Breathing and Exchange of Gases

Biology · Class 11

Lesson 11 of 11 · 12 min

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Must-know facts

19 facts

  1. 1Trachea divides into primary bronchi at the level of the 5th thoracic vertebra.
  2. 2Incomplete cartilaginous rings give support to the trachea, the bronchi (primary to tertiary) and the initial bronchioles.
  3. 3Epiglottis covers the glottis during swallowing; the larynx is the sound box.
  4. 4Conducting zone ends at terminal bronchioles; the exchange zone is the alveoli and their ducts.
  5. 5Inspiration: diaphragm contracts (antero-posterior axis) and external intercostals contract (dorso-ventral axis); intra-pulmonary pressure falls below atmospheric.
  6. 6Normal breathing rate: 12–16 breaths per minute.
  7. 7TV ≈ 500 mL; IRV 2500–3000 mL; ERV 1000–1100 mL; RV 1100–1200 mL.
  8. 8VC = IRV + TV + ERV; TLC = VC + RV; FRC = ERV + RV; IC = TV + IRV; EC = TV + ERV.
  9. 9A healthy person breathes in or out roughly 6000–8000 mL of air per minute (about 500 mL per breath × 12–16 breaths).
  10. 10pO₂ (mm Hg): atmosphere 159, alveoli 104, deoxygenated blood 40, oxygenated blood 95, tissues 40.
  11. 11pCO₂ (mm Hg): atmosphere 0.3, alveoli 40, deoxygenated blood 45, oxygenated blood 40, tissues 45.
  12. 12CO₂ is 20–25 times more soluble than O₂.
  13. 13Diffusion membrane = alveolar squamous epithelium + capillary endothelium + basement substance; much thinner than 1 mm.
  14. 14O₂ transport: 97% as oxyhaemoglobin, 3% dissolved in plasma. One haemoglobin carries at most 4 O₂.
  15. 15CO₂ transport: about 70% as bicarbonate, 20–25% as carbamino-haemoglobin, about 7% dissolved.
  16. 16Per 100 mL blood: about 5 mL O₂ delivered to tissues; about 4 mL CO₂ delivered to alveoli.
  17. 17Oxygen dissociation curve is sigmoid.
  18. 18Rhythm centre = medulla; pneumotaxic centre = pons; chemosensitive area responds to CO₂ and H⁺; O₂ has an insignificant role.
  19. 19Emphysema = alveolar walls damaged, less exchange surface; occupational dust exposure leads to fibrosis.

Common traps

Where marks are lost

Including Residual Volume in Vital Capacity.

VC = IRV + TV + ERV only. RV can never be breathed out, so it appears only in FRC and TLC.

Saying the diaphragm relaxes during inspiration.

It contracts and flattens during inspiration; normal quiet expiration is passive relaxation of the diaphragm and intercostals.

Mixing the axes: diaphragm with dorso-ventral, intercostals with antero-posterior.

Diaphragm increases the antero-posterior axis; external intercostals lift ribs and sternum to increase the dorso-ventral axis.

Assuming the pO₂ of oxygenated blood equals alveolar pO₂.

Alveolar pO₂ is 104 mm Hg but oxygenated blood leaving the lungs is 95 mm Hg; deoxygenated blood and tissues are both 40.

Guessing that most CO₂ rides on haemoglobin because most O₂ does.

Most CO₂ (about 70%) travels as bicarbonate; only 20–25% is carbamino-haemoglobin and about 7% is dissolved.

Believing that low O₂ is the main stimulus for breathing.

The chemosensitive area and peripheral receptors respond mainly to CO₂ and H⁺; O₂'s role in regulating rhythm is insignificant.

Swapping the roles of medulla and pons.

Medulla houses the respiratory rhythm centre that sets breathing; the pontine pneumotaxic centre only moderates it by shortening inspiration.

Thinking the conditions in tissues favour oxyhaemoglobin formation.

Formation needs high pO₂, low pCO₂, low H⁺ and lower temperature (alveoli). Tissues have the reverse, which favours O₂ release.

Assuming CO₂ diffuses more slowly because its pressure gradient (45 to 40 mm Hg) is small.

Its much higher solubility (20–25 times O₂) lets a lot of CO₂ cross even with a small gradient.

Key terms

18 terms

Glottis
The opening of the larynx, which the epiglottis closes during swallowing.
Epiglottis
A thin, elastic cartilage flap that stops food from entering the larynx.
Alveoli
Very thin, vascularised air sacs at the ends of terminal bronchioles where gas exchange occurs.
Pleura
The double membrane around each lung, with lubricating pleural fluid between its layers.
Conducting part
The airway from external nostrils to terminal bronchioles that carries air to the alveoli, clears it of particles, humidifies it and brings it to body temperature, without exchanging gases.
Intra-pulmonary pressure
The air pressure inside the lungs, whose difference from atmospheric pressure drives air movement.
Spirometer
An instrument for measuring volumes of air breathed in and out.
Tidal Volume
The volume of air moved in or out in one ordinary breath, about 500 mL.
Residual Volume
The air that stays in the lungs even after the strongest possible exhalation.
Vital Capacity
The largest volume a person can exhale after a maximal inhalation, or inhale after a maximal exhalation.
Partial pressure
The share of total pressure contributed by one gas in a mixture.
Oxyhaemoglobin
Haemoglobin with O₂ reversibly bound to it.
Carbamino-haemoglobin
Haemoglobin carrying bound CO₂.
Carbonic anhydrase
An enzyme, abundant in RBCs, that speeds up the interconversion of CO₂ + H₂O and HCO₃⁻ + H⁺.
Oxygen dissociation curve
The sigmoid graph of haemoglobin's percentage O₂ saturation against pO₂.
Pneumotaxic centre
A centre in the pons that moderates the medullary rhythm centre by shortening inspiration.
Chemosensitive area
A region beside the medullary rhythm centre that is highly sensitive to CO₂ and H⁺.
Fibrosis
Overgrowth of fibrous tissue in the lungs after long-term inflammation, as in dust exposure.
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