Final answer:
Increased contractility leads to a more forceful cardiac muscle contraction, resulting in a higher stroke volume, decreased end-systolic volume, and a steeper end-systolic pressure-volume relationship on the pressure-volume loop, indicating improved cardiac output.
Step-by-step explanation:
The question concerns the effect of increased contractility on the pressure-volume loop, which is a graphical representation of the cardiac cycle. In the setting of increased contractility, the heart muscle contracts more forcefully. This heightened contractile force leads to an increase in stroke volume (SV), which is the amount of blood ejected with each heartbeat. Consequently, with stronger contractions, the end-systolic volume (ESV) decreases, because more blood is being pumped out of the heart. In the pressure-volume loop, increased contractility will manifest as a steeper slope of the end-systolic pressure-volume relationship (ESPVR), indicating that for any given end-diastolic volume (EDV), the heart can generate a higher pressure, reflecting increased SV and improved cardiac output. Increased contractility is often mediated through sympathetic nervous system stimulation releasing epinephrine and norepinephrine, which are positive inotropic factors. These factors enhance the heart's pumping efficiency by increasing the strength of cardiac muscle contraction without necessarily increasing heart size or preload. In summary, an increase in contractility will typically result in improved cardiac performance, with a more pronounced ejection phase and a reduced ESV.