<P> The renal circulation receives around 20% of the cardiac output . It branches from the abdominal aorta and returns blood to the ascending vena cava . It is the blood supply to the kidneys, and contains many specialized blood vessels . </P> <P> The lymphatic system is part of the circulatory system . It is a network of lymphatic vessels and lymph capillaries, lymph nodes and organs, and lymphatic tissues and circulating lymph . One of its major functions is to carry the lymph, draining and returning interstitial fluid back towards the heart for return to the cardiovascular system, by emptying into the lymphatic ducts . Its other main function is in the adaptive immune system . </P> <P> The development of the circulatory system starts with vasculogenesis in the embryo . The human arterial and venous systems develop from different areas in the embryo . The arterial system develops mainly from the aortic arches, six pairs of arches which develop on the upper part of the embryo . The venous system arises from three bilateral veins during weeks 4--8 of embryogenesis . Fetal circulation begins within the 8th week of development . Fetal circulation does not include the lungs, which are bypassed via the truncus arteriosus . Before birth the fetus obtains oxygen (and nutrients) from the mother through the placenta and the umbilical cord . </P> <P> The human arterial system originates from the aortic arches and from the dorsal aortae starting from week 4 of embryonic life . The first and second aortic arches regress and forms only the maxillary arteries and stapedial arteries respectively . The arterial system itself arises from aortic arches 3, 4 and 6 (aortic arch 5 completely regresses). </P>

Which of the following is not a component of the circulatory system