Arterial blood pressure is the product of the driving force, the cardiac output, and the opposing force, the peripheral vascular resistance. The cardiac output depends upon the amount pumped per beat, the stroke volume and the number of beats per minute, the heart rate. In turn, the stroke volume is determined by the filling of the heart (the venous return) and the contractility. Probably the most important point to realise here is that arterial blood pressure is a very insensitive indicator of the actual blood volume status of the animal. This is 1) because of all these things that affect ABP and 2) because the physiological response to anything that tends to cause ABP to fall is to put it back to normal!