Hence, it has been suggested that the atria be considered endocrine organs containing endocrine fibers. These fibers have functional properties also seen in peptide- secreting cells of the gastrointestinal and respiratory tracts. Blood vessels that originate from the right and left ventricles are designated as arteries and have a distinctive structure. The exact structure and relative thickness of the three coats vary with the size of the artery. Be aware that there are regional structural variations.
Blood pressure falls sharply, and blood flow slows in arterioles.
Capillaries, because of their intimate relationship with the cells of the body and their special permeability characteristics, are functionally the most interesting of the blood vessels. Their thin walls and slow blood flow favor the exchange of nutrients and oxygen for metabolic wastes and carbon dioxide. In addition, hormones from endocrine glands enter and leave the vascular system through regionally specialized capillaries. The endothelium does not have fenestrae or open channels. This most common type is found in connective, muscle, nervous, and endocrine tissues.