Digestive System - Alimentary Canal


     The alimentary canal The Poo Machine (GI tract) is a continuous tube that winds and twists through the ventral body cavity. It is about 9 m (30 ft) in a cadaver but somewhat shorter in a living person. The organs that form the alimentary canal include:
Mouth (Oral Cavity)
     Food enters the alimentary canal through the mouth or oral cavity. The functions of the oral cavity include analysis, mechanical processing, digestion and lubrication. The anatomy of the mouth includes the following features:
Lips - protect the anterior opening of the mouth.
Cheeks - form the lateral walls of the mouth.
Hard palate - forms the anterior roof of the mouth.
Soft palate - forms the posterior roof of the mouth. The uvula is a fleshy extension that hangs from the posterior rim of the soft palate.
Vestibule - the space between the inner lining of the lips and cheeks and the teeth.
Oral cavity proper - the part of the oral cavity contained by the teeth.
Tongue - is a muscular organ that lies in the floor of the oral cavity proper. It is connected to the hyoid and the styloid process of the temporal bone.
Lingual frenulum - is a fold of mucous membrane that tethers the tongue to the floor of the mouth.
Palatine tonsils - lie within a recess formed by folds of mucous membrane that extend from the soft palate to the tongue and pharynx.
Lingual tonsils - are found at the base of the tongue.
Pharynx
     The pharynx is a passageway shared by both food and air. Muscles in the wall of the pharynx propel food into the esophagus.
Esophagus
     The esophagus is a muscular tube approximately 1 foot long that transports food and liquids from the pharynx to the stomach. The esophagus is where the alimentary tract becomes a tube which consists of four basic layers:
1. Mucosa
  The mucosa is essentially the mucous membrane that lines the cavity or lumen of the tube. It consists of a surface epithelium and its underlying connective tissue which is called the lamina propria.
2. Submucosa
  The submucosa underlies the mucosa and contains blood vessels, nerves, glands and lymphatic vessels.
3. Muscularis Externa
  The muscularis externa typically has an inner circular layer and an outer longitudinal layer of smooth muscle.
4. Serosa
  The serosa is the outermost layer of the tube when it is suspended in the ventral body cavity. It is lined by a serous membrane called the peritoneum and is suspended by a double fold of serous membrane called mesentery.
     The remainder of the alimentary canal will have these four basic layers with some modifications along the way. Review of Four Basic Layers
Stomach
     The functions of the stomach include:
  1. bulk storage of food
  2. mechanical breakdown of food
  3. chemical digestion of food
  Anatomy of the Stomach
     The stomach is an expanded J-shaped segment of the digestive tract. The medial surface of the stomach has a lesser curvature and the lateral surface has a greater curvature and the anterior and posterior surfaces are smooth and rounded.
     The stomach can be divided into 4 regions:
  1. Cardia – is the region of the stomach immediately surrounding the junction of the esophagus with the stomach.
  2. Fundus – is the region of the stomach superior to the gastroesophageal junction.
  3. Body – is the largest region of the stomach and lies between the fundus and the lower curving segment of the J.
  4. Pylorus – is the curve of the J. A muscular pyloric sphincter regulates the release of chyme into the duodenum.
     In the contracted stomach the mucosa is thrown into folds called rugae.  As the stomach expands the rugae flatten out to allow the stomach’s expansion.
  Mesenteries of Stomach
     The visceral peritoneum of the anterior and posterior surfaces fuse at the lesser and greater curvatures to form two prominent mesenteries:
  Greater omentumforms a pouch that hangs like an apron between the anterior abdominal wall and the abdominal viscera.
  Lesser omentum – is the mesentery between the lesser curvature and the liver.
  Musculature of the Stomach
     Both the muscularis mucosae and muscularis externa have extra muscle layers. The muscularis mucosae has an extra outer circular layer and the muscularis externa has an extra inner oblique layer.
  Histology of the Stomach
      The mucosal epithelium of the stomach is simple columnar and consists of mucus-secreting cells that produce a carpet of mucus. Gastric pits open on the surface of the mucosa and are connected to tubular gastric glands at their base. The neck is the place where the gastric glands and gastric pits join and the mucosal neck cells at this location continually divide to replace the cells that are lost.
  Gastric Secretory Cells
     The gastric glands produce gastric juice and have three secretory cells:
  1. Parietal cellssecrete hydrochloric acid and intrinsic factor. Intrinsic factor is necessary for absorption of vitamin B 12.
  2. Chief cells – secrete pepsinogen that is converted to the active proteolytic enzyme pepsin by the acids of the stomach.
  3. Enteroendocrine cells – G cells produce the hormone gastrin that stimulates gastric juice secretion and gastric motility.
Small Intestines
     The small intestines is where digestion and absorption of nutrients primarily occurs. The surface area of the intestinal mucosa is increased by transverse folds called plicae circulares. The intestines is about 6 m (about 20 ft in length) and is divided into three regions:
  Duodenum
  This initial segment is the shortest (about 25 cm or 10 inches long) and the widest segment. Except for a small segment where it is attached to the pylorus of the stomach, it is secondarily retroperitoneal. It receives chyme from the stomach and digestive secretions from the pancreas and the liver.
  The submucosa of the duodenum contains duodenal (Brunner’s) glands that produce copious mucus containing bicarbonate ion that rapidly changes the pH of the chyme coming from the stomach and protects the lining of the duodenum from its initial acidity.
  Jejunum
  The jejunum begins where the duodenum abruptly bends and becomes intraperitoneal. The jejunum is about 2.5 m (8 ft) long. The bulk of chemical digestion and nutrient absorption occurs here. Isolated lymphoid nodules are present in the lamina propria.
   Ileum
  The last segment of the small intestines is about 3.5 m (12 ft) long and ends where it joins the cecum at the sphincter called the ileocecal valve. Both the jejunum and ileum are supported by a mesentery called the mesentery proper. Lymphoid nodules are more numerous in the lamina propria of the ileum and may form aggregated lymphoid nodules (Peyer’s patches).
  Histology of the Small Intestines
     The mucosa of the small intestines has finger-like projections called villi (sing. villus). The epithelial cells of the villi have microvilli on their apical surface. Goblet cells that secrete mucins are found among the epithelial cells. The plicae circulares, villi and microvilli increase the surface area enormously.
     Tubular intestinal glands are found between the villi extending into the lamina propria. Cells in the intestinal glands continually divide and migrate onto the villi where they are eventually shed at the tips of the villi.
     The lamina propria of the small intestines have lymphatic vessels called lacteals that transport absorbed lipids to the circulation.
Large Intestines
     The large intestines is about 1.5 m (5 ft) long and frames the small intestines. Its major functions include:
  1. Reabsorption of water, electrolytes and compaction of feces.
  2. Absorption of vitamins.
  3. Storage of fecal material.
     The large intestines can be divided into three parts:
  Cecum
     The cecum is an expanded pouch that receives ingesta from the ileum through the ileocecal valve in its medial wall. The appendix is attached to the cecum and contains numerous lymphoid nodules.
  Colon (Colonoscopy)
     The colon makes up most of the large intestines and has several distinctive features:
  1. Haustra (sing. hastrum) – are pouches that permit distention and elongation.
  2. Taeniae coli – are three separate longitudinal ribbons of smooth muscle visible in the outer surface of the colon underneath the serosa.
  3. Fatty appendices of the colon (epiploic appendages) – are pockets of adipose tissue contained within pouch-like extensions of the serosa.
  Regions of the colon
     The colon can be divided into four regions, 1) ascending colon, 2) transverse colon, 3) descending colon, and 4) sigmoid colon.
     The ascending and descending colons are secondarily retroperitoneal on the right and left sides of the peritoneal cavity, respectively. The transverse and sigmoid colons are intraperitoneal and are suspended by mysentery.
  Rectum
     The rectum is the last 15 cm (6 in) of the large intestines. It temporarily stores feces prior to excretion. The last portion of the rectum is the anal canal which ends at the anus where the epithelium changes from simple columnar to stratified squamous. Distention of veins in the lamina propria and submucosa of the anal canal  are called hemorrhoids.
  Histology of Colon
     Distinctive histological features of the colon include:
  1. Absence of villi.
  2. Goblet cells are abundant
  3. Intestinal glands are deep and contain numerous goblet cells
  4. Presence of large, numerous lymphoid nodules
 

5. Longitudinal layer of muscularis externa have three thickened bands corresponding to the taeniae coli.