Respiratory System

Q1. (4 marks — 0.5 per correct structure in correct order, capped at 4) Nostrils → nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. Why: Air is filtered/warmed in the nasal cavity, passes the shared pharynx and voice-box (larynx), down the windpipe (trachea) which branches into bronchi → bronchioles → terminating in alveoli where gas exchange occurs.

Q2. (1 mark each) (a) Alveolus — a tiny air sac in the lung where gas exchange between air and blood occurs. (b) Diffusion — net movement of molecules from a region of higher to lower concentration (partial pressure) down a gradient. (c) Tidal volume — the volume of air inhaled or exhaled in one normal (resting) breath (~500 cm³). (d) Partial pressure — the pressure exerted by an individual gas in a mixture of gases.

Q3. (1 mark each, max 4)

• Large total surface area (many alveoli) — more area for diffusion.
• Thin wall (single squamous epithelial cell) — short diffusion distance.
• Rich/dense capillary network — maintains steep concentration gradient and carries gases away.
• Moist lining — allows gases to dissolve before diffusing.
• (Accept: good ventilation maintains gradient.)

Q4. (4 marks)

• Diaphragm contracts and flattens/moves down (1) increasing thoracic volume (1).
• External intercostal muscles contract, pulling the ribs up and out (1) further increasing thoracic (chest) volume (1). Why: Increasing volume lowers pressure so air flows in.

Q5. (2 marks each part) (a) Inhalation — pressure decreases (1). Because thoracic volume increases (diaphragm/intercostals contract), and by Boyle's law increased volume → decreased pressure → below atmospheric → air flows in (1). (b) Exhalation — pressure increases (1). Muscles relax, thoracic volume decreases, so pressure rises above atmospheric and air flows out (1).

Q6. (6 marks) Oxygen (3):

• Mainly (~97–98%) carried bound to haemoglobin as oxyhaemoglobin (1).
• Small amount (~2%) dissolved in plasma (1).
• Equation: Hb + 4O$_2$ ⇌ Hb(O$_2$)$_4$ (oxyhaemoglobin) (1).

Carbon dioxide (3):

• Majority (~70%) carried as hydrogencarbonate (bicarbonate) ions in plasma (1).
• Some (~23%) as carbaminohaemoglobin; small amount (~7%) dissolved in plasma (1).
• Equation: $CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$ (catalysed by carbonic anhydrase) (1).

Q7. (5 marks) (a) x-axis = partial pressure of oxygen (pO$_2$) (1); y-axis = percentage saturation of haemoglobin with oxygen (1). (b) In the middle region a small change in pO$_2$ causes a large change in saturation (1); because binding of one O$_2$ increases affinity for the next (cooperative binding) — steep region aids unloading in tissues (1). (c) It indicates a high affinity of haemoglobin for oxygen (1).

Q8. (4 marks)

• Bohr effect: an increase in CO$_2$ (and decrease in pH) reduces the affinity of haemoglobin for oxygen (1).
• The dissociation curve shifts to the right (1).
• In respiring tissues CO$_2$ is high, so haemoglobin releases (unloads) more oxygen (1) exactly where the actively respiring cells need it most (1).

Q9. (3 marks)

• The medulla oblongata (respiratory centre) in the brainstem (1).
• Rising CO$_2$ lowers blood pH; chemoreceptors (in medulla/aortic & carotid bodies) detect this (1).
• They send more nerve impulses to the diaphragm and intercostal muscles, increasing breathing rate and depth (1).

Q10. (1 mark each) (i) Gills. (ii) Tracheae / tracheal system (spiracles & tracheoles).

[
  {"claim":"Approximate O2-transport percentages (Hb-bound + dissolved) sum to 100",
   "code":"hb=98; dissolved=2; result = (hb+dissolved==100)"},
  {"claim":"Approximate CO2-transport percentages sum to 100",
   "code":"bicarb=70; carbamino=23; plasma=7; result = (bicarb+carbamino+plasma==100)"},
  {"claim":"Haemoglobin binds up to 4 O2 molecules (cooperative binding)",
   "code":"o2_per_hb=4; result = (o2_per_hb==4)"},
  {"claim":"Boyle's law: volume up gives pressure down for fixed nRT",
   "code":"P,V,k=symbols('P V k',positive=True); expr=P-k/V; V1,V2=1,2; P1=k/V1; P2=k/V2; result = (P2<P1)"}
]