The circulatory system in an animal basically consists of the blood, heart,
and blood vessels. (It's a little different in plants.) A "closed
circulatory system" is one in which the blood stays inside of blood
vessels (like veins, arteries, capillaries, etc.) at all times - i.e. the
blood never comes into direct contact with the rest of the cells in the
Ok, so if the blood never leaves, then how does all the important stuff like
oxygen and nutrients get to the rest of the body's tissues? Well, just
because the blood cells and plasma and stuff never leave doesn't mean that
the oxygen and nutrients can't. They go right out of the blood, through the
walls of the capillaries and into the other cells that need them. (This is
possible because capillaries are really tiny and their walls are very thin.
Also, capillaries are very close together, so the oxygen and stuff doesn't
have to go very far to get to the cells that need them.)
All vertebrate animals (animals with spines) and a few invertebrates
("echinoderms" - starfish, sand dollars, sea urchins, and sea
cucumbers) have closed circulatory systems. In contrast, most invertebrate
animals have what's called "open circulatory systems". This means
that the blood is not always held inside vessels like capillaries. Instead,
it goes through a only few blood vessels and then is basically pumped right
into the body cavity, making use of a special organ called a "
hemocoel". Without blood vessels to guide the blood along, it tends to
be pretty sluggish about getting back to the heart.
For more information on the difference between a "closed circulatory
system" and an "open circulatory system" check out these
There is another phrase that's used a lot in science that you should be
careful not to confuse with this one. A "closed system" with a
pump is one in which all the fluid coming out of the pump eventually comes
back to the input of the pump. That is, none is lost along the way
somewhere, and none is created anywhere.
Animals have circulatory systems with hearts as pumps. The circulatory
system is not quite a "closed system" by this definition -- new
blood cells and plasma are added continuously and old blood cells and fluids
are cleaned out of the bloodstream by the bone marrow and kidneys,
respectively. Water and nutrients are absorbed by the digestive tract and
gases, water and nutrients are exchanged with the lungs and cells. Plant
circulatory systems are also "open systems" since they exchange
fluids and nutrients with the soil.
The coolant and oil circulation systems in cars are much closer to
"closed systems" than those of living beings, but no one really
refers to them as "circulatory systems."
Tom & Tamara
(republished on 07/18/06)