Editing Octopus (section)

===Circulatory system===
Octopuses have a closed [[circulatory system]], in which the blood remains inside blood vessels. They have three hearts; a systemic or main heart that circulates blood around the body and two branchial or gill hearts that pump it through the two gills. The systemic heart becomes inactive when the animal is swimming. Thus, the octopus loses energy quickly and mostly crawls.<ref name=Wells/>{{rp|19–20, 31–35}}<ref name=Courage/>{{rp|42–43}} Octopus blood contains the [[copper]]-rich protein [[haemocyanin]] to transport oxygen. This makes the blood [[viscosity|viscous]] and it requires great pressure to pump it around the body; [[blood pressure]]s can surpass {{convert|75|mmHg|kPa|-1|abbr=on}}.<ref name=Wells/>{{rp|31–35}}<ref name=Courage/>{{rp|42–43}}<ref name="Schmidt">{{cite book |last=Schmidt-Nielsen |first=Knut |author-link=Knut Schmidt-Nielsen |year=1997 |title=Animal Physiology: Adaptation and Environment |publisher=Cambridge University Press |page=117 |isbn=978-0-521-57098-5}}</ref> In cold conditions with low oxygen levels, haemocyanin transports oxygen more efficiently than [[Hemoglobin|haemoglobin]].<ref name=Oellerman/> The haemocyanin is dissolved in the [[blood plasma]] instead of carried within blood cells and gives the blood a bluish colour.<ref name=Wells/>{{rp|31–35}}<ref name=Courage/>{{rp|42–43}}<ref name=Mather/>{{rp|22}}

The systemic heart has muscular contractile walls and consists of a single [[Ventricle (heart)|ventricle]] and two [[Atrium (heart)|atria]], which attach it to each of the two gills. The blood vessels consist of arteries, capillaries and veins and are lined with a cellular [[endothelium]] unlike that of most other [[invertebrate]]s. The blood circulates through the [[aorta]] and capillary system, to the [[Venae cavae|''venae cavae'']], after which the blood is pumped through the gills by the branchial hearts and back to the main heart. Much of the venous system is contractile, which helps circulate the blood.<ref name=Ruppert/>{{rp|358}}