OVERVIEW OF DIGESTIVE SYSTEM

I. Digestive tract - A continuous, coiled, hollow muscular tube open to the outside at both ends.  Portions of the tube become specialized into organs: mouth, pharynx, esophagus, stomach, small intestine, colon, rectum and anus. The interior lining is mucous membrane throughout.

II. Accessory organs of digestion - teeth, tongue and gall bladder facilitate digestion. Digestive glands, e.g., salivary glands, liver, pancreas and various glandular tissues lodged in the wall of the tract, produce secretions aiding digestion.

The Mouth: a cavity lined with stratified squamous (keratinized on gums, hard palate and dorsum of tongue).

The Vestibule - area bounded by lips, cheeks, teeth and gums.

The Red Margin - exposed, blood-flushed area of lips. Lacking sweat and oil glands. It must be moistened with saliva.

The Oral Cavity - area within the boundries of the teeth and gums.

The Palate - the roof of the mouth.

     Hard Palate - formed by the palatine processes of maxillary bones and the palatine bones.

     Soft Palate - soft fold containing skeletal muscles. It terminates posteriorly in the uvula.

The Tongue - a muscular organ anchored in the floor of the mouth. Bundles of skeletal muscle fibers confined to the tongue are found to run in longitudinal, transverse and vertical planes. These muscles change the shape of the tongue for speech and swallowing. The median septum divides the tongue into bilaterally symmetrical halves. The lingual frenulum secures the tongue to the floor of the mouth. A very short frenulum produces the condition "tongue-tied''.

The Extrinsic Muscles - Gross movements of the tongue are produced by the three extrinsic muscles which insert on the tongue and have their origins on portions of skull bones and the hyoid bone:

     1. Genioglossus - protrudes the tongue (origin is mandible)

     2. Hyoglossus - depresses the tongue (origin is hyoid bone)

     3. Styloglossus - Retracts and elevates the tongue (origin isthe styloid process of temporal bone.)

Lingual papillae - projections of the lamina propria on the dorsum of tongue. There are three types of  these projections:

1.   Filiform papillae - the most numerous they cover the anterior 2/3 of the dorsum. They give the tongue a        roughness needed in licking semisolid foods. Heavily keratinized, they give the tongue a "coated'' appearance.

2.  Fungiform Papillae - located on the sides of tongue interspersed among the filiform papillae. Taste buds are found around these papillae.

3.  Circumvallate Papillae - form a V - shaped formation near the posterior margin of the tongue. The largest number of taste buds are associated with these papillae.

The Lingual Tonsil - an unencapsulated cluster of lymphoid tissue located at the base of the tongue.

The Salivary Glands - Ducted exocrine glands producing saliva. Two types of secretory cells are found in the glandular tissue:

     1. Serous cells producing a watery secretion containing amylase.

     2. Mucous cells producing a viscous liquid containing the glycoprotein mucin.

Submandibular Glands - are bilaterally located at the median aspect of the mandibular angle. Their ducts bring saliva to the oral cavity at the base of the frenulum. They are mixed glands, containing approximately equal numbers of serous and mucous cells.

Sublingual Glands - are anterior to the submandibular glands under the tongue. Cells of these glands are mostly mucous producing. Very little amylase is found in this saliva.

Parotid Glands - are anterior and inferior to the external ears lying in a connective tissue capsule.  Parotid ducts bring saliva into the vestibule along side of the second upper molar. The glandular cells are mostly serous.

The Fauces - are the passageway from the mouth to the pharynx. This short corridor is guarded by four pillars; the two palatoglossal arches are more anterior followed by the two palatopharyngeal arches.  In between the two sets of arches on either side are the palatine tonsils. During swallowing, contraction of  the muscles in these arches constricts the pillars preventing food from reentering the mouth.

The Pharynx - receives food from the oral cavity via the fauces as the tongue moves up and back. The food enters the oropharynx first. Contractions of muscles in the soft palate raises this structure to prevent food from entering the nasopharvnx. The oropharynx and the more inferior larvngeopharynx are common passageways for food and air. These regions are lined with stratified squamous epithelium and well supplied with mucous glands.

Role in Swallowing - there are five pharyngeal muscles(skeletal) involved in swallowing: three constrictors and two elevators.

Constrictors

     1. Superior constrictors (from pharynx to mandible and pterygoid plate of spheroid).

     2. Middle constrictors (from pharynx to the hyoid bone)

     3. Inferior constrictors (from pharynx to the thyroid and cricoid cartilages).

Elevators

     1. Stylopharyngeus - raises the pharynx (origin is styloid process).

     2. Salpingopharyngeus - raises the pharynx (origin is Eustacian tube).

     As the larynx is raised due to the action of the elevators (muscles of the hyoid bone play a role here, too), it comes up to meet the epiglottis sealing the glottis and preventing food from entering the airway. The movements of the tongue and contraction of muscles of the fauces are considered to be voluntary. The movements of the soft palate and the actions of pharyngeal muscles are involuntary.

The Esophagus

A muscular tube about ten inches long. It receives food from the pharynx as a result of swallowing. The esophagus passes through the mediastinum and penetrates the diaphragm at the esophageal hiatus. Trauma in this area may allow the upper portion of the stomach to protrude above the diaphragm (hiatal hernia). This will produce regurgitation and ulceration of the esophageal wall. Normally, food passes through the esophagus by muscular contractions of the wall called peristalsis. The food enters the stomach through the cardiac orifice. This opening is guarded by a sling of muscle from the diaphragm called the cardiac sphincter (skeletal muscle).

Basic Design of the Wall of the Digestive Tract

From the esophagus to the anus, the wall of the digestive tract shows a basic plan or design. However, each region of the tract modifies this basic architecture for its own purposes. The overall design is as follows:

4. The inner lining or Mucosa of the tract is essentially a mucous membrane. Overall its functions are:

     a. Secretion - mucous, digestive enzymes and hormones.

     b. Absorption - end products of digestion, water, minerals.

     c. Protection from infection.

     It consists of:

a. An innermost epithelium - primarily simple columnar rich in goblet cells. Also contains enzyme producing      and hormone secreting cells.

b. Under the epithelium is a thin Lamina Propria consisting of areolar connective tissue. Small  blood vessels and lymphatics are found here. In the ileum of the small intestine lymphatic nodules called Peyer's patches can be seen.

c. Forming the boundary between the mucosa and the submucosa is the Muscularis Mucosa. This is a very thin layer of smooth muscle which alters the shape of the lumen and is responsible for the epithelial pleats and folds as seen in the jejunum.

     The esophagus shows this basic tract structure with the following exceptions:

     1. The epithelium lining the esophagus is stratified squamous (nonkeratinized).

     2. Upper 1/4 of the esophagus consists of skeletal muscle in the muscularis externa. This muscle is innervated by somatic motor fibers of cranial nerve X in a branch of the Vagus called the recurrent laryngeal nerve. This nerve also innervates the muscle of the pharyngeal wall and larynx. The lower 3/4 of the esophagus is innervated by parasympathetic fibers of the Vagus and the muscle is smooth.

The Stomach

The stomach is a greatly expanded portion of the digestive tract. It is J - shaped and operates as a food blender and food reservoir. It is greatly distendible, e.g., a newborn infant’s stomach, only the size of a lemon, can hold up to 30 ml of milk. It has two curvatures. The lesser curvature forms the concave border. The greater curvature forms the convex border.

Regions of the Stomach

1. Cardiac Portion - located near the cardiac orifice. The esophagus joins with the stomach here. It is called the cardiac portion because it lies next to the diaphragm just below the heart.

2. Fundus - is a rounded vault which is the most superior region of the stomach. It often contains a bubble of gas visible in an X-ray.

3. Body - is the major portion of the stomach.

44. Pyloric portion - is the most distal portion of the stomach. It terminates in the pylorus which separates the stomach from the duodenum. The middle layer of the muscularis externa (smooth muscle)is thickened here to form a sphincter. This sphincter is normally closed except when acid chyme is being propelled into the duodenum.

Gastric Mesenteries

The stomach is anchored in place between its surrounding organs by means of double thicknesses of peritoneum called mesenteries or omenta. Contained within these sheets are blood vessels, nerves and lymphatics.

1. The lesser omentum is a mesentery connecting the lesser curvature of the stomach with the liver.

2. The greater omentum connects the greater curvature of the stomach with the transverse colon.

Structural Design of the Stomach Wall

The stomach wall shows the standard digestive tract design with the following exceptions:

1. The epithelium is highly folded forming deep pits - gastric pits. The simple columnar epithelium contains a variety of other cell types:

a. Zymogenic cells (chief cells) - produce pepsinogen.

b. Parietal cells - produce HC1 and intrinsic factor.

c. Mucous cells near the lumen produce standard mucous. Those in the neck of the pit produce a thick, alkaline mucous.

d. Enteroendocrine cells - produce a variety of endocrine products, i.e., gastrin, serotonin, somatostatin.

2. The muscularis externa consists of three layers of smooth muscle; the circular, longitudinal and oblique layers.

Factors Protecting the Stomach Lining from Gastric Juice

1. Copious production or a thick, alkaline mucous.

2. Epithelial cells are joined at their apical surfaces by tight junctions.

3. The epithelium is shed and replaced every three days.

The Small Intestine - a major digestive organ about 21 feet in length and approximately 1 inch in diameter. It receives acid chyme from the stomach. The small bowel is divided up into three regions:

The Duodenum - this first region of the of the small intestine is approximately1, inches long and is subdivided into four portions which hug the head region of the pancreas:

Area #1 - is joined to the pylorus of the stomach. The qastroduodenal artery passes just posterior to this region(dangerous in the case of a perforated duodenal ulcer). This is the only area of the duodenum to be intraperitoneal.

Area #2 - is the location of the sphincter of Oddi (hepatopancreatic sphincter) which regulates the entrance of  both bile and pancreatic juice into the duodenum. This area, as well as, the remainder of the duodenum is retroperitoneal.

Area #3 - is the location where the superior mesenteric artery and vein pass just in front of the duodenum.

Area #4 - this last area of the duodenum is anchored to the diaphragm by the ligament of Treitz. The transition from duodenum to jejunum is indicated by the formation of a mesentery, i.e., the jejunum, as well as, the rest of the small bowel is intraperitoneal.

Structural Design of the Wall of the Duodenum

The duodenal wall shows the standard digestive tract design with the following exceptions:

2. The proximal portion of the duodenum shows very few foldings of the mucosal lining, i.e., few plicae and scant villi. As you approach the distal end of the duodenum, the number of foldings increase dramatically.

Remainder of the Small Bowel - The Jejunum ( 8 ft.) and the Ileum ( 12 ft.)

These regions of the small intestine are completely intraperitoneal. The mesentery or double layer of peritoneum anchoring the jejunum and ileum contains abundant blood vessels, lymphatics and nerves.  The demarcation between these two regions cannot be distinguished easily. However, certain features are more common in one area than in another.

1. abundant folding of the epithelial lining due to large numbers of plicae circularis and villi.

2. presence of intestinal crypts (Crypts of Lieberkuhn).

a. deep invaginations of the lamina propria

b. contain many different cell types producing mucous, enzymes and hormones.

c. stem cells at the base of crypt produce new epithelial cells

d. old cells exfoliate and bring to the surface enzymes, e.g., enterokinase, an enzyme which activates pancreatic enzymes.

                                                           Ileum - is characterized by:

1. Very few plicae or villi

2. large masses of lymphoid nodules called Peyer's patches (arrow) are found in the submucosa, especially at the distal end of the ileum.

Large Intestine (large bowel)

The ileum terminates in a smooth muscle sphincter, the ileo-caecal valve. As this sphincter opens, chyme enters the caecum, a blind pouch at the beginning of the large intestine. The entire large intestine has a length of about 5 ft. and a width of 3 inches. It consists of the following areas: the caecum, the ascending colon, the right colic flexure (hepatic flexure), the transverse colon, the left colic flexure (splenic flexure) and the descending colon. At the level, of the iliac fossa, the colon curves to the right and at the body's midline curves again downward. This curvaceous region is called the sigmoid colon (S-shaped). The appendix is a short, hollow extension from the caecum. Its walls are filled with lymphoid nodules. Usually twisted, it sometimes accumulates bacteria and becomes inflammed, a condition called appendicitis .

Structural Design of the Wall of the Colon

The wall of the Colon shows a number of departures from the basic design seen in the rest of the digestive tract:

1. Haustra are pocket-like sacs in the wall of the colon.

2. Three longitudinal bands of smooth muscle, the taenia coli, are visible on the outer surface of the colon. The steady tone in this muscle is responsible for the pouch-like haustra.

3. The epiploic appendages are small, fat-filled pockets protruding through the serosa of the colon.

4. The mucosa is folded but shows no evidence of villi.

5. The epithelium is simple columnar with an enormous number of mucous cells.

Remaining Portions of the Digestive Tract

1. The Rectum - the last 16 inches of the digestive tract, it has essentially the same structure as the colon.

2. At the end of the rectum, small longitudinal folds in the wall form the structures called rectal columns.

3. The Anorectal canal is just distal to the rectal columns. Here the epithelium changes to stratified squamous.