Final answer:
In the upright, PA projection of the chest, the CR enters the patient from the back and reaches the anterior chest wall.
Step-by-step explanation:
In the upright, PA projection of the chest, the CR (Central Ray) enters the patient from the back. It passes through the posterior aspect of the patient's body and reaches the anterior chest wall. This positioning allows for better visualization of the heart and lungs on the resulting radiograph.