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Gas Exchange
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  1. Gas Exchange Chapter 44

  2. Learning Objectives • Define Physiological Respiration, Ventilation and Perfusion • Diagram the human respiratory tract and explain functions of the organs • Define Tidal Volume, Vital Capacity and Residual Volume • Compare and contrast negative and positive pressure breathing • Describe how breathing is both a conscious and subconscious effort • Describe the physiological gradient that allows for CO2 and O2 diffusion within the body • Define COPD and health impact

  3. Physiological Respiration • Process by which animals exchange O2 and CO2 with the environment

  4. Cellular respiration Physiological respiration Respiratory surface (body surface, gills, or lungs) Mitochondrion Circulatory system Respiratory medium (air or water) Fig. 44.1, p. 998

  5. Breathing (Gas Exchange) • Two primary operating features of gas exchange • The respiratory medium, either air or water • The respiratory surface, a wetted epithelium over which gas exchange takes place

  6. Respiratory Surfaces • In some invertebrates, the skin is the respiratory surface • In other invertebrates and all vertebrates, gills or lungs are the primary respiratory surfaces

  7. Gas Exchange • Simple diffusion of molecules drives exchange of gases across the respiratory surface • From regions of higher concentration to regions of lower concentration • Area of respiratory surface determines total quantity of gases exchanged by diffusion

  8. Maximizing Gas Exchange Concentration gradients of O2 and CO2 across respiratory surfaces are kept at optimal levels by ventilation and perfusion Ventilation: Movement of respiratory media over the external respiratory surface Perfusion: Movement of circulatory fluid over the internal respiratory surface

  9. Air Breathers • Air is high in O2 content • Allows air-breathers to maintain higher metabolic levels than water breathers • Air has lower density and viscosity than water • Allows air breathers to ventilate respiratory surfaces with relatively little energy

  10. Insects: Tracheal System • Insects breathe by a tracheal system • Air-conducting tubes (trachea) lead from the body surface (through spiracles) and branch to all body cells • Gas exchange takes place in fluid-filled tips at ends of branches

  11. Lungs (Air Breathers) • Invaginations of the body surface • Allow air to become saturated with water before it reaches the respiratory surface • Reduce water loss by evaporation

  12. Lung Ventilation • Positive pressure breathing • Air is forced into lungs by muscle contractions • (Frogs do this) • Negative pressure breathing • Muscle contractions expand lungs, lowering air pressure inside • Allows air to be pulled into the lungs

  13. Mammalian Respiratory System • Air enters the respiratory system through the nose and mouth and passes through the pharynx, larynx, and trachea • Trachea divides into two bronchi leading to lungs • Within lungs, bronchi branch into bronchioles, leading into alveoli surrounded by networks of blood capillaries

  14. Nasal passages Chamber in which air is moistened, warmed, and filtered and in which sounds resonate Mouth Supplemental airway Pharynx (throat) Airway connecting nasal passages and mouth with larynx; enhances sounds; also connects with esophagus Epiglottis Closes off larynx during swallowing Pleura Double-layered membrane that separates lungs from the wall of the thoracic cavity; fluid between its two layers lubricates breathing movements Larynx (voice box) Airway where sound is produced; closed off during swallowing Trachea (windpipe) Airway connecting larynx with two bronchi that lead into the lungs Intercostal muscles Skeletal muscles between ribs that contract to fill and empty lungs Lung Lobed, elastic organ of breathing exchanges gases between internal environment and outside air Diaphragm Muscle sheet between the chest cavity and abdominal cavity that contracts to fill lungs Bronchi Increasingly branched airways leading to alveoli of lung tissue Alveoli (sectioned) Bronchiole Alveoli Alveoli Pulmonary capillaries Fig. 44.8, p. 1004

  15. Ventilation: Mammals • Negative pressure mechanism • Air is exhaled passively • Relaxation of diaphragm and external intercostal muscles between ribs • Elastic recoil of lungs (pleural membranes) • Deep and rapid breathing • Forceful expulsion of air driven by contraction of internal intercostal muscles

  16. Inward bulk flow of air Outward bulk flow of air Internal intercostal muscles External intercostal muscles Diaphragm Inhalation. Diaphragm contracts and moves down. The external intercostal muscles contract and lift rib cage upward and outward. The lung volume expands. Exhalation during breathing or rest. Diaphragm and external intercostal muscles return to the resting positions. Rib cage moves down. Lungs recoil passively. Fig. 44.9, p. 1005

  17. Measuring Lung Ventilation • Tidal volume • Amount of air moved in and out of lungs during an inhalation and exhalation • Vital capacity • Total volume of air a person can inhale and exhale by breathing as deeply as possible • Residual volume • Air remaining in the lungs after as much air as possible is exhaled

  18. Control of Breathing • Control mechanisms • Local chemical controls • Regulation centers in the brain stem • Control functions • Match rate of air and blood flow in lungs • Link rate and depth of breathing to body’s requirements for O2 uptake and CO2 release

  19. Interneurons Regulate Breathing • Basic rhythm of breathing • Produced by interneurons in the medulla • When more rapid breathing is required • Other interneurons in the medulla reinforce inhalation, produce forceful exhalation • Fine-tuned breathing • Two interneuron groups in the pons stimulate or inhibit the inhalation center in the medulla

  20. Blood Gas Control Sensory receptors in medulla detect changes in levels of O2 and CO2 in blood and body fluids (aortic and carotid, too) • Control centers in medulla and pons adjust rate and depth of breathing to compensate for changes in blood gases

  21. O2 Transport • O2 diffuses from alveolar air into blood • Partial pressure of O2 is higher in alveolar air than in blood in capillary networks surrounding alveoli • Most O2 entering the blood combines with hemoglobin inside erythrocytes

  22. Dry inhaled air Moist exhaled air 100 40 Alveolar sacs 160 0.04 120 27 Capillaries entering lungs 46 CO2 40 O2 Alveolar sacs O2 Pulmonary veins 100 40 Pulmonary arteries CO2 100 40 46 40 O2 CO2 Start of veins in body tissues Start of capillaries in body tissues 46 40 Cell 40 46 100 40 Capillaries entering tissues Cells of body tissues More than 46 Less than 40 100 O2 40 CO2 Fig. 44.11, p. 1008

  23. Hemoglobin and Oxygen • One hemoglobin molecule can combine with four O2 molecules • Large quantities of O2 combined with hemoglobin maintain a large partial pressure gradient between O2 in alveolar air and in blood

  24. a. Hemoglobin saturation level in lungs Saturation level in lungs Hemoglobin O2 Oxygen saturation (%) Body tissues PO2 (mm Hg) Alveoli In the alveoli, in which the PO2 is about 100 mm Hg and the pH is 7.4, most hemoglobin molecules are 100% saturated, meaning that almost all have bound four O2 molecules. Fig. 44.12a, p. 1009

  25. O2 Diffuses into Body Cells • O2 concentration in interstitial fluid and body cells is lower than in blood plasma • O2 diffuses from blood into interstitial fluid, and from interstitial fluid into body cells

  26. CO2 Transfer: Body Tissues • Partial pressure of CO2 is higher in tissues than in blood • About 10% of CO2 dissolves in blood plasma • 70% is converted into H+ and HCO3- (bicarbonate) ions • 20% combines with hemoglobin

  27. a. Body tissues Body cells CO2 Slow In body tissues, some of the CO2 released into the blood combines with water in the blood plasma to form HCO3– and H+. However, most of the CO2 diffuses into erythrocytes, where some combines directly with hemoglobin and some combines with water to form HCO3– and H+. The H+ formed by this reaction combines with hemoglobin; the HCO3– is transported out of erythrocytes to add to the HCO3– in the blood plasma. Capillary wall HCO3– + H+ CO2+ H2O Erythrocyte CO2 + H2O CO2 Fast Hemoglobin HCO3– + H+ Capillary Fig. 44.13a, p. 1009

  28. b. Lungs Slow HCO3– + H+ CO2 + H2O In the lungs, the reactions are reversed. Some of the HCO3– in the blood plasma combines with H+ to form CO2 and water. However, most of the HCO3– is transported into erythrocytes, where it combines with H+ released from hemoglobin to form CO2 and water. CO2 is released from hemoglobin. The CO2 diffuses from the erythrocytes and, with the CO2 in the blood plasma, diffuses from the blood into the alveolar air. HCO3– + H+ Hemoglobin Fast CO2 + H2O CO2 Capillary wall CO2 Alveolar air Alveolar wall CO2 Fig. 44.13b, p. 1009

  29. 44.5 Respiration at High Altitudes and in Ocean Depths • High altitudes reduce the PO2 of air entering the lungs • Diving mammals are adapted to survive the high partial pressures of gases at extreme depths

  30. High Altitudes: PO2 Decreases • When mammals move to high altitudes, the number of red cells and amount of hemoglobin per cell increase • These changes are reversed if the animals return to lower altitudes

  31. Adaptations to High Altitudes • Humans living at higher altitudes from birth develop more alveoli and capillary networks in the lungs • Some mammals and birds adapted to high altitudes have forms of hemoglobin with greater O2 affinity • Allows saturation at lower PO2

  32. Deep-Diving Marine Mammals • Blood (compared to other mammals) • Contains more red blood cells • Has higher hemoglobin content • Greater blood volume per unit of body weight • Muscles contain more myoglobin • Allows more O2 to be stored in muscle tissues

  33. Adaptations for Deep-Diving • During a dive • Heartbeat slows • Circulation is reduced to all parts of the body except the brain

  34. COPD • Chronic Obstructive Pulmonary Disorder • Number 4 leading cause of Death in US • 24 million adults affected • Causes include tobacco use and asthma • Asthma- Percent of noninstitutionalized adults who currently have asthma: 7.3% • Percent of children who currently have asthma: 9.4%