Think of the blood as electric current, it is a viscous fluid:

V = IR

is analogous to

P = Q * Resistance

Hypothetical:

So as your going through a circuit. You go through a slightly resistive wire the whole time... and at certain points you also hit resistor 1 2 then 3. There is a drop in voltage equal to V=IR at each resistor right? When you go through a wire, before an after these resistors, there is also a small loss of voltage to the wire itself. The whole LENGTH of the thin wire you are slowly losing voltage to the wire itself whether you hit the "resistors" or not. This loss is dependent on the resistivity of the wire which depends on its cross sectional area and a few other factors multiplied by the length of the this stuff your travel through. The "resistors" you pass can be viewed as specially coiled wires of specific length and radius... packaged into a "black box" that drops a specific (usually relatively high compared to circuit's wire) "resistance". So really the resistors we spoke of before are no different than the wire we are traversing along the way, but they are packaged into a cool black box.

So how does this relate to blood?

As you travel through the body's "wires", the blood loses its "voltage" (pressure) every bit of length you travel just like the charges passing through a circuit lose voltage. The voltage after a specific resistor is the original voltage minus that of the voltage loss over the resistor right? You can never get more voltage in the middle of a circuit unless you have a battery (an electron "pump") which is equivalent to a heart. The voltage of a circuit is analogous to the pressure for a circ. system.

The pressure when blood exits the heart is the "battery voltage". The voltage you lose over the wire is the pressure loss over the length of the vein/artery or even a system like all the capillaries (a resistor of sorts if you collect them and consider the system as a block box). The voltage along the journey of a circuit always goes from high to low as you go... so does the pressure in the circulatory system, THAT IS WHY THE PRESSURE DROPS. You start at the heart with 140mmhg (10Volts) let's say, you go through 10 meters of artery (the wire) then pass through something like the capillary network (a resistor which is really just a collection of wires)... plot the voltage in your head vs distance traveled. This a conceptual tool that can make blood flow seem more intuitive. Just remember that the resistance of the circulatory system obeys rules similar but not mathematically identical to the resistivity of wire.

Hope the analogy helps