves air in and out of the lungs to maintain steep concentration gradients of oxygen and carbon dioxide at the alveoli.

    Checked against the OCR H420 specification (opens in a new tab) on .

    • Point 4 of 7
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Ventilation moves air in and out of the lungs to maintain steep concentration gradients of oxygen and carbon dioxide at the alveoli. It works by pressure changes created by muscles altering the volume of the thorax. During inspiration, the diaphragm contracts and flattens while the external intercostal muscles contract, moving the ribs up and outwards. Thorax volume increases, pressure inside falls below atmospheric pressure, and air flows in. During expiration at rest, the diaphragm relaxes and domes upwards and the intercostals relax, so the ribs move down and inwards. Thorax volume decreases, pressure rises above atmospheric pressure, and air flows out. Forced expiration additionally uses the internal intercostal muscles and abdominal muscles to push the ribs down and compress the abdomen. Boyle's law links volume and pressure: as volume increases, pressure decreases.

    What earns the marks

    • Inspiration: diaphragm contracts and flattens; external intercostals contract, raising ribs up and outwards.
    • Thorax volume increases, so pressure inside the thorax falls below atmospheric pressure and air enters.
    • Expiration at rest: diaphragm relaxes and domes; intercostals relax, ribs move down and inwards.
    • Thorax volume decreases, pressure rises above atmospheric pressure and air leaves.
    • Forced expiration uses internal intercostals and abdominal muscles.
    • Pressure changes drive airflow, consistent with Boyle's law (volume up, pressure down).
    • Pleural membranes and fluid couple lung movement to thorax movement.

    Common mistakes and how to put them right

    • Saying the lungs actively suck air in: the lungs are passive; muscles change thorax volume and pressure drives airflow. Correction: describe muscles and pressure changes.

    • Reversing the pressure relationship.

      Put it right: State the direction of the pressure difference.

    • Confusing internal and external intercostals.

      Put it right: Name each muscle with its action.

    • Forgetting that quiet expiration is largely passive.

      Put it right: It results mainly from relaxation of muscles, not active contraction.

    Exam tips

    • Write ventilation answers as a chain: muscle action → volume change → pressure change → air movement.
    • Always compare thoracic pressure with atmospheric pressure to explain airflow direction.
    • Distinguish quiet breathing from forced breathing when describing expiration.

    Try the lesson

    The start of the Exchange surfaces: (d) the mechanism of ventilation in mammals lesson, made from checked teaching notes for this specification point.

    Have a go first

    During inhalation in a mammal, which action of the muscles directly increases the volume of the thorax?

    Listen

    Ventilation moves air in and out of the lungs to maintain steep concentration gradients of oxygen and carbon dioxide at the alveoli. It works by pressure changes created by muscles altering the volume of the thorax. During inspiration, the diaphragm contracts and flattens while the external intercostal muscles contract, moving the ribs up and outwards. Thorax volume increases, pressure inside falls below atmospheric pressure, and air flows in. During expiration at rest, the diaphragm relaxes and domes upwards and the intercostals relax, so the ribs move down and inwards. Thorax volume decreases, pressure rises above atmospheric pressure, and air flows out. Forced expiration additionally uses the internal intercostal muscles and abdominal muscles to push the ribs down and compress the abdomen. Boyle's law links volume and pressure: as volume increases, pressure decreases. A strong answer earns marks for these points. Inspiration: diaphragm contracts and flattens; external intercostals contract, raising ribs up and outwards. Thorax volume increases, so pressure inside the thorax falls below atmospheric pressure and air enters. Expiration at rest: diaphragm relaxes and domes; intercostals relax, ribs move down and inwards. Thorax volume decreases, pressure rises above atmospheric pressure and air leaves. A common slip: Saying the lungs actively suck air in: the lungs are passive; muscles change thorax volume and pressure drives airflow. Instead, describe muscles and pressure changes. Exam tip: Write ventilation answers as a chain: muscle action → volume change → pressure change → air movement.

    See it

    A flowchart with two parallel chains. The first chain starts with inspiration: the diaphragm contracts and flattens and the external intercostals contract, moving the ribs up and outwards. This leads to thorax volume increasing, then pressure inside the thorax falling below atmospheric pressure, then air flowing in. The second chain starts with expiration at rest: the diaphragm relaxes and domes and the intercostals relax, so the ribs move down and inwards. This leads to thorax volume decreasing, then pressure rising above atmospheric pressure, then air flowing out. A third starting box for forced expiration, using the internal intercostals and abdominal muscles to push the ribs down and compress the abdomen, also feeds into the thorax volume decrease step.
    Shows the chain from muscle action to volume change, pressure change and airflow for inspiration, quiet expiration and forced expiration.

    Key words

    Say what each word means before you turn the card.

    Mechanism of breathing

    Try answering aloud or in your head before turning the card.

    Pressure and volume relationships

    Try answering aloud or in your head before turning the card.

    Maintaining concentration gradients

    Try answering aloud or in your head before turning the card.

    Antagonistic muscle action

    Try answering aloud or in your head before turning the card.

    See every definition
    Mechanism of breathing
    Muscles alter the volume of the thorax, creating pressure changes that move air in and out of the lungs.
    Pressure and volume relationships
    Boyle's law links volume and pressure: as volume increases, pressure decreases.
    Maintaining concentration gradients
    Ventilation moves air in and out of the lungs to maintain steep concentration gradients of oxygen and carbon dioxide at the alveoli.
    Antagonistic muscle action
    External intercostals contract to raise the ribs in inspiration, while internal intercostals lower them in forced expiration.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart with two parallel chains. The first chain starts with inspiration: the diaphragm contracts and flattens and the external intercostals contract, moving the ribs up and outwards. This leads to thorax volume increasing, then pressure inside the thorax falling below atmospheric pressure, then air flowing in. The second chain starts with expiration at rest: the diaphragm relaxes and domes and the intercostals relax, so the ribs move down and inwards. This leads to thorax volume decreasing, then pressure rising above atmospheric pressure, then air flowing out. A third starting box for forced expiration, using the internal intercostals and abdominal muscles to push the ribs down and compress the abdomen, also feeds into the thorax volume decrease step.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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