This the left atrium, this is the right atrium.
Blood coming from the upper part of the body enters the right atrium by way of the superior vena cava. Blood coming from the lower part of the body enters it by way of the much larger inferior vena cava.
In a more intact dissection, here's the inferior vena cava, coming up through the diaphragm and almost immediately entering the right atrium.
In addition to the two venae cavae, blood from the heart itself enters the right atrium under here, by way of the coronary sinus, which we'll see later. From the upper part of the right atrium this blind pouch, the right auricle, or atrial appendage projects forwards.
The thin wall of the right atrium is formed largely of muscle. When the atrium contracts in diastole the blood in it passes forwards into the right ventricle, through the right atrio-ventricular valve, or tricuspid valve, which is here. The left atrium and the right atrium are in contact here, where they share a common wall, the inter-atrial septum, which lies quite obliquely.
To look at the inside of the right atrium, we'll remove this part of its wall. Here's the opening of the superior vena cava above, and of the inferior vena cava below. Here's the opening of the coronary sinus. This is the part of the atrial wall that's shared with the left atrium, the inter-atrial septum.
This thin oval patch in the septum is the fossa ovale: the remnant of the foramen ovale that connected the two atria in intrauterine life. Here, we're looking forwards into the tricuspid valve: we'll see more of it when we look at the right ventricle.
Now we'll move on, to look at the left atrium. Blood coming from the lungs enters the left atrium by way of the four pulmonary veins, two from the right lung, two from the left. The left atrium, like the right one, has a blind pouch, the left auricle or atrial appendage, which projects upwards and forwards. In diastole, the blood that's in the left atrium passes forwards into the left ventricle through the left atrio-ventricular valve, or mitral valve, which is here.
To see inside the left atrium we'll remove this part of its wall. With the four pulmonary veins removed, the inside of the left atrium is relatively featureless. Here's the inter-atrial septum again, and here's the remnant of the foramen ovale, seen from the left side. Here, we're looking forwards into the mitral valve.