Unlike fish gills, the lungs of most land vertebrates inflate and deflate to move fresh air with high levels of O₂ into the lungs and expire stale air with high levels of CO₂ out of the lungs. The low density and viscosity of air enables these animals to breathe by tidal ventilation without expending too much energy. In tidal respiration, air is drawn into the lungs during inhalation and then moved out during exhalation (Fig. 39.7a).

Mammals and reptiles expand their thoracic cavity to draw air inside their lungs on inhalation. The expansion of the lungs causes the air pressure inside the lungs to become lower than the air pressure outside the lungs. The resulting negative pressure draws air into the lungs. By contrast, amphibians inflate their lungs by pressure produced by their mouth cavity. In most animals, exhalation is passively driven by elastic recoil of tissues that were previously stretched during inhalation. Elastic recoil compresses the lungs, causing the air pressure inside the lungs to become higher than the air pressure outside the lungs. The resulting positive pressure forces air out of the lungs.

In mammals, inhalation during normal, relaxed breathing is driven by contraction of the diaphragm, a domed sheet of muscle located at the base of the lungs that separates the thoracic and abdominal cavities (Fig. 39.7a). Exhalation occurs passively by elastic recoil of the lungs and chest wall. During exercise, other muscles come into play to assist with inhalation and exhalation. For example, intercostal muscles, which are attached to adjacent pairs of ribs, assist the diaphragm by elevating the ribs on inhalation and depressing them during exhalation. The action of the intercostal muscles helps to produce larger changes in the volume of the thoracic cavity, increasing the negative pressure that draws air into the lungs during inhalation, and assisting elastic recoil of the lungs and chest wall to pump air out of the lungs during exhalation.

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At rest, we inhale and exhale about 0.5 L of air every cycle (Fig. 39.7b). This amount represents the tidal volume of the lungs. With a breathing frequency of 12 breaths per minute, the ventilation rate (breathing frequency × tidal volume) is 6 L per minute. When more O₂ is needed during exercise, both breathing frequency and tidal volume increase to elevate ventilation rate.

The fresh air inhaled during tidal breathing mixes with O₂-depleted stale air that remains in the airways after exhalation. As a result, the pO₂ in the lungs (approximately 100 mmHg in humans) is lower than the pO₂ of freshly inhaled air (approximately 160 mmHg at sea level). Consequently, the fraction of O₂ that can be extracted is lower than the fraction that can be extracted by the countercurrent flow of fish gills, which can reach 90% or more. Typically, mammalian lungs extract less than 25% of the O₂ in the air, and reptile and amphibian lungs extract even less. Nevertheless, the disadvantages of tidal respiration are offset by the ease of ventilating the lung at high rates and the high O₂ content of air.

Quick Check 3 What properties of air make it possible for humans and other mammals to breathe by tidal ventilation?