###### Created: 2024-01-13 14:54 ###### Areas & Topics: #medicine #anatomy ###### Note Type: #permanent ###### Connected to: [[Arterioles]] - Veins are blood vessels which carry deoxygenated blood from capillaries to the heart and lungs, where the blood can then be re-oxygenated and recirculated via the arteries - The exception to this is the pulmonary veins which carry oxygenated blood to the heart - Veins come in a variety of sizes, with the smaller veins being known as venules - The smallest form of veins are the post-capillary venules, which are formed immediately following [[Capillaries]]. ### Structure - Veins in general have thinner walls and wider lumens than arteries. - This allows them to hold more blood and expand more easily (which leads to them being known as capacitance vessels) - Veins generally are made up of three layers, similar to [[Arteries]]. - These layers are the tunica externa (tunica adventitia), tunica media and tunica intima. - The tunica externa is the outermost layer made up of thick connective tissue. - The tunica media is the middle layer made up of bands of smooth muscle. - The tunica intima is the innermost layer made up of a lining of endothelial cells. - The main difference between arteries and veins is that the veins do not experience the same levels of high blood pressure, meaning the muscle in their tunica media is significantly thinner and they contain valves to prevent backflow of blood (since the pressure is not high enough to continuously push blood forward). ![[Figure_40_03_07.jpg]] ### Function - Since veins are low pressure, the primary mechanisms of moving blood through them come from muscle pumps (also known as venous pumps), gravity and valves. - The contraction of muscle in the body (such as from skeletal muscle, smooth muscle or contraction of the diaphragm) presses on veins, pushing the blood forward towards the heart and lungs. - Additionally, the valves found inside of valves prevent blood flowing backwards as a way to compensate for the lack of high pressure. - Movement of the body and gravity can also assist in allowing venous blood to travel forward effectively. ![[Pasted image 20240113154046.png]] ### Types of Vein - Superficial veins are those found closer to the surface of the skin. - Deep veins are those found further from the surface of the skin. - Perforator veins are those which connect superficial and deep veins. - A portal venous system is where a series of veins or venules directly connect two capillary beds. - A vascular shunt is where a vein or venule bypasses a capillary bed completely to connect to another vein or venule (via the use of metarterioles which connect to hundreds of capillaries) ### Resources Essentials of Anatomy Handbook https://en.wikipedia.org/wiki/Vein#Function A venous plexus is