Concept 37.1 Fick's Law of Diffusion Governs Respiratory Gas Exchange

• Most cells require a constant supply of O₂ and continuous removal of CO₂. These respiratory gases are exchanged between an animal and its environment by diffusion.
   
• Fick's law of diffusion 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.
   
• 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.
   
• In air, the partial pressure of oxygen (P_O2) decreases as altitude increases.
   

Concept 37.2 Respiratory Systems Have Evolved to Maximize Partial Pressure Gradients

• Adaptations to maximize gas exchange include increasing the surface area for gas exchange and maximizing partial pressure gradients by ventilating the outer surface with the respiratory medium and perfusing the inner surface with blood.
   
• Insects distribute air throughout their bodies in a system of tracheae, tracheoles, and air capillaries. Review Figure 37.2
   
• The internal gills of fishes have large gas exchange surface areas that are ventilated continuously and unidirectionally with water. The countercurrent flow of blood helps increase the efficiency of gas exchange. Review Figures 37.3 and 37.4
   
• In all air-breathing vertebrates except birds, breathing is tidal, in which inhaled air is always mixed with some stale air. This is a less efficient form of gas exchange than that of fishes and birds.
   
• The gas exchange system of birds includes air sacs that supply fresh air to the lungs even during exhalation. Air flows unidirectionally and continuously through bird lungs. Review Figure 37.5, ANIMATED TUTORIAL 37.1, and WORKING WITH DATA 37.1
   

Concept 37.3 The Mammalian Lung Is Ventilated by Pressure Changes

• 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 37.6 and WEB ACTIVITY 37.1
   
• Inhalation occurs when contractions of the diaphragm pull on the pleural membranes and reduce the pressure in the thoracic cavity. Relaxation of the diaphragm increases pressure in the thoracic cavity and results in exhalation. Review Figure 37.7, ANIMATED TUTORIAL 37.2, and WORKING WITH DATA 37.2
   
• During strenuous exercise, the intercostal muscles and abdominal muscles increase the volume of air inhaled and exhaled.
   

Concept 37.4 Respiration Is under Negative Feedback Control by the Nervous System

• The breathing rhythm is generated by neurons in the medulla and modulated by higher brain centers.
   
• The most important feedback stimulus for breathing is the level of CO₂ in the blood, which is detected by the medulla indirectly via changes in the blood pH. Review Figure 37.9
   
• A large change in blood O₂ level can also affect respiration rate. O₂ is detected by chemosensors in the carotid and aortic bodies on the large vessels leaving the heart. Review Figure 37.10
   

Concept 37.5 Respiratory Gases Are Transported in the Blood

• O₂ is reversibly bound to hemoglobin in red blood cells. Each hemoglobin molecule can carry a maximum of four O₂ molecules.
   
• Hemoglobin's affinity for O₂ depends on the PO₂ to which the hemoglobin is exposed. Therefore, hemoglobin picks up O₂ as it flows through respiratory exchange structures and gives up O₂ in metabolically active tissues. The affinity of hemoglobin for O₂ is altered by the structure of the hemoglobin molecules, by the presence of H⁺ ions, and by the concentration of BPG in the red blood cells. Review Figure 37.11 and INTERACTIVE TUTORIAL 37.1
   
• Myoglobin serves as an O₂ reserve in muscle.
   
• Fetal hemoglobin has a higher affinity for O₂ than does maternal hemoglobin, allowing fetal blood to pick up O₂ from the maternal blood in the placenta. Review Figure 37.12 and WEB ACTIVITY 37.2
   
• CO₂ is transported in the blood principally as bicarbonate ions (HCO₃^-). Review Figure 37.13
   

See WEB ACTIVITY 37.3 for a concept review of this chapter.