Intestin vs. boyau
While both words relate to the digestive tract, intestin is the standard anatomical term, whereas boyau is a more versatile word used for sausage casings, tubes, narrow passages, or colloquial speech.
Intestin
B1Intestin is the formal, medical, and scientific term for the organ involved in digestion. You use this word when talking about anatomy, health, or biology regarding humans or animals in a neutral context.
Le médecin a examiné son intestin grêle avec attention.
(The doctor examined his small intestine carefully.)
Une consommation excessive de sucre peut irriter l'intestin.
(Excessive sugar consumption can irritate the intestine.)
Les requins ont un intestin très court par rapport aux mammifères.
(Sharks have a very short intestine compared to mammals.)
Boyau
C1Boyau originally refers to an animal gut used as a material (like sausage casings). By extension, it describes long flexible tubes (hoses, bicycle tires), narrow tunnels (military trenches, caves), or is used as a slang term for guts.
Le boucher utilise du boyau naturel pour faire ses saucisses.
(The butcher uses natural gut casing to make his sausages.)
N'oublie pas de ranger le boyau d'arrosage avant l'hiver.
(Don't forget to put away the garden hose before winter.)
Les soldats se cachaient dans un boyau étroit pour éviter les tirs.
(The soldiers were hiding in a narrow trench to avoid gunfire.)
Il a ri à s'en tordre le boyau.
(He laughed until his guts hurt.)
Ce vélo de course nécessite un boyau spécifique.
(This racing bike requires a specific tubular tire.)
Summary
Use intestin when discussing the body part in a standard or medical way. Use boyau when talking about sausage skins, garden hoses, narrow tunnels, tubular tires, or when referring to guts in a familiar or slang way.