Poitrine vs. thorax
While both poitrine and thorax can be translated as chest, they are not interchangeable. Poitrine is the common, everyday term used in most situations, whereas thorax is a precise scientific and medical term.
Poitrine
A2Poitrine is the general-purpose word for the chest. It refers to the upper front part of the torso in both men and women. It can also specifically refer to a woman's bust or be used in cooking to describe cuts of meat like breast or belly.
Après avoir couru, il sentait son cœur battre dans sa poitrine.
(After running, he could feel his heart beating in his chest.)
Le médecin a utilisé un stéthoscope pour écouter sa poitrine.
(The doctor used a stethoscope to listen to his chest.)
Cette robe met en valeur sa poitrine.
(This dress enhances her bust.)
Pour le barbecue, nous avons mariné de la poitrine de porc.
(For the barbecue, we marinated some pork belly.)
J'ai une forte toux et une douleur à la poitrine.
(I have a bad cough and a pain in my chest.)
Le bébé s'est endormi sur la poitrine de son père.
(The baby fell asleep on his father's chest.)
Thorax
B2Thorax is the technical, anatomical term for the chest. It describes the entire chest cavity, including the rib cage, spine, and the internal organs (heart, lungs). Its use is almost exclusively limited to medical, anatomical, or scientific contexts, such as zoology (e.g., the thorax of an insect).
La radiographie montre une fracture au niveau du thorax.
(The X-ray shows a fracture in the thorax area.)
En cours d'anatomie, on étudie les muscles du thorax.
(In anatomy class, we study the muscles of the thorax.)
Le patient a subi un grave traumatisme du thorax dans l'accident.
(The patient suffered severe trauma to the thorax in the accident.)
Le corps d'une fourmi est composé de la tête, du thorax et de l'abdomen.
(An ant's body is composed of the head, the thorax, and the abdomen.)
Le chirurgien se spécialise dans les opérations du cœur et du thorax.
(The surgeon specializes in operations of the heart and thorax.)
Le scanner a permis d'obtenir une image détaillée des organes du thorax.
(The CT scan provided a detailed image of the organs in the thorax.)
Summary
In essence, use poitrine in everyday conversation for anything related to the front of the chest, from general pain to cooking. Reserve thorax for formal scientific or medical discussions where anatomical precision is required to describe the entire chest region, including bones and organs.