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Digestive system. Roles. Digestion Break down food into particles that can be used by the cells Absorb these particles and bring them into the body Eliminate wastes Maintain blood homeostasis Deal with toxins Synthesize blood proteins Regulate blood components.

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roles
Roles
  • Digestion
    • Break down food into particles that can be used by the cells
    • Absorb these particles and bring them into the body
    • Eliminate wastes
  • Maintain blood homeostasis
    • Deal with toxins
    • Synthesize blood proteins
    • Regulate blood components
the gi tract
It is the tube that starts at the mouth and ends at the anus

The food is processed along this tube

This tube is formed by 3 layers. The layer components vary with the location of the track

The GI tract
gi accessory organs
Liver and pancreas

These organs secrete enzymes and juices that help digestion

Food particles do not enter these organs

GI accessory organs
gi tract structure
Four basic layers:

Mucosa (secretion) – include also a small ring of muscles (muscularis mucosae) with a nervous plexus (submucosal plexus = Meissner’s plexus)

Muscular layers (movement) – circular and longitudinal fibers with a plexus, the myenteric plexus (Auerbach plexus)  responsible for peristalsis

Serosa (protection), connective tissue, for protection is just an extension of the visceral peritoneum

GI tract structure
peristalsis

Involuntary

Peristalsis

Contraction of circular muscle behind the bolus

Contraction of longitudinal muscle IN FRONT of the bolus

Peristalsis: coordinated contraction of both circular and longitudinal muscle layers of the G.I. musculature as controlled by the enteric nervous system

enteric nervous system
Enteric Nervous System

Submucosal plexus: control secretion

Myenteric plexus: control peristalsis

unique features
Unique Features
  • Epithelia of the G.I. tract has a very high turnover/production rate.
    • Surface epithelia will renew itself every 2-6 days depending on the region of the intestinal tract.
    • Anti-cancer therapies (targeting rapidly dividing cells) often have the side-effect of targeting the gut as well.
mouth oral cavity
Mouth = oral cavity

Cheek, lips, hard & soft palate

Lips attached to gums via labial frenulum

Palate = roof of oral cavity

Hard palate = maxillae & palatine bones

Soft palate = muscle-reinforced region dorsal to the hard palate  palatine uvula

Tongue: move food around & helps for mastication

Lingual tonsils can be found at the base of the tongue

Mouth = Oral Cavity
salivary glands
Salivary glands
  • Saliva from 3 paired glands:
    • Parotid gland (between ear & masseter, largest gland) – innervated by glosspharyngeal (parasympathetic) + T1-T3 of the cervical ganglia of the sympathetic
    • Submandibular (under middle of mandible) - innervated by facial (VII) nerve (parasympathetic) & T1-T3 of the sympathetic
    • Sublingual (posterior to submandibular) - innervated by facial (VII) nerve (parasympathetic) & T1-T3 of the sympathetic
  • Parasympathetic = mucus rich secretion, sympathetic = watery
mastication chewing
Muscles involved in mastication:

Masseter

Temporalis

Medial & lateral pterygoid

ALL innervated by mandibular branch ofTrigeminal (V) nerve…V3

CN (V) palsy = inability to perform mastication

Voluntary

Mastication (chewing)
teeth
Humans have “heterodont” dentition (various types of teeth)

Incisors, canines, premolars & molars

Incisors & canines = single root teeth

Premolars & molars = multiple roots

Mapped using “dental formula” = # incisors, canines, premolars/molars in maxillary portion (assumes everything is equal) = (I 2/2, C 1/1, M 3/3) X 2

Teeth
teeth1
Humans are also “diphyodont”: 2 sets of teeth during their lifespan

Deciduous teeth (milk teeth) begin eruption at 6 months

Full set by 2.5 years (full set = 20 teeth)

(2/2, 1/1, 2/2) X 2 = 20 teeth

Permanent teeth replace deciduous teeth (all the way through late teens)

Final formula = (I 2/2, C 1/1, PM2/2, M 3/3) X 2 = 32 teeth

Teeth
teeth structure
Crown: above the gum

Neck: at the gum (gingiva) level

Root: within the bone

Outer layer: Enamel the hardest substance in the body, protects tooth. Subject to decay, on the crown only

Dentin: softer, throughout the entire tooth

Cementum: in the root, outer layer

Pulp: made of blood vessels and nerves, present in the pulp cavity

Periondontal ligament: attaches the tooth to the bone

Teeth - structure
esophagus
Stratified squamous epithelium (not a “simple” epithelium)

Combination of skeletal & smooth muscle (combination of voluntary & involuntary control)

Esophagus
stomach
Several regions: from lower esophageal sphincter Cardia – fundus – body – antrum

Several layers of muscles: longitudinal, oblique and circular

Epithelium folded and forming rugae

Stomach
slide20

Cardia

Fundus (storage)

Lower esophageal sphincter

3 muscle layers

longitudinal, circular, transverse/oblique

Body/corpus

(acid & pepsin secretion, storage)

Pylorus

(resistance)

Antrum

(acid control & mixing)

gastric epithelium
Chief cells: secrete pepsinogen (inactive precursor to pepsin; protein hydrolase) & prochymosin (precursor to chymosin; coagulates milk)

Parietal cells in body/corpus & antrum secrete HCl & intrinsic factor

G-cells in the antrum secrete gastrin (endocrine hormone) that triggers both parietal cells & Chief cells

Enterochromaffin-like cells (ECL) secrete histamine (works in to magnify gastrin response)

Goblet cells secrete mucus

Gastric epithelium

Epithelium folds and forms pits

Along the pits, several types of cells

small intestine
Small intestine
  • Three sections:
    • Duodenum
    • Jejunum
    • Ileum
  • Total length: about 25 feet
small intestine1
Small intestine

Small intestine

  • Surface area:  100-200 m2 ( tennis court) – 25 feet long
    •  greater area for nutrient absorption
    • cells)
  • Epithelial surface area is amplified by a number of structures:
    • Plica muscularis/circularis (2-5X increase in surface area)
    • Villi (10-30X increase in surface area)
    • Microvilli of each absorptive enterocyte (200-400X increase in surface area).
      • Folds of the apical plasma membrane)

Plica

Villi

Microvilli

duodenum
Duodenum

From stomach to jejunum

Shortest section, about 20 cm long.

Secretions from the pancreas and liver (bile) enter duodenum viaSphincter of Oddilocated in theampulla of Vater (=hepato-pancreatic duct)

Region of the intestine where most of food breakdown occurs

slide25

Jejunum

Important for nutrient absorption

Mucosa has many fold, the villi and microvilli  increase surface for reabsorption

Epithelium is columnar epithelium

Within each villus: blood capillaries and 1 lymphatic capillary, the lacteal  nutrient reabsorption

ileum
Ileum

Last section of the small intestine

2-4 m long

End at the cecal valve, point of entry into the large intestine

Site of absorption of remnants from jejunum, bile salts

ileocecal valve
Ileocecal Valve
  • Interface between ileum (terminal small intestine) & large bowel
    • Acts to limit amount/rate of nutrient entry into the large bowel
    • Initial “entrance” into large bowel = cecum
      • Enlarged region of the proximal large bowel
      • Appendix is located in the cecum
        • Dense collection of lymph nodes to allow immune system to sample/measure colonic microflora
large intestine colon
Large Intestine (colon)

Involuntary

  • Colon (ascending, transverse, descending & sigmoid) serves to absorb electrolytes, and remaining water from the chyme
  • As water is reabsorbed, the leftover material becomes thicker in consistency  stool formation
  • Colonic epithelia is generally crypt-like;
    • large capacity to absorb water and electrolytes through surface epithelia
    • crypt-like architecture also provides enormous secretory capacity
slide30

Colonic musculature is unique:

    • longitudinal muscle layer is “banded” into 2-3tanea coli
    • circular muscle contracts in “rings”haustraor haustrations
  • Musculature mixes colonic contents very well
rectum
Rectum
  • For fecal eliminatrion  defecation
  • Presence of 2 sphincters near the anus:
    • Anal smooth muscle sphincter (involuntary)
    • Anal striated muscle sphincter (voluntary)
slide32

Pelvic diaphragm: support intestines

    • Levator ani pubococcygeus (anal sphincter), puborectalis, and iliococcygeus.
    • Coccygeus
  • Urogenital diaphragm (perineum)
    • Sphincter urethrea (external sphinter)
    • Deep transverse perineus
accessory gastrointestinal organs liver
Accessory Gastrointestinal Organs: Liver
  • Largest internal organ
    • 4 lobes
      • Right&left lobesseparated byfalciform ligament
        • “round ligament/ligamentum teres” attached to falciform ligament = remnant of umbilical vein
      • Caudate lobeclosest to the IVC
      • Quadrate lobeadjacent to gall bladder
liver
Liver
  • Receives blood from the portal vein, from the intestines
  • Blood is too rich in nutrient and need to be dealt by the liver
    • Storage of nutrient
    • Synthesisis of various compact
    • Detoxification
  • Blood in homeostatic equilibrium then return to the circulation via the central vein  hepatic vein  inf. vena cava
  • Hepatic artery brings oxygen to the liver
liver hepatic lobule
Liver: hepatic lobule
  • The histological/functional units of the liver
  • Composed of rays of cells with sinusoidal capillaries
  • Cells = hepatocytes (liver cells)
    • Interspersed with “Kupffer cells” = resident macrophages
    • The hepatocytes draw nutrients from the portal vein and store/modify them
    • They detoxify toxins
    • They synthesize bile which is collected by the bile ducts
slide37

Liver and bile ducts

The small bile ducts fuse and form the left and right hepatic ducts

The hepatic ducts fuse to become the common hepatic duct

The bile, passing into the cystic duct, will be store in the gallbladder

When needed (after a meal, the gallbladder will contract and empty the bile into the common bile duct

The common bile duct passes around the duodenum, enters the pancreas and fuses with the pancreatic duct to form the hepatopancreatic duct (ampul of vater)

The short duct empties into the duodenum through a sphincter, the sphincter of Oddi

gall bladder
Gall Bladder
  • Stores & concentrates bile
    • Lined by smooth muscle to aid in contraction during bile mobilization
    • Can be afflicted by the formation of stones
slide39
ERCP

Diagnostic Tests: ERCP (Endoscopic Retrograde CholangioPancreatography)

To remove gallbladder stones

ERCP are done to remove gallbladder stones

pancreas
Pancreas
  • Mixed gland (endocrine & exocrine)
    • Islets of Langerhans= endocrine portion
    • Pancreatic acini= exocrine portion
    • Head is peritoneal
      • Body & tail are secondary retroperitoneal(similar to duodenum)
    • VERY rich in blood supply
      • Pancreatic artery & pancreaduodenal branch of the SMA
    • Exocrine secretions = proteases, carbohydrases, lipases, bicarbonate
slide41

Peritoneum

Two layer membrane surrounding most organs in the abdomen

Outer layer= parietal peritoneum, located against the abdominal wall

Inner layer: visceral peritoneum, surrounding the intestines

The space between the 2 layers is the peritoneal space has a thin layer of fluid which prevents bowel friction

Some abdominal organs are external to the peritoneum (kidneys)

slide42

Mesentery

Formed by a double layer of visceral peritoneum

Binds bowel loop

These layers contain blod and lymphatic capillaries as well as nerves going to the intestinal wall

The greater omemtum: Large fold over the abdomen

The lesser omentum: fold between stomach lesser curvature and liver

colostomy
Colostomy

Sometimes, fecal matter needs to be diverted into a bag because of diseased bowels  ileostomy – colostomy

hirschsprung disease
Congenital defect where the nerve plexuses within the intestinal wall are missing  no peristalsis  fecal matter is blocked and accumulates  the colon enlargesHirschsprung disease