The right atrium receives blood from the veins. This blood carries little oxygen and lots of carbon dioxide, because it's returning from the body's tissues, where much of the oxygen was removed and the carbon dioxide added.
Venous blood is darker in color than arterial blood because of the difference in dissolved gases.
While the heart is relaxed, venous blood flows through the open tricuspid valve to fill the right ventricle. An electrical signal starts the heartbeat by causing the atria to contract. Shortly after the atrium contracts, the right ventricle contracts. As this occurs, the tricuspid valve closes and the partially deoxygenated blood is pumped through the pulmonary valve, into the pulmonary artery and on to the lungs. In the lungs the blood gives up its carbon dioxide and gets oxygen before returning to the left atrium.