Question

multiple choice question
what are the right and left atria?
thin - walled upper chambers of the heart that receive blood returning to the heart
enlargements in the walls of the pulmonary trunk and aorta
thick - walled lower chambers of the heart that pump blood out of the heart
thick bundles of cardiac muscle that rim the chamber walls

Explanation:

The right and left atria are the thin - walled upper chambers of the heart that receive blood returning to the heart. The other options describe different structures or functions related to the heart but not the atria.

Answer:

Thin - walled upper chambers of the heart that receive blood returning to the heart