Overall, the tongue has five major functions:

Tongue – The tongue is formed mainly of interlacing skeletal muscle tissue. When the mouth is opened, only a small portion of the entire tongue is actually seen.

Visible portion of the tongue

1. The tongue is the main organ of taste.

2. The tongue plays a major role in speech.

4. The tongue cleans the teeth.

3. The tongue presses against the roof of the mouth and blocks off the oral cavity during the act of swallowing to prevent food from shooting back out the mouth.

5. The most important function of the tongue is in its use as a food manipulator. The tongue pushes the food between the teeth, shapes the chewed food into a nice glob (called a bolus) and then pushes the mass into the oropharynx.

Salivary glands –The three sets of salivary glands are:1. Parotid glands

The function of the salivary glands is to produce about one liter of saliva per day.

2. Submandibular glands3. Sublingual glands

Parotid gland

Submandibular gland

Sublingualgland

Saliva is a thin, watery, fluid about 99.5% water by composition. It has a somewhat slippery feel which is due to protein substance called mucin. It also contains inorganic salts, the digestive enzyme salivary amylase and a bactericidal enzyme called lysozyme. The pH of saliva is about 6.4 to 7.0 (or about neutral).

The functions of saliva:

1. It is a solvent, which permits the taste buds of the tongue to operate.2. Saliva keeps the lining of the mouth moist and along with the tongue helps to keep the teeth clean.

3. Mechanical digestion – Saliva helps to moisten and lubricate food as the teeth grind and the tongue manipulates it. This is important because this action helps to protect the inner surface of the esophagus from damage by food particles.

4. Chemical digestion – Saliva begins the chemical digestive process of carbohydrates. This is done by converting large, complex carbohydrate molecules to smaller, simpler ones through the action of Salivary Amylase.

The chemical digestive function of saliva is not very effective for several reasons:

1. If chewing is not adequate, food is not thoroughly mixed with saliva so the chemical digestion from salivary amylase does not get a chance to occur.

2. Beverages are often used to moisten food or “wash” food down which prevents adequate chewing, dilutes the saliva, thus not allowing amylase a chance to work.3. Many drinks are quite acidic and salivary amylase is deactivated by acid.

Teeth – An adult set is 32 teeth. This includes:

1. Eight (8) incisors for cutting.

incisors

Teeth – An adult set is 32 teeth. This includes:

1. Eight (8) incisors for cutting.

2. Four (4) canines or cuspids for ripping or tearing.

canines

Teeth – An adult set is 32 teeth. This includes:

1. Eight (8) incisors for cutting.

2. Four (4) canines or cuspids for ripping or tearing.3. Eight (8) bicuspids or premolars for tearing and crushing.

bicuspids

Teeth – An adult set is 32 teeth. This includes:

1. Eight (8) incisors for cutting.

2. Four (4) canines or cuspids for ripping or tearing.3. Eight (8) bicuspids or premolars for tearing and crushing. 4. Twelve (12) tricuspids or molars, four of which are the “wisdom teeth,” for crushing or grinding.

tricuspids

The function of the teeth is to masticate (chew) food which means to grind and/or pulverize food. The process of mastication (chewing) begins by grasping the food and cutting or tearing off a piece with the “front” teeth. The food is then ground to a mash between the “back” teeth. It ends with the tongue shaping the pulverized food into a bolus ready for deglutition (the process of “swallowing”).

Deglutition – “Swallowing” is a voluntary action in the beginning but involuntary once begun. In other words, once it begins, it cannot be stopped by voluntary mechanisms. It involves the tongue, pharynx, esophagus, brain and associated nerves and is divided into three stages:

Buccal Stage – this first stage is the voluntary phase where masticated & moistened food is shaped into a bolus and directed by the tongue to the back of the mouth. Through a series of voluntary muscle actions, the nasopharynx is closed off and the food is forced into the pharynx. At this point, reflex mechanisms take over and swallowing cannot be stopped.

Pharyngeal Stage – this second stage is an automatic phase where the food is passed through the pharynx to the esophagus. At this stage, several involuntary mechanisms controlled by reflex centers in the lower pons and medulla oblongata take over to assure that the food will pass into the esophagus rather than the trachea:1. The larynx rises and the epiglottis folds over the trachea opening.

2. The tongue closes off the oral cavity by pressing against the roof of the mouth.

3. The nasopharynx is blocked by the soft palate.

Esophageal Stage – this third stage is an automatic phase where food passes through the esophagus and to the stomach. After food enters the esophagus, peristaltic waves aided by gravity propel the food to the stomach. When the pressure in the esophagus increases to greater than that in the stomach the cardiac sphincter relaxes, allowing food to pass through and into the stomach.

The stomach then mixes food with gastric juice produced by gastric glands. There are actually three different types of cells in gastric glands, each secreting a different constituent of gastric juice.

Mucous or Goblet Cells – These secrete thin mucus which is mixed with the food.Chief cells – These secrete pepsinogen which is the inactive form of the enzyme, pepsin. Pepsin is an enzyme which splits protein molecules into smaller units called peptides. It is able to do this because the hydrochloric acid helps to unravel proteins and activate the enzyme. This means pepsinogen is transformed from inactive to active when it comes in contact with an acid environment of pH 3 or less. The stomach has a pH of 1 to 2 with an HCl concentration of approximately 0.017 molar.

****Note – The functions of HCl are to activate pepsinogen to pepsin, “unravels” large molecules, and kill bacteria.

Parietal cells – These cells secrete hydrochloric acid (HCl) and a substance called intrinsic factor, which helps in the body’s absorption of vitamin B12.

These four things together (mucus, pepsinogen/pepsin, hydrochloric acid, and intrinsic factor) make up the majority of the composition gastric juice. They are not the only components of gastric juice, however, just the most predominant.

Other enzymes in gastric juice are rennin (coagulates milk), gastric lipase (fat digestion), but these two enzymes are not very effective in acid environments so they are important in children, but not in adults.

The secretion of gastric juice is a continuous process, however it does increase and decrease according to need. The regulation of this secretion can be divided into 3 phases:Cephalic phase – During this stage the sight, smell, taste or even the thought of food (especially a favorite food) stimulates a flow of gastric secretions called appetite juice, which is high in HCl and pepsinogen.

Gastric phase – During this stage the food in the stomach stimulates the secretion of the hormone gastrin. This hormone starts a loop that stimulates additional secretions gastric juice until the volume of food diminishes, thus reducing all secretions.

Intestinal phase – During this stage the stomach is “empty” so only small amounts of secretions are produced. This secretion contains mostly mucus and lack HCl, pepsinogen, and intrinsic factor for the most part. This phase continues until the small intestine empties 6 to 8 hours later or until one thinks about eating again.

Thing that inhibit gastric juice flow:

1. Unappetizing food – The sight, smell, taste, or even suggestion of rotten, smelly, disgusting or disliked food immediately reduces appetite juice flow which shuts down the feeling of hunger.

2. Excess acid concentration in the stomach decreases gastric juice production by decreasing the release of gastrin.

3. Fat laden chyme in the duodenum causes the release of a hormone called cholecystokinin. This hormone inhibits both gastric secretions and gastric movements.

4. Strenuous exercise diverts blood to the heart and muscles and energy production is not used for digestion.

5. Cold temperatures inhibit stomach action by delaying peristaltic waves and stopping gastric juice secretion.

Anything that decreases gastric juice flow slows down or delays digestion. The draw back to delaying digestion is that microorganisms can ferment (“rot”) sugars in the food, producing large amounts of smelly gas (mostly hydrogen sulfide, H2S and methane, CH4) that can cause severe bloating, cramps, and diarrhea.

The three accessory organs not yet discussed are the liver, gall bladder, and pancreas.

Liver

Gall Bladder

Pancreas

Note – This picture gives a perspective on the location of these accessory organs with respect to each other and the stomach.

The Liver – The liver has only one function directly related to the mechanical or chemical digestion of food. That function is to produce bile and send it to the gall bladder.

Liver

Gall Bladder

Bile – This yellowish green liquid serves only one digestive purpose. This purpose is to emulsify fats.

This means that bile physically (mechanically NOT chemically) breaks up fat into smaller and smaller “blobs” which increases the total surface area of the fat and allows the lipase enzymes to chemically digest the fat more easily.

***Remember – Bile emulsifies fat

Gall Bladder – located “underneath” the liver. Has four functions which are all related to bile:

Gall Bladder1. Receives bile secretions from the liver.

2. Stores bile for secretion to the small intestine when dietary fat is consumed.

3. Concentrates bile by removing water

4. Discharges bile into the duodenum when fat laden chyme is present.

Pancreas – This organ has both an exocrine (digestive) and endocrine (hormonal) function.

PancreasThe endocrine portion of the pancreas consists of clusters of cells scattered along the organ called islets of Langerhans and they contain two types of secretory cells. Alpha cells secrete glucagon and Beta cells secrete insulin.

Glucagon stimulates the conversion of liver glycogen (animal starch), which is a large carbohydrate storage molecule, to blood glucose. In other words, glucagon raises blood sugar levels when necessary.

Insulin stimulates glycogen storage and cellular glucose use. In other words, insulin lowers blood sugar levels. These hormones are secreted directly into the blood stream (this is what distinguishes the function as endocrine – all hormones are secreted directly into the blood stream).

The excretory portion of the pancreas consists of structures called acini (pancreatic acinar cells)which are “grape like” clusters of secretory cells that produce and empty their secretions into microscopically sized ducts. These smaller ducts form a network that finally empty into a large common duct called the pancreatic duct, which empties into the duodenum.

This mixture of secretions called pancreatic juice contains digestive enzymes: carboxypeptidase, trypsin and chymotrypsin (“proteases” or protein digesting enzymes), Sucrase, Lactase, and Maltase (“amylases” or carbohydrate digesting enzymes), Pancreatic Lipase (which breaks down fats), RNase and DNase (“nucleases” which break down nucleic acids DNA and RNA), and sodium bicarbonate (Sodium Hydrogen Carbonate or NaHCO3) which

neutralizes the HCl from the stomach.The enzymes trypsin, chymotrypsin, and carboxypeptidase are secreted as trysinogen, chymotrypsinogen, and procarboxypeptidase.

Pancreatic juice actually consists of two portions: aqueous juice which contains mostly carbonate ions and enzyme juice which contain the enzymes.

The small intestine is lined with cells that secrete large amounts of mucus called goblet cells and are also lined with special mucus secreting glands that secrete large amounts of thick, alkaline mucus which helps to neutralize the acidic secretions of the stomach.

Some final thoughts on digestion:

Enzymes that are active in the small intestine are not active in acid environments and those that are in the small intestine itself are some peptidases, sucrase, maltase, lactase and intestinal lipase.