Biology • Year 11 • Module 2 • Lesson 15
Gas Exchange Between Internal and External Environments
Apply Fick’s law and partial pressure concepts to real data, clinical scenarios, and a six-step pathway analysis.
1. Interpret the partial pressure graph
The graph below shows the partial pressure of O₂ (solid line) at five locations along the gas exchange pathway. Study the graph, then answer the questions. 6 marks
Figure: O₂ partial pressure (mmHg) at five locations along the gas exchange pathway.
1.1 Describe the overall trend in O₂ partial pressure from atmospheric air to tissue cells. 1 mark
1.2 The largest single drop in O₂ partial pressure occurs between which two locations? Calculate the size of this drop. 2 marks
1.3 Alveolar O₂ (100 mmHg) is lower than atmospheric O₂ (159 mmHg). Using lesson content, explain why. 2 marks
1.4 Predict how the graph would change for a person during intense exercise. Describe which data point(s) would change and in which direction. 1 mark
2. Fick’s law applied to clinical scenarios
For each condition, identify which Fick variable is affected, state the direction of change, and predict the overall effect on gas exchange rate. 8 marks (2 each)
| Condition | Fick variable affected | Direction (increases / decreases) | Effect on gas exchange rate and patient impact |
|---|---|---|---|
| Emphysemaalveolar walls break down; many alveoli merge into fewer, larger spaces. Total SA may fall from ~250 m² to ~30 m². | |||
| Pulmonary oedemafluid accumulates in the interstitial space between the alveolar epithelium and capillary endothelium. | |||
| Hyperventilationbreathing rate dramatically increases, continuously flushing alveoli with fresh air faster than normal. | |||
| Anaemiareduced haemoglobin concentration means less O₂ is removed from blood plasma, so blood pO₂ remains higher after passing alveoli. |
3. The six-step gas exchange pathway
The six steps below are listed in the wrong order. Number them 1–6 in the correct sequence, then classify each as diffusion (D) or bulk flow (BF). 6 marks
| Order (#) | Step description | D or BF? |
|---|---|---|
| O₂ binds haemoglobin in red blood cells, forming oxyhaemoglobin, which removes free O₂ from plasma and maintains the alveolar–blood gradient. | ||
| Diaphragm and intercostal muscles expand the thorax; air moves by pressure difference from atmosphere into alveoli. | ||
| O₂ moves from cytoplasm into mitochondria; used in oxidative phosphorylation to produce ATP; CO₂ produced as byproduct. | ||
| Left ventricle pumps oxygenated blood through the aorta and arteries to systemic capillaries throughout the body. | ||
| O₂ diffuses from alveolar air (pO₂ 100 mmHg) across the 0.5 μm alveolar–capillary membrane into pulmonary blood (pO₂ 40 mmHg arriving). | ||
| O₂ diffuses from capillary blood (pO₂ ~95 mmHg) into tissue cells (pO₂ ~20–30 mmHg); CO₂ diffuses in the opposite direction. |
4. Apply to a new scenario, mountain climber at altitude
At high altitude (e.g. 5000 m), atmospheric pressure falls and the partial pressure of O₂ in the air drops from 159 mmHg to approximately 80 mmHg. Alveolar pO₂ falls to roughly 45 mmHg and blood pO₂ arriving at the alveoli is ~30 mmHg. 4 marks
4.1 Calculate the O₂ diffusion gradient across the alveolar membrane at altitude. Compare it with the sea-level gradient. 2 marks
4.2 Using Fick’s law, predict the effect on gas exchange rate and explain what acclimatisation responses (e.g. increased ventilation rate, increased red blood cell production) would help restore effective gas exchange. 2 marks
Q1.1, Overall trend (1 mark)
O₂ partial pressure decreases continuously from atmospheric air (159 mmHg) to tissue cells (~20–30 mmHg), following the direction of diffusion from high to low partial pressure at each step. [1]
Q1.2, Largest drop (2 marks)
The largest single drop occurs between venous blood (40 mmHg) and arterial blood (returning from tissues, so between arterial blood at 95 mmHg and venous blood at 40 mmHg) = a drop of 55 mmHg across the systemic circuit. [1 for locations; 1 for value]
Accept also: from atmospheric air (159) to alveolar air (100) = 59 mmHg drop if well justified.
Q1.3, Why alveolar O₂ is lower than atmospheric (2 marks)
Inhaled fresh air mixes with residual (dead-space) air already present in the airways that was not exhaled [1]. This stale air has already lost some O₂ to the blood and gained CO₂, so mixing dilutes the incoming O₂ and raises CO₂, alveolar pO₂ is never as high as atmospheric pO₂ [1].
Q1.4, Exercise prediction (1 mark)
The tissue cell / venous blood pO₂ data points would shift significantly lower (venous pO₂ may fall to ~15 mmHg during intense exercise) because muscles consume O₂ much faster, increasing the gradient that drives O₂ from blood into tissues. [1]
Q2, Fick’s law clinical scenarios (2 marks each)
Emphysema: Surface area; decreases dramatically; gas exchange rate decreases proportionally (by Fick: rate ∝ SA), patient cannot load sufficient O₂ despite normal breathing and normal membrane thickness. [2]
Pulmonary oedema: Membrane thickness; increases (fluid layer added to the normal ~0.5 μm barrier); gas exchange rate decreases (Fick: rate inversely proportional to thickness), patient develops hypoxaemia even with normal alveolar pO₂. [2]
Hyperventilation: Concentration gradient; increases (alveolar pO₂ rises and pCO₂ falls because stale air is cleared faster); gas exchange rate increases (Fick: rate ∝ concentration gradient), blood pO₂ rises; pCO₂ drops. [2]
Anaemia: Concentration gradient; decreases (O₂ not removed from plasma as efficiently, so blood pO₂ rises toward alveolar pO₂, shrinking the gradient); gas exchange rate decreases, O₂ delivery to tissues is impaired even though the lungs are structurally normal. [2]
Q3, Six-step pathway order and classification
Correct order and classification:
- 1Diaphragm/intercostal muscles bulk flow into alveoli → BF
- 2O₂ diffuses across alveolar–capillary membrane → D
- 3O₂ binds haemoglobin → D (short-distance diffusion into RBC + chemical binding)
- 4Left ventricle pumps blood through arteries → BF
- 5O₂ diffuses from capillary blood into tissue cells (internal gas exchange) → D
- 6O₂ used in mitochondria (cellular respiration) → D (into mitochondria)
Marking notes: 1 mark per correctly ordered step (all 6 correct = 3 marks); 1 mark per correct D/BF classification (3 marks). Total 6.
Q4.1, Gradient at altitude (2 marks)
Alveolar pO₂ at altitude = 45 mmHg; arriving blood pO₂ = 30 mmHg. Gradient = 45 − 30 = 15 mmHg [1]. At sea level the gradient is 100 − 40 = 60 mmHg. The altitude gradient is 4× smaller [1].
Q4.2, Fick’s law prediction and acclimatisation (2 marks)
By Fick’s law, rate of diffusion ∝ concentration gradient; a 4× smaller gradient means gas exchange rate is greatly reduced [1]. Acclimatisation responses that help: increased ventilation rate raises alveolar pO₂ (increases gradient); increased red blood cell / haemoglobin production means more O₂ is removed from blood faster, maintaining the gradient; both responses act to keep the driving gradient as steep as possible. [1]