Digestion: Physiology and Hormonal Control

Fibre is undigestible and moves through the digestive tract without being broken down by enzymes. Many foods contain both soluble and insoluble fiber. Soluble fibre dissolves easily in water and takes on a soft, gel-like texture in the intestines. Insoluble fibre, on the other hand, passes essentially unchanged through the intestines.

All the  hormones control digestion .

Foods rich in carbohydrates include bread, potatoes, dried peas and beans, rice, pasta, fruits, and vegetables. The digestible carbohydrates—starch and sugar—are broken into simpler molecules by enzymes in the saliva, in juice produced by the pancreas, and in the lining of the small intestine. Starch is digested in two steps. Sugars are digested in one step. An enzyme in the lining of the small intestine digests sucrose, into glucose and fructose, which are absorbed through the intestine into the blood. Milk contains another type of sugar, lactose, which is changed into absorbable molecules by another enzyme in the intestinal lining.

Protease and bromelain are the two enzymes, along with the gastric acid contained in the stomach, responsible for the breakdown of protein. The protein found in meat products is very difficult to breakdown and takes the digestive system a long time to perform the task. When protein is sufficiently broken down into its smaller molecules called amino acids, the body can then utilize them to rebuild soft tissue.

Ghrelin is produced in the stomach and upper intestine in the absence of food in the digestive system and stimulates appetite.

Amylase is an enzyme that catalyses the breakdown of starch into sugar. Amylase is present in human saliva, where it begins the chemical process of digestion. Foods that contain much starch but little sugar, such as rice and potato, taste slightly sweet as they are chewed because amylase turns some of their starch into sugar in the mouth.

A diastase is any one of a group of enzymes which catalyses the breakdown of starch into maltose.

Taking food into the mouth is called ingestion. We use lips, tongue and teeth. The tongue is also important in tasting food, to tell  whether it is good to eat; if not it will be ejected from the mouth rather than swallowed.

Lingual lipase is a member of a family of digestive enzymes called lipases, that use the catalytic triad of Aspartate (Asp), Histidine (His), and Serine (Ser) to hydrolyze long-chain triglycerides into partial glycerides and free fatty acids. The enzyme catalyzes the first reaction in the digestion of dietary lipid, with diglycerides being the primary reaction product.

Papain is the enzyme found in papaya that is valuable for increased immune system function and has a soothing effect on the stomach by aiding in protein digestion. It can also help alleviate gas, bloating, diarrhea and cramps associated with incomplete digestion

Rugae are the extensive folds in the stomach lining. These folds can stretch to accommodate an increase in stomach volume during consumption of a big meal.

The Suspensory muscle of duodenum or Ligament of Treitz (named after Vلclav Treitz) connects the duodenum of the small intestines to the diaphragm. It contains a slender band of skeletal muscle from the diaphragm and a fibromuscular band of smooth muscle from the horizontal and ascending parts of the duodenum.

All of the above control the digestive process.

Amino-acids are absorbed into the villus cells by active transport and pass out of the opposite side by diffusion. In a fetus a newly born baby , some entire undigested proteins can be absorbed by pinocytosis. This is how babies are able to absorb some of their mother's antibodies from the milk. This can also happen to a small extent in adults.

The gall bladder stores and concentrates bile which emulsifies fats making them easier to break down by the pancreatic juices.

The inner layer of the stomach wall, the mucosa, is specialized to produce large quantities of gastric juice, up to 2 dm each day. Gastric juice contains protease and lipase, as well as hydrochloric acid (HCL).To protect the cells in the wall from damage by the acid and proteases, they are covered with a slimy coat of mucus containing hydrogen carbonate ions which neutralize the acid.

The appendix is a blind-ended tube connected to the cecum (or caecum), from which it develops embryologically. The cecum is a pouchlike structure of the colon. The appendix is located near the junction of the small intestine and the large intestine.

Defecation is the final act of digestion by which organisms eliminate solid, semisolid or liquid waste material (feces) from the digestive tract via the anus.

The stomach lies below the diaphragm and to the left of the liver. It is the widest part of the alimentary canal and acts as a reservoir for the food where it may remain for between 2 and 6 hours. Here the food is churned over and mixed with various hormones, enzymes including pepsinogen which begins the digestion of protein, hydrochloric acid, and other chemicals; all of which are also secreted further down the digestive tract.

The pharynx is part of the gastrointestinal tract, lying between the mouth and the esophagus. The oropharynx is the first region food enters when it is swallowed. The opening from the oral cavity into the oropharynx is called the fauces. Masses of lymphoid tissue - the palatine tonsils - are near the fauces. The lower region, posterior to the larynx, is the laryngopharynx, or hypopharynx. The laryngopharynx opens into both the esophagus and the larynx.

Secretin is produced by cells of the duodenum. Its production is stimulated by acid chyme from stomach. It stimulates the pancreas to produce sodium bicarbonate, which neutralizes the acidic chyme. It also stimulates the liver to secrete bile.

The small intestine is approximately 6 m long. Like the stomach, it contains numerous ridges and furrows. In addition, there are numerous projections called villi that function to increase the surface area of the intestine. Individual villus cells have microvilli which greatly increase absorptive surface area.

Chewing breaks food into smaller particles so that chemical digestion can occur faster. Salivary amylase breaks starch (a polysaccharide) down to maltose (a disaccharide). Bicarbonate ions in saliva act as buffers, maintaining a pH between 6.5 and 7.5. Mucins (mucous) lubricate and help hold chewed food together in a clump called a bolus. The tongue contains chemical receptors in structures called taste buds.