The Digestive System

I.               Anatomy of the Digestive System- The alimentary canal is a tube where the organs of the digestive system are located.  It is also known as the gastrointestinal tract.  It begins with the mouth and ends with the anus.  The digestive system has four functions:

1.     Ingest the food

2.     Break food down into small molecules that can cross plasma membranes

3.     Absorb these nutrient molecules

4.     Eliminate nondigestible wastes

 

A.   Mouth-  Receives  food and is bounded externally by the lips and cheeks.  The space between the lips and cheeks and the teeth is the vestibule.  The tonge is skeletal muscle whose contraction changes the shape of the tongue.  The tongue is anchored by the hyoid bone.  There are three pairs of salivary glands that send juices (saliva) through ducts to the mouth.  Saliva contains bicarbonate and an enzyme called salivary amylase which begins the process of digesting starch.  The roof of the mouth has two parts:  a hard palate and a soft palate.

1.     Hard palate:  contains bones

2.     Soft palate:  muscular only and ends in a finger-shaped projection called the uvula.

 

B.    The Pharynx-  The tonsils are embedded in the mucous membrane of the pharynx.  Tonsils help protect the body against infection.  The pharynx has three parts:  nasopharynx, oropharynx, and the laryngopharynx.

1.     Nasopharynx:  posterior to the nasal cavity, serves as a passageway for air.

2.     Oropharynx:  Posterior to the soft palate, is a passageway for both air and food.

3.      Laryngopharynx:  Just inferior to the esophagus, is a passageway for food entering the esophagus.

 

C.    Swallowing-  When swallowing, food usually enters the esophagus since other possible avenues are blocked.  Swallowing is considered a reflex action since it is performed automatically.  When we swallow the soft palate moves back to close off the nasopharynx, and the trachea moves up under the epiglottis so that food is less likely to enter it.  The tongue presses against the soft palate, sealing off the oral cavity, and the esophagus opens to receive a food bolus.

 

D.   The Esophagus-  The esophagus is a muscular tube that passes from the pharynx through the thoracis cavity and diaphragm into the abdominal cavity, where it joins the stomach.  Peristalsis (a rhythmic contraction) pushes food along the alimentary canal.  Peristalsis begins in the sophagus and continues throughout the alimentary canal.  Sphincters (muscles that encircle tubes and act as valves) are on the esophagus. 

 

E.    The Wall of the Digestive Tract-  Has several layers

1.     Mucosa- This layer contains glandular epithelial cells that secrete digestive enzymes and goblet cells that secrete mucus.

2.     Submucosa-  Band of loose connective tissue that contains blood vessels.  Helps protect the body from disease.

3.     Muscularis-  Has two layers of smooth muscle.

4.     Serosa-  Thin, outermost layer of squamous epithelium supported by connective tissue.  It secretes a serous fluid that keeps the outer surface of the intestines most so that the organs of the abdominal cavity slide against one another.

 

F.    The Stomach-  A thick-walled, J-shaped organ that lies on the left side of the abdominal cavity.

 

1.     Regions of the stomach-  There are four regions of the stomach

A.   Cardiac region-near the heart, surrounds the lower esophageal sphincter where food enters the stomach. 

B.    Fundic Region-holds food temporarily, is an expanded portion above the cardiac region.

C.    Body Region- the main part.

D.   Pyloric Region-  narrows to become the pyloric canal.

 

2.     Digestive Functions of the Stomach-  The stomach physically and chemically acts on food.  The muscular wall moves food along and churns the food allowing it to mix with gastric juice which breaks it down into small pieces.

 

 

3.  Peritoneum-  A serous membrane that covers the abdominal wall as well as the organs of the abdomen.    The portion that lines the wall is called the parietal peritoneum and the portion that covers the organs is called the visceral peritoneum.  It supports the visceral organs, including the blood vessels, nerves, and lymphatic vessels. 

 

4,  The Small Intestine-  extends from the pyloric valve of the stomach to the ileocecal valve.  All the contents of food, such as fats, proteins, and carbs, are digested in the small intestine to soluble molecules that can be absorbed. 

 

A.   Regions of the small intestine

1.     Duodenum-  The first 25 cm. which contains distinctive glands that secrete mucus and receive the pancreatic secretions.

2.     Jejunum-  The next 1 M. contains folds and villi.

3.     Lleum-  the last 2 M. contains Peyer patches, aggregates of lymph nodules.

 

B.    Functions of the Small Intestine-  Complete the process of food digestion.  It also absorbs the nutrients in food.  It moves the nondigested remains to the large intestine.

 

5.     The Large Intestine-  Includes the cecum, the colon, the rectum, and the anal canal.  It absorbs water, salts, and some vitamins, as well as store indigestible material until it is eliminated at the anus. 

 

 

II.             Accessory Organs of Digestion

A.   The Pancreas-  Lies deep in the abdominal cavity, behind theperitoneum.  Pancreatic cells produce pancreatic juice which help digest food.

B.    The Liver-  Largest organ of the body.  It removes pathogens and debris that may have entered the hepatic portal vein.  It helps remove poisonous substances and detoxify the body. 

C.    The Gallbladder-  Excess bile produced by the liver are stored here. 

 

III.           Chemical Digestion-  The digestive enzymes are hydrolytic enzymes, which break down substances by the introduction of water at specific bonds. 

 

Warnings

 

  1. Heartburn-  Feels like a burning pain rising up into the throat.  It occurs during reflux when some of the stomach contents escape into the esophagus.
  2. Ulcer-  An open sore in the wall caused by the gradual disintegration of tissue.  Most ulcers are due to a bacterial infection that impairs the ability of mucous cells to produce protective mucus. 
  3. Diarrhea-  Caused by infection of the lower intestinal tract and nervous stimulation.  Can lead to dehydration and disturbances in heartŐs contraction.
  4. Constipation-  Feces are dry and hard and can lead to hemorrhoids.
  5. Polyps-  Small growths arising from the epithelial lining.  Whether they are benign or cancerous they can be removed surgically along with a portion of the colon if necessary. 

 

 

Dangers

 

  1. Peritonitis- generalized infection of the lining of the abdominal cavity.  It can lead to death if not properly treated. 
  2. Appendicitis-  The appendix becomes infected and can burst which can be fatal if not taken care of.
  3. Diverticulosis-  Saclike pouches in the colon.  Usually they cause no problems, but some people may develop an inflammation known as diverticulitis.  It can cause fever, loss of appetite, nausea, and vomiting.