Since vertebrates are large and multicellular, they must have a means of shuttling materials from one part of the animal to another. If this system involves the movement of fluid through tubes, then it can be considered a circulatory system . Vertebrates have a closed circulatory system, where blood is always surrounded by blood vessels. The vessels of the circulatory system vary. As you saw in the chicken wing, arteries have thicker, more muscular walls than veins.
Veins also have valves to prevent the backflow of blood.
The biggest pressure drop, and hence the greatest point of control of blood pressure or blood flow to different organs, is across the arterioles. In addition to these functions, the blood is also the site of gas exchange. Regardless of the site, a thin epithelial tissue and many capillaries are involved. Gas exchange occurs by diffusion through moist surfaces. The skin of an amphibian is moist and has many capillaries underneath.