No distinct elastic laminae are seen, but sparse elastic networks are found throughout the wall. 6 Comparison between a vein and its companion muscular artery Both are tubes lined by endothelium and may contain RBCs. Veins in general are very variable in their structure. 2 Cerebral arteries are thin walled and have no external elastic layer. 4 Arterial intimal cushions are present in arteries to erectile tissue, kidneys, etc.
Some vessels have a high protruding endothelium, e.g., fetal stem arteries.
Arterial anastomoses , e.g., circle of Willis to the brain. In the periphery, arteries and veins run together with nerves bound in CT as a neurovascular bundle . In the brain, arterial and venous distributions are separate. Vasa vasorum are blood vessels serving the adventitia and media of larger arteries and veins. Portal systems exist to the liver and pituitary gland, where venous blood drained from one organ is fed as a supply to the sinusoids or capillaries of another.