Intravenous cannulation is a technique in which a cannula is placed inside a vein to provide venous access. Venous access allows sampling of blood as well as administration of fluids, medications, parenteral nutrition, chemotherapy, and blood products. Veins have a three-layered wall composed of an internal endothelium surrounded by a thin layer of muscle fibers that is surrounded by a layer of connective tissue. Venous valves encourage unidirectional flow of blood and prevent pooling of blood in the dependent portions of the extremities they also can impede the passage of a catheter through and into a vein. Venous valves are more numerous just distal to the points were tributaries join larger veins and in the lower extremities.