Chapter 48 Lecture 17 Gas exchange in animals Dr. Alan McElligott

Gas exchange in animals Aims: To examine the principles behind gas exchange in animals To examine how different animals maximise respiratory gas exchange To examine how human lungs work

Gas exchange in animals Aims: To examine the principles behind gas exchange in animals To examine how different animals maximise respiratory gas exchange To examine how human lungs work These lecture aims form part of the knowledge required for learning outcomes 3 and 4. Describe mechanisms for life processes (LOC3). Appreciate how the physiology of an organism fits it for its environment (LOC4).

48 Gas Exchange in Animals 48.1 What Physical Factors Govern Respiratory Gas Exchange? 48.2 What Adaptations Maximize Respiratory Gas Exchange? 48.3 How Do Human Lungs Work? Essential reading Pages

48.1 What Physical Factors Govern Respiratory Gas Exchange? O 2 and CO 2 are respiratory gases exchanged 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 O 2 (P O 2 ) is 159 mm Hg

48.1 What Physical Factors Govern 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 P 1 – P 2 L

48.1 What Physical Factors Govern Respiratory Gas Exchange? A: the area where diffusion occurs P 1 and P 2 : partial pressures of the gas at two locations L: the path length between the locations (P 1 – P 2 )/L is a partial pressure gradient. Q = DA P 1 – P 2 L

48.1 What Physical Factors Govern Respiratory Gas Exchange? Oxygen is easier to obtain from air than from water: O 2 content of air is higher than that of water. O 2 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.

48.1 What Physical Factors Govern 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, central cavities, or specialized respiratory systems.

Figure 48.1 Keeping in Touch with the Medium

48.1 What Physical Factors Govern Respiratory Gas Exchange? An aquatic animal’s body temperature and metabolic rate rise with an increase in water temperature. The animal’s need for oxygen is increased while the available oxygen decreases in the warmer water. An increase in altitude reduces available oxygen for air breathers due to the lower partial pressure of oxygen at high altitudes.

Figure 48.2 The Double Bind of Water Breathers

48.1 What Physical Factors Govern Respiratory Gas Exchange? CO 2 diffuses out of the body as O 2 diffuses in. The concentration gradient of CO 2 from air-breathers to the environment is always large. CO 2 is very soluble in water and is easy for aquatic animals to exchange.

48.2 What Adaptations Maximize Respiratory Gas Exchange? Some respiratory systems have adaptations to maximize the exchange of O 2 and CO 2. Increased surface area Maximized partial pressure gradients Minimized diffusion path length

48.2 What Adaptations Maximize Respiratory Gas Exchange? Surface area (A) is increased by: External gills: also minimize the diffusion path length (L) of O 2 and CO 2 in water Internal gills: protected from predators and damage Lungs: internal cavities for respiratory gas exchange with air Tracheae: air-filled tubes in insects

48.2 What Adaptations Maximize Respiratory Gas Exchange? Partial pressure gradients are increased by: Minimization of the diffusion path length (L) of O 2 and CO 2 Ventilation: active moving of the respiratory medium over the gas exchange surfaces Perfusion: circulating blood over the gas exchange surfaces

48.2 What Adaptations Maximize Respiratory Gas Exchange? A gas exchange system is made up of the gas exchange surfaces and mechanisms for ventilation and perfusion of those surfaces. Examples: tracheal system in insects, fish gills, lungs in birds and humans.

Figure 48.3 Gas Exchange Systems

48.2 What Adaptations Maximize Respiratory Gas Exchange? Insects have a tracheal system: Spiracles in the abdomen open to allow gas exchange and close to limit water loss. Spiracles open into tracheae, that branch to tracheoles, that end in air capillaries.

Figure 48.4 The Tracheal Gas Exchange System of Insects

48.2 What Adaptations Maximize Respiratory Gas Exchange? 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 48.5 Fish Gills

48.2 What Adaptations Maximize Respiratory Gas Exchange? Constant water flow maximizes P O 2 on the external gill surfaces and blood circulation minimizes P O 2 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.

48.2 What Adaptations Maximize Respiratory Gas Exchange? 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 P O 2 gradient.

Figure 48.6 Countercurrent Exchange Is More Efficient

48.2 What Adaptations Maximize Respiratory Gas Exchange? Bird lungs use unidirectional air flow to maintain a high P O 2 gradient. Birds also have air sacs that receive inhaled air but are not sites of gas exchange. Air enters through the trachea, which divides into bronchi, then into parabronchi, and then into air capillaries.

Figure 48.7 The Respiratory System of a Bird

48.2 What Adaptations Maximize Respiratory Gas Exchange? Air sacs keep air moving through the lungs in a continuous and unidirectional flow: Air flows unidirectionally through the parabronchi. Inhalation expands the air sacs and exhalation compresses them: fresh air is forced out and passes over the lungs.

Figure 48.8 The Path of Air Flow through Bird Lungs

Figure 48.8 The Path of Air Flow through Bird Lungs (Part 1)

Figure 48.8 The Path of Air Flow through Bird Lungs (Part 2)

48.2 What Adaptations Maximize Respiratory Gas Exchange? Ventilation in lungs is tidal: air flows in and out by the same path. Tidal volume: the amount of air that moves in and out per breath, at rest is measured by a spirometer. Inspiratory and expiratory reserve volumes are the additional amounts of air that we can inhale or exhale.

Figure 48.9 Measuring Lung Ventilation

48.2 What Adaptations Maximize Respiratory Gas Exchange? The vital capacity is the sum of the tidal volume, the inspiratory reserve volume, and the expiratory reserve volume. The residual volume is the air that cannot be expelled from the lungs. The total lung capacity is the sum of the vital capacity and the residual volume.

48.2 What Adaptations Maximize Respiratory Gas Exchange? Tidal breathing reduces P O 2 and does not permit countercurrent gas exchange. Two features offset the inefficiency of tidal breathing in mammals: An enormous surface area A very short path length for diffusion

48.3 How Do Human Lungs Work? Air enters the human lung through the oral cavity or nasal passage via the trachea. The trachea branches into two bronchi, then into bronchioles, and then into alveoli—the sites of gas exchange.

Figure The Human Respiratory System

48.3 How Do Human Lungs Work? Mammalian lungs produce two secretions that affect ventilation: mucus and surfactant. Mucus: lines the airways and captures dirt and microorganisms. The mucus escalator is a group of cells with cilia that sweep the mucus and particles out of the airways.

48.3 How Do Human Lungs Work? A surfactant reduces the surface tension of a liquid. The fluid covering the alveoli has surface tension that makes the lungs elastic. Lung surfactant is released by cells in the alveoli when they are stretched— less force is needed to inflate the lungs.

48.3 How Do Human Lungs Work? Premature babies may not have developed the ability to make lung surfactant. Without it, they have great difficulty breathing, known as respiratory distress syndrome.

48.3 How Do Human Lungs Work? Human lungs are inside a right and left thoracic cavity. The diaphragm is a sheet of muscle at the bottom of the cavities. The pleural membrane lines each cavity and covers each lung, and encloses the pleural space.

48.3 How Do Human Lungs Work? The pleural space contains fluid to help the membranes slide past each other during breathing. A negative pressure is created in the pleural space when the volume increases in the thoracic cavity. The slight negative pressure present in between breaths keeps the alveoli inflated.

48.3 How Do Human Lungs Work? Inhalation begins when the diaphragm contracts. The diaphragm pulls down on the thoracic cavity and on the pleural membranes. The pleural membranes pull on the lungs, air enters through the trachea, and the lungs expand.

48.3 How Do Human Lungs Work? Exhalation begins when the diaphragm stops contracting and relaxes. The elastic lung tissues pull the diaphragm back up and push air out of the airways.

48.3 How Do Human Lungs Work? The intercostal muscles, located between the ribs, can also change the volume of the thoracic cavity. 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.

Figure Into the Lungs and Out Again

Into the Lungs and Out Again Endoscopic view of trachea, bronchi, and bronchioles

Check out 48.1 Recap, page CHAPTER SUMMARY, page Recap, page CHAPTER SUMMARY, page 1041, See WEB/CD Activity Recap, page CHAPTER SUMMARY, page 1041, See WEB/CD Activity 48.2 Self Quiz page : Chapter 48, questions 1-5 For Discussion page 1042: Chapter 48, question 3 Gas exchange in animals

Key terms: afferent, alveoli, bronchiole, bronchus (pl. bronchi), countercurrent, diaphragm, efferent, expiratory reserve volume, Fick’s law, gills, inspiratory reserve volume, intercostal, lamellae, larynx (voice box), lungs, mucus, mucus escalator, opercular flaps, parabronchi, perfusion, pharynx, plearual space, spiracles, spirometer, surfactant, thoracic cavity, tidal ventilation, trachea, ventilation, vital capacity Gas exchange in animals