Gastro-intestinal tract parts
- Pharynx
- Esophagus
- Stomach
- Small intestine
- Duodenum
- Jejunum
- Ileum
- Large intestine
- Anus
Accessory structures:
- Oral cavity and
accessory parts - Salivary glands
- Liver
- Gall bladder
- Pancreas
FUNCTIONS OF THE
DIGESTIVE SYSTEM
Ingestion
Chemical
Peristalsis
Digestion
Absorption
Defecation
Ingestion
* Taking in food.
* Mechanical:
* Mastication = chewing.
* Co-ordinated movements of the mandible, teeth, lips, tongue.
* Chemical - Serous secretion moistens the food:
* Mucous secretion binds the food bolus ready for deglutition = swallowing.
- Peristalsis = continuous involuntary muscle movements to move the bolus of food into the stomach.
- Digestion
- Breaking food down both physically and chemically.
- Chyme = semi-digested liquid.
- Sphincters – serve as “gates” between different parts of the digestive system.
- Absorption
- Digestive enzymes from the pancreas and bile from the liver will enter into the duodenum to facilitate fat digestion.
- Movement of nutrients into the bloodstream.
Defecation
* Rids the body of indigestible waste.
ORAL MUCOSA
- Epithelium – SSNK
- Exceptions – lips and dorsal tongue (SSK).
- Epithelium supported by lamina propria – DICT.
- Many sensory receptors.
Saliva produced by salivary glands:
* Serous secretions
* Mucous secretions
* Seromucous secretions
LIPS
Exterior surface (S)
* SSK
* Sebaceous and sweat glands
Transitional zone (V)
* SS
* Occasional sebaceous glands (no sweat glands or hair follicles).
- Interior (O)
- SSNK
- Lamina propria
- Skeletal muscles (M)
- Salivary glands (G)
TONGUE
parts anterior and posterior
- Muscular organ covered by oral mucosa.
- Solid mass of skeletal muscles
– multidirectional orientation of muscle fibers.
Specialized in:
* Manipulating of food
* Sensory function of taste
* Speech
* Divided into anterior 2/3 and posterior 1/3 by sulcus terminalis
Anterior tongue is made up of papillae:
* Filiform
Threadlike bristlesm-Tastebuds absent
* Fungiform
Mushroom- shaped- Tastebuds present
* Foliate
Leaf-life- Tastebuds present
* Circumvallate
Serous glands at the base of papillae
Numerous tastebuds
Posterior tongue is composed of:
* SSNK epithelium
* Lymphoid tissue
* Waldeyer’s ring – made up of lingual tonsil, pharyngeal tonsil and palatine tonsils
SALIVA
Thin watery suspension.
* Secreted by:
* Serous cells-Enzymes, mucous, inorganic
ions, antibodies.
* Mucous cells -Mucinous substances.
Functions:
* Cleansing action on the teeth.
* Moistens and lubricates food during mastication and swallowing.
* Dissolves certain molecules so foods can be tasted.
* Initiates chemical digestion of starches with the help of an enzyme called salivary amylase.
SALIVARY GLANDS
Parotid glands
* Submandibular glands
* Sublingual glands
PAROTID GLANDS
Morphology: Compound acinar.
Secretion type: Mostly serous substances.
SUBMANDULAR GLANDS
Morphology: Compound tubuloacinar.
Secretion type: Serous and mucous
substances
SUBLINGUAL GLANDS
Morphology: Compound tubular
Secretion type: Mostly mucous substances
- all have Compound glands with lobules separated by septae (made up of collagen fibers).
- With branching ducts
- Intralobular ducts Ducts “within the lobes”
smaller diameter - Interlobular ducts Ducts “between the lobes” Larger diameter.
SALIVARY SECRETORY UNIT
Serous cells
Mucous cells
Myoepithelial cells
Serous cells
* Secrete serous substances.
* Shaped like pyramids.
* Nuclei are round
Mucous cells
* Secrete mucous substances.
* Shaped like pyramids.
* Nuclei are flat.
Myoepithelial cells
* Located on the surface of the secretory units.
* resemble smooth muscle cells.
* As myoepithelial cells contract, they press on the acinus, releasing secretory product into the duct of the gland.
* They lie between the basal lamina and the epithelial cells of the acini.
LIVER
- Largest gland in the body
- Lies beneath the diaphragm.
- Lobes: left and right separated by falciform ligament.
- Functional unit: Hepatocytes.
- Exocrine function: Bile formation and secretion.
- Endocrine function: Production of some plasma proteins and hormones.
- Detoxification.
- Metabolism of food (fat,
carbohydrate and protein). - Storage of iron, glycogen and vitamins A, B12 and D.
- Site of fetal hematopoiesis
- Glisson’s capsule – DICT.
- Lined by mesothelium – simple cuboidal.
HEPATOCYTES
- Functional units of the liver.
- Large, polyhedral cells with round nuclei and prominent nucleoli; sometimes exhibiting binucleation (2 nuclei).
- Structures in between the hepatocytes are sinusoids.
- Presences of lipofuscin (wear and tear pigment).
- Supported by reticular connective tissue.
- Store glycogen.
- Form bile.
- Hepatocytes + phagocytic Kupffer cells - remove bacteria and worn out RBCs from the blood.
- Absorb nutrients and secrete products into the bloodstream.
CELLS LINING THE SINUSOIDS in the liver
- Endothelial cells : squamous’ cells lining the endothelium
- Kupffer cells
- AKA Reticuloendothelial cells - monocytemacrophage defense system + removal of old RBCs
- Stellate/Ito cells or hepatic lipocytes
- Produce lipid droplet with vitamin A and produce ECM + collagen
LIVER LOBULES
- Pentagon or hexagon shaped cylinder made of hepatocytes arranged as thin plates separated by sinusoids.
- Both hepatocytes and sinusoids radiate towards the center of the lobule.
- At the center of each lobule is a centrilobular vein/central.
- Portal triad at each corner of the lobules:
- Bile duct
- Portal vein
- Hepatic artery
GALL BLADDER
- Muscular sac like structure.
- Hollow organ
- Stores, concentrates and releases bile that the liver produces.
- Carries bile into the small intestine to aid in digestion
HISTOLOGICAL FEATURES of the gall bladder
- MUCOSA = Epithelium + Lamina propria.
* Epithelium: Simple columnar with microvilli.
* Lamina propria: LOCT.
* Mucosa has infoldings called rugae. - SUBMUCOSA
* LOCT, vascular, elastin and collagen fibers, lymphatic vessels. - MUSCULARIS EXTERNA
* Smooth muscles in all directions, not layered. - SEROSA/ADVENTITIA
* LOCT.
PANCREAS
Large gland ~15cm long.
* Located in the abdomen in the curve of the duodenum.
Has both exocrine and endocrine functions:
EXOCRINE
* Majority of the pancreas.
* Cells of acinar units secrete digestive enzymes and pancreatic juices.
* Secretions drain into a duct system.
* Morphology: Compound acinar
* Lobulated by CT septae
* Intralobular (within the same lobule) & interlobular ducts (between lobules).
ENDOCRINE.
* Cells secrete hormones essential for control of carbohydrate metabolism.
* Secretions go directly into the capillaries.
* Islets of Langerhans (pancreatic islets)
* Morphology: Clump and cord
Cells:
* Alpha cells – glucagon.
* Beta cells – insulin.
* Delta cells.
* Polypeptide cells.
- Three components of mucosa and its transitional changes
- Surface epithelium: either SSNK or simple columnar types.
- Glandular epithelium: either simple branched tubular or simple tubular.
- Lamina propria
- LOCT supporting the epithelium.
- Lymphoid tissues found here – MALTs (GALTs)
- Muscularis mucosae
- Narrow band of smooth muscles.
- Gastroesophageal junction:
* Between esophagus and stomach
* SSNK → Simple columnar (squamocolumnar junction). - Gastroduodenal junction:
* Between stomach and duodenum.
* Simple branched tubular → Simple tubular. - llecocecal junction:
* Between ileum and cecum
* With villi → without villi. - Rectoanal junction:
* Between rectum and anus
* Simple tubular → SSK.
SUBMUCOSA
- “Just beneath the mucosa”.
- LOCT
- Thick bundles of collagen fibers.
- Many blood vessels + lymphatics + nerves.
- Meissner’s plexus (nerves)
- Innervate the muscularis mucosa and mucosal glands.
- Submucosal glands
- Compound tubular glands of the esophagus and duodenum.
MUSCULARIS EXTERNA
- Inner and outer layers of smooth or skeletal muscles.
- Exceptions:
- Upper third of the esophagus has two layers of skeletal muscles.
- Stomach has three layers of smooth muscles.
- Auerbach’s plexus (nerves)
- Located between the muscle layers.
- Provides innervation to these smooth muscles.
SEROSA/ADVENTITIA
- AKA serous membrane.
- The outermost layer
- LOCT - supporting blood vessels, nerves and adipose tissue.
- Serosa
- Responsible for covering external surfaces of the organs that are faced with the body cavities.
- Adventitia
- Responsible for attaching the organs to the surrounding tissues.
ORGANS OF THE GIT
- Pharynx
- Esophagus
- Stomach
- Small intestine
- Large intestine
- Appendix
- Anus
ANATOMY OF PHARYNX
- Nasopharynx
* Not part of the digestive system. - Oropharynx
* Posterior to the oral cavity.
* Involved in swallowing. - Laryngopharynx
* Below the oropharynx and connected to the esophagus
- Serves as passageway for food and air.
- Passageway for food:
- Mouth → oropharynx → esophagus → stomach.
- Food is propelled to the esophagus by two skeletal muscle layers:
- Inner longitudinal layer
- Outer circular layer
- Food movement is facilitated by peristalsis
Mucosa
* Epithelium – SSNK.
* Lamina propria – LOCT + increased amount of elastin fibers
- Submucosa- Absent.
Muscularis externa
* Skeletal type – striated, voluntary (SV).
* Inner longitudinal
* Outer circular
- Outer layer – buccopharyngeal fascia
- LOCT + nerves + blood vessels
ANATOMY OF ESOPHAGUS
- Conducts food by peristalsis.
- Passageway for food only epiglottis cartilage flaps up or down for eating and breathing passageways
- First segment of the digestive tract.
- Has 3 regions:
1. Upper 3rd
2. Middle 3rd
3. Lower 3rd - Has 2 sphincters:
1. Upper esophageal sphincter – skeletal muscles
2. Lower esophageal sphincter – smooth muscles
Mucosa
* Protective type.
* Epithelium – SSNK.
* Lamina propria – LOCT
* Muscularis mucosa – skeletal/smooth muscle.
* Lymphoid tissues (MALTs).
Submucosa
* Mucous secreting esophageal glands (compound tubular)
* LOCT
* Meissner’s plexus
Muscularis externa
* 2 layers:
* Inner circular
* Outer longitudinal
* Skeletal – upper 3rd
* Skeletal + smooth – middle 3rd
* Smooth – lower 3rd
* Auerbach’s plexus.
- Adventitia
- LOCT.
GASTROESOPHAGEAL
JUNCTION AKA: Squamocolumnar
junction.
- Food - Enters at the lower esophageal sphincter.
- Empties into the small intestine at the pyloric sphincter (valve).
- Stomach is located on the left side of the abdominal cavity
- Abrupt change in mucosa
from SSNK (esophagus) →simple columnar (stomach). - This junction provides a physiological sphincter to prevent reflux
- Affected by gastric reflux.
- If metaplasia occurs – Barrett’s esophagus.
- High risk for developing dysplasia and invasive carcinoma.
