VEIN ANATOMY

The vein wall consists of three layers: The tunica intima, the tunica media and the tunica adventitia.

The innermost layer of the vein is the tunica intima. This layer consists of flat epithelial cells. These cells allow fluid to flow smoothly and are interspersed with valves that ensure the flow continues in one direction. This continuous layer of epithelial cells holds cells and fluid within the vessel lumen. Any trauma to the tunica intima can initiate an inflammatory response leading to platelet aggregation and thrombosis (11,12).

The middle layer, the tunica media, is the thickest part of the wall. It consists of mostly smooth muscle and is innervated by the sympathetic nervous system. This sympathetic nervous system is what causes venospasms following changes in temperature or irritation within the vein (11,12).

The tunica adventitia is the outermost layer and consists of connective tissue and elastic fibers that provide the vessel's strength. In large veins, this may be the thickest layer. The tunica adventitia contains sympathetic nerves and capillaries that supply blood to the vessel wall (11,12).

Understanding what a healthy vein looks like and how it functions are fundamental to placing central venous catheters. An ultrasound scan and assessment of the veins takes place prior to insertion of a catheter to select the site that offers the best/healthiest veins allowing for the most positive patient outcomes. When performing the scan you must fully comprehend what a healthy vein looks like on the ultrasound screen in order to select the best access site (11).