How is the human gas-exchange system adapted to load oxygen and remove carbon dioxide, and how is breathing measured?
Structure of the human gas-exchange system, the mechanism of ventilation, gas exchange at the alveoli, lung volumes and capacities measured by spirometry, and the effects of smoking and disease on the lungs.
A CCEA Life and Health Sciences answer on the human respiratory system: the structure of the gas-exchange system, the mechanism of ventilation, alveolar gas exchange, lung volumes and capacities measured by spirometry, and the effects of smoking on the lungs.
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What this dot point is asking
CCEA wants you to describe the structure of the human gas-exchange system, explain the mechanism of ventilation (breathing in and out), explain how the alveolus is adapted for efficient gas exchange, define and interpret lung volumes and capacities measured by a spirometer, and describe the effects of smoking and lung disease. It pairs with the cardiovascular dot point: together they show how oxygen reaches respiring tissues.
Structure and ventilation
Air enters through the nose and mouth, passes down the trachea, which branches into two bronchi, then into finer bronchioles, ending in clusters of alveoli. The trachea and bronchi are held open by rings of cartilage and lined with ciliated epithelium and goblet cells that trap and sweep out dust and microbes. During inspiration, the external intercostal muscles contract to raise the ribs and the diaphragm contracts and flattens; this increases the volume of the thorax, so the pressure falls below atmospheric pressure and air is drawn in. During expiration at rest, these muscles relax, the elastic lungs recoil, the thorax volume falls and pressure rises, pushing air out. Forced expiration also uses the internal intercostal muscles and abdominal muscles.
Gas exchange at the alveoli
Oxygen diffuses from the alveolar air (high oxygen concentration) across the alveolar and capillary walls into the blood, where it binds to haemoglobin in red blood cells to form oxyhaemoglobin. Carbon dioxide diffuses the opposite way, from blood (high carbon dioxide) into the alveolar air, to be breathed out. The steep gradients are maintained because blood flow constantly removes oxygenated blood and brings deoxygenated blood, while ventilation constantly refreshes the air in the alveoli.
Lung volumes and the effects of smoking
A spirometer measures lung volumes. Tidal volume is the air moved in one normal breath (about 0.5 cubic decimetres at rest); inspiratory and expiratory reserve volumes are the extra air that can be forcibly breathed in or out; vital capacity is the maximum that can be exhaled after a maximum inhalation; and residual volume is the air that always remains in the lungs. Pulmonary ventilation rate is tidal volume multiplied by breathing rate.
Smoking harms the lungs in several ways: tar paralyses and destroys the cilia, so mucus and microbes are not removed, causing chronic bronchitis; tar and inflammation break down alveolar walls, reducing surface area and causing emphysema with breathlessness; carbon monoxide binds irreversibly to haemoglobin, reducing oxygen transport; and carcinogens in tar can cause lung cancer.
Examples in context
Example 1. Exercise and breathing. During exercise, respiring muscles produce more carbon dioxide, which is detected by chemoreceptors. The breathing centre increases the rate and depth of breathing, raising tidal volume and breathing rate so the ventilation rate climbs. This delivers more oxygen to the blood and removes carbon dioxide faster, matching gas exchange to demand.
Example 2. Emphysema and surface area. In emphysema the walls between alveoli break down, merging many small alveoli into fewer large air spaces. This greatly reduces the total surface area for gas exchange, so less oxygen can diffuse into the blood and the person becomes breathless even with mild activity. It shows directly why a large surface area matters for efficient exchange.
Try this
Q1. Explain how contraction of the diaphragm and external intercostal muscles causes air to enter the lungs. [3 marks]
- Cue. They increase thorax volume, so pressure falls below atmospheric, and air flows in down the pressure gradient.
Q2. State two ways smoking reduces the efficiency of gas exchange. [2 marks]
- Cue. Destroys alveolar walls (less surface area, emphysema); carbon monoxide reduces oxygen carried by haemoglobin; tar destroys cilia.
Q3. Define vital capacity. [1 mark]
- Cue. The maximum volume of air that can be exhaled after a maximum inhalation.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA AS 26 marksExplain how the structure of an alveolus is adapted for efficient gas exchange, and describe the path taken by a molecule of oxygen from the alveolar air to the blood.Show worked answer →
The answer needs the adaptations linked to fast diffusion, then the diffusion path in order.
Adaptations: the alveolar wall is one cell thick (squamous epithelium) and the capillary wall is one cell thick, giving a very short diffusion distance. There are millions of alveoli, giving a large total surface area. The walls are moist so oxygen dissolves before diffusing. A dense capillary network and continuous blood flow keep the concentration gradient steep, as does ventilation refreshing the alveolar air.
Diffusion path of oxygen: oxygen dissolves in the film of moisture lining the alveolus, diffuses across the alveolar epithelium, then across the capillary endothelium, into the blood plasma, and into a red blood cell where it binds to haemoglobin to form oxyhaemoglobin.
Markers reward at least three adaptations each linked to its effect on the rate of diffusion, and the diffusion path named in the correct order ending at haemoglobin.
CCEA AS 25 marksUsing a spirometer trace, explain what is meant by tidal volume and vital capacity, and calculate the pulmonary ventilation rate for a person with a tidal volume of 0.5 cubic decimetres breathing 15 times per minute.Show worked answer →
Define the two volumes from the trace, then apply the ventilation-rate equation.
Tidal volume is the volume of air moved into or out of the lungs in one normal (resting) breath, seen as the small regular up-and-down on the trace. Vital capacity is the maximum volume that can be exhaled after a maximum inhalation, seen as the largest single deflection on the trace; it is the sum of tidal volume and the inspiratory and expiratory reserve volumes.
Pulmonary ventilation rate is tidal volume times breathing rate:
Markers reward correct definitions of both volumes referenced to the trace and the correct ventilation rate with units.
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Sources & how we know this
- CCEA GCE Life and Health Sciences specification — CCEA (2016)