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Chapter 23

Chapter 23. The Respiratory System: Physiology . Respiratory System Anatomy. Functionally , the respiratory system is divided into the conducting zone and the respiratory zone. The conducting zone - n ose , pharynx, larynx, trachea, bronchi, bronchioles and terminal bronchioles.

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Chapter 23

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  1. Chapter 23 The Respiratory System: Physiology

  2. Respiratory System Anatomy • Functionally, the respiratory system is divided into the conducting zone and the respiratory zone. • The conducting zone - nose, pharynx, larynx, trachea, bronchi, bronchioles and terminal bronchioles. • The respiratory zone is the main site of gas exchange and consists of the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli.

  3. Functions of Respiratory System • The respiratory system functions to supply the body with oxygen and dispose off carbon dioxide • Four processes accomplish this: • Pulmonary ventilation – moving air into and out of the lungs • External respiration – gas exchange between the lungs and the blood • Internal respiration – gas exchange between blood and tissues • Transport of oxygen and carbon dioxide between the lungs and tissues- by blood

  4. Pulmonary ventilation • Pulmonary ventilation is the movement of air between the atmosphere and the alveoli • Inspiration – air flows into the lungs • Expiration– air flows out of the lungs

  5. Pressure Relationships in the Thoracic Cavity • Respiratory pressures are described relative to atmospheric pressure • Atmospheric pressure • Pressure exerted by the air surrounding the body • At sea level the atmospheric pressure is 760mmHg= 1atm

  6. Pressure Relationships in the Thoracic Cavity • Intrapulmonary pressure– pressure within the alveoli • Intrapulmonary rises & falls with the phases of breathing, but always equalizes itself with atmospheric pressure- 760mmHg

  7. Pressure Relationships in the Thoracic Cavity • Intrapleural pressure– pressure within the pleural cavity • Intrapleural pressure is less than intrapulmonary pressure= 756mmHg

  8. Pulmonary Ventilation • A mechanical process that depends on volume changes in the thoracic cavity • Volume changes lead to pressure changes, which lead to the flow of gases to equalize pressure • Boyle’s law– the pressure of a gas varies inversely with its volume • The larger the volume the lesser the pressure- V ∝ 1/P Volume = 1/2 liter Pressure = 2 atm Volume = 1 liter Pressure = 1 atm

  9. Pulmonary Ventilation • Muscles of inspiration ( inhalation): • Diaphragm ( primary muscle of inspiration) • External intercostals • Normal expiration is a passive process • Muscles of forced expiration (exhalation): • Internal intercostals • Abdominal muscles

  10. The recruitment of accessory muscles depends on whether the respiratory movements are quiet (normal), or forced

  11. Inspiration • Inspiratory muscles contract: diaphragm descends, rib cage rises • Thoracic cavity volume increases • Lungs stretched- intrapulmonary volume increases • Intrapulmonary pressure drops by 2mmHg • Air flows into lungs down the pressure gradient, till intrapulmonary pressure equalizes atmospheric pressure

  12. Expiration • Inspiratory muscles relax; diaphragm rises, rib cage descends • Thoracic cavity volume decreases • Elastic lungs recoil passively • Intrapulmonary volume decreases • Intrapulmonary pressure rises by 2mmHg • Air flows out of the lungs, down the pressure gradient, till intrapulmonary pressure equalizes atmospheric pressure

  13. Factors affecting Pulmonary Ventilation • 3 factors affect the ease with which we ventilate: • Surface tension of alveolar fluid • Lung compliance • Airway resistance

  14. Factors affecting Pulmonary Ventilation • The surface tension of alveolar fluid causes the alveoli to assume the smallest possible diameter The alveoli would collapse each expiration • Surfactant reduces tension- prevents the collapse of alveoli • Clinical connection: Infant respiratory distress syndrome ( IRDS) • .

  15. Factors affecting Pulmonary Ventilation • Lung compliance means the ease with which lungs and chest wall expand. • Related to two main factors • Elasticity of the lung tissue • Surface tension of the alveoli • Lungs of healthy people have a high compliance • Compliance is decreased in: • Lung fibrosis, IRDS, intercostal muscle paralysis, emphysema

  16. Factors affecting Pulmonary Ventilation 3. Airway resistance • Gas flow is inversely proportional to resistance (friction)- mainly determined by diameter of airways • The smaller the diameter the more the resistance • Sympathetic stimulation dilates bronchi & decreases resistance • Airway resistance increases in: • Asthma attacks, chronic bronchitis-when bronchioles are constricted -decreases ventilation

  17. Measuring Ventilation- Ventilation can be measured using spirometry.Lung volumes and Capacities can be measured Old and new spirometers used to measure ventilation.

  18. Lung Volumes • Tidal Volume (VT)is the volume of air inspired (or expired) during normal quiet breathing (500 ml). • Inspiratory Reserve Volume (IRV) is the volume inspired during a very forced inhalation (3100 ml – height and gender dependent).

  19. Lung Volumes • Expiratory Reserve Volume (ERV) is the volume expired during a forced exhalation (1200 ml). • Residual Volume (RV) is the air still present in the lungs after a force exhalation (1200 ml). • The RV is a reserve for mixing of gases but is not available to move in or out of the lungs.

  20. Lung Capacities • Inspiratory capacity: Is the total volume of air that can be inspired after a tidal expiration IC=TV+IRV • Functional residual capacity: Is the volume of air that remains in the lungs at the end of normal tidal expiration FRC= RV+ ERV • Vital Capacity (VC) : the total amount of exchangeable air Is all the air that can be exhaled after maximum inspiration. It is the sum of the inspiratory reserve + tidal volume + expiratory reserve (4800 ml) • Total lung capacity- Is the sum of all lung volumes-6000ml

  21. A graph of spirometer volumes and capacities

  22. Forced vital capacity (FVC)– the volume of air forcibly & rapidly expelled after taking a deep breath • Forced expiratory volume (FEV1)– the volume of air expelled during 1sec (healthy person can expel 80% of FVC in 1sec) in the FVC test • COPD decreases FEV1, because it increases resistance to flow of air

  23. Only about 350 ml of the tidal volume reaches the respiratory zone – the 150ml remains in the conducting zone (called the anatomic dead space). • If a single VT breath = 500 ml, only 350 ml will exchange gases at the alveoli. • With a respiratory rate of 12/min, the minute ventilation rate= 12 x 500 = 6000 ml/min. • The alveolar ventilation rate(volume of air/min that actually reaches the alveoli) = 12 x 350 = 4200ml/min.

  24. Respiration • Respirationis the exchange of gases. • Externalrespiration (pulmonary) is gas exchange between the alveoli and the blood. • Internalrespiration (tissue) is gas exchange between the systemic capillaries and the tissues of the body.

  25. Exchange of O2 and CO2 • The respiratory system depends on the medium of the earth’s atmosphere to extract the oxygen necessary for life. • The atmosphere is composed of these gases: • Nitrogen (N2) 79% • Oxygen (O2) 21% • Carbon Dioxide (CO2) 0.04% • Water Vapor variable, but on average around 1%

  26. Exchange of O2 and CO2 • Using gas laws we can understand the principals of respiration • Dalton’s Law states that each gas in a mixture of gases exerts its own pressure- its partial pressure Pp. • Total pressure is the sum of all the partial pressures. • The partial pressure of each gas is directly proportional to its percentage in the mixture

  27. Exchange of O2 and CO2 • The partial pressures determine the direction of movement of gases • Each gas diffuses across a permeable membrane from high to low partial pressure • There is a higher PO2in the alveoli than in the pulmonary capillaries O2 moves from the alveoli into the blood. • Since there is a higher PCO2 in the pulmonary capillaries CO2moves into the alveoli

  28. Exchange of O2 and CO2 • Henry’s law deals with gases and solutions: The quantity of a gas that will dissolve in a liquid is proportional to the partial pressures of the gas and its solubility. • Increasing the partial pressure of a gas in contact with a solution will result in more gas dissolving into the solution • How much it dissolves also depends on solubility • CO2 is 24 times more soluble in blood (and soda !) than O2

  29. Clinical connections • Hyperbaric oxygen- high pressures of O2 are used to treat anaerobic bacterial infections such as tetanus, gangrene • Decompression sickness (“the bends”) • Air is mostly N2, but very little dissolves in blood due to its low solubility • Insoluble N2is forcedto dissolve into the blood and tissues because of breathing compressed air in scuba diving • By ascending too rapidly, the N2bubbles out of the tissues and blood

  30. Alveolar air is different in composition from Atmospheric air • The atmosphere is mostly oxygen and nitrogen, while alveoli contain in comparison more carbon dioxide and less oxygen • These differences result from: • Gas exchanges in the lungs • Mixing of alveolar air that remains, with newly inspired air Atmospheric air: Alveolar air: PO2 = 159 mmHg PO2 = 105 mmHg PCO2= 0.3 mmHg PCO2= 40 mmHg

  31. External Respiration (Pulmonary gas exchange) • O2diffuses down its steep PO2gradient in the alveoli (105mmHg) to pulmonary capillary blood (40mmHg) • CO2diffuses down its gentler PCO2 gradient from pulmonary capillary blood ( 45mmHg) to alveoli (40mmHg)- exhaled • Blood in the pulmonary veins entering the left atrium has: • PCO2 40mmHg • PO2100mmHg (due to mixing of blood from bronchial veins)

  32. Internal Respiration • As in gas exchange between blood & alveoli, the gas exchange betweenblood & tissue cells occurs by simple diffusion, driven by partial pressure gradients • Tissue cells constantly use O2 & produce CO2 • PO2 in tissue is 40mmHg- O2 moves into tissues from blood capillaries • PCO2 is 45 mm Hg in tissues- CO2 moves into blood • PO2of venous blood draining tissues is 40 mm Hg and PCO2is 45 mm Hg

  33. CO2 exhaled Atmospheric air: PO2 = 159 mmHg PCO2 = 0.3 mmHg O2 inhaled Alveolar air: PO2 = 105 mmHg PCO2 = 40 mmHg Alveoli CO2 O2 Pulmonary capillaries (a) External respiration: pulmonary gas exchange To lungs To left atrium Deoxygenated blood: PO2 = 40 mmHg PCO2 = 45 mmHg Oxygenated blood: PO2 = 100 mmHg PCO2 = 40 mmHg To right atrium To tissue cells (b) Internal respiration: systemic gas exchange Systemic capillaries O2 CO2 Systemic tissue cells: PO2 = 40 mmHg PCO2 = 45 mmHg

  34. Factors affecting gas exchange • Factors influencing the movement of oxygen and carbon dioxide across the respiratory membrane • Partial pressure gradients and gas solubilities • Surface area for gas exchange & thickness of the respiratory membrane • Matching of alveolar ventilation(airflow)to alveoli and pulmonary perfusion (blood flow)

  35. Partial pressure gradients and gas solubility • The more the partial pressure differences, the more is the rate of gas diffusion • During exercise greater differences in PCO2and PO2between alveolar air and pulmonary blood- greater rate of gas diffusion • Decreased alveolar PO2at high altitudes – decreases oxygen diffusion • Solubility: • CO2 diffuses out faster compared to O2 diffusing in

  36. Surface area & respiratory membrane • Respiratory membranes are only 0.5 to 1 m thick- allows efficient gas exchange • Thicken in pulmonary edema- gas exchange is inadequate • The greater is the surface area, the more gases can be exchanged- normally huge • Decrease in surface area: • emphysema, when walls of adjacent alveoli break • mucus, tumors block gas flow into alveoli

  37. Ventilation-Perfusion Matching Ventilation and perfusion must be matched for efficient gas exchange In the lungs, pulmonary vasoconstriction occurring in response to hypoxia diverts pulmonary blood from poorly ventilated areas of the lungs to well-ventilated regions pulmonary vasodilation in response to increased ventilation

  38. Transport of O2 • In the blood, some O2 is dissolved in the plasma as a gas (only about 1.5%) • Most O2(about 98.5%) is carried attached to Hb. • Oxygenated Hb is called oxyhemoglobin(Hb-O2)

  39. Transport of O2 • The amount of Hb saturated with O2 is called percent saturation of hemoglobin • Each Hb molecule can carry 1 to 4 molecules of O2. Blood leaving the lungs has Hb that is almost fully saturated- the percent saturation is close to 98% Partially saturated hemoglobin – when 1-3 heme groups are bound to oxygen

  40. Factors affecting saturation of Hb • Most important factor is PO2 • The relationship between the amount of PO2 in plasma and the saturation of Hb is called the oxygen-hemoglobin dissociation curve. • The higher the PO2dissolved in the plasma, the higher the Hb. saturation • With PO2 100mmHg in arterial blood saturation is 98%

  41. PO2 and percent saturation contd. In the venous blood at PO2 40mmHg -percent saturation is 75% - only 25% has O2 been unloaded to tissues With PO2 between 60-100mmHg, Hb is 90% or more saturated with oxygen So even with PO2 as low as 65mmHg Hb saturation is not so low- (important for those with lung diseases or living at high altitudes

  42. PO2 and percent saturation contd. • Between 40 and 20mmHg a small decrease in PO2 causes a large drop in Hb saturation -with release of oxygen • In actively contracting muscles PO2 may drop to 20mmHg – saturation 35%- with oxygen release to muscles

  43. Transport of O2 • Measuring hemoglobin saturation is common in clinical practice- done by Pulse oximeters 3660 Group, Inc/NewsCom

  44. Factors influencing the affinity of Hb binding with O2 -Affect percent saturation of Hb

  45. Bohr Effect • Metabolically active tissues produce H+ • H+ bind to Hb- change its shape- decreasing affinity of Hb for oxygen- enhancing unloading of O2 to tissues • The pH decrease shifts the O2–Hb saturation curve “to the right” • This is called the Bohr effect

  46. Transport of CO2 • CO2 is transported in the blood in three different forms: • 7% is dissolved in the plasma, as a gas. • 70% is transported as bicarbonate ions (HCO3–) through the action of an enzyme called carbonic anhydrase. • CO2 + H2O H2CO3 H+ + HCO3- • 23% is attached to Hb(to the amino acids) as carbaminohemoglobin( HbCO2)

  47. Transport of CO2 At the level of tissues: Carbon dioxide diffuses into RBCs, combines with water to form H2CO3, (catalyzed by carbonic anhydrase),which quickly dissociates into hydrogen ions and bicarbonate ions Cl–) • Bicarbonate diffuses from RBCs into the plasma • The chloride shift – to balance the outrush of negative bicarbonate ions from the RBCs, chloride ions (Cl–) move into the erythrocytes

  48. Transport of CO2 At the lungs, these processes are reversed Cl–)

  49. Control of Respiration- Respiratory Center • The medullary rhythmicity area,has centers that control basic respiratory rythm • The inspiratory center stimulates the diaphragm via the phrenic nerve, and the external intercostal muscles via intercostal nerves. Inspirationnormally lasts about 2s.

  50. Control of Respiration-Respiratory Center • Expirationis a passive process- nerve impulses cease for about 3 sec, causing relaxation of inspiratory myscles • The expiratory center is inactive during quiet breathing • During forced exhalation, however, impulses from this center stimulate the internal intercostal and abdominal muscles

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