The mouth, esophagus, stomach and small intestine are all part of what system?

Food must be broken down by the digestive system in order for the blood to absorb it. (The word digest refers to the process of breaking something down into smaller pieces; for example, reader's digest breaks up long stories into smaller, more manageable pieces.)

The food is broken down into smaller pieces by the mouth's chewing process.

Smaller food particles that can be slowed down and passed down the esophageal tube to the stomach are carried by the esophagus.

The stomach dissolves the smaller particles into a slurry by mixing them with acids and enzymes, which further reduces their size.

Additional enzymes in the small intestine break the slurry's particles into molecules that can cross the villi in the small intestine and enter the blood.

These, then, are all components of the digestive system that reduce food to tiny enough pieces for the blood to carry to the cells.

They are all part of the digestive system.