All four of the heart's chambers squeeze blood out into the corresponding chamber or artery. The right atrium pumps blood from the body into the right ventricle, and the right ventricle pumps that blood out of an artery that leads to the lungs. The blood then returns to the heart and is pumped by the left atrium to the left ventricle, which pumps the blood to the rest of the body.
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The heart of a lamprey looks similar to that of modern fish in a sense. It is a two chambered heart with a single atrium and a single ventricle. The atrium of the heart is proceeded by a sinus venosus, which isn't considered an actual chamber but rather pre-heart chamber. Then after the ventricle we have a conus arteriosus that again isn't considered to be a chamber of the heart but rather a post-heart chamber. The blood flow through the heart is a one way path like that of modern day fishes.
endocardium
The fox has a 4 chambered heart.
The left atrium is the chamber that receives oxygenated blood returning from the lungs.
atrium