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Lec 2

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University of Toronto St. George
Mc Kay+ French

39 Pulmonary MechanicsJanuary 09 2008 Divisions of lung air ventilation rates How is air moved into and out of the lungs 1 The main muscle of breathing is the diaphragm It is a domeshaped sheet of muscle separating the thoracic and abdominal cavitiesThe diaphragm is controlled by the phrenic nerve from spinal segments C3 C4 and C5 It is inserted into the lower ribs and moves downwards as it contractsforces the abdominal contents down and forward and increases the thoracic volumeNOTE As the diaphragm descends it flattens and becomes less effective at producing a downward motion and inspiration o The diaphragm starts contracting in the horizontal direction pulls the ribs in o It is the vertically oriented sides of the diaphragm muscle that produce the downward motion not the flat horizontal centre section which pull the ribcage up and forward 2 Inspiration is assisted by the external intercostal muscles They increase the thoracic cavity volume like a bucket handlemay assist inspiration 3 During heavy breathing induced by exercise accessory muscles The Scalene elevates the first two ribsThe Sternomastoids elevate the sternum process requiring muscular workNOTE Inspiration is an active due to the recoil of the elastic systemNOTE Expiration is usually passiveo When breathing is heavy or under voluntary control expiration may be active using the abdominal muscles to force the abdomen in and the diaphragm up and the internal intercostal muscles to force the rib cage inwards How does the movement of the diaphragm and rib cage expand the lungs We can model this process as a balloon in a box How to expand a balloonneed high pressure inside balloon relative to outside balloon o Increase pressure inside balloonBalloon is stretchyresists expansionVectors of contractionproduce an inward pressure to resist the expansion force of the pressure inside the balloonReach equilibrium between outward force of expansion of balloon and inward pressure of the stretchiness o Lower pressure outside intrapleural pressure keep pressure inside balloon constantBalloon expandsExpanding the box expands the balloonHowever the air inside the box and outside the balloon also expands air is compressible so some of the box volume change is not transmitted to the balloonThis problem can be remedied by replacing the air in the box with liquid which is incompressible looks more like this So the real system The outer surface of the lungs is covered by a pleural membrane as is the inner surface of the thoracic cavity The space between is the intrapleural spaceThe intrapleural space is filled with a few ml of fluid which lubricates the sliding tissues as the lungs expandthe lungs follow the volume changes of the thorax even thoughSince liquid is incompressiblethe lungs are not fastened to the thoracic wall1
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