By Amanda Neufeld & Sheri Mullen

The digestive system is a complex system consisting of the oral cavity, pharynx, esophagus, stomach, small intestine and the large intestine; also includes the salivary glands, hepatobiliary apparatus( liver and gallbladder), and pancreas which includes accessory organs The development of this system starts at about 6-8 wks after fertilization. This complex system is responsible for our ability to live. Each part of the digestive system is important to the bodies over homeostasis.
The muscle of the digestive system are mainly smooth muscle in nature, however they can also consist of skeletal as well.
The mouth and the pharynx are skeletal and develop from the branchial mesoderm. The esophagus is part smooth as well as skeletal, leaving the rest of the GI tract as smooth muscle that develops from the surrounding mesoderm.
The lining of the structures first arises for the primitive gut tube. This tube consists of three parts; the forgut, midgut, and the hindgut.

* forgut : gives rise to the region from the oral cavity to the proximal half of the duodenum.
* midgut :gives rise to the region from the distal half of the duodenum to the right 2/3rds of the transverse colon.
* hindgut: gives rise to the region from the left 1/3 of transverse colon to the superior portion of the anal canal.
All the digestive organs arise from the primitive gut and while developing they develop next to the peritoneum cavity. This cavity consist of two layers the visceral, and parietal. The parietal layers surrounds the entire digestive forgut midgut hindgut

Peritoneum
Most of the digestive system organs resides in the abdominal cavity and are contained in the peritoneum. The visceral peritoneum covers the external surfaces of most digestive organs and is continuous with the parietal peritoneum that lines the body wall. As the organs develop they invaginate into the visceral peritoneum, others are either partly covered or lie behind. The organ that have invaginated into the visceral peritoneum have folds or two layers called mesentery which the blood vessels and nerves pass through. Organs that lie behind the visceral peritoneum are called retroperitoneal.
Organs inside the visceral peritoneum.
•Stomach
•Small intestine
•Sigmoid colon
•Transverse colon
•Jejunum and ileum visceral parietal
Retroperitoneal organs
*pancreas
*ascending colon
*descending colon
*rectum
*aorta
*duodenum

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Musclaris Externa: is just deep to the submucosa and also called muscularis. This layer is responsible for segmentation and peristalsis. It typically has two layers. The inner circular layer and the outer longitudinal layer of smooth muscle cells. In several places along the tract the circular layer thickens forming sphincters. In between these two layer there are nerve plexus called Myeriteric nerve plexus, which feed the meissner’s nerve plexus.
circular longitudinal
Serosa / Adventitia: This layer is named depending on whether or not it’s covered by the visceral peritoneal cavity. ( exp. Esophagus) If it is covered it is referred to as serosa.
* serosa- areolar connective tissue covered with mesothelium.
* adventitia- fibrous connective tissue.
Submucosa: is just external to the mucosa
It’s made up by connective tissue and contains blood and lymphatic vessels, lymphoid follicles, and nerve plexuses. This layer is lax providing elasticity for mobility. Nerve plexus in this area is known as Meissner’s plexus (parasympathetic in nature) And found mainly in the esophagus and duodenum.
Submucosa gland
Mucosa: is the inner most layer and goes from the mouth to the anus. It functions to secret mucus, digestive enzymes and hormones; it helps with the absorption of end products of digestion into the blood; and protects against infectious disease. The mucosa contains three sub layers.
* lining epithelium
* lamina propria
* muscularis mucosa

Celiac axis:
Left gastric
Common hepatic artery splenic
The splanchnic circulation includes the arteries that branch off the abdominal aorta to serve the digestive organs.
The arterial supply ( hepatic, splenic, and left gastric branches of the celiac trunk serves the spleen liver and stomach.
Sup. and Inf. mesenteric Abdominal aorta
Sup. mesenteric
The mesenteric arteries serve both the large and small intestines .
Inf. mesenteric

Salivary Glands
Secrete saliva into the oral cavity, lubricating, dissolving and beginning the chemical digestion of food that has been ingested. They arise from the forgut and receive blood through the celiac axis. They consist of rounded acini which can be ether mucus and serous secreting. The epithelium of larger ducts are lined by stratified cuboidal epithelium, and small ducts are lined by cuboidal epithelium.
Sublingual
Submandibular
Parotid

Parotid:
Is serous gland containing serous acini.
Anterior to the tragus of the ear and covered by thick fascia.
Submandibular: both serous and mucus secreting glands. Lies in the mandibular fossa, ducts opening on the floor of the mouth.
Sublingual: mainly mucus secreting. They are
Ant. and sup. to the submandibular gland opening through multiple ducts on the floor of the mouth.

Pancreas: contains acini darkly staining, and ducts from the exocrine portion. The endocrine portion contains the islets of langerhans which show up as a cluster of cells, surrounded by lots of capillaries and blood vessels.
Pancreas
Portal triad
Liver: Is made up of hepatocytes arranged as plates around the central vein. The hepatocytes receive nutrients and oxygen through the sinusoids, in which the hepatic arterial and portal veins drain.
Both the sinusoid and bile canaculi lie in between each hepatocyte. – portal triad consists of a branch of the hepatic artery, and portal vein, also including a bile ductule.
Liver

Pharynx and Esophagus
Through progression of coordinated contraction and relaxation (peristalsis) food is propelled down to the stomach.
Pharynx Esophagus
The muscular layer is skeletal and
Consists of an inner longitudinal and outer circular.
Is contained mainly in the thoracic but passes through the diaphragm
Into the abdomen. pharynx esophagus
Gastro-esophageal or cardio-esophageal junction guarded by the physiological sphincter. (smooth muscle)

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Lies in left hypochondriac, epigastric & umbilical region.
Cardia- surrounds the superior opening of the stomach
Fundus- The rounded portion superior and to the left of the cardia
Body- inferior to the fundus . Large central portion on the stomach
Pylorus-communicates with the duodenum via the pyloric sphincter. Has two parts.
Pyloric antrum connects to the body of the stomach. Pyloric canal leads into the duodenum.
Lesser curvature-concave medial border.
Greater curvature-convex lateral border.

The lobes of the liver are made up of many functional units called Lobules. A lobule is typically a six-sided structure that consists of specialized epithelial cells, called
hepatocytes , arranged in irregular, branching, interconnected plates around a
central vein. Instead of capillaries, the liver has larger, endothelium-lined spaces called sinusoids, through which blood passes. Also present in the sinusoids are fixed phagocytes called stellate reticuloendothelial (KUPFFER) cells, which destroy worn-out white and red blood cells, bacteria, and other foreign matter in the venous blood draining from the gastrointestinal tract.
Bile, which is secreted by hepatocytes, enters Bile canaliculi, which are narrow intercellular canals that empty into small bile ductules. The ductules pass bile into bile ducts at the periphery of the lobules. The bile ducts merge and eventually form the larger right and left hepatic ducts, which unite and exit the liver as the
common hepatic duct. Farther on the common hepatic duct joins the cystic duct from the gallbladder to form the common bile duct.

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Is both and exocrine and endocrine gland
It is retroperitoneal
Head, uncinate process, body and tail
Head lies in the C of the duodenum.
Tail is related to the spleen
Large pancreatic duct runs through the length of the gland and usually joins the bile duct to drain into the duodenum at the major duodenal papilla
Minor or accessory duct may also be seen

DUODENUM- The shortest region, is retroperitoneal and C shaped. It starts at the pyloric sphincter of the stomach and extends about
25cm (10 in.) until it merges with the jejunum.
Jejunum- is about 1 m (3 ft) long and extends to the ileum. Possess mesentery.
Ileum-Longest region of the small intestine. It measures about 2 m (6 ft) and joins at the large intestine at the ileocecal sphincter. Possess mesentery.

SMALL INTESTINES epithelium is thrown into evaginations called villi which increase surface area for absorption and also secrete a few enzymes to complete digestion

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The overall function of the large intestine is the completion of absorption of water , and production of certain vitamins, the formation of feces, and the expulsion of feces from the body.
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Extends from the ileocecal valve to the anal opening.
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Is about 1.5 m long in a cadaver