What are veins?
All blood vessels in the body, the arteries, veins and capillaries, have the same basic structure. Each have three layers in their walls:
- Intima – The innermost layer
- Media – the middle layer that is variable thickness and made up of smooth-muscle cells
- Adventitia – the outer layer that supplies nutrients to the living cells of the vessel wall
It is the job of the intima to support the endothelial cells and to provide a structure for the endothelial cells to hold onto and to obtain nutrition from.
The intima and adventitia are very similar in all blood vessels – the difference between the vessels is mainly the thickness of the media layer.
It is the colour of the blood within the arteries and veins that give them their traditional colours. Most arteries in the body carry oxygenated blood which is bright red and most veins carry deoxygenated blood which is darker and more blue.
The circulation is split into two parts – the circulation of the body, legs, arms and head which is called the “systemic” circulation and the circulation of the lungs which is called the “pulmonary” circulation.
For the rest of this website we are only going to talk about the systemic circulation. If you are interested about the circulatory system and want to know in more detail, please see The College of Phlebology’s textbook “Understanding Venous Reflux – the cause of varicose veins and venous leg ulcers“.