Chapter 22 the respiratory system physiology.doc

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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 body
-Patm is 760 mmHg / 1atm
Intrapulmonary Pressure
-intra-alveolar pressure (Ppul)
-the pressure in the alveoli
-pressure rises and falls with the phases of breathing but always equalizes with atmospheric
pressure eventually
Intrapleural Pressure
-pressure in the pleural cavity
-fluctuates with breathing processes
-Pip is always negative relative to Ppul
-2 forces act to pull the lungs (visceral pleura) away from the thorax wall (parietal pleura)
to cause the lung to collapse
1) The Lung's Natural Tendency to Recoil
-lungs 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
-these lung-collapsing 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
-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 lymphatics
-if fluid accumulated, a positive pressure would be produced in the pleural cavity
-any condition that equalizes intrapleural pressure with intrapulmonary / atmospheric
pressure causes lung collapse
-transpulmonary pressure (the difference between intrapulmonary and intrapleural
pressures) keeps the air spaces in the lungs open
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-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 plugged
-the 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
-lung volume is changeable and can be increased by enlarging its dimensions, thus
decreasing the gas pressure inside it
-a 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 activated
1) Diaphragm Action
-when the dome-shaped diaphragm contracts, it moves inferiorly and flattens out
-as a result, the superior-inferior (vertical / height) dimensions of the thoracic cavity
-the diaphragm is more important in producing volume changes that lead to normal quiet
2) Intercostal Muscle Action
-contraction of the external intercostal muscles lifts the rib cage and pulls the sternum
-when rib are raised and drawn together, they swing outward and expand the diameter of
the thorax
-this 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
-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 pressure = atmospheric pressure
-intrapleural pressure declines to -6 mmHg relative to atmospheric pressure during
this time
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-during deep / forced inspiration that occurs during exercise the thoracic volume is further
increased by activity of accessory muscles
-scalene muscles, sternocleidomastoid muscles of the neck and chest raise the ribs
even more than during quiet expiration
-the back extends as the thoracic curvature is straightened by the spine muscles
-quiet expiration is a passive process
-quiet expiration depends on lung elasticity rather than muscle contraction
-as inspiratory muscles relax and resume their resting length, the rib cage descends and the
lungs recoil
-thoracic and intrapulmonary volumes decrease to about 1 mmHg above
atmospheric pressure
-when pulmonary pressure is greater than atmospheric pressure, gas is forced to
flow out of the lungs
-forced expiration is an active process
-forced expiration depends on the contraction of abdominal wall muscles
-these contractions:
1) increase the intra-abdominal pressure and forces abdominal organs superiorly
against the diaphragm (causing diaphragm to contract and expiration to occur)
2) depress the rib cage (decreasing thoracic volume) with the help of intercostal
-control of accessory muscles is important for the precise regulation of air flow from the
lungs (i.e.: important for vocalists)
Physical Factors Influencing Pulmonary Ventilation
1) Airway Resistance
-friction is a major nonelastic source of resistance to gas flow
Flow = Change in Pressure / Resistance
-the factors determining gas flow in the respiratory passages and blood flow in the
cardiovascular system are equivalent
-the amount of gas flowing into and out of the alveoli is directly proportional to difference
in pressure between the external atmosphere and the alveoli
-gas flow is inversely proportional to resistance
-airway resistance is insignificant in healthy individuals because:
1) airway diameter in the first part of the conducting zone is large, relative to the
low viscosity of air
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