A slow heart rate affects how often the heart ejects blood, which affects the amount of blood ejected. If the volume of blood ejected decreases, blood pressure falls. If the volume of blood ejected rises, blood pressure rises.
Heart rate is the number of times the heart beats per minute, usually between 60 and 100 beats per minute, and when it is below 60 beats per minute it is called bradycardia. Blood pressure is the pressure of the blood against the walls of the blood vessels and is related to the amount of circulating blood. Various factors cause the amount of circulating blood to rise, which raises the blood pressure.
When bradycardia occurs, there are two scenarios. In one scenario, bradycardia causes the heart to hold more blood during diastole, which causes systolic ejection to rise, resulting in an increase in systolic blood pressure and an unchanged or lower diastolic blood pressure. The other is that the systolic phase of bradycardia does not eject all of the blood, leaving some of the blood in the heart, and the amount of blood circulating through the blood vessels decreases, resulting in a drop in blood pressure.