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Atmospheric pressure Patm is Pressure exerted by air surrounding the body. Respiratory pressures described relative to Patm. Negative respiratory pressure: less than Patm .Positive respiratory pressure: greater than Patm. Zero respiratory pressure: equal to Patm Intrapulmonary pressure P P U L is Pressure in alveoli. Also called intra-alveolar pressure. Fluctuates with breathing. Always eventually equalizes with Patm Intrapleural pressure (Pip). Pressure in pleural cavity. Fluctuates with breathing Always a negative pressure <Patm and <Ppul .Usually always 4 mm Hg less than Ppul. Fluid level must be kept at a minimum. Excess fluid pumped out by lymphatic system. If fluid accumulates, positive Pip pressure develops and Lung collapses. Two inward forces promote lung collapse one Lungs’ natural tendency to recoil Because of elasticity, lungs always try to assume smallest size and Two Surface tension of alveolar fluid since Surface tension pulls on alveoli to try to reduce alveolar size. Negative Pip is affected by these opposing forces but is maintained by strong adhesive force between parietal and visceral pleurae Transpulmonary pressure is Paul  Pip Pressure that keeps lung spaces open keeps lungs from collapsing Greater transpulmonary pressure, the larger the lungs will be Lungs will collapse if intrapleural pressure equals intrapulmonary or intrapleural equals atmosphere. Negative Pip must be maintained to keep lungs inflated Atelectasis is when lung collapse due to Plugged bronchioles, which cause collapse of alveoli, or Pneumothorax, air in pleural cavity which Can occur from either wound in parietal pleura or rupture of visceral pleura Treated by removing air with chest tubes When pleurae heal, lung reinflates Volume changes lead to pressure changes Pressure changes lead to flow of gases to equalize pressure Boyle’s law is relationship between pressure and volume of a gas Gases always fill the container they are in. If amount of gas is the same and container size is reduced, pressure will increase. So pressure (P) varies inversely with volume (V). Mathematically P1V1 = P2V2 Inspiration Action of the diaphragm: when dome-shaped diaphragm contracts, it moves inferiorly and flattens out Results in increase in thoracic volume Action of intercostal muscles: when external intercostals contract, rib cage is lifted up and out Results in increase in thoracic volume As thoracic cavity volume increases, lungs are stretched as they are pulled out with thoracic cage Sequence of Inspiration 1. Inspiratory muscles contract diaphragm descends rib cage rises 2. Thoracic cavity volume increases 3. Lungs are stretched intrapulmonary volume increases 4. Intrapulmonary pressure drops 5. Air gases flow into lungs down its pressure until intrapulmonary pressure is 0 equal to atmospheric Pressure and volume have an inverse relationship. Inspiration increases lung volume by enlarging all of its dimensions; this lowers gas pressure inside the lungs. Expiration Quiet expiration normally is passive process inspiratory muscles relax, thoracic cavity volume decreases, and lungs recoil Volume decrease causes intrapulmonary pressure Forced expiration is an active process that uses oblique and transverse abdominal muscles, as well as internal intercostal muscles EXPIRATION SEQUENCE 1. Inspiratory muscles relax 2. Thoracic cavity volume decreases 3. Elastic lungs recoil passively; intrapulmonary volume decreases 4. Intrapulmonary pressure rises 5. Air gases flows out of lungs till intrapulmonary pressure is 0 Three physical factors influence the ease of air passage and the amount of energy required for ventilation – Airway resistance – Alveolar surface tension – Lung compliance Airway resistance. Friction: major nonelastic source of resistance to gas flow; occurs in airways Relationship between flow pressure and resistance is flow equals Pressure between atmosphere and alveoli over Resistance. Gas flow changes inversely with resistance As airway resistance rises, breathing movements become more strenuous. Severe constriction or obstruction of bronchioles Can prevent life-sustaining ventilation and can occur during acute asthma attacks and stop ventilation • Epinephrine dilates bronchioles, reduces air resistance Alveolar surface tension Surface tension: the attraction of liquid molecules to one another at a gas-liquid interface. Tends to draw liquid molecules closer together and reduce contact with dissimilar gas molecules. Resists any force that tends to increase surface area of liquid. Decreased alveolar surface area may be caused by tumors, mucus, or inflammatory material Surfactant is body’s detergent-like lipid and protein complex that helps reduce surface tension of alveolar fluid. Prevents alveolar collapse. Produced by type II alveolar cells Insufficient quantity of surfactant in premature infants causes infant respiratory distress syndrome Results in collapse of alveoli after each breath Treatment: spraying natural or synthetic surfactant into newborn’s air passages. Positive pressure devices also help to keep alveoli open between breaths. Severe cases may require mechanical ventilation. Survivors of mechanical ventilation may develop bronchopulmonary dysplasia, chronic childhood lung disease Lung compliance: measure of change in lung volume that occurs with given change in transpulmonary pressure. Measure of how much “stretch” the lung has. Normally high because of Distensibility of lung tissue .Surfactant, which decreases alveolar surface tension .Higher lung compliance means it is easier to expand lungs Gas exchange occurs between lungs and blood as well as blood and tissues. External respiration: diffusion of gases between blood and lungs. Internal respiration: diffusion of gases between blood and tissues. Both processes are subject to Basic properties of gases and Composition of alveolar gas. Dalton’s law of partial pressures Total pressure exerted by mixture of gases is equal to sum of pressures exerted by each gas Partial pressure Pressure exerted by each gas in mixture. Directly proportional to its percentage in mixture • At high altitudes, partial pressures declines, but at lower altitudes (under water), partial pressures increase significantly Henry’s law is For gas mixtures in contact with liquids: Each gas will dissolve in the liquid in proportion to its partial pressure. At equilibrium, partial pressures in the two phases will be equal. Temperature: as temperature of liquid rises, solubility decreases. Example of Henry’s law: hyperbaric chambers Alveoli contain more CO2 and water vapor than atmospheric air because of: Gas exchanges in lungs lungs (O2 diffuses out of lung, and CO2 diffuses into lung) Humidification of air by conducting passages Mixing of alveolar gas with each breath External Respiration External respiration (pulmonary gas exchange) involves the exchange of O2 and CO2 across respiratory membranes. Exchange is influenced by: – Partial pressure gradients and gas solubilities – Thickness and surface area of respiratory membrane – Ventilation-perfusion coupling: matching of alveolar ventilation with pulmonary blood perfusion • Partial pressure gradient for CO2 is less steep. Though gradient is not as steep, CO2 still diffuses in equal amounts with oxygen Thickness and surface area of the respiratory membrane Respiratory membranes are very thin Large total surface area of the alveoli is 40 the surface area of the skin. Effective thickness of respiratory membrane increases dramatically if the lungs become waterlogged and edematous. Seen in pneumonia or left heart failure Perfusion: blood flow reaching alveoli Ventilation: amount of gas reaching alveoli Ventilation and perfusion rates must be matched for optimal, efficient gas exchange. Both are controlled by local autoregulatory mechanisms • PO2 controls perfusion by changing arteriolar diameter • PCO2 controls ventilation by changing bronchiolar diameter Where alveolar O2 is high, arterioles dilate Where alveolar O2 is low, arterioles constrict Directs blood to go to alveoli, where oxygen is high, so blood can pick up more oxygen Internal respiration Internal respiration involves capillary gas exchange in body tissues. Partial pressures and diffusion gradients are reversed compared to external respiration. Tissue PO2 is always lower than in arterial blood PO2 so oxygen moves from blood to tissues. Tissue PCO2 is always higher than arterial blood PCO2 so CO2 moves from tissues into blood. The relationship between the pressure and volume of a gas: At constant temperature, the pressure of a gas varies inversely with its volume.