Main features of circulatory systems.
Body fluids
The fluid compartments of animals consist of intracellular
and extracellular components. The intracellular component
includes the body cells and, where present, the blood
cells, while the extracellular component includes the
tissue fluid, coelomic fluid, and blood plasma. In all
cases the major constituent is water derived from the
environment. The composition of the fluid varies markedly
depending on its source and is regulated more or less
precisely by homeostasis.
Blood and coelomic fluid are often physically separated
by the blood-vessel walls; where a hemocoel (a blood-containing
body cavity) exists, however, blood rather than coelomic
fluid occupies the cavity. The composition of blood
may vary from what is little more than the environmental
water containing small amounts of dissolved nutrients
and gases to the highlycomplex tissue containing many
cells of different types found in mammals.
Lymph essentially consists of blood plasma that has
left the blood vessels and has passed through the tissues.
It is generally considered to have a separate identity
when it is returned to the bloodstream through a series
of vessels independent of the blood vessels and the
coelomic space. Coelomic fluid itself may circulate
in the body cavity. In most cases this circulation has
an apparently random nature, mainly because of movements
of the body and organs. In some phyla, however, the
coelomic fluid has a more important role in internal
distribution and is circulated by ciliary tracts.
