The way to the heart
An important collection point for blood from the abdominal cavity is the portal vein, a vein that brings oxygen-poor but nutrient-rich blood from the abdominal organs to the liver – the central metabolic organ.
However, not all veins carry “used”, i.e. oxygen-poor, blood. The exception is the four pulmonary veins, which bring blood that has been oxygenated in the lungs back to the heart (to the left atrium).
Vein structure
Veins have about the same circumference as arteries, but a thinner wall (because there is less pressure in them) and therefore a larger lumen. Unlike arteries, they have only a thin layer of muscle in their middle wall layer (media or tunica media). Another difference from arteries: Many veins have valves built into them (see below).
Superficial and deep veins
Deep veins run in deeper tissue layers of the body, mostly surrounded by muscles. They contain most of the blood volume of the venous system (about 90 percent) and transport the blood from the muscles back to the heart. Superficial and deep veins are in contact with each other through connecting veins.
Veins store a lot of blood
Impeded blood transport
The low internal pressure in venous vessels and the slow blood flow make it difficult to transport blood back to the heart. Particularly when standing, venous blood must be transported upward from below against the force of gravity. To do this, it needs support.
Venous valves
Muscle pump
In addition to the valve system, the skeletal muscles around the veins support their work – but only when we move. When we sit or stand for a long time, the muscle pump in the legs is hardly active. Then the legs can swell and feel heavy.
