Concept 31.1   Respiratory Gas Exchange Depends on Diffusion and Bulk Flow

• Most cells require a constant supply of O₂ and continuous removal of CO₂. These respiratory gases are exchanged between an animal’s cells and the animal’s environment by a combination of diffusion and bulk flow. Review Figure 31.1 and Figure 31.3
• A gas diffuses from where its partial pressure is high to where its partial pressure is low. The law of diffusion for gases (Fick’s law) shows how various physical factors influence the diffusion rate of gases. Adaptations to maximize respiratory gas exchange influence one or more variables of Fick’s law. Review Figure 31.2
• As they move between an animal’s internal tissues and the environment, respiratory gases diffuse across gas exchange membranes.
• Breathing organs are classified based on their structure: lungs fold inward into the body; gills fold outward from the body. Review Figure 31.4
• In water, gas exchange is limited by the low diffusion rate and low solubility of O₂ in water. O₂ becomes less soluble in water as water temperature increases. Review Figure 31.5

Concept 31.2   Animals Have Evolved Diverse Types of Breathing Organs

• Adaptations to maximize gas exchange include increasing the surface area for gas exchange and maximizing both ventilation with the respiratory medium and perfusion with blood. The gas exchange membranes are also very thin, which minimizes the distance gases must diffuse.
• Cocurrent gas exchange occurs when perfusion flows in the same direction as ventilation; flows in opposite directions result in countercurrent gas exchange. The most efficient transfer of gases is achieved by countercurrent systems, such as those found in fish. Review Figure 31.6 and Figure 31.7
• Internal partitioning of the lungs is greater in mammals than in non-avian reptiles like lizards, and greater in these reptiles than in amphibians. Ventilation in all these groups is tidal.
• The breathing system of birds includes air sacs that are filled and emptied tidally and act as bellows to ventilate the lungs. Air flows unidirectionally and continuously through the parabronchi (the principal gas exchange parts) of bird lungs. Review Figure 31.8, Figure 31.9 and ANIMATED TUTORIAL 31.1
• Insects distribute air throughout their bodies in a system of tracheae and tracheoles. The circulatory system is not involved in gas exchange because all cells are supplied directly by the tracheal system. Review Figure 31.10

Concept 31.3   The Mammalian Breathing System Is Anatomically and Functionally Elaborate

• In mammalian lungs, the gas exchange surface area provided by the millions of alveoli is enormous, and the diffusion path length is short. Review Figure 31.11 and ACTIVITY 31.1
• The tidal volume is the amount of air that moves in and out of the lungs per breath. The respiratory minute volume is the total volume of air that is inhaled and exhaled per minute. Review Figure 31.12
• Inhalation occurs when contractions of the diaphragm and some of the intercostal muscles expand the thoracic cavity. In a resting person, when the diaphragm and intercostal muscles relax, the thoracic cavity contacts by elastic recoil, resulting in exhalation. During exercise, greater muscular forces are used, increasing the volume of air inhaled and exhaled per breath and the rate of breathing. Review Figure 31.13 and ANIMATED TUTORIAL 31.2
• The breathing rhythm is generated by neurons in rhythm-generating centers in the medulla oblongata. Review Figure 31.14
• The most important feedback stimulus for breathing is the level of CO₂ in the blood, which is detected by chemosensitive neural centers on the surface of the medulla oblongata. These centers detect both the blood CO₂ partial pressure and the blood pH, which indirectly reflects the level of CO₂. Review Figure 31.15
• Breathing rate can also be affected by the onset of exercise (sensed by receptors that detect motion in muscles and joints) and by large changes in blood O₂ partial pressure (detected by chemosensitive carotid and aortic bodies on the large vessels leaving the heart).

See ACTIVITY 31.2 for a concept review of this chapter.