are two atria of the heart, left and right.
The right atrium receives unoxygenated blood from the veins
called the inferior and superior vena cavae.
It sends this blood through the tricuspid valve into the right
ventricle where it is pumped through the pulmonary valve into
the pulmonary artery and on into the lungs.
The left atrium receives oxygenated blood from the lungs and
sends it through the mitral valve to the left ventricle, where
it is pumped through the ascending aorta to the remaining
parts of the body.