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Lung Pressure

When I started at Homershams 11 years ago, pressure was the parameter I knew almost nothing about.  I knew tyres were pressurised to about 30 psi (206 kPa) and that was about it!

In the intervening years, it’s become one of my favourite measures, not least because of the fact that I sleep with a CPAP machine to relieve sleep Apnoea*.  (*from the Greek word, meaning ‘without breath’)


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My Fisher & Paykel CPAP machine, generates around 1 KPa of warmed, humidified air, to keep the soft pallet, uvula etc from collapsing, resulting in buzz-saw snoring and low O2 levels.

My sleep study, shown here, (2014) showed several significant periods of non-breathing during the deepest phase of sleep (REM).  Note "14 minutes of hyponea at 3:17am"

Corona Virus

And then, of recent times, the COVID-19 pandemic has had me thinking about the function of the lungs.

The lungs are ostensibly a pressure pump and a gas-exchange system, exchanging CO2 for Oxygen in the blood.  Within the tiny sacks, called Alveoli, at the end of the bronchi, gasses pass in and out of capillaries. Oxygen is forced into the bloodstream and Carbon Dioxide is forced out.

The most serious complication of Corona is Pneumonia, where these sacks fill with fluid and prevent oxygenation of the blood which leads to pneumonia and worse.

The Lung Pump

Like a bellows pump, the diaphragm moves up and down filling (inhalation) and emptying (exhalation) the lungs.  The lungs are themselves housed within a ‘bag’ called the Pleura. There are differing pressures between the lung outer wall and the pleura (interpleural) and within the alveoli.


Breathing in: Interpleural pressure is around -1.1 kPa Alveoli pressure around -0.4 kPa
Breathing out: Interpleural pressure is around -0.2 kPa Alveoli pressure around +0.4 kPa

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Many of my readers will know the horror of asthma. New Zealand has one of the highest rates of it in the world (1 in 7 NZ children which is double the OECD average).  Asthma is a disease of enzymes. ATP (the fuel of the body) is converted by an enzyme (adenylate cyclase) into Cylic AMP.  C-AMP  leads to Bronchodialtion which is good.  However C-AMP breaks down to 3-prime,  5-prime AMP which causes bronchoconstriction which is bad.
So we can either make more good Cyclic AMP with the drug Ventolin/Salbutamol, which mimics Adenylate Cylase, or prevent it's breakdown with Theophylline, which blocks the enzyme Phosphodiesterase.



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