Final answer:
In the systemic and pulmonary circulations of a healthy individual, the preload and stroke volume are the most similar characteristics because both ventricles fill with and eject similar volumes of blood per beat.
Step-by-step explanation:
The characteristics that are most similar between the systemic and pulmonary circulations of a healthy 22-year-old man are the preload and the stroke volume. Preload refers to the initial stretching of the cardiac myocytes prior to contraction, which is essentially the end-diastolic volume (EDV). Preload is similar in both circulatory systems because each ventricle fills with a nearly identical volume of blood during diastole, assuming there is no heart disease present. Additionally, the stroke volume, which is the volume of blood each ventricle pumps out with each beat, is typically equal for both the right and left ventricles in a healthy heart.
Meanwhile, peak systolic pressure and mean diastolic pressure are generally higher in the systemic circulation due to the greater resistance and distensibility of the systemic vasculature compared to the pulmonary circulation. Afterload, which refers to the pressure the heart must work against to eject blood, is generally lower in the pulmonary circulation for the same reason. For resistance, despite the fact that each ventricle pumps against different pressures, the total peripheral resistance in the systemic circulation is far greater than the resistance in the pulmonary circulation.