The mouth is the beginning of the digestive tract and, digestion starts here when taking the first bite of food.

The Teeth are 32 small, hard, living organs that contain blood vessels and nerves under the dentin in the soft region of the pulp. They are found along the anterior and lateral edges of the mouth. Each tooth is made of a bone-like substance called dentin and covered in a layer of enamel, and include the incisors, canine teeth, premolars, and molars are found next to the premolars. Teeth are necessary for biting, tearing, grinding, and crushing food into
 smaller particles.

The salivary glands produce saliva and contains amylase 
enzymes, which break down starches (complex carbohydrates) to simpler carbohydrates. Saliva lubricates the food so it can be swallowed, dissolves food particles, and makes it possible to taste what is being eaten. It also protects ones teeth against bacteria, and aids in the digestion of food. The amylase enzymes also help break down complex carbohydrates to simple carbohydrates. The 3 major pairs of salivary glands are the parotid glands on the insides of the cheeks, the submandibular glands at the floor of the mouth, and the sublingual glands under the tongue.

 The stomach is J-shaped and has numerous ridges that allow it to expand so that it can store about 1.5 L of food. Partial digestion of the food takes place here, and the churning action of the stomach muscles physically breaks down the food.

 The stomach releases acids and enzymes for the chemical breakdown of food. Gastric fluid includes substances such as mucus, hydrochloric acid, and pepsinogens. Mucus provides a protective coating. Hydrochloric acid kills many harmful substances that are ingested with food and also converts pepsinogen into its active form, pepsin, which is a protein-digesting enzyme. Pepsin is responsible for protein breakdown and breaks the  the long amino acid chains in proteins into shorter chains, called polypeptides. pH is also located inside the stomach and normally ranges between 2.0 and 3.0, but may approach pH 1.0. When one eats, the stomach releases enzymes called proteases and hydrochloric acid to aid in digestion. After a high-protein meal, their stomach pH may drop to as low as 1 or 2, but soon quickly raises the pH back to 3 or 4 and after the meal has been digested, their
stomach pH would return to a resting level of about 4 or 5. The stomach then releases food into the small intestine in a controlled and regulated manner.

The small intestine can be divided into 3 major regions: the duodenum, jejunum, and ileum.

The pancreas is an abdominal organ that is located behind the stomach and is surrounded by other organs and plays an important role in digestion and in regulating blood sugar.  It makes enzymes to digest proteins, fats, and carbs in the intestines and produces the hormones insulin and glucagon.

 Enzymes and hormones play a large role in the pancreas, such as the secretin, a hormone that stimulates pancreatic and bile secretions. The pancreas releases lipases, which are lipid-digesting enzymes. There are 2 different types of lipid-digesting enzymes, the pancreatic lipase, that breaks down fats into fatty acids and glycerol, and phospholipase acts on phospholipids. Trypsin is a protein-digesting enzyme, enterokinase, which is an enzyme of the small intestine that converts trypsinogen to trypsinerepsins, and erepsins that complete protein digestion by converting small-chain peptides to amino acids. Pancreas secretions (such as trypsinogen and erepsins) play a large role in protein digestion. The pancreas also secretes amylase enzymes, which continue the digestion. Trypsinogen is released from the pancreas, and once it reaches the small intestine, enterokinase converts the inactive trypsinogen into trypsin,which acts on the partially digested proteins. Trypsin then  breaks down long-chain polypeptides into shorter-chain peptides.

When acids enter the small intestine, the chemical of prosecretin is converted into secretin, and is then asorbed into the bloodstream and carried to the pancreas, where it signals the release of a solution containing bicarbonate ions. The ions are then are carried to the small intestine, where they neutralize in HCl in gastric fluid and the pH raises from about 2.5 to 9.0. The now basic pH inactivates pepsin. Thus, the small intestine is protected from stomach acids by the release of secretin of carbohydrates that was started in the mouth.