Mechanics of respiration physiology | Alveolar and pleural pressures | Respiratory physiology

Physiology lecture on respiratory system physiology explains mechanics of respiration

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The term mechanics of respiration refers to the forces, pressure and volume changes that occur in the thoracic cage and the lungs so that the air can move into the lungs during inspiration and out during expiration.

The lungs are housed inside the thoracic cage. Both lungs and thoracic cage are elastic structures, that is they can expand and recoil back to their respective resting positions. When seen independently the resting position of lungs is collapsed while thoracic cavity is in much more expanded position. But in an intact human body, lungs are housed inside the thoracic cavity separated by pleural membranes. Since the natural tendency of thoracic cage is to move outwards and that of lungs is to move inwards, these opposite forces act on each other. This keeps the lungs a bit expanded and the thoracic cage a bit collapsed from their resting positions respectively. Because of these opposing forces, layers of pleura also tend to move to opposite sides, parietal pleura with thoracic cage and visceral pleura with lungs. This creates a negative pressure inside the pleural cavity of -5 cm H2O.

How inspiration occurs ?

Since lungs are housed inside the thoracic cage which is a closed cavity, any changes which occur in the volume and pressure of the thoracic cage are communicated to the lungs. The expansionn of thoracic cage is provided by the contraction of the skeletal muscles which line the thoracic cage. For quiet inspiration, these muscles are diaphragm, and external intercostal muscles. With increase in thoracic cage volume, intrapleural pressure decreases upto -8 cm H2O. This causes a pull on the lungs, thus increasing the volume of the lungs and hence decreasing the alveolar pressure. During tidal inspiration in healthy people, the decrease in alveolar pressure is up to -1 cm of H2O. Due to the pressure difference generated between atmosphere and the alveoli, air moves into the alveoli until the alveolar pressure becomes equal to atmospheric pressure. With this much pressure difference, approximately 500 ml of air moves into the alveoli in healthy people.

How tidal expiration occurs ?

For tidal expiration, The changes in thoracic cavity and lung volume caused during tidal inspiration are simply reversed. So the diaphragm and external intercostal muscles relax during expiration. All the events reverse i.e decrease in thoracic cage volume, then increase in pleural pressure to the resting posting i.e -5 cm H2O, causing decrease in lung volume. This brings the thoracic cage and the lungs to their equilibrium position. With the increased volume of air inside the lungs from inspiration, the pressure does not become zero despite the breathing apparatus coming back to its equilibrium position. Instead it increases to +1 cm H2O. This causes the exhalation of the same amount of air which entered the lungs during inspiration, at the end of which the alveolar and the atmospheric pressures again become equal. . So basically tidal expiration is a passive process brought about by relaxation of inspiratory muscles.

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Chapters:
00:00 Introduction
01:23 Position of lungs and thoracic cage
02:42 Negative pleural pressure creation
04:47 Volumes and pressure at equilibrium
05:10 How inspiration occurs?
06:24 Volumes and pressure during inspiration
07:22 How expiration occurs?
08:40 Forced inspiration and expiration


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