Introduction:

The purpose of digestion is to absorb nutrients in the body. The digestive tract breaks down food so that our intestines can absorb it.

There are several stages:

• upper aerodigestive tract
• digestive tract
• accessory organs

The inside of the digestive system is considered part of the external environment.

Everything that the intestines secrete is first absorbed by the liver.

It functions in various ways:

• digestion
• secretion
• absorption
• motility

DIGESTION:

To reduce food into absorbable nutrients. Proteins into amino acids, lipids into glycerol and fatty acids and sugars into monosaccharides.

SECRETION:

Foods need to be dissolved before they are cut into pieces using several tools.

• saliva
• hydrochloric acid in the stomach
• bile in the liver

To chop food (enzymes):

• salivary amylase in the mouth
• pepsin and lipase in the stomach
• trypsin, chymotrypsin, lipase, and amylase in the duodenum
• maltase, lactase, etc. in the small intestine.

ABSORPTION:

Through the intestinal villi where there are blood capillaries and the chyliferous duct, nutrients are directed via the portal vein to the liver, filtered, and sent to various tissues.

MOTILITY:

There are numerous contractions of the smooth muscles of the gastrointestinal tract:

• Mixing luminal content with various secretions.
• Moving content along the digestive tube.

Enzymes:

An enzyme is a protein that accelerates a chemical reaction.

They can be anabolic, allowing the creation of a molecule from two pieces. A catabolic enzyme allows the breakdown of a substance into smaller pieces (like small scissors cutting). All digestive enzymes are catabolic.

Digestive enzymes: amylase, lipase, pepsin, trypsin, chymotrypsin, sucrase, lactase. (when there's "-ase" at the end of the word, it's an enzyme).

Each digestive enzyme operates within a specific PH range (e.g., pepsin only works at a PH of 1.5 to 1.6).

The Mouth

Mastication is a mechanical step that destroys the structures of plant cells.

It conditions the bioavailability of nutrients.

Saliva

Saliva is produced by major salivary glands at structures called acini. It has an acidic PH, its role is to moisten and lubricate food.

Saliva has digestive activity, particularly on starch. It contains an enzyme (salivary amylase) that digests starch and transforms it into disaccharide maltose (2 glucose molecules).

It contains water, enzymes, as well as proteins that are necessary for digestion:

• mucins that produce mucus
• other proteins dedicated to immune defense (lysozyme, defensins, antibodies IGA).

Oral Cavity and Microbiota

The mouth hosts a microbiota.

When this microbiota is out of balance = cavities and dental plaque.

The Esophagus

Swallowing:

Involves the contraction of 22 groups of muscles controlled by cranial nerves.

Peristalsis = involuntary movement of food in the esophageal conduit.

Dysphagia = difficulty swallowing.

The Stomach

The stomach stores food and starts the digestion of proteins. It mixes its content with gastric juice to form gastric chyme.

The contractions allow the chyme to be pushed towards the pylorus and the duodenum to continue the digestion of food.

The stomach is lined with a simple columnar epithelium.

The epithelium is penetrated by orifices or crypts that lead to gastric glands.

Mucous Cells:

They secrete mucus and protect the gastric mucosa from acidity. They are located at the level of surface epithelium and at the level of crypts and the neck.

Parietal Cells:

They secrete hydrochloric acid (protein) and intrinsic factor (absorption of vitamin B12).

Hydrochloric acid = a denaturant for proteins meaning it "untangles the proteins."

Intrinsic Factor:

It is a glycoprotein secreted by parietal cells.

The intrinsic factor binds to B12 (in the duodenum, not in the stomach!) and allows its absorption in the ileum.

Physiology of Digestion

Main Cells:

Chief cells: they will release pepsin.

KCL = hydrochloric acid, 1 = it denatures proteins meaning "untangles proteins"

2 = it is important for the intrinsic factor, 3 = it activates pepsinogen into pepsin.

Chief cells will release pepsinogen which will become pepsin upon contact with hydrochloric acid.

Pepsin:

Once activated, it will allow the first step of protein digestion.

Endocrine Cells:

They will secrete all the necessary hormones for digestion such as:

• gastrin
• histamine
• serotonin
• somatostatin.

Regulation of gastric secretion:

This secretion has three phases:

• Cephalic: the stomach prepares for the arrival of food, stimulating the production of mucus, hydrochloric acid, and enzymes.
• Gastric: intense stimulation of secretion, homogenization, and acidification of chyme, digestion of proteins.
• Intestinal: Chyme enters the duodenum after the pylorus, slowing gastric secretion.