Physiology of The Respiratory System I: Mechanics of Ventilation

Although I generally find physiology a tad on the dry side, a good foundation in this is vital for success in Surgical exams and patient care. I thought we could start with the respiratory system as I'm currently doing a Resp job.

Components of the Respiratory system

1. Nasal passages
2. Olfactory system
3. Conducting airways
• Nasopharynx
• larynx
• trachea
• bronchi
• bronchioles
• Alveoli

Functions of the respiratory system

1. Cleaning, humidification and warming/cooling of air: this is achieved by the nose hairs, mucociliary escalator and air flow through the conchae.
2. Respiratory gas exchange: flow of gases depends on pressure gradient between atmosphere and alveoli which can be represented as V (rate of air flow)  = Palveoli - Patmosphere/R (resistance). Thus bronchoonstriction leads to reduced air flow due to increased resistance.
3. Facilitation  of olfaction and sound production

Mechanics of Ventilation

Inspiration is an active process. At the start of inspiration the intrapleural pressure is about -4cmH2O. This decreases to around -9cmH2O when respiratory muscles contract to increase increase chest volume. This change in intrapleural pressure causes lung expansion and generation of of a negative intralaveolar pressure. The result of this is that atmospheric pressure is higher leading to air inhalation.  (NB at rest around 500mL of air is inhaled, during excercise pressure can decrease down to -30cmH2O and thus 2-3L of air can be inhaled).

Expiration is passive due to elastic recoil of the lung. However, during excercise, contraction of the accessory muscles of respiration (internal intercostals and abdominal muscles) can generate intrapleural pressures of up to +20cmH2O to expel air more quickly.


Pressures and forces acting on the Lung
Three forces act on the lung:

1. Elasticicity of the lungs: under normal conditions this keeps the lungs stretched whic results in a force that pulls inwards on the visceral pleura.
2. Surfactant: lines alveolar surfaces and produces surface tension thus producing an inward pressure which accounts for 2/3 of elastic recoil. Surfactant also increases lung compliance thus reducing work of breathing, prevents fluid accumulation in alveoli and reduces alveolar instability [ΔP (alveolar distending pressure) T (tension)/r (radius)] by stopping them from collapsing.
3. Negative intrapleural pressure: opposes the above two forces and is created by the chest wall and diaphragm pulling the parietal pleaura outwards. This results in the two layers of the pleura being pulled in opposite directions leading to a negative pressure.

 The pressure in the alveoli is equal to atmospheric pressure which is 0cmH2O. As intrapleural pressure is between -4 and -9cmH2O this results in a transmural pressure which keeps the lungs distended.

Compliance
This is the ease with which lungs can be inflated and can be expressed as:

Compliance = ΔV (change in volume)/ΔP(change in pressure)

It is governed by elsticity of the lung parenchyma and surface tension. Thus compliance is reduced in scarring or fibrosis of parenchyma, pulmonary oedema, deficiency of surfactant, reduced lung expansion (e.g. motor neurone disease/muscular paralysis), supine position, mechanical ventilation (due to reduced pulmonary blood flow), age and breathing 100% O2. Conversely, emphysema increases lung compliance to destruction of elastic fibres in the lung parenchyma. 

Regional differences in Ventilation

In the upright position the apices are less ventilated than the bases. This is due to gravity and the fact that the pressure-volume curve is sigmoid shaped and thus the two parts lie on diferent areas of the curve. This is because the bases lie on the diaphragm and are compressed whereas the apices re already stretched by their own weight this inflation begins further along the pressure-volume curve.

Well that's it for now, I shall be writing another two posts on Respiratory phyiology. The next shall be on Lung Function tests and the final on Blood flow, Gas exchange and the regulation of Ventilation. For more info, I found the following webites useful: www.acbrown.com/lung/www.acbrown.com/lung/Lectures/RsVntl and www.medicine.mcgill.ca/physio/resp-web. Hope this has been useful.

Amel