Breathing and Exchange of Gases

Lung volumes and capacities: naming the air a lung actually holds

A human lung's total air capacity is divided into four named volumes, and any two or more volumes added together form a named capacity, a distinction worth holding precisely since "volume" and "capacity" are not interchangeable terms in this chapter. Tidal volume, the volume of air moved in a single normal, unforced breath, is roughly 500 mL, a genuinely small fraction of the lung's total working range. Inspiratory reserve volume, the additional air that can still be forcibly inhaled beyond a normal tidal breath, runs to roughly 2,500 to 3,000 mL. Expiratory reserve volume, the additional air that can still be forcibly exhaled after a normal tidal breath out, is roughly 1,000 to 1,100 mL. Residual volume, the air that remains trapped in the lungs even after the most forceful possible exhalation and can never be voluntarily expelled, is roughly 1,100 to 1,200 mL. Vital capacity, the maximum air a person can forcibly exhale after the deepest possible inhalation, sums the tidal volume plus both reserve volumes; total lung capacity adds the residual volume on top of vital capacity, the single largest figure in this set precisely because it is the only one that includes air the lungs can never actually expel.

Gas transport: oxygen rides on haemoglobin, carbon dioxide mostly travels as bicarbonate

Oxygen is carried through the blood almost entirely bound to haemoglobin inside red blood cells, with only a small fraction (roughly 2 to 3 percent) simply dissolved in plasma, since haemoglobin's own affinity for oxygen is what makes blood capable of carrying far more oxygen than plain dissolved gas ever could on its own. Carbon dioxide's transport is split three ways, and the majority route is worth knowing precisely because it is not simple dissolution: roughly 70 percent of transported carbon dioxide travels as bicarbonate ions (HCO3-), formed inside red blood cells when the enzyme carbonic anhydrase rapidly converts carbon dioxide and water into carbonic acid, which then dissociates into bicarbonate and a hydrogen ion. A further roughly 20 to 23 percent travels bound directly to haemoglobin as carbamino compounds, and only the remaining roughly 7 percent travels as carbon dioxide simply dissolved in the blood plasma, the same minor transport route oxygen also uses.

Regulating breathing: a chemical, not a purely mechanical, control

Breathing rate is regulated chiefly by a respiratory rhythm centre in the medulla oblongata, and the primary signal that centre actually responds to is a genuinely counter-intuitive one: it is driven far more strongly by rising carbon dioxide concentration in the blood than by falling oxygen concentration, since specialised chemoreceptors are considerably more sensitive to shifts in carbon dioxide (and the resulting change in blood pH) than to oxygen level directly, which is exactly why breath-holding becomes urgently uncomfortable primarily from carbon dioxide build-up rather than from oxygen running low.

Quick revision points

• Lung volumes: tidal volume (~500 mL, normal unforced breath), inspiratory reserve volume (~2,500-3,000 mL), expiratory reserve volume (~1,000-1,100 mL), residual volume (~1,100-1,200 mL, never voluntarily expelled).
• Lung capacities (sums of volumes): vital capacity = tidal + inspiratory reserve + expiratory reserve volumes. Total lung capacity = vital capacity + residual volume (the largest figure, since it includes air that can never be expelled).
• Oxygen transport: almost entirely bound to haemoglobin in red blood cells; only about 2-3% simply dissolved in plasma.
• CO2 transport: roughly 70% as bicarbonate ions (via carbonic anhydrase converting CO2 to carbonic acid, which dissociates), roughly 20-23% as carbamino compounds bound to haemoglobin, roughly 7% simply dissolved in plasma.
• Breathing regulation: controlled by the respiratory rhythm centre in the medulla oblongata, driven primarily by rising blood CO2 (and the resulting pH shift) rather than by falling oxygen level, which is why breath-holding becomes urgent mainly from CO2 build-up.