19. D. Lymphatic system is composed of all of the following
except the vena cavae.
Vena cavae are the structures into which one of the two
lymphatic ducts empty its contents. So, it is not part of the
20. E. The lymphocytes are most important in fighting infection.
Lymphocytes are of two types B and T cells. Both are part of the
immune system and help in producing antibodies that fight the
21. A, B, C, D. Compared to blood capillaries, lymph capillaries
exhibit all of the following except that they have no basement
membrane, have walls of endothelial cells that overlap like
shingles, and are smaller in diameter.
22. A. Lymphatic vessels are located in almost all portions of
the body except the central nervous system. Lymphatic vessels are
present all over the body except in the bone marrow and central
23. A, B. Most of the lymph returns to the venous circulation by
way of the right lymphatic duct and thoracic duct. There are two
ducts by which lymphatic vessels empty the contents. They empty the
contents into right and left subclavain veins respectively.