Thymus
The thymus is found in the upper chest. Chemicals made by the thymus help the production
of certain infection-fighting cells. The thymus is where certain white blood cells called
lymphocytesmature. Thesecellsmovefromthebonemarrowtothethymustofinishgrowing.
The thymus grows to its largest size near puberty, and gets smaller as a person ages. If a
person’s thymus is surgically removed or damaged by disease while they are young, the
person will be very prone to infections.Spleen
The spleen is in the abdomen, as shown inFigure18.10. In an area of the spleen called
red pulp, materials are filtered from the blood, including old and dead red blood cells. The
spleen also makes red blood cells. Areas calledwhite pulphelp fight infections by making
white blood cells. If a person’s spleen is surgically removed, or does not work properly, the
person is prone to certain infections.You can learn more about the roles of the lymphatic system and white blood cells in the
Diseases and the Body’s Defenseschapter.Table 18.1: Structures and Functions of the Cardiovascular and Lymphatic Sys-
tems
System Structure (organs and
tissues)FunctionLymphatic Lymph vessels Transports fluid (lymph)
from between body cells
back to blood
Lymph nodes Traps invading microbes,
foreign particles, cancerous
cells
Spleen, tonsils, andadenoids Trapsinvadingmicrobesand
foreign particles
Thymus Siteofwhitebloodcell(lym-
phocyte)maturation
Cardiovascular Blood vessels Transportsbloodaroundthe
body
Blood Transport of oxygen and
nutrients; transports white
blood cells to sites of infec-
tion and inflammation
Heart Pumps blood around the
body