The heart pumps blood into arteries and pulls blood out of veins. Blood seeps from arteries into veins through huge number of very thin capillaries. Exchange of oxygen, nutrients, etc. between the blood and tissues goes in that area under control of nerves. This works the same way in lungs, muscles, brain, etc.
Arteries are active - they have muscles, which help to move blood through. Veins are passive - they just have “pockets”, which favor movement of the blood in the direction toward the heart.
In different parts of the body, speed of blood transfer through capillaries from the arteries to veins differs from time to time. To compensate for that, veins diameter increases or decreases locally, as needed.
One pump of the heart pumps blood from veins into the lungs, and the other pumps oxygenated blood from lungs into the arteries.
Blood pressure, at a particular point in the arteries or veins, depends on the current location relative to the ground. Pressure in lower points is higher.
When there is imbalance between the heart pumping blood into the arteries and pumping it out of veins, the veins compensate by corresponding expansion, or contraction.
Arteries have muscles, pumping blood through them in concert with actions of the heart. Veins do not have such mechanism. The body relies on skeletal muscles to squeeze veins. Pockets in the veins prevent movement of the blood back, when squeezing is eased. When there is insufficient “squeezing” of veins by skeletal muscles, veins’ diameter could increase to compensate for imbalance between pumping action of the heart into the arteries, and pumping action of the heart out of veins.
When veins are not evacuated quickly enough, pressure in them increases and walls get more permeable. This could cause minor leakage of blood from veins into tissues.
This, in turn, could cause various health problems, like skin ulceration.
To avoid these problems, one should reduce time spent sitting, or standing, and walk more.
Alexander Liss 1/18/2021