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Chapter 22 the respiratory system physiology.doc

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University of Ottawa
Anatomy and Physiology
Jacqueline Carnegie

Chapter 22 The Respiratory System Physiology breathing or pulmonary ventilation consists of inspiration when air flows into the lungs and expiration when gases exit the lungs respiratory pressures are always described relative to atmospheric pressure Patm is the pressure exerted by gases surrounding the bodyPatm is 760 mmHg1atm Intrapulmonary Pressureintraalveolar pressure Ppul the pressure in the alveoli pressure rises and falls with the phases of breathing but always equalizes with atmospheric pressure eventually Intrapleural Pressurepressure in the pleural cavity fluctuates with breathing processes Pip is always negative relative to Ppul2 forces act to pull the lungs visceral pleura away from the thorax wall parietal pleura to cause the lung to collapse1 The Lungs Natural Tendency to Recoillungs always assume the smallest size possible due to their elasticity 2 The Surface Tension of the Alveolar Fluid molecules of the fluid lining the alveoli attract each other and produces surface tension that constantly acts to draw the alveoli to their smallest possible dimension these lungcollapsing forces are opposed by the natural elasticity of the chest wall and tends to pull the thorax outward and enlarge the lungs pleural fluid secures the pleurae together the way a drop of water holds 2 glass slides together pleurae slides from side to side easily but are closely apposed this results in a negative intrapleural pressure the amount of pleural fluid in the pleural cavity must remain minimal for intrapleural pressure to be maintained pleural fluid is actively pumped out of the pleural cavity into the lymphaticsif fluid accumulated a positive pressure would be produced in the pleural cavity any condition that equalizes intrapleural pressure with intrapulmonaryatmospheric pressure causes lung collapsetranspulmonary pressure the difference between intrapulmonary and intrapleural pressures keeps the air spaces in the lungs open the size of transpulmonary pressure determines the size of the lungs at any point in time the greater the transpulmonary pressure the larger the lungs atelectasis or lung collapse occurs when a bronchiole becomes pluggedthe associated alveoli absorb all their air and then collapse may also occur when air enters the pleural cavity since lungs are in separate cavities one lung can collapse without interfering with the function of the other Pulmonary Ventilation consists of inspiration and expiration a mechanical process that depends on volume changes in the thoracic cavity volume changes lead to pressure changes and pressure changes lead to the flow of gases in order to equalize pressure Inspiration lung volume is changeable and can be increased by enlarging its dimensions thus decreasing the gas pressure inside ita drop in pressure causes air to rush in from the atmosphere because gases flow down their concentration gradients during normal quiet inspiration the inspiratory muscles made up of the diaphragm and external intercostal muscles are activated1 Diaphragm Action when the domeshaped diaphragm contracts it moves inferiorly and flattens outas a result the superiorinferior verticalheight dimensions of the thoracic cavity increases the diaphragm is more important in producing volume changes that lead to normal quiet inspiration 2 Intercostal Muscle Action contraction of the external intercostal muscles lifts the rib cage and pulls the sternum superiorly when rib are raised and drawn together they swing outward and expand the diameter of the thoraxthis increases the volume by 500 mL the usual volume of air that enters the lungs during normal quiet inspiration as thoracic dimensions increase the lungs are stretched and intrapulmonary volume increases intrapulmonary pressure drops by about 1 mmHg relative to atmospheric pressure and air rushes into the lungs along the pressure gradient inspiration ends when intrapulmonary pressureatmospheric pressure intrapleural pressure declines to 6 mmHg relative to atmospheric pressure during this time
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