Breathing and Exchange of Gases — Mechanism, Lung Volumes and How the Blood Carries O₂ and CO₂
Three things get examined here and it helps to keep them apart. The mechanism of breathing turns on one idea — air moves because the diaphragm and rib muscles change the volume of the chest and hence its pressure, and no muscle "pushes" air. The lung volumes and capacities are pure arithmetic: learn four volumes and build the capacities by adding them, and half the questions are just sums. The transport of gases is the conceptually rich part: oxygen rides on haemoglobin along an S-shaped curve, while most carbon dioxide travels as bicarbonate, not bound to haemoglobin — a fact students routinely get backwards. Keep those three compartments separate and the chapter is very scoring.
Lung Volumes and Capacities
| Term | Meaning | Value / formula |
|---|---|---|
| Tidal volume (TV) | Air moved in or out in a normal breath | ~500 mL |
| IRV / ERV | Extra air taken in / breathed out forcibly | IRV ~2500–3000; ERV ~1000–1100 mL |
| Residual volume (RV) | Air left after the most forceful expiration | ~1100–1200 mL |
| Vital capacity (VC) | Max out after a max in — the key clinical figure | ERV + TV + IRV |
| Total lung capacity | Everything the lungs can hold | VC + RV (~5800 mL) |
🧠 Build every capacity by adding volumes — never memorise them separately
A "capacity" is always a sum of two or more "volumes", and the whole family follows from that. Inspiratory capacity = TV + IRV (a normal breath in, then all the extra you can take). Expiratory capacity = TV + ERV. Functional residual capacity = ERV + RV (what stays after a normal breath out). Vital capacity = IRV + TV + ERV — everything except the residual air, which is why VC can never include RV. Total lung capacity = VC + RV. The one that catches students is why residual volume exists at all: some air must remain so the alveoli never fully collapse and gas exchange can continue even between breaths. Treat any numerical question as one of these additions and there is nothing to memorise beyond the four base volumes.Transport of Oxygen and Carbon Dioxide
- About 97% of oxygen is carried bound to haemoglobin as oxyhaemoglobin (each Hb binds up to four O₂); only ~3% is dissolved in plasma. The oxygen dissociation curve is sigmoid, which is what lets Hb load fully in the lungs and unload readily in the tissues.
- About 70% of carbon dioxide travels as bicarbonate (HCO₃⁻) in plasma, ~20–25% bound to Hb as carbamino-haemoglobin, and ~7% dissolved. The enzyme carbonic anhydrase in red cells drives the fast conversion CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻.
- The direction of exchange at every surface is set by the partial pressure gradient: O₂ is high in alveoli and low in tissues, so it diffuses inward then outward; CO₂ is high in tissues and low in alveoli, so it moves the other way. Nothing is pumped — it is all diffusion down gradients.
🎯 The sigmoid curve and the Bohr effect — why blood unloads O₂ exactly where it is needed
If you plot haemoglobin's oxygen saturation against the partial pressure of oxygen you get an S-shaped curve, and its shape is the elegant part. In the lungs, where pO₂ is high, the curve is nearly flat near the top, so haemoglobin saturates almost fully even if lung pO₂ drops a little. In active tissues, where pO₂ is low, the curve is steep, so a small fall in pO₂ releases a large amount of oxygen — precisely where cells are demanding it. The Bohr effect sharpens this: a low pH, high CO₂ and high temperature — exactly the conditions in a hard-working muscle — shift the curve to the right, making haemoglobin give up even more oxygen. So the blood is self-targeting: the more active and acidic a tissue, the more oxygen it extracts, with no regulation needed beyond the chemistry of the molecule itself.Key takeaways
- Gas exchange happens only in alveoli; everything above (nose to bronchioles) is a conducting passage.
- Inspiration is active (diaphragm + external intercostals raise chest volume, drop pressure); normal expiration is passive.
- Breathing rate is driven mainly by CO₂ and H⁺ sensed near the medulla, NOT by falling oxygen.
- A capacity is a sum of volumes: VC = IRV + TV + ERV; TLC = VC + RV; FRC = ERV + RV.
- Residual volume can never be exhaled; it keeps alveoli from collapsing and sustains exchange between breaths.
- O₂: ~97% on haemoglobin (sigmoid curve). CO₂: ~70% as bicarbonate, ~20–25% carbamino-Hb, ~7% dissolved.
- Carbonic anhydrase in RBCs speeds CO₂ + H₂O ⇌ H⁺ + HCO₃⁻, enabling bicarbonate transport.
- The Bohr effect: low pH, high CO₂ and heat shift the O₂ curve right, so active tissues extract more oxygen.
Practice questions (8)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
The single most important factor that increases the rate of breathing in a healthy person is a rise in:
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Answer: A — Carbon dioxide (and H⁺) concentration in the blood
The chemosensitive area near the medulla responds strongly to CO₂ and H⁺; when they rise, breathing quickens to expel the excess. Oxygen has only a minor, back-up role, which is why "falling oxygen" is the classic wrong answer here.Vital capacity of the lungs is equal to:
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Answer: A — IRV + TV + ERV
Vital capacity is the maximum you can exhale after a maximum inhalation, so it sums the inspiratory reserve, tidal and expiratory reserve volumes but excludes residual volume, which can never be exhaled. Adding RV gives total lung capacity, and ERV + RV is functional residual capacity.Most of the carbon dioxide in blood is transported in the form of:
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Answer: A — Bicarbonate ions in the plasma
About 70% of CO₂ travels as bicarbonate formed in the red cells by carbonic anhydrase; only ~20–25% is carbamino-haemoglobin and ~7% is dissolved. The enzyme drives the reaction but is not itself the transport form, so that option is a trap.During normal inspiration, air enters the lungs because:
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Answer: A — Thoracic volume increases and intrapulmonary pressure falls below atmospheric
Contraction of the diaphragm and external intercostals enlarges the thorax, lowering the pressure inside so air flows in down the gradient. The other options describe expiration (diaphragm relaxing, pressure rising) or the wrong muscle set, so they invert the mechanism.The oxygen dissociation curve of haemoglobin shifts to the right when:
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Answer: A — pH falls and CO₂ and temperature rise, as in an active tissue
The Bohr effect: low pH, high CO₂ and high temperature — the signature of a working tissue — shift the curve right, so haemoglobin releases more oxygen exactly where it is needed. The lung conditions (high pH, low CO₂) do the opposite and favour loading.The exchange of O₂ and CO₂ across the alveolar surface occurs by:
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Answer: A — Simple diffusion along partial pressure gradients
Gases move purely by diffusion from higher to lower partial pressure — O₂ into blood, CO₂ out — with no energy expended and nothing pumped. The gradients are set by the differing partial pressures in alveolar air, blood and tissues.The volume of air remaining in the lungs after the most forceful expiration is called:
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Answer: A — Residual volume
By definition the air that cannot be expelled even by the strongest effort is the residual volume (~1100–1200 mL); it keeps the alveoli inflated. ERV is the extra air you can force out beyond a normal breath, and vital capacity excludes RV entirely.Carbonic anhydrase, important in CO₂ transport, is found in high concentration in:
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Answer: A — Red blood cells
The enzyme sits inside red blood cells, where it rapidly converts CO₂ and water to carbonic acid, which dissociates to bicarbonate for transport. Plasma carries the bicarbonate but lacks the enzyme, so naming plasma inverts where the reaction is catalysed.