Fig. 1
The inspiratory flow-generation pathway and the feedback mechanisms affecting it, in a normal subject during passive (no expiratory muscles activity) and active (expiratory muscles activity) expiration. For simplicity and demonstration purpose, RCOI always begins when expiratory muscles activity ceases. Assuming that PmusE is able to lower lung volume below FRC (negative PEE), rapid relaxation of expiratory muscles (rapid decrease in PmusE) passively generates inspiratory flow. When PmusE decreases to zero, FRC is reached. At this point PmusI increases and actively generates inspiratory flow. Notice, compared to passive expiration, the higher VT with active expiration, which corresponds to higher RCO during the whole breath (respiratory drive). Gate: the effects of afferent signals (inputs) on respiratory centers vary, depending on the breath phases (inspiratory, post-inspiratory, expiratory); RCO: total respiratory centers output during the breath (respiratory drive); RCOI, RCOE: respiratory centers output to inspiratory and expiratory muscles, respectively; EAI, EAE: electrical activity of inspiratory and expiratory muscles, respectively; PmusI, PmusE: pressure generated by inspiratory and expiratory muscles, respectively; PEE: elastic recoil pressure of respiratory system at end-expiration (zero at FRC, and positive and negative at volume above and below FRC, respectively); Ers: respiratory system elastance; Rrs: respiratory system resistance; ΔV: volume above end-expiratory lung volume; VT: tidal volume; VI: inspired volume; blue areas: RCOI, EAI and PmusI; red areas: RCOE, EAE and PmusE; I, PI, E: inspiratory, post-inspiratory and expiratory phases, respectively; black double edges vertical arrow: VT; blue and red dashed double edges vertical arrows: contribution of inspiratory and expiratory muscle activity to VT