Pressure-volume curve: Elastic Recoil Pressure don't make sense

[brownie_souffl]

From pressure-volume curve of the lung and chest wall (attached photo), I don't understand why would the elastic recoil pressure of the lung is initially negative then becomes positive above 30% of vital capacity when the lung volume increases from residual volume?

What I initially thought was as the lung volume increases, the elasticity of the lung would increases (thus increasing elastic recoil pressure) because the lung is being stretched more. However I don't understand as to why should the elastic recoil pressure starts at such a negative value in the beginning?

Can someone please help me clear up this confusion?

 

[256bits]

Wrong beginning.
See the line resting respiratory level - approx 35% lung repiratory capacity, 48% total.
That is where you are right now with your lungs, diaphram relaxed and not breating in or out.

Exhale - you go below the line. Inhale you go above. Try some breathing to see where you are on the curve(s) and what you or your breathing apparatus has to do.