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This lesson covers the structure and function of the human gas exchange system as required by the Edexcel A-Level Biology specification (9BI0). You need to understand the gross anatomy of the lungs, the structure of the alveoli, the mechanism of ventilation, and how gas exchange occurs at the alveolar surface. You should also be able to describe the Edexcel required practical investigating the effect of exercise on breathing rate.
The human gas exchange system consists of the following structures, arranged in the order through which air passes:
| Structure | Description |
|---|---|
| Nasal cavity | Warms, moistens, and filters incoming air; lined with mucus and cilia |
| Pharynx | Shared passage for air and food at the back of the throat |
| Larynx | Contains the vocal cords; also called the voice box |
| Trachea | The windpipe; approximately 12 cm long; supported by C-shaped rings of cartilage |
| Bronchi (singular: bronchus) | Two main branches of the trachea, one entering each lung |
| Bronchioles | Smaller branches of the bronchi; walls contain smooth muscle but no cartilage |
| Terminal bronchioles | The smallest airways; lead to alveolar ducts |
| Alveoli | Tiny air sacs where gas exchange takes place; approximately 480 million in both lungs |
Exam Tip: Be prepared to label a diagram of the gas exchange system. Know the difference between the trachea, bronchi, and bronchioles — and which structures have cartilage rings.
Each alveolus is a tiny, thin-walled air sac approximately 200–300 µm in diameter. The alveoli are the site of gas exchange.
| Adaptation | Explanation |
|---|---|
| Very thin walls — single layer of squamous (flat) epithelial cells | Short diffusion pathway (approximately 0.2 µm) |
| Huge total number — approximately 480 million | Enormous total surface area (approximately 70 m²) |
| Rich capillary network | Each alveolus is closely wrapped by capillaries, maintaining steep concentration gradients |
| Moist lining | Gases dissolve in the thin layer of moisture before diffusing across the epithelium |
| Elastic fibres in walls | Allow alveoli to stretch during inhalation and recoil during exhalation |
| Surfactant | Reduces surface tension of the fluid lining the alveoli, preventing them from collapsing |
Ventilation is the mechanical process of moving air into and out of the lungs. It maintains a steep concentration gradient for oxygen and carbon dioxide at the alveolar surface.
During vigorous exercise or forced breathing, the internal intercostal muscles contract, actively pulling the rib cage downwards and inwards. The abdominal muscles also contract, pushing the diaphragm upwards. This increases the rate and depth of exhalation.
Key Definition: Tidal volume — the volume of air inhaled and exhaled in a single normal breath (approximately 500 cm³ at rest). Vital capacity — the maximum volume of air that can be exhaled after a maximum inhalation.
Gas exchange occurs by diffusion across the respiratory surface — the combined barrier of the alveolar epithelium, the basement membrane, and the capillary endothelium.
Two processes maintain the steep concentration gradients:
Exam Tip: A common exam question asks you to explain how the alveoli are adapted for efficient gas exchange. Use Fick's Law and refer to large surface area, thin barrier, and maintenance of the concentration gradient by ventilation and blood flow.
To investigate the effect of exercise on breathing rate and tidal volume.
During exercise, the rate of aerobic respiration increases in muscle cells. This produces more CO₂, which is detected by chemoreceptors in the medulla oblongata and in the carotid and aortic bodies. These chemoreceptors trigger an increase in breathing rate and depth (tidal volume) via the autonomic nervous system, ensuring more O₂ is delivered and more CO₂ is removed.
| Variable | Detail |
|---|---|
| Independent variable | Intensity or duration of exercise |
| Dependent variables | Breathing rate (breaths per minute), tidal volume (cm³) |
| Control variables | Type of exercise, duration, age/fitness of subject, room temperature |
Exam Tip: In practical questions, always state how you would ensure reliability (e.g. repeat measurements, use the same subject, standardise the exercise protocol) and how you would present results (e.g. a line graph of breathing rate against time after exercise).
Several conditions impair gas exchange by affecting the properties of the exchange surface:
| Feature | Detail |
|---|---|
| Gas exchange surface | Alveoli |
| Total number | ~480 million |
| Total surface area | ~70 m² |
| Barrier thickness | ~0.5–1 µm (alveolar epithelium + basement membrane + capillary endothelium) |
| Gradient maintained by | Ventilation (breathing) and blood flow (perfusion) |
| Key diseases | Emphysema (reduced area), fibrosis (thicker barrier), asthma (narrowed airways) |
A thorough understanding of the structure and function of the human gas exchange system is essential — it forms the basis for exam questions on adaptations, disease, and the application of Fick's Law.
This material sits squarely in Edexcel 9BI0 Topic 7 (Run for your life — Exchange and Transport), which expects candidates to describe the gross structure of the human gas exchange system, the cellular structure of the alveolar–capillary barrier, the mechanism of pulmonary ventilation (pressure changes driven by intercostal and diaphragmatic action), and the application of Fick's law to alveolar gas exchange. It is heavily synoptic with Topic 5 (Energy for Biological Processes) because alveolar oxygen supply must match cellular respiratory demand, with Topic 4 (Biodiversity and Natural Resources / Microbiology) through Mycobacterium tuberculosis pathology of the alveolar wall, and back with Topic 2 (Membranes and Transport) because the alveolar epithelium is a model membrane-transport system (simple diffusion of dissolved gases across a phospholipid bilayer). Examiners pair anatomical recall with quantitative work on tidal volume, alveolar ventilation and minute ventilation, and with stretch questions on the multi-step oxygen cascade. Refer to the official Pearson Edexcel 9BI0 specification document for the exact wording of statements 7.1–7.5.
Question (8 marks):
A resting adult has a tidal volume of 500 cm3 and a respiratory rate of 12 breaths per minute. Anatomical dead space (the volume of conducting airways in which no gas exchange occurs) is 150 cm3.
(a) Calculate the minute ventilation in dm3 min−1. (2)
(b) Calculate the alveolar ventilation in dm3 min−1. (3)
(c) During moderate exercise, tidal volume rises to 1500 cm3 and respiratory rate to 20 breaths per minute. Compare the fractional gain in minute ventilation with the fractional gain in alveolar ventilation, and explain why the difference matters for cellular respiration. (3)
Solution with mark scheme:
(a) Step 1. Minute ventilation = tidal volume × respiratory rate =500 cm3×12 min−1=6000 cm3 min−1=6.0 dm3 min−1.
M1 — correct multiplication. A1 — value with correct units (dm3 min−1). A common pitfall is leaving the answer in cm3 when the question requests dm3.
(b) M1 (AO1) — recognising that only the alveolar portion of each breath participates in gas exchange: alveolar tidal volume = tidal volume − dead space =500−150=350 cm3.
M1 (AO2) — alveolar ventilation =350 cm3×12 min−1=4200 cm3 min−1=4.2 dm3 min−1.
A1 — value with correct units. Many candidates lose marks here by computing minute ventilation and reporting it as alveolar ventilation; the dead-space subtraction is the discriminator.
(c) M1 (AO2) — exercise minute ventilation =1500×20=30000 cm3 min−1, a 5-fold increase. Exercise alveolar ventilation =(1500−150)×20=27000 cm3 min−1, a 6.4-fold increase.
M1 (AO2) — alveolar ventilation rises more than minute ventilation because deeper breaths dilute the fixed-volume dead-space penalty: dead space is the same 150 cm3 whether tidal volume is 500 or 1500 cm3, so its proportional cost falls.
A1 (AO3) — conclude that deeper, slower breathing is more efficient per unit of muscular effort than shallow, rapid panting because a larger fraction of each breath reaches the alveoli, supporting the elevated rate of aerobic respiration in working muscle. A common pitfall is comparing only the absolute values without computing the ratios, which forfeits the AO3 mark.
Total: 8 marks (M5 A3).
Question (6 marks): Patients with pulmonary fibrosis have alveolar walls that are thickened and stiffened by deposition of collagenous scar tissue. Despite breathing more rapidly than healthy individuals, these patients become hypoxic on exertion.
Using Fick's law and your knowledge of the oxygen cascade, explain why elevated respiratory rate fails to compensate for fibrotic alveolar walls.
Mark scheme decomposition by AO:
| Mark | AO | Earned by |
|---|---|---|
| 1 | AO1.1 | Stating Fick's law: rate of diffusion ∝ (surface area × concentration gradient) / diffusion distance |
| 2 | AO1.2 | Identifying that fibrosis increases the thickness d of the alveolar–capillary barrier |
| 3 | AO2.1 | Explaining that elevated ventilation steepens the alveolar O2 gradient (numerator) but cannot reduce d (denominator) |
| 4 | AO2.7 | Recognising that the diffusion step is only one resistance in a multi-step cascade (alveolar gas → plasma → haemoglobin → tissue) |
| 5 | AO3.1 | Concluding that exertion raises tissue O2 demand faster than the thickened barrier can transfer O2, so arterial saturation falls |
| 6 | AO3.2 | Synthesis: ventilation and perfusion can be upregulated, but barrier thickness is structurally fixed in fibrosis — Fick's denominator is the rate-limiting step |
Total: 6 marks (AO1 = 2, AO2 = 2, AO3 = 2). Edexcel disease-application questions of this type reliably allocate AO marks roughly 30/35/35 across AO1/AO2/AO3.
Lesson 1 (this course) — SA:V and exchange-surface principles. The 480 million alveoli and 70 m2 of surface area in the human lung are the textbook resolution to the SA:V problem identified for whole organisms in lesson 1. Alveolar folding restores effective local SA:V at the gas-exchange surface even though whole-body SA:V in an adult human is approximately 0.025 m−1 — far too low to support diffusive gas exchange across the skin.
Lesson 5 (Cardiac Cycle) and Lesson 6 (Blood Vessels). Cardiac output couples directly to alveolar ventilation through the ventilation–perfusion (V/Q) ratio. A healthy resting V/Q is approximately 0.8 (alveolar ventilation 4.2 dm3 min−1 ÷ pulmonary blood flow 5.0 dm3 min−1). Mismatch — for example, a pulmonary embolus reduces Q while V continues — collapses gas exchange even with normal lungs.
Lesson 7 (Haemoglobin and Oxygen Transport). Once O2 has crossed the alveolar–capillary barrier, haemoglobin's cooperative binding pulls dissolved O2 out of plasma, keeping plasma PO2 low and the alveolus-to-plasma gradient steep. Haemoglobin is the partner of the alveolus: without it, dissolved O2 alone could supply only about 1.5% of resting metabolic demand.
Topic 5 (Cellular Respiration). Alveolar oxygen supply must match the rate of mitochondrial O2 consumption. At rest, a 70 kg adult consumes roughly 250 cm3 O2 min−1; during heavy exercise this can rise tenfold. Pulmonary ventilation, cardiac output and haemoglobin saturation are all upregulated together through medullary chemoreceptor feedback.
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