Respiratory Physiology

1. Respiratory Physiology Chapter 16

2. Chapter 16 Outline • The Respiratory System • Physical Aspects of Ventilation • Mechanics of Breathing • Gas Exchange in the Lungs • Regulation of Breathing • Hemoglobin and Oxygen Transport • CO2 Transport • Acid-Base Balance of the Blood • The Effect of Exercise and High Altitude on Respiratory Function 16-2

3. Respiration • Encompasses 3 related functions: • Ventilation • gas exchange • O2 utilization (cellular respiration) • Ventilation moves air in and out of lungsfor gas exchange with blood (external respiration) • Gas exchange between blood and tissues, and O2 use by tissues is internal respiration • Gas exchange is passive via diffusion 16-3

4. Respiratory Structures 16-4

5. Structure of Respiratory System • Air passes from mouth to trachea to right and leftbronchi to bronchioles to terminal bronchioles to respiratorybronchioles to alveoli • Gas exchange occurs only in respiratory bronchioles and alveoli (= respiratory zone) • All other structures constitute the conducting zone • Are polyhedral in shape and clustered at ends of respiratory bronchioles, like units of honeycomb

6. Structure of Respiratory System

7. Structure of Respiratory System • Gas exchange occurs across the 300 million alveoli (60-80 m2 total surface area) • Only 2 thin cells are between lung air and blood: 1 alveolar and 1 endothelial cell

8. Conducting Zone • Warms and humidifies inspired air • Mucus lining filters and cleans inspired air • Mucus moved by cilia to be expectorated

9. Thoracic Cavity • Is created by the diaphragm, a dome-shaped sheet of skeletal muscle • Contains heart, large blood vessels, trachea, esophagus, thymus, and lungs • Below diaphragm is abdominopelvic cavity • Contains liver, pancreas, GI tract, spleen, and genitourinary tract

10. Thoracic Cavity • Intrapleural space is thin fluid layer between visceral pleura covering lungs and parietal pleura lining thoracic cavity walls

11. Physical Aspects of Ventilation

12. Physical Aspects of Ventilation • Ventilation results from pressure differences induced by changes in lung volumes • Air moves from higher to lower pressure • Ease of Ventilation influence by: • Compliance • Elasticity • Surface tension

13. Intrapulmonary and Intrapleural Pressures • Visceral and parietal pleurae normally adhere to each other so that lungs remain in contact with chest walls • And expand and contract with thoracic cavity • Intrapleural space contains a thin layer of lubricating fluid • During inspiration, intrapulmonary pressure is about -3 mm Hg pressure; during expiration is about +3 mm Hg • Positive transmural pressure/transpulmonary (intrapulmonary minus intrapleural pressure) keeps lungs inflated

14. Intrapulmonary and Intrapleural Pressures

15. Intrapulmonary and Intrapleural Pressures 16-16

16. Boyle’s Law (P = 1/V) • Implies that changes in intrapulmonary pressure occur as a result of changes in lung volume • Pressure of gas is inversely proportional to volume • Increase in lung volume decreases intrapulmonary pressure to subatmospheric levels; air goes in • Decrease in lung volume raises intrapulmonary pressure above that of the atmosphere; air expelled from lungs 16-17

17. Compliance • Is how easily lung expands with pressure • Or change in lung volume per change in transmural pressure (V/P) • Is reduced by factors that cause resistance to distension – such as pulmonary fibrosis

18. Elasticity • Is tendency to return to initial size after distension • Due to high content of elastin proteins • Elastic tension increases during inspiration and is reduced by recoil during expiration • Factors that cause reduced elasticity – such as Emphysema

19. Surface Tension (ST) • ST and elasticity are forces that promote alveolar collapse and resist distension • Lungs secrete and absorb fluid, normally leaving a thin film of fluid on alveolar surface • Fluid absorption occurs by osmosis driven by Na+ active transport • Fluid secretion is driven by active transport of Cl- out of alveolar epithelial cells • This film causes ST because H20 molecules are attracted to other H2O molecules • ST acts to collapse alveoli; thus inc. pressure of air within alveoli 16-20

20. Surfactant • Consists of phospholipids secreted by Type II alveolar cells • Lowers ST by getting between H2O molecules, reducing their ability to attract each other via hydrogen bonding

21. Mechanics of Breathing 16-24

22. Mechanics of Breathing • Pulmonary ventilation consists of inspiration (= inhalation) and expiration (= exhalation) • Accomplished by alternately increasing and decreasing volumes of thorax and lungs 16-25

23. Quiet Breathing • Inspiration occurs mainly because diaphragm contracts, increasing thoracic volume vertically • Parasternal and external intercostal contraction contributes a little by raising ribs, increasing thoracic volume laterally • Expiration is due to passive recoil 16-26

24. Deep Breathing • Inspiration involves contraction of extra muscles to elevate ribs: scalenes, pectoralis minor, and sternocleidomastoid muscles • Expiration involves contraction of internal intercostals and abdominal muscles 16-27

25. Mechanics of Pulmonary Ventilation 16-28

26. Pulmonary Function Tests • Assessed clinically by spirometry, a method that measures volumes of air moved during inspiration and expiration • Anatomical dead space is air in conducting zone where no gas exchange occurs 16-29

27. Pulmonary Function Tests • Tidal volume is amount of air expired/breath in quiet breathing • Vital capacity is amount of air that can be forcefully exhaled after a maximum inhalation • = sum of inspiratory reserve, tidal volume, and expiratory reserve

28. Gas Exchange in the Lungs 16-40

29. Partial Pressure of Gases • Partial pressure is pressure that a particular gas in a mixture exerts independently • Dalton’s Law states that total pressure of a gas mixture is the sum of partial pressures of each gas in mixture • Atmospheric pressure at sea level is 760 mm Hg • PATM = PN2 + PO2 + PCO2 + PH2O = 760 mm Hg

30. Gas Exchange in Lungs • Is facilitated by enormous surface area of alveoli, short diffusion distance between alveolar air and capillaries, and tremendous density of capillaries 16-43

31. Blood PO2 and PCO2 Measurements • Provide good index of lung function • At normal arterial blood has about PO2= 100mmHg • PO2 = 40mmHg in systemic veins • PCO2 = 46mmHg in systemic veins

32. Pulmonary Circulation • Rate of blood flow through pulmonary circuit equals flow through systemic circulation • But is pumped at lower pressure (about 15 mm Hg) • Pulmonary vascular resistance is low • Low pressure produces less net filtration than in systemic capillaries • Avoids pulmonary edema • Pulmonary arterioles constrict where alveolar PO2 is low and dilate where high • This matches ventilation to perfusion 16-46

33. Lung Ventilation/Perfusion Ratios • Normally, alveoli at apex of lungs are underperfused and overventilated • Alveoli at base are overperfused and underventilated 16-47

34. Regulation of Breathing 16-50

35. Brain Stem Respiratory Centers • Automatic breathing is generated by a rhythmicity center in medulla oblongata • Consists of inspiratory neurons that drive inspiration and expiratory neurons that inhibit inspiratory neurons • Their activity varies in a reciprocal way and may be due to pacemaker neurons 16-51

36. Brain Stem Respiratory Centers • Inspiratory neurons stimulate spinal motor neurons that innervate respiratory muscles • Expiration is passive and occurs when inspiratory neurons are inhibited

37. Pons Respiratory Centers • Activities of medullary rhythmicity center are influenced by centers in pons • Apneustic center promotes inspiration by stimulating inspiratory neurons in medulla • Pneumotaxic center antagonizes apneustic center, inhibiting inspiration

38. Chemoreceptors • Automatic breathing is influenced by activity of chemoreceptors that monitor blood PCO2, PO2, and pH • Central chemoreceptors are in medulla • Peripheral chemoreceptors are in large arteries near heart (aortic bodies) and in carotids (carotid bodies)

39. CNS Control of Breathing

40. Effects of Blood PCO2 and pH on Ventilation • Chemoreceptors modify ventilation to maintain normal CO2, O2, and pH levels • PCO2 is most crucial because of its effects on blood pH • H2O + CO2 H2CO3 H+ + HCO3- • Hyperventilation causes low CO2 (hypocapnia) • Hypoventilation causes high CO2 (hypercapnia)

41. Effects of Blood PCO2 and pH on Ventilation

42. Effects of Blood PCO2 and pH on Ventilation • Brain chemoreceptors are responsible for greatest effects on ventilation • H+ can't cross BBB but CO2 can, which is why it is monitored and has greatest effects • Rate and depth of ventilation adjusted to maintain arterial PCO2 of ~40 mm Hg • Peripheral chemoreceptors do not respond to PCO2, only to H+ levels

43. Effects of Blood PCO2 and pH on Ventilation A rise in blood CO2 inc. the H+ concentration (lowers pH) of CSF and thereby stim. chemoreceptor neurons in the medulla oblongata.

44. Effects of Blood PO2 on Ventilation • Low blood PO2 (hypoxemia) has little effect on ventilation • Does influence chemoreceptor sensitivity to PCO2 • PO2 has to fall to about half normal before ventilation is significantly affected • Emphysema blunts chemoreceptor response to PCO2 • Oftentimes ventilation is stimulated by hypoxic drive rather than PCO2 16-60

45. Comparison of PCO2 and PO2 Effects on Ventilation