A frog's circulatory system has three chambers - one ventricle and two atria. Deoxygenated blood enters one atrium from the body and oxygenated blood enters the other atrium from the lungs. The single ventricle then pumps this mixed blood out through the truncus arteriosus to circulate through the body and lungs before returning to the heart, making it a closed double loop system. Though the blood mixes somewhat, valves help direct the blood flows to minimize mixing and supply the tissues with sufficient oxygen.

1. Frog’s Circulatory
System

3. A Frog's Heart Is Central to Its Circulation
 It has two atria and a single ventricle, making it havea total of three chambers.
Here's howit works.
 Deoxygenated blood is sentto the atrium from various organs in the frog's body
through blood vessels and veins. This is the blood being drained from organs, thus
starting the cleansing process. Oxygenated blood then comes from the lungs and
the skin, and enters the left atrium. (This also serves as a gas exchangein most
amphibians.)
 Both atria dump their blood into the single ventricle. Theventricle is divided into
two narrowly separated chambers, which cleverlyreduces the mixing of
oxygenated and deoxygenated blood together. The ventricle contracts, sending
oxygenated blood tothe brain from the left ventricle. It reaches the head by flowing
through the carotid arteries.
 This is an almost pure blood that reaches the brain.

4.  Theskinandlungsreceive nearlypurebloodfromtherightatriumthrough
pulmocutaneousarteries. Fromtheskinandlungs,freshoxygencanbe
pickedup.Thebloodthatpassesthroughtheaorticarchesisverymixedup,
butstillhasa lotofoxygeninit.Thereisenoughtosupplytherestofthe
bodywhatitneeds.
 Interestingnote:Boththegas exchangeorgansand theinteriortissuesofthe
bodygettheir bloodunderfullpressure.

5.  TheCirculatorysystemofa frog consistsoftheheart,bloodvessels,and
blood.Frogshavea closeddoubleloopcirculatorysystem.
 Aclosedcirculatorysystemmeansthatthefrog’sbloodiscontainedwithin
bloodvesselsandtheheart.Thedoubleloopsystemmeansthattheblood
goesbacktothehearttwotimesbeforethebloodgoesthroughthewhole
systemofthefrog’sbody.
 Thereare threechambers consistingofoneventricleandtwoatria.
 Theventriclepumpsbloodthrougha singleartery(truncusarteriosus)witch
distributesbloodthoughtthebody.
 Oxygenatedbloodcomingbacktotheheartfromlungsenterstheheartinto
differentatria than thede-oxygenatedblooddoescomingfrom thebody
capillaries.

6.  Thevalvehelpstokeepbloodfromonlymixingas minimallyas possible.The
valvedirectstheoxygenatedbloodtotheaorta,whichwillleaveheartandgo
tothebodycapillariesandthedeoxygenatedbloodoutthepulmonaryvein
andtothelungs.Oxygenatedandnon-oxygenatedbloodsstillmix,because
Frogs lacka septumthatwouldseparateblood.
 TheFrog'scirculatorysystemworksisbecausefrogsdonotneedas much
oxygenrichbloodintheirbodybecausetheyare gettingoxygenfromtwo
sources,theirlungs,andthroughtheir skin.Bothoxygenandcarbondioxide
are abletotransferthroughtheskinoffrogs.Anotherreasonwhytheir three
chamberedheart worksforfrogsisbecausefrogsare ectoderm'swhich
meansthemaintaintheirbodytemperature externallyandthisrequiresa lot
lessoxygen.