Summary

This chapter surveyed how gaseous exchange is achieved across the animal kingdom and, in detail, how it is achieved in humans. Different animal groups rely on very different respiratory organs, named here without going into their internal mechanism: general body-surface diffusion in the simplest animals, moist vascularised skin in earthworms, a branching tracheal system in insects, gills in aquatic arthropods, molluscs and fish, a genuine combination of moist skin, buccal-cavity lining and lungs in amphibians (whose larvae instead use gills), and a pair of lungs alone in reptiles, birds and mammals.

The human respiratory system itself forms one continuous passage -- external nostrils, nasal cavity, pharynx, larynx, trachea, bronchi, progressively finer bronchioles, and finally the alveoli where gas exchange actually occurs -- with the paired, pleura-enclosed lungs housed within a thoracic cavity whose floor is the diaphragm and whose wall carries the intercostal muscles. Breathing itself is driven purely by the resulting cyclical change in thoracic, and so pulmonary, volume: the diaphragm and external intercostal muscles contracting to enlarge the thorax and lower pulmonary pressure below atmospheric for inspiration, then relaxing (very largely passively, under quiet breathing) to raise pulmonary pressure above atmospheric for expiration, with forced breathing additionally calling on further accessory and internal intercostal/abdominal muscles. This range of movement is captured quantitatively in the standard respiratory volumes and capacities -- tidal volume, inspiratory and expiratory reserve volumes, residual volume, and the combined capacities (inspiratory capacity, functional residual capacity, vital capacity, total lung capacity) built from them.

At the alveolar membrane, oxygen and carbon dioxide are each exchanged by simple diffusion down their own partial-pressure gradient, a process repeated in reverse at the tissue level. Oxygen is then transported chiefly bound to haemoglobin as oxyhaemoglobin (with only a small dissolved fraction), its cooperative, sigmoid dissociation curve shifted rightward by the Bohr effect (raised CO2, H+ and temperature) so that actively respiring tissue automatically receives a disproportionately larger share of the oxygen delivered to it. Carbon dioxide is transported by three parallel routes of sharply unequal size -- chiefly as bicarbonate (about 70%, via the chloride shift), then bound to haemoglobin as carbaminohaemoglobin (about 20-25%), and a small fraction simply dissolved in plasma (about 7%). …