There are two atria of the heart, left and right.

figure 105A

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.