Gas Exchange in Animals 37 Gas Exchange in Animals

Chapter 37 Gas Exchange in Animals Key Concepts 37.1 Fick’s Law of Diffusion Governs Respiratory Gas Exchange 37.2 Respiratory Systems Have Evolved to Maximize Partial Pressure Gradients 37.3 The Mammalian Lung Is Ventilated by Pressure Changes

Chapter 37 Gas Exchange in Animals Key Concepts 37.4 Respiration Is under Negative Feedback Control by the Nervous System 37.5 Respiratory Gases Are Transported in the Blood

Chapter 37 Opening Question How are bar-headed geese able to sustain the high metabolic cost of flight at altitudes higher than Mount Everest?

Concept 37.1 Fick’s Law of Diffusion Governs Respiratory Gas Exchange Organisms must exchange O2 and CO2— respiratory gases exchanged only by diffusion along their concentration gradients. Partial pressure is the concentration of a gas in a mixture. Barometric pressure—atmospheric pressure at sea level is 760 mm Hg. Partial pressure of O2 (PO2) is 159 mm Hg. LINK Concept 5.2 discusses the diffusion of molecules in solution, and Concept 6.2 describes aerobic cellular respiration and the production of ATP

In-Text Art, Ch. 37, p. 730

Concept 37.1 Fick’s Law of Diffusion Governs Respiratory Gas Exchange Fick’s law of diffusion applies to all gas exchange systems. Q = the rate of diffusion D = the diffusion coefficient: A characteristic of the diffusing substance, the medium, and the temperature Q = DA P1– P2 L

Concept 37.1 Fick’s Law of Diffusion Governs Respiratory Gas Exchange A = the area where diffusion occurs. P1 and P2 = partial pressures of the gas at two locations. L = the path length between the locations. (P1 – P2)/L is a partial pressure gradient. Q = DA P1– P2 L

Concept 37.1 Fick’s Law of Diffusion Governs Respiratory Gas Exchange Oxygen is easier to obtain from air than from water: O2 content of air is higher than that of water O2 diffuses much faster through air Air and water must be moved by the animal over its gas exchange surfaces— requires more energy to move water than air

Concept 37.1 Fick’s Law of Diffusion Governs Respiratory Gas Exchange The slow rate of diffusion of oxygen in water limits the size and shape of species without internal systems for gas exchange. These species have evolved larger surface areas, or central cavities, or specialized respiratory systems.

In-Text Art, Ch. 37, p. 731 (1)

In-Text Art, Ch. 37, p. 731 (2)

Concept 37.1 Fick’s Law of Diffusion Governs Respiratory Gas Exchange O2 availability is limited in some environments due to temperature. For water-breathers, body temperature and metabolic rate rise with an increase in water temperature—need for oxygen increases while the available oxygen decreases. For air-breathers, increase in altitude reduces available oxygen due to lower partial pressure of oxygen at high altitudes. APPLY THE CONCEPT Fick’s law of diffusion governs respiratory gas exchange

Concept 37.1 Fick’s Law of Diffusion Governs Respiratory Gas Exchange Respiratory gas exchange is a two-way process: CO2 diffuses out of the body as O2 diffuses in. The concentration gradient of CO2 from air- breathers to the environment is always large. CO2 is very soluble in water and is easy for aquatic animals to exchange.

Adaptations to maximize the exchange of O2 and CO2: Concept 37.2 Respiratory Systems Have Evolved to Maximize Partial Pressure Gradients Gas exchange systems are made up of surfaces and the mechanisms that ventilate and perfuse those surfaces. Adaptations to maximize the exchange of O2 and CO2: Increase surface area Maximize partial pressure difference Minimize diffusion path length Minimize the diffusion that takes place in an aqueous medium

Surface area (A) is increased by: Concept 37.2 Respiratory Systems Have Evolved to Maximize Partial Pressure Gradients Surface area (A) is increased by: External gills—also minimize the diffusion path length (L) of O2 and CO2 in water Internal gills—protected from predators and damage Lungs—internal cavities for respiratory gas exchange with air Tracheae—air-filled tubes in insects

Figure 37.1 Gas Exchange Systems

Transporting gases optimizes partial pressure gradients—increased by: Concept 37.2 Respiratory Systems Have Evolved to Maximize Partial Pressure Gradients Transporting gases optimizes partial pressure gradients—increased by: Minimization of the diffusion path length (L) of O2 and CO2 Ventilation—active moving of the respiratory medium over the gas exchange surfaces Perfusion—circulating blood over the gas exchange surfaces

Insects have a tracheal system throughout their bodies. Concept 37.2 Respiratory Systems Have Evolved to Maximize Partial Pressure Gradients Insects have a tracheal system throughout their bodies. Spiracles in the abdomen open to allow gas exchange and close to limit water loss. Spiracles open into tracheae that branch to tracheoles, which end in air capillaries.

Figure 37.2 The Tracheal Gas Exchange System of Insects (Part 1)

Figure 37.2 The Tracheal Gas Exchange System of Insects (Part 2)

Figure 37.2 The Tracheal Gas Exchange System of Insects (Part 3)

Fish gills use countercurrent flow to maximize gas exchange. Concept 37.2 Respiratory Systems Have Evolved to Maximize Partial Pressure Gradients Fish gills use countercurrent flow to maximize gas exchange. Gills are supported by gill arches that lie between the mouth and the opercular flaps. Water flows unidirectionally into the mouth, over the gills, and out from under the opercular flaps.

Figure 37.3 Countercurrent Exchange Is More Efficient

Figure 37.4 Fish Gills (Part 1)

Concept 37.2 Respiratory Systems Have Evolved to Maximize Partial Pressure Gradients Constant water flow maximizes PO2 on the external gill surfaces and blood circulation minimizes PO2 on the internal surfaces. Gills are made up of gill filaments that are covered by folds, or lamellae. Lamellae are the site of gas exchange and minimize the diffusion path length (L) between blood and water.

Figure 37.4 Fish Gills (Part 2)

The countercurrent flow optimizes the PO2 gradient. Concept 37.2 Respiratory Systems Have Evolved to Maximize Partial Pressure Gradients Afferent blood vessels bring blood to the gills and efferent vessels take blood away. Blood flows through the lamellae in the direction opposite to the flow of water. The countercurrent flow optimizes the PO2 gradient. LINK Countercurrent flow is also important for thermoregulation in some animals and in mammalian kidney function See Figures 29.10 and 40.8

Lungs and airways are never completely empty—contain some dead space Concept 37.2 Respiratory Systems Have Evolved to Maximize Partial Pressure Gradients Most terrestrial vertebrates use tidal ventilation in lungs—air flows in and out by the same path. Lungs and airways are never completely empty—contain some dead space The residual volume (RV) is the air that cannot be expelled from the lungs and contains “stale” (low O2) air. Each inhalation brings a mixture of outside air and stale air to the exchange area.

Concept 37.2 Respiratory Systems Have Evolved to Maximize Partial Pressure Gradients Bird lungs use unidirectional air flow to maintain a high PO2 gradient. Air enters through the posterior end of the lung and flows through parabronchi, and then into air capillaries—the sites of gas exchange. Birds have air sacs that receive inhaled air but are not sites of gas exchange. Posterior air sacs store fresh air and release it to lungs during exhalation—anterior sacs receive air from lungs. ANIMATED TUTORIAL 37.1 Airflow in Birds

Figure 37.5 The Respiratory System of a Bird (Part 1)

Figure 37.5 The Respiratory System of a Bird (Part 2)

Concept 37.3 The Mammalian Lung Is Ventilated by Pressure Changes In mammals, air enters the lung through the oral cavity and nasal passage, which join in the pharynx. Below the pharynx, the esophagus directs food to the stomach, and the trachea leads to the lungs—at the beginning is the larynx, or voice box. The trachea branches into two bronchi, then into bronchioles, and then into alveoli— the sites of gas exchange. VIDEO 37.1 Endoscopic view of trachea, bronchi, and bronchioles

Figure 37.6 The Human Respiratory System (Part 1)

Figure 37.6 The Human Respiratory System (Part 2)

Concept 37.3 The Mammalian Lung Is Ventilated by Pressure Changes Capillaries surround and lie between the alveoli—diffusion path between blood and air is less than two micrometers. Mammalian lungs produce two secretions that affect ventilation—mucus and surfactant. Mucus lines the airways and captures dirt and microorganisms. A surfactant reduces the surface tension of liquid lining the alveoli. VIDEO 37.2 Involvement of blood vessels in gas exchange

Figure 37.6 The Human Respiratory System (Part 3)

Figure 37.6 The Human Respiratory System (Part 4)

Figure 37.6 The Human Respiratory System (Part 5)

Concept 37.3 The Mammalian Lung Is Ventilated by Pressure Changes Tidal volume (TV)—the amount of air that moves in and out per breath, at rest. Inspiratory (IRV) and expiratory (ERV) reserve volumes are the additional amounts of air that we can forcefully inhale or exhale. The vital capacity (VC) is the sum of TV + IRV + ERV.

Figure 37.7 Measuring Lung Ventilation

Concept 37.3 The Mammalian Lung Is Ventilated by Pressure Changes Mammalian lungs are suspended inside a thoracic cavity. The diaphragm is a sheet of muscle at the bottom of the cavity. The pleural membrane covers each lung and lines the thoracic cavity. The space between the membranes contains fluid to help them slide past each other during breathing.

Concept 37.3 The Mammalian Lung Is Ventilated by Pressure Changes Inhalation begins when the diaphragm contracts—it pulls down, expanding the thoracic cavity and pulling on on the pleural membranes. The pleural membranes pull on the lungs, which expand and draw air in from outside. Exhalation begins when the diaphragm relaxes. The elastic lung tissues pull the diaphragm up and push air out of the airways. ANIMATED TUTORIAL 37.2 Airflow in Mammals

Figure 37.8 Into the Lungs and Out Again (Part 1)

Figure 37.8 Into the Lungs and Out Again (Part 2)

Concept 37.3 The Mammalian Lung Is Ventilated by Pressure Changes Additional muscles are used during exercise. The external intercostal muscles lift the ribs up and outward, expanding the cavity. The internal intercostal muscles decrease the volume by pulling the ribs down and inward.

Breathing is controlled in the medulla oblongata, in the brain stem. Concept 37.4 Respiration Is under Negative Feedback Control by the Nervous System Breathing is controlled in the medulla oblongata, in the brain stem. Groups of respiratory motor neurons increase their firing rate just before an inhalation. The breathing rate is modulated to meet demands for O2 supply and CO2 elimination.

Concept 37.4 Respiration Is under Negative Feedback Control by the Nervous System In humans and mammals, the breathing rate is more sensitive to increases in CO2 than to falling levels of O2. The PCO2 of blood is the primary metabolic feedback for breathing. When breathing or metabolism changes, it alters PO2 and PCO2 in the blood. Ventilation increases rapidly with exercise, in anticipation of a rise in PCO2. LINK Review the discussion of feedforward information in Concept 29.2

Figure 37.9 Sensitivity of Respiratory Control System Changes With Exercise (Part 1)

Figure 37.9 Sensitivity of Respiratory Control System Changes With Exercise (Part 2)

Concept 37.4 Respiration Is under Negative Feedback Control by the Nervous System The major site of sensitivity to PCO2 is on the ventral surface of the medulla. Cells respond to H+ ions produced when CO2 diffuses from blood into extracellular fluid. CO2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3– H+ ions stimulate cells to increase respiratory gas exchange—respiration is controlled by pH.

Concept 37.4 Respiration Is under Negative Feedback Control by the Nervous System Sensitivity to PO2 is monitored by the carotid and aortic bodies in the blood vessels leaving the heart. If PO2 falls, chemoreceptors in these bodies send nerve impulses to the brain stem to stimulate breathing.

Figure 37.10 Feedback Information Controls Breathing

Concept 37.5 Respiratory Gases Are Transported in the Blood O2 is non-polar and does not dissolve in the blood plasma, or liquid part of blood. In most animals blood contains transport molecules that bind reversibly to O2. One such O2 transporter is hemoglobin—a protein in red blood cells

Concept 37.5 Respiratory Gases Are Transported in the Blood Hemoglobin is a protein with four polypeptide subunits. Each polypeptide surrounds a heme group that can bind a molecule of O2. One molecule of hemoglobin can bind up to four molecules of O2. O2 is picked up where PO2 is high and is released where PO2 is lower. LINK The systems that circulate blood throughout the bodies of animals are described in Concepts 38.1 and 38.2

Figure 37.11 Binding of O2 to Hemoglobin Depends on PO2

Concept 37.5 Respiratory Gases Are Transported in the Blood The relationship between PO2 and the amount of O2 that binds is S-shaped. Hemoglobin will pick up or release O2 depending on the PO2 of the environment. If PO2 of the plasma is high, as in the lungs, hemoglobin will pick up its maximum of four O2 molecules. As blood circulates through tissues with lower PO2, hemoglobin will release only some O2.

Concept 37.5 Respiratory Gases Are Transported in the Blood Myoglobin is a single polypeptide molecule in muscles and can bind one molecule of O2. It has a higher affinity for O2, binds it at low PO2 values when hemoglobin molecules would release their O2. It provides a reserve for high metabolic demand for O2. INTERACTIVE TUTORIAL 37.1 Hemoglobin: Loading and Unloading

Concept 37.5 Respiratory Gases Are Transported in the Blood The affinity of hemoglobin for O2 varies. Three factors are: Hemoglobin composition pH—in the Bohr effect, blood circulating through active tissues has a lower pH and H+ ions bind to the hemoglobin molecule in place of O2 2,3-bisphosphoglyceric acid (BPG)—also lowers the affinity for O2 See Concept 6.2

Figure 37.12 Oxygen-binding Adaptations

Concept 37.5 Respiratory Gases Are Transported in the Blood Besides delivering O2, blood also transports CO2 away from the tissues. In the blood plasma, CO2 is slowly converted into bicarbonate ions (HCO3–). In endothelial cells and red blood cells, carbonic anhydrase speeds up the conversion.

Concept 37.5 Respiratory Gases Are Transported in the Blood The conversion keeps PCO2 low and facilitates diffusion away from the tissues. Some CO2 binds to hemoglobin molecules. In the lungs the conversion reaction is reversed—CO2 diffuses from the blood into the alveoli and is exhaled. APPLY THE CONCEPT Respiratory gases are transported in the blood LINK To review the properties of enzymes and the rules that determine the direction of chemical reactions, see Concepts 3.3 and 3.4

Figure 37.13 Carbon Dioxide Is Transported as Bicarbonate Ions

Answer to Opening Question Bar-headed geese have adaptations in respiration that give them an advantage: As birds, they have a continuous, unidirectional air flow. Because they live at high altitude, their respiration is driven by increases in low O2. They also have a point mutation in their hemoglobin gene that gives their hemoglobin a higher affinity for O2.