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Lecture 21

Anatomy Lecture 21.docx

Anatomy and Physiology
Course Code
ANP 1105
John Copeland

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Anatomy Lecture 21
Breathing is autoprogrammed – we don’t have to think about it. Controlled by skeletal
muscle not smooth muscle
760mmHg allows for regular breathing – air flowing from high pressure outside lungs
(760mmHg) to low pressure in the lungs (756mmHg)
Air pressure is the intrapulmonary pressure and rises and falls with breathing but
eventually equalizes
-756mmHg is the intrapleural pressure, between the lung and the diaphragm. Lines the
outside of the lung and the inside of the thoracic cavity
Lungs stick to the pleura in the thoracic cavity – lung adheres to the pleura
Lung expands because thoracic cavity is increasing and the lung is stuck to the pleura
which is stuck to the thoracic cavity
- volume inside thoracic cavity increases because diaphragm is contracting
- diaphragm decreases in size when it is constricted/contracted
- Causes the lung to expand and draw air in
Intrapleural pressure is typically 4mmHg less than the pressure of the alveoli which is
usually the same as the atmospheric pressure
Three factors at work:
- natural tendency for the lung to recoil (wants to expand and bring in more air)
- surface tension of the alveoli fluid
- Surface tension of the pleural fluid and the elasticity of the chest wall
Net result is a negative intrapleural pressure
And condition equalizing intrapleural pressure with intrapulmonary (or atmospheric)
pressure causes immediate lung collapse! Because of a break between the lung and the
intrapleural fluid
Lungs are in different pleural cavities so if one collapses the other one still works usually
Diaphragm and accessory muscles
Divided into to physiological categories – quiet and forced
Quiet inspiration:
- diaphragm contracts to increase the height of the thoracic cavity (contraction of
diaphragm causes the floor to flatten…greater space from top to bottom) (caused
by musclular contraction)
- 0.5 L is the approximate volume change during quiet inspiration
- Air rushes in; inspiration ends when intra-pulmonary pressure is equal to the
atmospheric pressure
- At the same time, the intrapleural pressure drops to about -6mmHg
Quiet expiration:
- Depends on the elastic recoil of the lung more than muscular contraction
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