Animal Biology

The Digestive System

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 Introduction

 Your digestive system is made up of two major parts and it has two major functions: digestion and absorption.  This complex system of breaking down food and absorbing the vital nutrients is something you should have a good understanding of in order to understand human physiology.

 Functions of the Digestive System

 The function of the digestive system is digestion and absorption.  Digestion is the breakdown of food into small molecules, which are then absorbed into the body.  The digestive system is divided into two major parts:

·        The gastrointestinal (GI) tract (alimentary canal) is a continuous tube with two openings, the mouth and the anus.  It includes the mouth, pharynx, esophagus, stomach, small intestines, and large intestines.  Food passing through the internal cavity, or lumen, of the GI tract does not technically enter the body until it is absorbed through the walls of the GI tract and passes into blood or lymphatic vessels.

·        Accessory organs include the teeth and tongue, salivary glands, liver, gallbladder and pancreas.

 The treatment of food in the digestive system involves the following seven processes:

·        Ingestion is the process of eating.

·        Propulsion is the movement of food along the digestive tract.  The major means of propulsion is peristalis, a series of alternating contractions and relaxations of smooth muscle that lines the walls of the digestive organs and that forces food to move forward.

·        Secretion of digestive enzymes and other substances  liquefies, adjusts the pH of, and chemically breaks down the food.

·        Mechanical digestion is the process of physically breaking down food into smaller pieces.  This process begins with the chewing of

food and continues with the muscular churning of the stomach.  Additional churning occurs in the small intestines through muscular constriction of the intestinal wall.  This process, called segmentation, is similar to peristalsis, except that the rhythmic timing of the muscle constrictions forces the food backward and forward rather than just forward only.

·        Chemical digestion is the process of chemically breaking down food into simpler molecules.  The process is carried out by enzymes in the stomach and small intestines.

·        Absorption is the movement of molecules (by passive diffusion or active transport) from the digestive tract to adjacent blood and lymphatic vessels.  Absorption is the entrance of the digested food into the body.

·        Defecation is the process of eliminating undigested material through the anus.

 Structure of the GI Tract Wall

The digestive tract, from the esophagus to the anus, is characterized by a wall with four layers, or tunics.  Here are the layers, from the inside of the tract to the outside:

·        The mucosa is a mucous membrane that lines the inside of the digestive tract from mouth to anus.  Depending on the section of the digestive tract, it protects the GI tract wall, secretes substances, and absorbs the end products of digestion.  It is composed of three layers.

The mucosa (mucous membrane layer) is a layer of epithelial cells supported by connective tissue and smooth muscle lines the lumen (central cavity) and contains glandular epithelial cells that secrete digestive enzymes and goblet cells that secrete mucus.

The submucosa (submucosal layer) is a broad band of loose connective tissue that contains blood vessels and lies beneath the mucosa.  Lymph nodules, called Peyer's patches, are in the submucosa.  Like the tonsils, they help protect us from disease.

The muscularis (smooth muscle layer) is two layers of smooth muscle.  the inner, circular layer encircles the gut; the outer, longitudinal layer lies in the same direction as the gut.  (The stomach also has oblique muscles.)

The serosa (serous membrane layer) is found in most of the digestive tract and has a very thin, outermost layer of squamous epithelium supported by connective tissue.  The serosa secretes a serous fluid that keeps the outer surface of the intestines moist so that the organs of the abdominal cavity slide against one another. 

The esophagus has an outer serosa layer composed only of loose connective tissue called the adventitia. The visceral peritoneum is the serous membrane that lines the stomach, large intestine, and small intestine. 

The mesentery is an extension of the visceral peritoneum that attaches the small intestine to the rear abdominal wall. 

The mesocolon is an extension of the visceral peritoneum that attaches the large intestines to the rear of the abdominal wall.  The parietal peritoneum lines the abdominopelvic cavity (abdominal and pelvic cavities).  The abdominal cavity contains the stomach, small intestines, large intestines, liver, spleen, and pancreas.  The pelvic cavity contains the urinary bladder, rectum and internal reproductive organs.

Digestive Enzymes

During digestion, four different groups of molecules are commonly encountered.  Each is broken down into its molecular components by special enzymes:

·        Complex carbohydrates ( polysaccharides) such as starches, are broken down into oligosaccharides (consisting of two to ten linked monosaccharides), disaccharides ( such as maltose), or individual monosaccharides (such as glucose of fructose).  Enzymes called amylases break down starch.

·        Proteins are broken down into short chains of amino acids (peptides) or individual amino acids by enzymes called proteases.

·        Fats (lipids) are broken down into glycerol and fatty acids by enzymes called lipases.

·        Nucleic acids are broken down into nucleotides by enzymes called nucleases.

Following is a summary of enzymes and their substrate (substances upon which the enzymes operate).

Enzymes and Their Substrates