Professor Victoria Claydon, Simon Fraser University, Burnaby, BC answered on May 9, 2008, A:If you found this answer useful, please consider making a small donation to science.ca.
The systolic blood pressure largely represents the maximum force generated by the heart, whereas the diastolic pressure is a better indicator of the resistance (degree of constriction) of the peripheral blood vessels, especially the arterioles. The mean arterial pressure is, as its name suggests, an amalgamation of the two, but because the heart spends more time in diastole than in systole, the mean pressure is biased towards the diastolic pressure [mean pressure = diastolic pressure + 1/3 x (systolic pressure – diastolic pressure)]. Looking at all three pressures gives the most comprehensive assessment of blood pressure changes and can indicate whether the changes in blood pressure are due to changes in resistance, pumping force, or both. Furthermore, the baroreceptors (regions that detect changes in blood pressure and bring about reflex responses to control it) are differentially sensitive to systolic, diastolic and mean blood pressures.