Editing Circulatory system (section)

===Open circulatory system===
{{See also|Hemolymph}}
In [[arthropod]]s, the open circulatory system is a system in which a fluid in a [[body cavity|cavity]] called the '''hemocoel''' or '''haemocoel''' bathes the organs directly with oxygen and nutrients, with there being no distinction between blood and interstitial fluid; this combined fluid is called [[hemolymph]] or haemolymph.<ref>{{cite web|last=Bailey|first=Regina|title=Circulatory System|url=https://biology.about.com/od/organsystems/a/circulatorysystem.htm|work=biology.about.com|access-date=2022-02-23|archive-date=2016-11-29|archive-url=https://web.archive.org/web/20161129050935/http://biology.about.com/od/organsystems/a/circulatorysystem.htm|url-status=live}}</ref> Muscular movements by the animal during [[Animal locomotion|locomotion]] can facilitate hemolymph movement, but diverting flow from one area to another is limited. When the heart relaxes, blood is drawn back toward the heart through open-ended pores (ostia).

Hemolymph fills all of the interior hemocoel of the body and surrounds all [[Cell (biology)|cells]]. Hemolymph is composed of [[water]], [[Inorganic chemistry|inorganic]] [[Salt (chemistry)|salts]] (mostly [[sodium]], [[chloride]], [[potassium]], [[magnesium]], and [[calcium]]), and [[Organic chemistry|organic compounds]] (mostly carbohydrates, [[protein]]s, and [[lipid]]s). The primary oxygen transporter molecule is [[hemocyanin]].

There are free-floating cells, the [[Hemocyte (invertebrate immune system cell)|hemocytes]], within the hemolymph. They play a role in the arthropod [[immune system]].

[[File:Pseudoceros liparus - Blue Pseudoceros Flatworm.jpg|thumb|left|Flatworms, such as this ''[[Pseudoceros bifurcus]]'', lack specialized circulatory organs.]]