Airways, Breathing and Gas Transport

Air passes: nasal cavity, pharynx, larynx, trachea, two bronchi, many bronchioles, alveoli.

• The alveoli are the site of gas exchange. They suit it because they have a huge total surface (about 70 m²), a wall of flat squamous cells one cell thick lying against capillary endothelium (a diffusion path under 1 µm), and steep concentration gradients kept by breathing and by blood flow.
• Surfactant, made by alveolar cells, lowers surface tension so the alveoli do not collapse when you breathe out.
• The epiglottis is a flap of elastic cartilage that covers the larynx during swallowing, so food goes down the oesophagus.
• Smooth muscle in the bronchioles can narrow them; in asthma it contracts too strongly.

Gas pressure falls as volume rises (Boyle's law). Air always flows from higher to lower pressure.

• The lungs are wrapped in two pleural membranes with a thin fluid between them, so the lungs follow the chest wall.
• If air gets into the pleural space (a pneumothorax), the lung on that side collapses, because nothing holds it against the chest wall.
• Quiet exhalation is passive: the muscles relax and the stretched elastic fibres recoil. Forced exhalation uses the internal intercostal and abdominal muscles.
• The breathing rate is set by the medulla oblongata, mainly in response to blood $\mathrm{CO_2}$ (sensed through its effect on pH).

• Tidal volume: the volume of one normal breath, about 500 mL at rest.
• Minute ventilation: the volume breathed in per minute.
• Dead space: the volume of the conducting airways, about 150 mL in an adult. This air does not take part in gas exchange.
• Alveolar ventilation: the fresh air reaching the alveoli each minute. Slow deep breathing gives more alveolar ventilation than fast shallow breathing with the same minute ventilation.
• Vital capacity is the largest volume that can be breathed out after the deepest breath in; some air (the residual volume) always stays in the lungs.

Haemoglobin has four polypeptide chains, each with a haem group containing iron (Fe²⁺), so one molecule carries up to four $\mathrm{O_2}$.

• Binding is cooperative: once one oxygen binds, the next binds more easily. This gives an S-shaped (sigmoid) oxygen dissociation curve of % saturation against partial pressure of oxygen.
• Bohr effect: more $\mathrm{CO_2}$, more $\mathrm{H^+}$ (lower pH) and higher temperature lower haemoglobin's affinity for $\mathrm{O_2}$. The curve shifts to the right and more $\mathrm{O_2}$ is released in active tissues; fewer haemoglobin molecules stay oxygenated.
• $\mathrm{CO_2}$ transport: about 70% as hydrogencarbonate ions $\mathrm{HCO_3^-}$ in the plasma (made inside red cells by carbonic anhydrase), about 20% bound to haemoglobin as carbaminohaemoglobin, and about 10% dissolved.
• Muscles respiring aerobically release carbon dioxide; lactate builds up only when they respire anaerobically.