Digestive System is a collective term that includes a set of organs which do the following functions:
1. ingestion
2. Secretion
3. transportation or the mixing and propulsion
4. digestion
5. absorption
- .elimination
The organs of the Digestive system is divided into two group:
1. Gastrointestinal tract – also called the alimentary tract. mouth, esophagus, stomach, small intestine, and large intestine including anus.
2.accessory organs – includes teeth, tongue, salivary glands, liver, pancreas, and gallbladder.
Layers of the GIT
1. Mucosa – is the innermost lining of the GIT. It is consists of epithelial cells, loose connective tissues and a circular muscle that separates it from the next layer. Epithelial cells are continueously slough off and replaced every 5-7 days
2. Submucosa – compose of connective tissues, blood vessels, lymphatic vessels, and nerve fibers (Meissener’s plexus) which is responsible for controlling the secretion of GIT.
3. Muscularis – these are layers of skeletal muscles (mouth and esophagus), smooth muscles ( rest of the tract), and nerve fibers that mostly controls the motility (myenteric or Auerbach’s plexus)
4. Serosa or Adventitia – it is the outermost layer of the GIT. Compose of connective tissues and epithelium in most of esophagus and rectum. Connective tissues and visceral peritoneum in other part of the tract
- Also called buccal or oral cavity
- Formed by the cheek, hard and soft palate lips and the tongue
- Contains accessory organs: teeth and salivary glands
- Ptyalin is a salivary amylase secreted with the saliva that helps in the breakdown of starch
- Both mechanical and chemical digestion occurs in the mouth
Esophagus
- Is a muscular tube that is 2.5cm in diameter and about 25 cm long extending from pharynx to stomach
- Posterior to trachea and anterior to vertebral column
- Pierces to the diaphragm through hiatus
- Contains 2 sphincter:
UES- tracheoesophageal sphincter
LES – cardiac sphincter
Stomach
- J- shaped enlargement of the GIT inferior to the diaphragm
- Regions of the stomach: cardia, fundus, body,antrum, pylorus
- Pyloric sphincter – connects stomach to small intestines
- Rugae – folds in the lining of stomach when empty
- Lesser and greater curvature
Gastric glands cells
- Mucous neck cells- Secretes mucous that serves as the protection of the stomach from HCl
- Chief cells- Pepsinogen and gastric lipase
- Parietal cells- Intrinsic factor and HCL
- G cells- Gastrin – stimulates parietal cells to secrete HCl and chief cells to secrete pepsinogen; contracts lower esophageal sphincter, increase motility of stomach and relaxes pyloric sphincter
Functions:
1. Form chyme by mixing saliva, food and gastric juices
2. Temporary reservoir of food
3. Secretes gastric juice which is composed of HCl, gastric lipase, pepsin, intrinsic factor
4. Chemically digests protein by pepsinogen, lipids by lipase
5. Aids in absorption of Vit B12 by intrinsic factor
6. Killing of some bacteria from food
7. Secretion of gastrin
Small Intestines
- Averages of 2.5 cm in diameter and 3m long
- Extends from the pyloric sphincter to the ileocecal sphincter
- Divided into 3: duodenum, jejunum and ileum
- Mechanical digestion involves segmenting and migrating motility
- Parasympathetic increases motility thus sympathetic decreases
- Absorption of nutrients, electrolytes, vitamins and water
- Structural features:
o Villi- functional units of the small intestines; fingerlike projections located in the mucous membrane which contains goblet cells that secretes mucus and absorptive cells that absorb digested food
o Cryps of lieberkuhn – produces secretions containing digestive enzymes
o Brunner’s glands – found in the submucosa of the duodenum that secretes mucus
Large Intestines
- Extends from ileocecal sphincter to the anus
- Regions are cecum, colon (ascending, transverse, descending), rectum, anal canal
- Haustral churning and peristalsis
- Bacteria of large intestines produces some B vitamins and vitamin K
- Absorption of some water, electrolytes
- Process of defacation
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