Respiratory System

Question 1 (10 marks)

(a) Pathway (6 marks) — 1 mark per correctly ordered group (max 6): Nostril/nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveolar ducts → alveoli. Why: air is filtered/warmed/moistened in the nose, the larynx/trachea are held open by cartilage rings, and branching increases into terminal alveoli where exchange occurs. Award 1 mark each for: nasal cavity, pharynx, larynx, trachea, bronchi/bronchioles, alveoli.

(b) Correcting the misconception (4 marks)

• Lungs are passive; they do not actively pull (1).
• Diaphragm contracts and flattens; external intercostals contract raising the ribs up and out → thoracic volume increases (1).
• By Boyle's Law, increased volume → decreased intrapulmonary pressure below atmospheric (1).
• Air flows down the pressure gradient into the lungs — inflation is a consequence, not a cause (1).

Question 2 (10 marks)

(a) Boyle's Law: at constant temperature, pressure is inversely proportional to volume ($P \propto 1/V$) (1). If thoracic volume increases, pressure of gas in the lungs decreases (2).

(b) Using $P_1V_1 = P_2V_2$: $P_2 = \frac{P_1 V_1}{V_2} = \frac{101.3 \times 2.4}{2.5} = 97.25 \text{ kPa}$ (3) Pressure gradient = $101.3 - 97.25 = 4.05$ kPa (1). Since intrapulmonary pressure < atmospheric, air flows into the lungs (1).

(c) As air enters, lung pressure rises back toward atmospheric, abolishing the gradient — so equilibrium is quickly re-established and flow stops (2).

Question 3 (10 marks)

Marking: up to 2 marks per feature correctly linked to Fick (max 8) + 2 for coherent Fick framing.

Model chain:

• Fick states diffusion rate rises with surface area and concentration gradient and falls with distance (2).
• Millions of alveoli provide an enormous total surface area (~70 m²) → increases area term → faster diffusion (2).
• Wall one cell thick (squamous epithelium) + thin capillary endothelium → very short diffusion distance → increases rate (2).
• Dense capillary network + continuous blood flow maintains a steep concentration gradient by constantly removing oxygenated blood/bringing deoxygenated blood (2).
• Moist thin film of surfactant/fluid allows gases to dissolve and diffuse; ventilation refreshes alveolar air keeping the gradient high (2).

Question 4 (12 marks)

(a) CO₂ transport (6 marks) — 2 marks each:

• As bicarbonate ions ($\text{HCO}_3^-$) in plasma — ~70% (2).
• Bound to haemoglobin as carbaminohaemoglobin — ~23% (2).
• Dissolved directly in plasma — ~7% (2).

(b) (3 marks) $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$ (2) Enzyme: carbonic anhydrase (1).

(c) Bohr effect (3 marks)

• The reaction releases H⁺, lowering pH in respiring tissue (1).
• Increased H⁺/CO₂ decreases haemoglobin's affinity for oxygen (shifts curve right), promoting O₂ release (1).
• Benefit: more O₂ is unloaded exactly where metabolic demand is highest (1).

Question 5 (10 marks)

(a) Axes: x = partial pressure of O₂ (kPa), y = % saturation of Hb (2). S-shaped curve with lungs point at high pO₂/~98% and tissues point at lower pO₂/lower saturation labelled (2).

(b) Binding of the first O₂ to a haem group changes Hb conformation, increasing affinity of remaining sites (cooperative/positive cooperativity) (2). This makes uptake slow at first then rapid, producing the sigmoid shape rather than linear binding (1).

(c) Bohr shift: curve shifts to the right (increased CO₂/H⁺/temperature) (1). At any given pO₂ saturation is lower (1), so more oxygen is released to actively respiring tissues (1).

Question 6 (8 marks)

(a) Gills vs tracheae (5 marks)

• Fish gills: exchange with water (medium) (1); O₂ diffuses into blood which transports it around the body (1); countercurrent flow of water and blood maintains a gradient along the whole lamella maximising uptake (1).
• Insect tracheae: air-filled tubes deliver O₂ directly to tissues via tracheoles (1); no blood involved in gas transport — diffusion straight to cells, no countercurrent system (1).

(b) Regulation (3 marks)

• Rising CO₂ lowers blood pH, detected by chemoreceptors (in medulla/aorta/carotid bodies) (1).
• The medulla oblongata (respiratory centre) increases the rate and depth of breathing via the phrenic/intercostal nerves (1).
• More CO₂ is exhaled, pH returns to normal — negative feedback (1).

[
  {"claim":"Q2b intrapulmonary pressure after expansion is 97.25 kPa","code":"P1=101.3; V1=2.4; V2=2.5; P2=P1*V1/V2; result = abs(P2-97.248)<0.01"},
  {"claim":"Q2b pressure gradient is about 4.05 kPa","code":"P1=101.3; V1=2.4; V2=2.5; P2=P1*V1/V2; grad=P1-P2; result = abs(grad-4.052)<0.01"},
  {"claim":"Q4a CO2 transport percentages sum to 100","code":"result = (70+23+7)==100"},
  {"claim":"Boyle inverse relation: volume increase lowers pressure","code":"P1=101.3; V1=2.4; V2=2.5; P2=P1*V1/V2; result = P2<P1"}
]