Varicose veins are veins that have become enlarged and twisted, and this most commonly happens in the veins of the leg.
How do they form? Well - the arterial circulation, going away from the heart, is a high pressure system, meaning the movement of blood is dependent on high pressures that essentially push it through the arteries, on the order of 120 mmHg.
The venous circulation going back to the heart, on the other hand, is a low-pressure system with a central venous pressure around 5mm Hg.
This means that the movement of blood has to rely heavily on what’s called the skeletal muscle pump, which is just a way of saying that it relies on the contraction of surrounding skeletal muscles, which compresses the vein and propels blood through the vessels.
But let’s think about the veins in the legs when you’re standing, now to get to the heart the blood has to go up, right, which is working against gravity.
So if your calf muscles contract and squeeze the blood inside, some blood gets propelled downward, while some gets propelled upward, but then gravity pushes that upward-moving blood back down, and it doesn’t seem like much gets accomplished...and it wouldn’t, but that’s not the whole story—most veins also have one-way valves.
These valves only let blood move in one direction, toward the heart.
So now, as the skeletal muscles contract, it squeezes the veins, and this lower valve stays closed to prevent blood from going downward, while the upper valve lets blood through, but even though gravity wants to push it back down, that blood isn’t allowed to fall back down through the upper valve, right?
For some people, the downward gravitational pull on blood causes the walls of the leg veins to stretch apart over time, which tends to also pull apart those valves.
If these valves fail to close properly, they can allow blood to leak backward and pool in the veins, which can lead to more valves stretching out and failing.
The veins have now become varicose veins, and from this additional blood, they start becoming tortuous, or twisted.