Digestive System 1&2

Front 1

alimentary tract

Back 1

A muscular tube lined internally by an epithelium

• Epithelium varies in type along tract dependent upon function

• Structure of musculature is similar throughout

Front 2

functions of alimentary tract

Back 2

• Ingestion

• Processing

• Digestion

• Absorption

• Excretion

Front 3

oral cavity

Back 3

• Cavity lined by stratified squamous epithelium

• Ingestion and fragmentation of food

• Salivary glands- lubricating fluid + enzymes that break down carbs

• Tongue – highly muscular + squamous epithelium
  - manipulates food for mastication
  - taste

• 3 major types of papillae:
  a. circumvallate (bitter taste buds)
  b. fungiform (sweet and salty taste buds)
  c. filiform (most numerous, no taste buds)

Front 4

transport passages of alimentary tract

Back 4

Pharynx

Oesophagus

Anal canal

• Simple muscular transport tubes

• Lined with stratified squamous epithelium

Some mucous glands – lubrication

Front 5

digestive tract

Back 5

Stomach

Small intestine (duodenum, jejunum, ileum)

Large intestine (caecum, appendix, colon, rectum)

• Mucosa - most variable component of tract

• Many epithelial cell types - both absorptive & secretory

• Efficiency of absorption improved by increasing surface area of contact between epithelia and lumen
  - Intrusions/folding of epithelium (villi or plicae)
  - Inversions (tubular structures), the lumen of which
  communicate with main lumen

• Formation complex glands

Front 6

4 layers of alimentary canal

Back 6

• Mucosa (innermost)

-epithelium

-lamina propria

-muscularis mucoase

• submucosa
• muscularis externa

-longitudinal and circular muscle

• Serosa

-epithelium

-connective tissue

Front 7

Mucosa

Back 7

• Epithelium (epithelial lining) e.g. mucous membrane
  - Mainly columnar epithelium
  - Moistened by glandular secretions

• Lamina propria
  - Loose connective tissue
  - Small blood vessels, lymphatics, nerve fibres
  - Immune cells e.g. macrophages and lymphocytes

• Muscularis mucosae
  - Thin muscle layer

Front 8

submucosa

Back 8

• 2nd layer of loose connective tissue

• Between mucosa and main muscle layers

• Blood vessels & lymphatics and nerves

• Neural tissue – submucosal plexus
  - Regulates contractions & glandular secretions

Front 9

Muscularis externa

Back 9

• Smooth muscle typically divided into two differentially orientated layers
  - Inner circular layer
  - Outer longitudinal layer

• Arranged in layers which orientate in different directions → Contractions propel materials along tract
  Actually arranged spirally with circular level being a compact spiral and longitudinal level being an elongated helix

• Second nerve plexus located between muscle layers – myenteric plexus

Front 10

serosa/adventitia

Back 10

• defines the alimentary canal and covers the muscularis externa

• Called serosa – when the outermost layer lies adjacent to the peritoneal cavity
  - connective tissue with simple squamous epithelium (mesothelium)
  stomach, majority of small intestine & large intestine covered by mesothelium
  but not part of duodenum and an area of colon
  retroperitoneal digestive tract (oral cavity, pharynx, oesophagus, rectum) has no serosa
  - same composition as mesentery
  mesentery = sheets of connective tissue holding the loops of the gastrointestinal tract
  - comprises the visceral peritoneum
  cf. parietal peritoneum lines peritoneal cavity

• Called adventitia – when layer is attached to surrounding tissue
  - fibrous connective tissue

Front 11

increasing surface area improves....

Back 11

efficiency of absorption

(plica circulares)

Front 12

oesophagus

Back 12

~25cm long, leads through diaphragm to stomach

Mucosa
- Stratified squamous epithelium
- Below diaphragm (~2cm) lined by columnar epithelium
= oesophago-gastric junction
- Well defined lamina propria and muscularis mucosae

Muscularis externa
- Striated muscle in upper one third
- Mixture of striated and smooth muscle in middle
- Smooth muscle lower one third

Adventitia
- Layer of loose connective tissue
- Serosa present on short intraperitoneal segment

Front 13

stomach

Back 13

• Reservoir and digestive organ
  4 anatomical regions:
  - Cardia
  - Fundus
  - Body (Corpus)
  - Pylorus

• Mucosa
  Muscularis mucosae - circular & longitudinal muscle fibres

• Muscularis externa
  3 muscle layers:
  - Inner oblique
  - Middle circular
  - Outer longitudinal

Front 14

gastric mucosa (stomach)

Back 14

• Columnar epithelium

• Mucosa folded (gastric folds = rugae)
  Rugae disappear when stomach distended

• Shallow depressions on surface (= gastric pits)

• Entire mucosa occupied by simple tubular gastric glands – open into gastric pits

• Contains mucus producing cells, forms secretory sheath (~1 mm thick) to protect against acid

Front 15

gastric glands (stomach)

Back 15

• Cardiac glands (at cardia)
  Heavily branched tubular mucus-producing glands

• Principal (corpus-fundic) glands (at corpus & fundus)
  Contain 4 cell types:
  - Mucous neck cells
  - Chief (zymogenic) cells – enzyme producing
  - Parietal (oxyntic) cells – acid secreting
  - Endocrine cells (e.g. G-cells) – stimulate secretion of other cells

• Pyloric glands (at pylorus)
  - More coiled than principal glands
  - Endocrine cells more frequent, no chief cells

Front 16

gastric gland cells (stomach)

Back 16

Front 17

subdivisions of small intestine

Back 17

• Duodenum

• Jejunum

• Ileum

Front 18

small intestine- mucosa

Back 18

• Plicae - folds of mucosa and submucosa
  - Increase surface area by x3
  - Permanent structures (not distension dependent)
  - Absent from start of duodenum and at the distal ileum

• Intestinal villi – entire intestinal mucosa
  - Increase surface area by x10
  - Main cell type – enterocytes → absorptive function

• Microvilli on enterocytes
  - Increase surface area by x20

• Tubular glands (Crypts of Lieberkühn) from base of villi through lamina propria to muscularis mucosae
  - Secretion of “intestinal juice”
  - Paneth cells at base of crypt – secretion, control of infection

• Goblet cells – secretion

• Muscularis mucosae – two layers and extends into villi

• Regeneration of intestinal epithelia
  - Epithelial cell turnover time is less than one week
  - Undifferentiated stem cells at base of crypt
  - Stem cells divide and ‘move up’ crypt and villi

Front 19

small intestine-submucosa

Back 19

• Contains glands only in the duodenum

• Submucosal glands in duodenum = Brunner’s glands
  - Mucous secretion to protect duodenal lumen

Front 20

small intestine

Back 20

Front 21

large intestine- 3 main sections

Back 21

Three main sections:

Anal Canal:

-epithelium changes from columnar to stratified squamous

• Caecum (including appendix)

• Appendix:

  
  

  Thickened wall
  - Large levels of lymphoid tissue
  - Muscularis externa thinner
  than colon

  
  

• Colon (ascending, transverse, descending)

• Rectum (including anal canal)

Front 22

large intestine- mucosa

Back 22

-Surface relatively smooth – no plicae or villi

-Crypts of Lieberkühn – present and longer than S.I.

-Columnar epithelium

• Goblet (mucous) cells more numerous than S.I.

• Endocrine cells: few in number

• Thin lamina propria

• Muscularis mucosae – 2 layers

Muscularis Externa

-difference to small intestine

-longitudinal layers form 3 flattened strands (taenia coli)

Serosa

-forms small pouches pilled with adipose along the large intestine

Front 23

accessory glands of digestive system

Back 23

Salivary glands

-secretes lubricating fluid +enzymes that break down carbs

Liver

bile- lipid digestion. bile=steroid acid from cholic acid/cholesterol act as surfactant emulsifying fats in food.

Gall bladder

-stores and concentrates bile

Pancreas

-exocrine cells= fluids and enzymes

-endo cells- hormones

Front 24

dietary fluid input

Back 24

2000mL

Front 25

Alimentary tract secretions

Back 25

-saliva= 1500 ml

-gastric secretions=1500 ml

-liver (bile)- 1000 ml

-pancreas (pan juices)- 1000 ml

-Intestinal secretions- 2000 ml

-Colon mucus secretions 200ml

Front 26

water reabsorption

Back 26

-small intestine 7800ml

-colon- 1250ml

Front 27

salivary glands

Back 27

Saliva is a mixed secretion, derived from numerous large and small salivary glands

• All tubuloacinar glands

​

Small salivary glands

Large Salivary Glands

• Situated in submucosa of oral cavity and tongue

• Grouped as: lingual, labial, buccal, molar, palatine

• (3 main groups)

• Sublingual - beneath tongue, many ducts

• Submandibular - floor of mouth along inner surface mandible, ducts behind teeth

• Parotid - largest, empties at the 2nd molar

Front 28

glands located close to oral cavity secrete________

Back 28

mucous secreting

Front 29

glands distant to cavity secrete_______

Back 29

serous

Front 30

sublingual salivary gland are mainly what type of gland

Back 30

mucous acini

Front 31

submandibular gland secrete___________

Back 31

both serous and mucous acini

Front 32

parotid gland

Back 32

almost exclusively serous acini

Front 33

serous secretions consist mainly of

Back 33

enzymes like alpha-amylase

Front 34

mucous secretions secrete

Back 34

mucins- heavily glycosylated protein

-involved in lubrication

Front 35

The liver

Back 35

• Largest gland of body and largest internal organ

• Highly vascularised

• Red / brown colour

• 4 Lobes: 2 Major (left & right) – largest
  2 Minor (caudate & quadrate)

• Surrounded by well defined thin capsule of connective tissue

• Functions as exocrine gland – bile secretion

• Gallbladder (bile storage) lodged within recess under right lobe

Front 36

Liver vasculature

Back 36

• Liver receives blood from two vessels
  - Hepatic artery (oxygenated blood from aorta)
  - Hepatic portal vein (from digestive system and spleen)

• Artery (25% blood supply), Vein (75% blood supply)

• Hepatic artery and portal vein discharge into system of small vascular channels - Sinusoids

• No capillaries - endothelium-lined spaces: Sinusoids

• Kupffer cells destroy bacteria, worn-out blood cells

• Sinusoids → central vein → hepatic vein → inferior vena cava

  
  

Front 37

liver lobule

Back 37

• Basic functional unit of liver

• Specialised epithelial cells - Hepatocytes

• Lobule composed of
  - Central terminal hepatic venule
  - Interconnecting plates of hepatocytes
  - Peripherally arranged portal triads
  (terminal branches of hepatic artery and portal vein and bile duct)

• Lobule delimited by connective tissue in animals, but very little in humans

• Hepatocyte zones - centrilobular, periportal, mid zone

Front 38

liver zones

Back 38

Front 39

hepatc bile production

Back 39

• Bile contains both organic (e.g. bilirubin) and inorganic (bile salts) components.

• In the small intestine, bile salts facilitate digestion and absorption of fat

• Bile produced by hepatocytes passes into bile canaliculi

• Bile canaliculi carry bile back to the portal triad (opposite direction to blood flow)

Front 40

bile secretion

Back 40

- Terminal bile ducts (lined by cuboidal epithelium)

- Intrahepatic bile ducts (columnar)

- Coalesce into left and right hepatic ducts (columnar)

- Unite to form common hepatic duct (columnar)

- Cystic duct (to/from gallbladder) & bile duct (to duodenum)

Front 41

Gall bladder

Back 41

• Ovoid sac with muscular wall

• Capable of moderate distension

• Concentrates and stores bile
  - H2O & ions absorbed by mucosa

• Receives watery bile from hepatic duct and empties thick, concentrated bile into common bile duct

Mucos

- Columnar epithelium, microvilli (resorption of water from bile)a

- No mucus-producing cells

- No muscularis mucosae

• Arranged in plicae which flatten when distended

• Concentrated bile expelled from gallbladder by contraction of smooth muscle wall via cystic duct into common bile duct

Front 42

Cholecystokinin (CCK )

Back 42

• Cholecystokinin (CCK ), a peptide hormone of the gastrointestinal system. Stimulates digestion of fat and protein.

• Synthesized by I-cells in the mucosal epithelium of the small intestine and secreted in the duodenum. Causes release of bile from the gallbladder and digestive enzymes from the pancreas.

  
  

Front 43

The pancreas

Back 43

• Behind stomach, elongate, pink/grey

• Retroperitoneal on posterior wall of abdominal cavity

• No distinct capsule, covered by thin layer of connective tissue

• Both exocrine and endocrine gland

• Endocrine accounts for ~1% of pancreas, consists of cells of islets of Langerhans (pancreatic islets)

Front 44

Exocrine functions

pancreas

Back 44

• Exocrine pancreas – tubuloacinar glands
  - Secrete pancreatic juice, a clear alkaline fluid containing precursors of digestive enzymes

• Single layer epithelial cells form secretory acini
  - Acini duct – 1st part of intercalated ducts (cuboidal)
  - Intercalated ducts empty to interlobular ducts (columnar)
  - Interlobular ducts empty to pancreatic duct (columnar)
  - Pancreatic duct joins bile duct from liver & gallbladder
  - Secretes into duodenum

• Bulk of fluid and bicarbonate ions of juice secreted by cells of intercalated ducts rather than acini

Front 45

Endocrine functions

Back 45

• Islets of Langerhans scattered throughout pancreas

• Islets contain different neuroendocrine cell types

• 4 main cell types

• 65-80% beta-cells which secrete insulin & amylin
  - Stimulate glycogen, protein and fatty acid synthesis
  - Facilitate glucose uptake

• 10-20% alpha-cells which secrete glucagon
  - Generally opposite to insulin

• 3-10% delta-cells which secrete somatostatin
  - Locally acting hormone, inhibits other endocrine cells

• 3-5% PP cells which secrete pancreatic polypeptides
  - Stimulate gastric chief cells, inhibits bile secretion