Link to glossary: Glossary Symbols of Respiratory Physiology
To achieve ventilation you need to be able to cycle the air in and out of the lungs. We achieve this via changing the volume of the lungs. This in turn will change the pressure leading to inflow and outflow
Inspiration and expiration
Inspiration
Diaphragm and external intercostal muscles contract which expand the thoracic volume. this pulls the lungs out leading to more lung volume. ledd pressure lead to air being pushed into the lungs by air pressure.
Expiration
everything happens in reverse
Pleural space
the pleura are membranes which line the connection between thoracic cavity wall and lung surface. as the lungs want to collapse the intrepleural pressure is about -5 cm h2o compared to outside. this is vital for inspiration as it couples expanding the thoracic cavity with expanding the lungs
Passive vs forced
Inhalation always requires effort to perform however exhalation can be forced or passive. Passive exhalation relies on the elastic recoil of the lungs. forced exhalation uses accessory muscles (and internal intercostals).
Pneumothorax
pneumothorax occurs when the pressure in the pleural space does not equal below atmospheric pressure (gas gets into the pleura). This means that expansion of the thorax is no longer coupled with expansion of the lung (not as well at least).
External breathing
we can also cycle air in and out of the lungs by changing external air pressure eg intubation, pump etc. An iron lung is like this, but it uses suction to expand and release the thorax
Work of breathing
work of breathing is determined by 2 factors: 1. how easy it is to stretch the lungs (compliance), and how much resistance there is in air moving into or out of the lungs (airways resistance).
1. Compliance
Lung compliance is the same as lung stiffness (elasticity). CL is varied by lung volume (as the lungs stretch elasticity say they want to snap back and its harder to expand), disease (fibrosis and loss of elastic tissue), age(lungs get more compliant but thorax get less compliant), and Alveolar surface tension.
Alveolar surface tension
The surface tension in the alveoli is like an added elasticity. water alone would make it too hard to expand the lungs so the lungs make surfactant, which lessens surface tension. this allows us to expand our lungs. :)
2. Airways resistance
Determined by poiseuilles’s law:
As you can see increasing (r)adius means that (R)esistance massively decreases and viceversa. Therefore bronchoconstriction and bronchodilation are massively important is determining airway resistance. Additionally most of the resistance is in the upper airway and the first 6 generations of the lower airway. Small airways do not contribute much to the resistance because there are so many of them.
Dynamic compression of airways.
This is an effect that can happen when you have bronchioles with very little rigidity. the increased lung pressure (compared to atmosphere) in exhalation pinches the bronchioles shut, causing difficulty in exhalation. To combat this people with this purse their lips to lessen relative pressure difference in bronchioles allowing normal respiration. recheck