How can the pressure during contraction vary between the left and right ventricles?
The key is vascular resistance.
Pumps like the heart don't move fluid, they exert force (&quot;push&quot;) on it, increasing the pressure. The movement happens subsequently because there is somewhere for the fluid to go, so it moves down the pressure gradient from a place of high pressure to a place of lower pressure. If you clamped their outputs (increased resistance to infinity), neither ventricle would push any blood out.
We can say that the flow rate is proportional to the pressure difference and resistance:
Flow ~ ${\Delta}$P/R
If we assume that the two ventricles push out over the same time frame, you could easily get equal &quot;Flow&quot; with different &quot;${\Delta}$P&quot; if RLungs is different from RBody, and this is exactly the case: pulmonary vascular resistance is much lower than systemic vascular resistance. It's easier to pump the same volume through the lungs than through the rest of the body, so less pressure is needed.