The y axis is the pressure given in millimeters of mercury and the x axis basically are the different types of blood vessels. So if we look at our largest artery array order and the smaller ones are arteries this is where our pressure is at a highest value. Now, when we go from the arteries to the tiniest arteries called arterioles we have a very sharp drop in blood pressure as shown by the slope of this line. The question is why? Why is there a drop in blood pressure when we go from the arteries to the arterios and what is the benefit of that to our body? So the reason there is a drop in blood pressure is because our arterios have a relatively high area to volume ratio.