OCR · A-Level · Biology

    Course version: H420 · Version from Sept 2025

    (h) the examination of microscope slides to show the histology of exchange…

    Full specification wording

    Exchange surfaces: (h) the examination of microscope slides to show the histology of exchange surfaces.

    This practical involves using a light microscope to examine prepared slides showing the histology of exchange surfaces, such as gill lamellae, alveoli or insect tracheae.

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

    • Point 7 of 7
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    This practical involves using a light microscope to examine prepared slides showing the histology of exchange surfaces, such as gill lamellae, alveoli or insect tracheae. You set up the microscope correctly, select an appropriate magnification, focus the specimen and identify the tissue layers visible. For each slide, note features such as thin walls, extensive blood supply, large surface area and the arrangement of cells that suit efficient gas exchange. You should relate each histological feature to its function, for example a thin epithelium shortening the diffusion distance. Accurate observation means distinguishing real structures from artefacts and recording what is actually seen at the chosen magnification. Assessment checks that you can operate the microscope, identify exchange-surface tissues and explain how their structure supports gas exchange.

    What earns the marks

    • Sets up and focuses the light microscope correctly, selecting an appropriate magnification to view the tissue.
    • Identifies the tissue layers of the exchange surface shown on the slide, such as epithelium, capillaries or air spaces.
    • Describes histological features that aid gas exchange, including thin walls, large surface area and a rich blood supply.
    • Relates each observed feature to its function in efficient gas exchange.
    • Distinguishes genuine structures from artefacts and records observations accurately at the stated magnification.

    Common mistakes and how to put them right

    • Confusing magnification with resolution.

      Put it right: Is to recognise that magnification enlarges the image while resolution distinguishes fine detail.

    • Assuming every dark mark is a structure.

      Put it right: Is to distinguish artefacts from genuine tissue features.

    • Describing features without linking them to function.

      Put it right: Is to explain how each feature aids gas exchange.

    Exam tips

    • Focus at low power first, then increase magnification to examine detail.
    • Record the magnification used and describe only what is visible at that magnification.
    • Link each histological feature directly to its role in gas exchange.

    Try the lesson

    The start of the Exchange surfaces: (h) the examination of microscope slides to show the histology of exchange surfaces. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A student examines a prepared slide of mammalian lung tissue under a light microscope. Which histological feature would indicate that the tissue is from the alveolar region?

    Listen

    This practical involves using a light microscope to examine prepared slides showing the histology of exchange surfaces, such as gill lamellae, alveoli or insect tracheae. You set up the microscope correctly, select an appropriate magnification, focus the specimen and identify the tissue layers visible. For each slide, note features such as thin walls, extensive blood supply, large surface area and the arrangement of cells that suit efficient gas exchange. You should relate each histological feature to its function, for example a thin epithelium shortening the diffusion distance. Accurate observation means distinguishing real structures from artefacts and recording what is actually seen at the chosen magnification. Assessment checks that you can operate the microscope, identify exchange-surface tissues and explain how their structure supports gas exchange. A strong answer earns marks for these points. Sets up and focuses the light microscope correctly, selecting an appropriate magnification to view the tissue. Identifies the tissue layers of the exchange surface shown on the slide, such as epithelium, capillaries or air spaces. Describes histological features that aid gas exchange, including thin walls, large surface area and a rich blood supply. Relates each observed feature to its function in efficient gas exchange. A common slip: Confusing magnification with resolution. Instead, is to recognise that magnification enlarges the image while resolution distinguishes fine detail. Exam tip: Focus at low power first, then increase magnification to examine detail.

    See it

    A top-down flowchart. It starts with a node for setting up and focusing the light microscope, which leads to selecting an appropriate magnification. That leads to examining a prepared slide of an exchange surface, then to identifying tissue layers: epithelium, capillaries and air spaces. This leads to noting histological features, which branches into three nodes: thin walls, large surface area and rich blood supply. All three of these lead into a single node for relating each feature to its function in gas exchange. That finally leads to a node for distinguishing genuine structures from artefacts and recording observations at the stated magnification.
    The sequence of examining a prepared slide of an exchange surface, from setting up the microscope to identifying features and relating them to gas exchange.

    Key words

    Say what each word means before you turn the card.

    Correct use of the light microscope

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    Histology of exchange surfaces

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    Structure–function relationships in gas exchange

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    Accurate observation and recording

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    See every definition
    Correct use of the light microscope
    Setting up and focusing the light microscope correctly, selecting an appropriate magnification to view the tissue.
    Histology of exchange surfaces
    The tissue layers of the exchange surface shown on the slide, such as epithelium, capillaries or air spaces.
    Structure–function relationships in gas exchange
    Relating each observed histological feature, such as a thin epithelium shortening the diffusion distance, to its function in efficient gas exchange.
    Accurate observation and recording
    Distinguishing genuine structures from artefacts and recording observations accurately at the stated magnification.

    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 top-down flowchart. It starts with a node for setting up and focusing the light microscope, which leads to selecting an appropriate magnification. That leads to examining a prepared slide of an exchange surface, then to identifying tissue layers: epithelium, capillaries and air spaces. This leads to noting histological features, which branches into three nodes: thin walls, large surface area and rich blood supply. All three of these lead into a single node for relating each feature to its function in gas exchange. That finally leads to a node for distinguishing genuine structures from artefacts and recording observations at the stated magnification.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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