afterload
Afterload is the wall the heart has to push against. It is the resistance and pressure the ventricle must overcome to open its outlet valve and force blood out — like the effort needed to push a door open against a strong spring.
For the left ventricle, the biggest contributor to afterload is the pressure in the aorta and the resistance of the body's small arteries; high blood pressure and a narrowed (stenotic) aortic valve both raise it. For the right ventricle, the relevant resistance is in the lungs.
When afterload climbs, the ventricle has to work harder for each beat, and stroke volume tends to fall unless the muscle compensates. Sustained high afterload makes the muscle thicken over time, which can eventually impair function.
Lowering afterload is a core treatment strategy: drugs that relax arteries reduce the resistance the heart faces, easing its workload and often improving forward flow in conditions like heart failure and severe hypertension.
In severe aortic stenosis the narrowed valve sharply raises afterload, so the left ventricle must generate very high pressures to push the same blood through a smaller opening.
A stenotic aortic valve is a classic cause of high left-ventricular afterload.
Afterload and preload are different loads: preload is the stretch from filling before contraction, afterload is the load the heart pushes against during ejection. Don't confuse the two.