I've always understood how and why the aortic pressure contributes to afterload on the LV (higher aortic pressure means a greater pressure that the LV must overcome to eject blood; makes sense)
However, I recently came across the idea that the mean LV transmural pressure also contributes to the afterload... I don't understand why the LV has to 'overcome' this pressure? It's trying to eject blood out into the aorta, for which it needs to generate a certain pressure... why does the LV transmural pressure have anything to do with this?
I know that the the transmural pressure is the difference between the internal chamber pressure and the external (pleural) pressure, (i.e., how much higher the internal LV pressure is compared to the outside); I can also then understand the application of this, that increasing pleural pressure via positive pressure ventilation can reduce the transmural pressure and help the LV push blood out; but I just don't see why the transmural pressure contributes to afterload
Edit: after thinking about this for a little longer, I've tried to reason it with the following idea: the LV is trying to contract inwards to eject blood; to do this, it must push against its own internal pressure; it can either be hindered in this task by outward suction due to negative pleural pressure or helped in this task by positive pressure from outside. If we consider the situation in which the transmural pressure goes up, this can occur either due to more internal pressure (which makes it harder for the LV to contract inwards) or a lower outer pressure (which also makes it harder to contract inwards); hence, increasing LV transmural pressure increases afterload... is this thought process along the right lines?