Final answer:
Afterload is the resistance that the heart's ventricles must overcome to eject blood. It's affected by arterial pressure, blood viscosity, and valve conditions. A higher afterload means greater resistance and more work for the heart.
Step-by-step explanation:
The afterload is a term used in cardiovascular physiology to refer to the level of resistance that the ventricles of the heart must overcome to successfully eject blood during systole. When the ventricles contract, they must generate enough pressure to open the semilunar valves the aortic and pulmonary valves and push the blood out into the arteries.
The resistance encountered is influenced by factors such as arterial pressure, blood viscosity, and the condition of the valves. Any increase in this resistance results in a higher afterload, making the heart work harder to pump blood. Conversely, a decrease in resistance will lead to a lower afterload, easing the heart's workload.