Chapter Summary
48.1 Respiratory Gas Exchange Is Governed by Physical Factors
Most cells require a constant supply of O2 and continuous removal of CO2. These respiratory gases are exchanged between an animal’s body fluids 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-
breathing animals, gas exchange is limited by the low diffusion rate and low amount of O2 in water. If water temperature rises, water- breathing animals face a double bind: while O2 in water decreases, the amount of work required to move water over the gas exchange surfaces increases. Review Figure 48.2 In air, O2 partial pressure (PO2) decreases with increasing altitude.
48.2 Enhancing Diffusion Maximizes Respiratory Gas Exchange
Adaptations to maximize gas exchange include increasing the surface area for gas exchange and maximizing partial pressure gradients across those exchange surfaces. Partial pressure gradients are established both by ventilating the outer surface with the respiratory medium, and perfusing the inner surface with blood.
External and internal gills are adaptations for gas exchange with water. Lungs and tracheae are adaptations for gas exchange with air. Review Figures 48.3, 48.4
The gills of fish 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 48.5, 48.6
The gas exchange system of birds includes air sacs that communicate with the lungs but are not used for gas exchange. Air flows unidirectionally through bird lungs; gases are exchanged in air capillaries that run between parabronchi. Review Figure 48.7
Each breath of air remains in a bird’s respiratory system for two breathing cycles. The air sacs work as bellows to supply the air capillaries with a continuous unidirectional flow of fresh air. Review Figure 48.8, Animation 48.1
In all air-
breathing vertebrates except birds, breathing is tidal. Although the volume of air exchanged with each breath can vary considerably in tidal breathing, the inhaled air is always mixed with stale air. Review Figure 48.9
48.3 Humans Have Tidal Respiration
In mammalian lungs, the gas exchange surface area provided by the millions of alveoli is enormous, and the diffusion path length between the air and perfusing blood is short. Surface tension in the alveoli would make inflation of the lungs difficult if the alveoli did not produce surfactant. Review Focus: Key Figure 48.10, Activity 48.1
Inhalation occurs when contractions of the diaphragm increase volume and reduce pressure in the thoracic cavity, thereby pulling on the pleural membranes. Relaxation of the diaphragm increases pressure in the thoracic cavity and results in exhalation. Review Figure 48.11, Animation 48.2
During periods of heavy metabolic demands such as strenuous exercise, the intercostal muscles, located between the ribs, increase the volume of air inhaled and exhaled.
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48.4 Respiratory Gases Are Transported by the Blood
O2 is reversibly bound to hemoglobin in red blood cells. Each hemoglobin molecule can carry a maximum of four O2 molecules. Because of positive cooperativity, 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. Review Figure 48.12, Activity 48.2
Myoglobin serves as an O2 reserve in muscle.
There is more than one type of hemoglobin. Fetal hemoglobin has a higher affinity for O2 than does adult hemoglobin, allowing fetal blood to pick up O2 from the maternal blood in the placenta. Review Figure 48.13, Activity 48.3
CO2 is transported in the blood principally as bicarbonate ions (HCO3–). Review Figure 48.14
48.5 Breathing Is Homeostatically Regulated
The basic breathing rhythm is an involuntary function 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. Review Figures 48.15, 48.16
The breathing rhythm is sensitive to feedback from chemosensors on the ventral surface of the medulla and in the carotid and aortic bodies on the large vessels leaving the heart. Review Figure 48.17
See Activity 48.4 for a concept review of this chapter.
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