Final answer:
The right atrium wall is composed of a smooth portion and an area with pectinate muscles. The smooth portion covers the posterior and medial wall, while the pectinate muscles are found anteriorly, around the fossa ovalis.
Step-by-step explanation:
The two parts of the right atrium wall are distinguished by their distinct features. The first part is the smooth walled portion, which makes up the majority of the atrium's internal surface. The second part is characterized by the presence of pectinate muscles, which are prominent muscular ridges found on the anterior surface of the right atrium and within the right auricle.
Regarding the borders of these parts, the fossa ovalis, a medial depression that is a remnant of the foramen ovale from fetal development, marks a significant internal landmark. The delineation between the smooth wall and the muscle-ridged areas is around the fossa ovalis, with the smooth part making up the posterior and medial wall, and the pectinate muscles primarily located in the anterior section adjacent to the right auricle.