Digestive system process, function.

Digestive system process, function.

The process and function of the digestive system are:

1. Ingestion. Food must be placed into the mouth before it can be acted on; this is an active, voluntary process called ingestion.

2. Propulsion. If foods are to be processed by more than one digestive organ, they must be propelled from one organ to the next; swallowing is one example of food movement that depends largely on the propulsive process called peristalsis (involuntary, alternating waves of contraction and relaxation of the muscles in the organ wall).

3. Food breakdown: mechanical digestion. Mechanical digestion prepares food for further degradation by enzymes by physically fragmenting the foods into smaller pieces, and examples of mechanical digestion are: mixing of food in the mouth by the tongue, churning of food in the stomach, and segmentation in the small intestine.

4. Food breakdown: chemical digestion. The sequence of steps in which the large food molecules are broken down into their building blocks by enzymes is called chemical digestion.

5. Transport of digested end products from the lumen of the GI tract to the blood or lymph is absorption, and for absorption to happen, the digested foods must first enter the mucosal cells by active or passive transport processes.

6. Defecation is the elimination of indigestible residues from the GI tract via the anus in the form of feces.

 

Anatomy of the Digestive System

The organs of the digestive system can be separated into two main groups: those forming the alimentary canal and the accessory digestive organs.

Organs of the Alimentary Canal

The alimentary canal, also called the gastrointestinal tract, is a continuous, hollow muscular tube that winds through the ventral body cavity and is open at both ends. Its organs include the following:

 

Mouth

Food enters the digestive tract through the mouth, or oral cavity, a mucous membrane-lined cavity.

• Lips. The lips (labia) protect its anterior opening.

• Cheeks. The cheeks form its lateral walls.

• Palate. The hard palate forms its anterior roof, and the soft palate forms its posterior roof.

• Uvula. The uvula is a fleshy finger-like projection of the soft palate, which extends inferiorly from the posterior edge of the soft palate.

• Vestibule. The space between the lips and the cheeks externally and the teeth and gums internally is the vestibule.

• Oral cavity proper. The area contained by the teeth is the oral cavity proper.

• Tongue. The muscular tongue occupies the floor of the mouth and has several bony attachments- two of these are to the hyoid bone and the styloid processes of the skull.

• Lingual frenulum. The lingual frenulum, a fold of mucous membrane, secures the tongue to the floor of the mouth and limits its posterior movements.

• Palatine tonsils. At the posterior end of the oral cavity are paired masses of lymphatic tissue, the palatine tonsils.

• Lingual tonsil. The lingual tonsils cover the base of the tongue just beyond.

 

Pharynx

From the mouth, food passes posteriorly into the oropharynx and laryngopharynx.

• Oropharynx. The oropharynx is posterior to the oral cavity.

• Laryngopharynx. The laryngopharynx is continuous with the esophagus below; both of which are common passageways for food, fluids, and air.

 

Esophagus

The esophagus or gullet, runs from the pharynx through the diaphragm to the stomach.

• Size and function. About 25 cm (10 inches) long, it is essentially a passageway that conducts food by peristalsis to the stomach.

• Structure. The walls of the alimentary canal organs from the esophagus to the large intestine are made up of the same four basic tissue layers or tunics.

• Mucosa. The mucosa is the innermost layer, a moist membrane that lines the cavity, or lumen, of the organ; it consists primarily of a surface epithelium, plus a small amount of connective tissue (lamina propria) and a scanty smooth muscle layer.

• Submucosa. The submucosa is found just beneath the mucosa; it is a soft connective tissue layer containing blood vessels, nerve endings, lymph nodules, and lymphatic vessels.

• Muscularis externa. The muscularis externa is a muscle layer typically made up of an inner circular layer and an outer longitudinal layer of smooth muscle cells.

• Serosa. The serosa is the outermost layer of the wall that consists of a single layer of flat serous fluid-producing cells, the visceral peritoneum.

• Intrinsic nerve plexuses. The alimentary canal wall contains two important intrinsic nerve plexuses- the submucosal nerve plexus and the myenteric nerve plexus, both of which are networks of nerve fibers that are actually part of the autonomic nervous system and help regulate the mobility and secretory activity of the GI tract organs

Stomach

Different regions of the stomach have been named, and they include the following:

• Location. The C-shaped stomach is on the left side of the abdominal cavity, nearly hidden by the liver and the diaphragm.

• Function. The stomach acts as a temporary “storage tank” for food as well as a site  for food breakdown.

• Cardiac region. The cardiac region surrounds the cardioesophageal sphincter, through which food enters the stomach from the esophagus.

• Fundus. The fundus is the expanded part of the stomach lateral to the cardiac region.

• Body. The body is the midportion, and as it narrows inferiorly, it becomes the pyloric antrum, and then the funnel-shaped pylorus.

• Pylorus. The pylorus is the terminal part of the stomach and it is continuous with the small intestine through the pyloric sphincter or valve.

• Size. The stomach varies from 15 to 25 cm in length, but its diameter and volume depend on how much food it contains; when it is full, it can hold about 4 liters (1 gallon) of food, but when it is empty it collapses inward on itself.

• Rugae. The mucosa of the stomach is thrown into large folds called rugae when it is empty.

• Greater curvature. The convex lateral surface of the stomach is the greater curvature.

• Lesser curvature. The concave medial surface is the lesser curvature.

• Lesser omentum. The lesser omentum, a double layer of peritoneum, extends from the liver to the greater curvature.

• Greater omentum.  The greater omentum, another extension of the peritoneum, drapes downward and covers the abdominal organs like a lacy apron before attaching to the posterior body wall, and is riddled with fat, which helps to insulate, cushion, and protect the abdominal organs.

• Stomach mucosa. The mucosa of the stomach is a simple columnar epithelium composed entirely of mucous cells that produce a protective layer of bicarbonate-rich alkaline mucus that clings to the stomach mucosa and protects the stomach wall from being damaged by acid and digested by enzymes.

• Gastric glands. This otherwise smooth lining is dotted with millions of deep gastric pits, which lead into gastric glands that secrete the solution called gastric juice.

• Intrinsic factor. Some stomach cells produce intrinsic factor, a substance needed for the absorption of vitamin b12 from the small intestine.

• Chief cells. The chief cells produce protein-digesting enzymes, mostly pepsinogens.

• Parietal cells. The parietal cells produce corrosive hydrochloric acid, which makes the stomach contents acidic and activates the enzymes.

• Enteroendocrine cells. The enter endocrine cells produce local hormones such as gastrin, that are important to the digestive activities of the stomach.

• Chyme. After food has been processed, it resembles heavy cream and is called chyme.

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