Laplace law explains why the pressure is the same in aorta and arteries (100). I am thinking why there is so big drop in pressure when going from arteries to arterioles (40-60).
Why there is a great drop in pressure from arteries to arterioles?
While correct, @AndroidPenguin 's answer is only part of the explanation. By far the main reason of the pressure difference is that the peripheral circulatory system has two main compartments:
The reason for the abrupt pressure fall from arterioles onward is precisely that this is the main function of the arterioles: they contract and dilate to adjust capillary pressure. Since a good part of capillary function depends on exchanges along oncotic pressure gradients, hydrostatic pressure cannot be allowed to stay too high in the capillary compartment. So in conclusion, @AndroidPenguin gave you the physical reason, but mine is the physiological reason.
The vessel diameter is the most principal determinant to control resistance. Compared to other smaller vessels in the body, the artery has a much bigger diameter (4 mm), therefore the resistance is low.
In addition, flow rate (Q) is also the product of the cross-sectional area of the vessel and the average velocity (Q=AV). Flow rate is directly proportional to the pressure drop in a tube or in this case a vessels, although there are other factors of blood vessels that contribute towards the difference in pressure drop in bifurcations of blood vessels. These include viscosity, length of the vessel, and radius of the vessel.
Factors that determine the flow's resistance as described by Poiseuille’s relationship: ∆P=8µlQ/πr4:
∆P: Pressure drop/gradient µ: Viscosity l: length of tube. In the case of vessels with infinitely long lengths, l is replaced with diameter of the vessel. Q: flow rate of the blood in the vessel r: radius of the vessel
Assuming steady, laminar flow in the vessel, the blood vessels behavior is similar to that of a pipe. For instance if p1 and p2 are pressures are at the ends of the tube, the pressure drop/gradient is (p1−p2)/l =∆P.
In the arterioles blood pressure is lower than in the major arteries. This is because velocity of flow is increased with decrease in diameter and vice versa.
This is important as for efficient exchange at the capillary bed pressure and flow need to be much lower.