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                                      Topic 9: Gas exchange

8.1 The circulatory system

Students should be able to:

1) describe the structure of the human gas exchange system
2) describe the distribution in the gas exchange system of cartilage, ciliated epithelium, goblet cells, squamous epithelium of alveoli, smooth muscle and capillaries
3) recognise cartilage, ciliated epithelium, goblet cells, squamous epithelium of alveoli, smooth muscle and capillaries in microscope slides, photomicrographs and electron micrographs
4) recognise trachea, bronchi, bronchioles and alveoli in microscope slides, photomicrographs and electron micrographs and make plan diagrams of transverse sections of the walls of the trachea and bronchus
5) describe the functions of ciliated epithelial cells, goblet cells and mucous glands in maintaining the health of the gas exchange system
6) describe the functions in the gas exchange system of cartilage, smooth muscle, elastic fibres and squamous epithelium
7) describe gas exchange between air in the alveoli and blood in the capillaries

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1. Structure of the Human Gas Exchange System

The human gas exchange system is designed to provide a large surface area and short diffusion distance for efficient exchange of oxygen and carbon dioxide between air and blood. It consists of the lungs, trachea, bronchi, bronchioles, alveoli, and an extensive capillary network.

Air enters through the nose or mouth and passes down the trachea, a flexible tube supported by C-shaped rings of cartilage to prevent collapse during inhalation. The trachea divides into two bronchi, each leading to a lung. Inside the lungs, the bronchi branch repeatedly into narrower bronchioles, which end in clusters of tiny air sacs called alveoli.

Each lung contains millions of alveoli, greatly increasing the surface area available for gas exchange. The walls of alveoli are extremely thin (one cell thick) and are closely surrounded by a dense network of capillaries, ensuring a very short diffusion pathway between the air in the alveoli and the blood in the capillaries. The entire system is adapted to maintain a steep concentration gradient of oxygen and carbon dioxide through continuous ventilation and blood flow.

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2. Distribution of Tissues in the Gas Exchange System

Different parts of the gas exchange system contain specific tissues suited to their function.

  • Cartilage is present in the trachea and bronchi, forming C-shaped rings in the trachea and irregular blocks or plates in the bronchi. These provide structural support and prevent the airways from collapsing while still allowing flexibility.

  • Ciliated epithelium lines the trachea, bronchi, and larger bronchioles. The cilia beat rhythmically to move mucus and trapped particles upward toward the throat for removal.

  • Goblet cells, interspersed among the ciliated cells, secrete mucus that traps dust, microbes, and other debris.

  • The squamous epithelium forms the walls of the alveoli and the capillaries. These extremely thin, flat cells provide a minimal diffusion distance for gases.

  • Smooth muscle is found in the walls of the bronchi and bronchioles, allowing them to constrict or dilate and control airflow.

  • Elastic fibres (not listed in this point but closely associated) are found throughout the bronchioles and alveolar walls, allowing them to stretch during inhalation and recoil during exhalation.

  • Capillaries form a dense network around the alveoli, providing a rich blood supply to maintain steep diffusion gradients for oxygen and carbon dioxide.

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3. Recognition of Tissues in Microscopy

When viewed under a microscope or in photomicrographs, these tissues have distinct appearances that allow identification.

  • Cartilage appears as pale-staining, semi-transparent tissue with rounded chondrocytes in lacunae, often arranged in clusters.

  • Ciliated epithelium shows columnar cells with hair-like projections (cilia) visible on the apical surface.

  • Goblet cells appear as lighter, mucus-filled cells interspersed among the ciliated cells.

  • Squamous epithelium of alveoli appears as an extremely thin layer of flattened cells forming the alveolar walls.

  • Smooth muscle shows elongated, spindle-shaped cells with central nuclei, forming bands around bronchi and bronchioles.

  • Capillaries appear as thin-walled tubes, often seen in close association with alveoli, containing red blood cells within the lumen.

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4. Recognition of Trachea, Bronchi, Bronchioles, and Alveoli in Sections

The trachea can be identified by its large open lumen, thick wall, and the presence of C-shaped cartilage rings. It also has ciliated epithelium and goblet cells lining the mucosa. The bronchi resemble the trachea but have smaller lumens and irregular cartilage plates instead of complete rings. Bronchioles are smaller still, lack cartilage, and have relatively thicker smooth muscle layers, with simple cuboidal or ciliated epithelium but few goblet cells. Alveoli appear as clusters of small, thin-walled sacs with very little connective tissue between them and a rich capillary network.

In plan diagrams of transverse sections, students should clearly represent structural layers — the mucosa, cartilage, smooth muscle, and epithelial lining — but not attempt to draw individual cells.

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5. Functions of Ciliated Epithelial Cells, Goblet Cells, and Mucous Glands

The lining of the trachea and bronchi is protected by the mucociliary escalator, a system that maintains the cleanliness and health of the gas exchange system. Goblet cells and mucous glands secrete mucus, which coats the epithelium and traps dust, pollen, and pathogens. Ciliated epithelial cells then move the mucus upward toward the pharynx by the coordinated beating of their cilia. The mucus is swallowed and digested in the stomach, removing the trapped particles. This mechanism prevents the accumulation of harmful substances in the lungs and protects the delicate alveoli from infection and blockage.

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6. Functions of Cartilage, Smooth Muscle, Elastic Fibres, and Squamous Epithelium

  • Cartilage maintains the patency (openness) of large airways such as the trachea and bronchi, preventing collapse during inhalation when air pressure in the thoracic cavity falls.

  • Smooth muscle allows constriction and dilation of bronchioles, regulating the amount of air entering the alveoli. This is especially important in controlling airflow and protecting the lungs from harmful particles or irritants.

  • Elastic fibres enable alveoli and bronchioles to stretch during inhalation to accommodate increased air volume and then recoil during exhalation to help force air out of the lungs. This elastic recoil is essential for passive exhalation.

  • Squamous epithelium, which lines the alveoli, provides an extremely thin barrier (approximately 0.1–0.5 µm thick) to allow rapid diffusion of gases. Together with the endothelial layer of capillaries, it forms the respiratory membrane through which gas exchange occurs efficiently.

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7. Gas Exchange Between Air in the Alveoli and Blood in the Capillaries

Gas exchange occurs by simple diffusion across the thin alveolar and capillary walls. Oxygen from the air in the alveoli diffuses across the alveolar epithelium, the capillary endothelium, and into the red blood cells where it binds to haemoglobin to form oxyhaemoglobin. At the same time, carbon dioxide produced by cellular respiration diffuses from the blood plasma into the alveolar air to be exhaled.

Several features make this process highly efficient:

  • Large surface area: Millions of alveoli provide a combined surface area of about 70 m² in adult humans.

  • Short diffusion distance: The total barrier between air and blood is only about two cell layers thick.

  • Steep diffusion gradients: Maintained by continuous ventilation (airflow in and out of alveoli) and a constant blood flow through the pulmonary capillaries.

  • Moist surfaces: The alveoli are lined with a thin layer of fluid that allows gases to dissolve before diffusing, facilitating gas exchange.

  • Surfactant: A phospholipid secretion that reduces surface tension, preventing alveolar collapse and making inflation easier.

As a result of these adaptations, oxygen uptake and carbon dioxide removal are rapid and efficient, supplying tissues with oxygen for aerobic respiration while preventing the build-up of carbon dioxide.

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