8.3 Gas Exchange

Diffusion rate rises with surface area and concentration or partial-pressure difference, and falls as diffusion distance increases. Effective gas-exchange surfaces are therefore extensive, thin and moist. A transport system carries absorbed gas away and delivers gas from internal tissues, while ventilation renews the external medium. Large organisms need both processes because diffusion alone is too slow across long distances.

In mammalian lungs, repeatedly branching bronchioles terminate in millions of alveoli. Type I pneumocytes form a very thin diffusion surface next to capillary endothelium. Type II pneumocytes secrete surfactant, which reduces surface tension and makes alveoli less likely to collapse during exhalation. Elastic fibers allow expansion and recoil. Macrophages remove particles, and ciliated airways move mucus toward the throat before particles reach the delicate exchange surface.

During inspiration, the diaphragm contracts and flattens while external intercostal muscles lift the ribs. Thoracic volume increases, intrapulmonary pressure falls below atmospheric pressure and air enters. Quiet expiration is mainly passive elastic recoil. Forced expiration recruits internal intercostal and abdominal muscles. Air moves because of pressure differences; muscles do not pull oxygen molecules selectively into the lung.

Ventilation rate and depth respond strongly to carbon dioxide. Carbon dioxide hydration changes hydrogen-ion concentration, and chemoreceptors in the brainstem and major arteries signal respiratory centers. Rising carbon dioxide increases ventilation, removing carbon dioxide and opposing the disturbance. Oxygen becomes a particularly strong ventilatory stimulus when its partial pressure is very low. Perfusion must also match ventilation: an unperfused alveolus cannot load blood efficiently even if it contains fresh air.

Alveolar oxygen diffuses into blood because alveolar partial pressure exceeds that in arriving venous blood. Carbon dioxide moves in the opposite direction. Continuous blood flow prevents capillary blood from remaining at equilibrium with alveolar air, while ventilation refreshes the alveolar side. At tissues the gradients reverse because respiration consumes oxygen and produces carbon dioxide.

A broad, thin leaf exposes a large light-capturing area while keeping diffusion paths short. The palisade mesophyll is rich in chloroplasts near the upper surface; irregular spongy mesophyll creates interconnected air spaces and a large moist cell-wall surface. Stomata connect those spaces to the atmosphere. Carbon dioxide diffuses toward photosynthesizing cells while oxygen and water vapor can diffuse outward. During net respiration, or in darkness, the direction of oxygen and carbon dioxide exchange can reverse.

Guard cells regulate pore width by changing turgor. Ion uptake lowers their water potential, water enters, and unequal wall thickness plus radial cellulose arrangement curves the cells apart. Water loss reverses opening. Stomatal density and placement reflect a tradeoff between carbon dioxide supply and dehydration risk. Many terrestrial leaves concentrate stomata on the lower surface, where air is cooler and less exposed to wind.