Blood vessels run through the entire human body as proverbial lifelines. Two different types of vessels are distinguished, namely arteries and veins. See also: blood circulation.
What are veins?
Veins are vessels that carry blood to the heart, as opposed to arteries, which carry it to the periphery. There is less pressure inside veins than in arteries. The supposed definition according to which oxygen-rich blood flows in the arteries and oxygen-poor blood in the veins only applies to the systemic circulation, but not to the pulmonary circulation. The vena cavae of the heart are about 2 cm in diameter, whereas most peripheral veins are much smaller, down to tiny venules about 15 μm across.
Anatomy and structure
The wall of a vein is usually thinner than that of an artery of the same size because of lower internal pressure and is composed of three layers: The inner layer is called the tunica intima. It consists of single-layer squamous epithelium, i.e., a bandage of flat mucosal cells overlying a basement membrane. This layer forms the venous valves in many veins remote from the heart. Below this is the middle layer or tunica media, which is composed mainly of bundles of smooth muscle arranged in a ring or spiral pattern. This muscular layer is weaker in veins than in arteries. The outer tunica adventitia is a connective tissue layer that fixes the vein in its environment. Within it run nerves and, in very large veins, smaller blood vessels or vasa vasorum, which serve to nourish the large vessels.
Functions and tasks
The heartbeat that drives blood through the arteries has little influence after the blood in the body’s periphery has passed through the tiny capillaries with their high vascular resistance and finally enters the veins. Blood transport in the veins is made possible by several other mechanisms, collectively called the venous pump: Veins that run near a muscle are compressed when the muscle contracts, allowing blood to be transported further. Many veins attach themselves as companion veins to an artery, the pulse waves of which also cause the veins to be compressed and thus the blood to flow forward. Blood flow in the veins near the heart is also influenced by the respiratory pressure changes in the thoracic cavity. In the abdominal cavity, intestinal peristalsis takes over this task. Since all these factors are based on compression of the veins, the blood could theoretically also be pushed in the wrong direction – this is prevented by the venous valves. These pocket valves act as directional valves and open only when blood flows through them in the right direction – toward the heart – while blocking backflow in the other direction. They thus counteract gravity and are therefore particularly numerous in the veins of the arms and legs. Furthermore, veins serve as capacity vessels, i.e. they can absorb and store large quantities of blood by expanding their elastic walls. When needed, they release this blood to regulate circulation.
Diseases
A common vein condition is varicosis, or varicose veins. It results from congenital or acquired connective tissue weakness and/or venous valve insufficiency and manifests as blood stasis with venous dilation. Depending on the localization, a distinction is made between spider veins (skin veins), reticular varicosis (subcutaneous veins) and truncal varicosis (deep veins). In mild cases, varicose veins are only a cosmetic problem, but swelling, cramps or rupture may also occur. Varices can be sclerosed, treated with laser beams, or surgically removed. Congested veins can lead to venous edema over time, which is an accumulation of water in the surrounding tissue. Another possible late consequence is ulcus cruris or lower leg ulcer. A particularly dangerous venous disease is thrombosis, in which a detached blood clot blocks a vein. If the thrombus reaches the lungs, a life-threatening pulmonary embolism occurs. In addition, veins can become inflamed for various reasons – this is called phlebitis. If it is accompanied by thrombus formation, it is usually benign thrombophlebitis (superficial veins) or a more dangerous phlebothrombosis (deep veins).
Typical and common conditions
- Thrombosis
- Varicose veins
- Weakness of the veins (venous disease)
- Vein inflammation
