Editing Circulatory system (section)

==Other animals==
[[File:Open Circulatroy.gif|thumb|The open circulatory system of the grasshopper – made up of a heart, vessels and hemolymph. The hemolymph is pumped through the heart, into the aorta, dispersed into the head and throughout the hemocoel, then back through the ostia in the heart and the process repeated.]]

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While humans, as well as other [[vertebrates]], have a closed blood circulatory system (meaning that the blood never leaves the network of arteries, veins and capillaries), some [[invertebrate]] groups have an open circulatory system containing a heart but limited blood vessels. The most primitive, [[diploblastic]] animal [[phylum|phyla]] lack circulatory systems.

An additional transport system, the lymphatic system, which is only found in animals with a closed blood circulation,  is an open system providing an accessory route for excess interstitial fluid to be returned to the blood.<ref name="Sherwood, Lauralee 2011 401"/>

The blood vascular system first appeared probably in an ancestor of the [[triploblasts]] over 600 million years ago, overcoming the time-distance constraints of diffusion, while [[endothelium]] evolved in an ancestral vertebrate some 540–510 million years ago.<ref>{{cite journal|doi=10.1111/jth.12253|pmid=23809110|pmc=5378490|title=Evolutionary origins of the blood vascular system and endothelium|year=2013|last1=Monahan-Earley|first1=R.|last2=Dvorak|first2=A. M.|last3=Aird|first3=W. C.|journal=Journal of Thrombosis and Haemostasis|volume=11|issue=Suppl 1 |pages=46–66}}</ref>

===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.]]

===Closed circulatory system===

[[File:Two chamber heart.svg |thumb |left |Two-chambered heart of a fish]]

The circulatory systems of all vertebrates, as well as of [[annelid]]s (for example, [[earthworm]]s) and [[cephalopod]]s ([[squid]]s, [[octopus]]es and relatives) always keep their circulating blood enclosed within heart chambers or blood vessels and are classified as ''closed'', just as in humans. Still, the systems of [[fish]], [[amphibian]]s, [[reptile]]s, and [[bird]]s show various stages of the [[evolution]] of the circulatory system.<ref>{{cite journal |doi=10.1016/j.ydbio.2004.09.026 |pmid=15572135 |title=The evolutionary origin of cardiac chambers |year=2005 |last1=Simões-Costa |first1=Marcos S. |last2=Vasconcelos |first2=Michelle |last3=Sampaio |first3=Allysson C. |last4=Cravo |first4=Roberta M. |last5=Linhares |first5=Vania L. |last6=Hochgreb |first6=Tatiana |last7=Yan |first7=Chao Y.I. |last8=Davidson |first8=Brad |last9=Xavier-Neto |first9=José |journal=Developmental Biology |volume=277 |issue=1 |pages=1–15 |doi-access=}}</ref> Closed systems permit blood to be directed to the organs that require it.

In fish, the system has only one circuit, with the blood being pumped through the capillaries of the [[gill]]s and on to the capillaries of the body tissues. This is known as ''single cycle'' circulation. The heart of fish is, therefore, only a single pump (consisting of two chambers).{{cn |date=January 2025}}

In amphibians and most reptiles, a double circulatory system is used, but the heart is not always completely separated into two pumps. Amphibians have a three-chambered heart.{{cn |date=January 2025}}

In reptiles, the [[ventricular septum]] of the heart is incomplete and the [[pulmonary artery]] is equipped with a [[sphincter muscle]]. This allows a second possible route of blood flow. Instead of blood flowing through the pulmonary artery to the lungs, the sphincter may be contracted to divert this blood flow through the incomplete ventricular septum into the left ventricle and out through the [[aorta]]. This means the blood flows from the capillaries to the heart and back to the capillaries instead of to the lungs. This process is useful to [[ectothermic]] (cold-blooded) animals in the regulation of their body temperature.{{cn |date=January 2025}}

Mammals, birds and [[crocodilia]]ns show complete separation of the heart into two pumps, for a total of four heart chambers; it is thought that the four-chambered heart of birds and crocodilians evolved independently from that of mammals.<ref>{{cite web | title=Crocodilian Hearts | date=October 24, 2008 | access-date=October 3, 2015 | website=National Center for Science Education | url=https://ncse.com/creationism/analysis/crocodilian-hearts | archive-date=September 26, 2015 | archive-url=https://web.archive.org/web/20150926104854/http://ncse.com/creationism/analysis/crocodilian-hearts | url-status=live }}</ref> Double circulatory systems permit blood to be repressurized after returning from the lungs, speeding up delivery of oxygen to tissues.{{cn |date=January 2025}}

===No circulatory system===
Circulatory systems are absent in some animals, including [[flatworm]]s. Their [[body cavity]] has no lining or enclosed fluid. Instead, a muscular [[pharynx]] leads to an extensively branched [[digestive system]] that facilitates direct [[diffusion]] of nutrients to all cells. The flatworm's dorso-ventrally flattened body shape also restricts the distance of any cell from the digestive system or the exterior of the organism. [[Oxygen]] can diffuse from the surrounding water into the cells, and carbon dioxide can diffuse out. Consequently, every cell is able to obtain nutrients, water and oxygen without the need of a transport system.

Some animals, such as [[jellyfish]], have more extensive branching from their [[gastrovascular cavity]] (which functions as both a place of digestion and a form of circulation), this branching allows for bodily fluids to reach the outer layers, since the digestion begins in the inner layers.