Section 8: Digestive System Flashcards
Gut - structure
A soft muscular tube
~5m long
Gut - function
Where food is stored, broken down and absorbed
Glands associated with gut tube
Pancreas
Liver
Tooth: Enamel
Crystalline rods/prisms of calcium phosphate and carbonate
No cells –> difficult to replace
No sensation
What is the hardest tissue of the body
Enamel of teeth
Tooth: Dentin
Similar to bone, but cells (odontoblasts) are localised at junction between dentin and pulp instead of scattered
Tooth: Pulp
Core of tooth
Soft tissue
BVs, nerves, lymphatics
Tooth: Cementum
Calcified CT surrounding dentin in root region
Tooth: Periodontal ligament
Collagen fibres linking alveolar bone to cementum
Rapid turnover
Tooth: Alveolar bone
Bone of the socket
Tooth: Enamel - what does it cover
Covers dentin in the crown region
Tooth: Periodontal ligament - size
Short in terms of distance, but has large X-sectional area
So distance between cementum and alveolar bone is v short
Tooth: Periodontal ligament - mechanoreceptors - functions
Protection, e.g. biting a fork
Force transducers, e.g. braces
Tooth: Periodontal ligament - rapid turnover
Lots of collagen synthesis
Vit C is crucial for this, so a poor vit C supply –> scurvy –> teeth can fall out
Tooth: Why is it important to remove infections in the pulp
Because there’s a blood supply there –> bacteria can travel into blood vessel
How many roots do most teeth have
3-4
Tongue has both _____ and ______ muscles
Intrinsic
Extrinsic
Tongue: Intrinsic muscles
Longitudinal - along length of tongue
Vertical
Horizontal/transverse
Tongue: Intrinsic muscles - contraction
Longitudinal - shortens tongue
Vertical - flattens tongue
Horizontal/transverse - narrows tongue
Tongue: Papillae
Projections towards surface of tongue
Tongue: Papillae - types
Filiform
Fungiform
Vallate
Tongue: Papillae - filiform
Sharp-like
No taste buds
Flexible
Tongue: What is the most common form of papillae
Filiform
Tongue: Papillae - fungiform
Mushroom-like projections
Some have taste buds but not many
Tongue: Papillae - vallate
Most important for taste buds
Surrounded by a groove like a moat - protection
Tongue: Papillae - different regions
Depending on the region of tongue, have diff frequencies of types of papillae
Salivary glands - major types
Parotid - serous only
Sublingual - mixed, mostly mucous
Submandibular - mixed
Salivary glands: Types of cells
Serous cells (watery) Mucous cells (viscous)
Salivary glands: Serous cells
Contribute a watery component towards saliva that is enzyme-rich
Salivary glands: Serous cells - enzymes
Amylase
Lysozyme
Salivary glands: Enzymes - amylase
Breaks down carbohydrate/starchy debris around teeth
Contained within zymogen granules and released in batches when you chew food to make sure there’s not abundant starch for bacterial growth
Salivary glands: Enzymes - lysozyme
Antibacterial properties
Salivary glands: Mucous cells
Viscous/sticky
Good for lubricating
Contain mucin granules
Salivary glands: Types of cells - staining
Serous cells = quite dark-staining
Mucous cells = quite light-staining
Salivary glands: Parotid glands - location
In front of ears
Digestive system - functions
Digestion
Absorption
Secretion
Transport
Digestive system: Functions - digestion
Chemical breakdown of ingested food into absorbable molecules
Requires secretion
Digestive system: Functions - absorption
Movement of nutrients, water and electrolytes through epithelial lining of gut into blood or lymph
Digestive system: Functions - secretion
Saliva - enzymes and mucous
Gaining surface area - methods
Gross convolutions
Luminal folds/wrinkles
Projections towards lumen = evagination
Projections away from lumen = glands
Gaining surface area: Luminal folds - examples
Circular in small intestine = plicae circulares
Longitudinal, e.g. stomach = rugae
Gaining surface area: Evagination - examples
Villus/villi, small intestine
Gaining surface area: Glands - examples
Stomach and small intestine
Gaining surface area - must take what into account?
Time
Tunics of gut tube - what are they
Mucosa (mucous membrane)
Submucosa
Muscularis externa (external smooth muscle)
Serosa
Tunics of gut tube: Mucosa - components
Epithelium
Lamina propria
Muscularis mucosae
Tunics of gut tube: Mucosa - epithelium
Specialised for protection, absorption, secretion, or combination of all 3
Physical barrier between outside world and internal systems
What happens if there’s a breach of the epithelium
Results in ulceration
Tunics of gut tube: Mucosa - lamina propria
Soft fibrous bed of loose CT where the epithelium rests
Carries nerves and blood capillaries
Populated with defense cells and collagen
Support
Tunics of gut tube: Mucosa - lamina propria - lymphocytes
Scattered throughout gut to control pathogens / harmful agents coming through
Tunics of gut tube: Mucosa - muscularis mucosae
2 thin layers of smooth muscle; inner circular and outer longitudinal
Provides mucosa with some movement independent of external muscle coat
Tunics of gut tube: Submucosa
Thick bed of loose CT carrying larger BVs, lymphatic vessels and nerves (submucosal plexus)
Connects mucosa to external muscle coat, but allows some movement between them
Tunics of gut tube: Muscularis externa
2 layers to produce peristalsis (transport)
Inner circular layer and outer longitudinal layer
Tunics of gut tube: Muscularis externa - myenteric nerve plexus
Occurs between the two layers of the muscularis externa
Responsible for coordinating the 2 layers
Enteric nervous system is made up of…
Submucosal nerve plexus + myenteric nerve plexus
Enteric nervous system: SNS and PNS
PNS excites/stimulates enteric NS
SNS inhibits function of enteric NS
Enteric nervous system and ANS
Enteric NS is thought of as a separate entity, but has many interactions with ANS
Tunics of gut tube: Serosa
A slippery outer covering for gut tube (except oesophagus)
Tunics of gut tube: Serosa - layers
Two layered;
Outer mesothelial boundary
CT layer
Tunics of gut tube: Serosa - AKA…
Visceral peritoneum
Tunics of gut tube: Serosa - adventia
Where a structure is not in contact with the body cavity, the outermost CT layer is referred to as the adventitia
Mesothelial cells secrete…
Serous fluid
Oesophagus - structure
Muscular tube
Extends from pharynx to stomach
Normally empty with lumen collapsed, and expands to accommodate food/water
Oesophagus - functions
Transport (fast travel time, due to peristalsis)
Protection
No absorption or digestion, little secretion
Oesophagus: Epithelium
Thick, stratified squamous, with sacrificial outer layers for protection against abrasive fragments of food
Protects against friction and abrasion
Oesophagus: Epithelium - how are cells replaced
By division in basal layers at BM, then slow migration outwards
Oesophagus: The entire epithelium is renewed every ______
7 days
What is the most variable throughout the gut tube
Epithelium
Serosa vs adventitia
Serosa: when an organ is not in contact with a neighbouring organ, and instead is in contact with a body cavity
Adventitia: glues one organ to its adjacent organs
Omentum
A fatty apron-like structure that hangs off the transverse colon
Must cut this away to expose underlying organs
Fatty tissue - function
Protection
Omentum - lymphatic tissue
Helps control localised infection by migrating to quarantine the infection
Swallowing food - pathway
Food is pushed backwards by tongue –> oropharynx –> laryngopharynx –> oesophagus
Swallowing food - epiglottis
Bolus comes in contact with epiglottis, pushing it backward to seal off the glottis (opening to trachea)
Adventitia between trachea and cartilage
Loose because it needs to move/expand
Oesophagus - luminal surface
Many layers of stratified squamous epithelium
Tunics of gut tube: Mucosa - lamina propria - blood vessels
Supply epithelium with nutrients and O2, and take waste away from epithelium
Oesophagus: External muscle (Muscularis externa)
Contains additional skeletal muscle in upper third of oesophagus to allow rapid contraction and voluntary control of swallowing
Oesophagus: Serosa
Majority of oesophagus doesn’t lie in a body cavity so lacks a serosa
Instead is mostly covered with a fibrous adventitia
Only when it passes the diaphragm into abdominal cavity where it starts to have serosa
Bolus of food travels down the oesophagus by _____
Peristalsis
Oesophagus: Peristalsis
Involves coordinated contraction of inner circular and outer longitudinal muscle layers (muscularis externa)
Inner = narrows tube
Outer = shortens tube –> squeezing motion
Stomach - structure
J-shaped bag on left side
Enlargement of gut tube
Stomach - capacity
~1.5L
Stomach - primary function
Storage, since food is eaten quicker than its digested and absorbed
Stomach - regions
Cardia
Fundus
Body
Pylorus
Stomach - rugae
Transient structures - only when empty, it’s lined with longitudinal folds called rugae
When stomach is full, wrinkles flatten
Increases SA
Stomach: Pyloric sphincter
Well-developed muscular sphincter at outlet
Controls rate of chyme flow into duodenum
Stomach: Pyloric sphincter is a thickening of…
Inner circular layer of muscles
Stomach: Sphincter at inlet
Less developed
If not well-developed enough, can result in gastric reflux
Stomach: Epithelium
Forms many pits lined with mucus-secreting cells
Gastric glands open into pits
Stomach: External muscle / Muscularis externa
Additional layer (3 in total) with addition of innermost oblique layer - gives churning motion –> mixing waves
Stomach: Functions
Storage Secretion of acids, enzymes, mucous Digestion of proteins by pepsin Absorption of water, ions, some drugs Protection Transport (mixing waves)
Chyme = ?
Food + gastric juice
Mucosa of stomach: Gastric pits
Pores
Mucosa of stomach: Gastric glands
Increase SA
Mucosa of stomach: Parietal cells
Pumps out H+ and Cl- separately, which recombine in lumen instead of killing the cell - kills microbes and living cells
Secretes intrinsic factor
Autodigestion
When a cell makes pure HCl - we don’t want this!
Mucosa of stomach: Parietal cells - what is HCl important for
Sterilisation of food
Acidifying environment –> indirectly helps with digestion
Mucosa of stomach: Parietal cells - intrinsic factor
Important for B12 absorption, which is critical for haematopoiesis / RBC formation
Lack of RBCs leads to…
Anaemia
Mucosa of stomach: Chief cells
Secrete pepsinogen into lumen –> travels up past the acidic environment by parietal cells and become activated into pepsin –> digests luminal contents
Mucosa of stomach: Chief cells - structure
Have apical granules in their cytoplasm which contain pepsinogen
Mucosa of stomach: Chief cells - what is pepsin
A protein-splitting enzyme
Mucosa of stomach: Surface mucous cells
Secrete insoluble alkaline mucus which protects the mucosa from acid and pepsin, i.e. protects stomach from digesting itself
But some cells will still die
Mucosa of stomach: Undifferentiated cells
Stem cells dividing to generate new epithelium
Migrate upwards to replace superficial cells which died
Mucosa of stomach: Mucous neck cells
Secrete soluble acid mucus at mealtimes
Mucosa of stomach: Enteroendocrine cells
Secretes gastrin (hormone) into bloodstream
Mucosa of stomach: Enteroendocrine cells - stimuli
When you eat food:
- stomach becomes distended/stretched
- pH of stomach changes
Mucosa of stomach: Enteroendocrine cells - what does gastrin stimulate
Secretion of acid and pepsinogen
Muscular contractions of stomach
Relaxes pyloric sphincter
Mucosa of stomach: Enteroendocrine cells - structure
Granules are basolaterally located
Liver is made of…
Epithelial cells called hepatocytes
Liver - hepatocytes
Multi-talented cells, performing many metabolic functions
Liver: Every hepatocyte requires…
Access to:
- nutrient-laden blood drained from intestinal wall
- oxygenated blood from systemic circuit
- ducts which drain bile to gall bladder
Liver: Hepatocytes - structure
Stacked up like a wall with canals passing through them
Liver: Hepatocytes - Bile canaliculus
Little canals that carry bile
Liver: Hepatocytes - capillaries
Hepatocytes are flat either side by capillaries, which are v leaky
Thick enough to let through lymph, but exclude RBCs
Liver: Hepatocytes - microvilli
Increases SA available for transport proteins to be embedded in membrane to allow substrate to be absorbed into mepatocyte
Liver: Hepatocytes - sinusoid
Can accommodate more than 1 RBC
Wide and leaky (fenestrated)
Liver: Hepatocytes - sinusoid fenestration
Allow passage of lymph but exclude RBCs
Lymph
Watery part of blood
Liver lobules
Formed by plates of hepatocytes stacked tgt
Hexagonal in X-section
Liver lobules: Portal triads
Found at the edges of each lobule
Liver lobules: Hepatic portal vein
Carries nutrient-rich but oxygen-poor blood to feed into lobule
Liver lobules: Hepatic artery
Carries systemic oxygen-rich blood to supply hepatocytes
Liver lobules: Blood in sinusoids
Mixed; both nutrient and O2 rich, which is consumed by hepatocytes until eventually drained by central vein away from liver –> IVC –> heart
Liver lobules: Where is bile carried
In canaliculi between adjacent hepatocytes
Transported through ductules that connect canaliculi and feed into bile duct
Liver lobules: Triad - components
Hepatic portal vein
Hepatic artery
A branch of the bile duct
Liver lobules: Bile - direction
Opposite direction to blood
Sinusoids - leakiness
More leaky than typical capillary because have fenestrations - allows more watery components of blood to pass through to hepatocytes
Bile ductules
Combine into a bile duct which converge and feed into gallbladder
Bile - functions
Helps emulsify fats –> increases SA for things to act on them, e.g. pancreatic enzymes
Once pancreatic enzymes have digested smaller fat globules, bile helps absorb the fats
Emulsification
Breaking down large fat globules into smaller ones
Where is bile released and stored
Released from liver and transported and stored in gallbladder
When/where is bile released during a meal
Into duodenum (small intestine)
Pancreas
Dual-function organ
Both endocrine and exocrine
Pancreas: Exocrine gland
Manufactures precursors of digestive enzymes and secrete them as alkaline ‘pancreatic juice’ via duct system leading to duodenum
Pancreas: Exocrine - why are enzymes released as precursors
To prevent autodigestion
Pancreas: Exocrine gland - when are precursors converted to their active form
Once they arrive in duodenum
Most food substances are digested by…
Pancreatic enzymes
Pancreas - tree
Leaf = acinus
Twigs and branches = duct system
Trunk = main pancreatic duct
What organ makes bile and what happens to the bile
Liver makes bile, can either be stored in gallbladder or ejected directly through common bile duct into duodenum
Stomach: Chyme - where does it go
Travels through lumen of duodenum
Pancreas - inputs
3 inputs;
Liver - bile
Stomach - chyme
Pancreas - pancreatic juice / enzyme precursors
Pancreas - secretory unit
Acinus
Pancreas: What does the secretory cell contain
Lots of mitochondria and organelles - helps with production
Pancreas: What type of cell is the secretory cell
Serous type
Pancreas: Secretory cell - cytoplasm
Has secretory granules / acinar cells containing enzyme precursors, which travel through out of acinus –> ducts –> converge and feed into bigger ducts
Small intestine - parts
Duodenum (shortest)
Jejunum
Ileum
Small intestine - size
3cm diameter
3m long
Where does most digestion and absorption occur
Small intestine
What does the small intestine receive exocrine secretions from
Liver (bile) and pancreas (pancreatic juice)
Small intestine: Duodenum
Shortest part of SI C-shaped Not suspended by mesentery Receives biliary and pancreatic ducts Close to gallbladder, stomach, pancreas
Small intestine: Jejunum
Longer ~1m long
Coiled
Small intestine: Ileum
Longer ~2m long
Coiled
Small intestine: Mucosa
Specialised to increase SA available for secretion and absorption
Small intestine: Submucosa
Just downstream of pyloric sphincter contains mucous glands / Brunner glands
Small intestine: Submucosa - Brunner/mucous glands
Acidic chyme from stomach (mixed with HCl, so pH 1-2)
Secretes HCO3- rich mucous that helps buffer against acidic content coming in
Simultaneously changes pH to optimise environment in duodenum for pancreatic enzymes
Small intestine: Increasing SA - methods
Gross convolutions (coiled)
Plicae circularis
Villi
Microvilli
Small intestine: Increasing SA - Plicae circularis
Circular folds
Each is covered with mucosa and has a core of submucosa
Permanent structures
Small intestine: Increasing SA - villi
Mini finger-like projections / evaginations
Covering is epithelium, core is lamina propria
Small intestine: Increasing SA - microvilli
Microscopic projections
Form a brush border on surface of individual absorptive cells
Each is covered with cell membrane and filled with cytoplasm
Small intestine: Increasing SA - size
Gross convolutions = largest, visible with naked eye
Plicae = visible for naked eye
Villi = much smaller, visible with naked eye
Microvilli = microscopic, can’t see with naked eye
Small intestine: Nutrient-laden blood in veins goes back to…
The liver
Small intestine: Lacteals
Lymph vessel within each villus
Small intestine: Lacteals - function
Absorb broken-down products from lumen of SI
Lymph is ‘milked’ along lacteal by contraction of smooth muscle fibres in lamina propria which shorten the villus
Products transported in blood system back to CVS
Fibres arise from muscularis mucosae
Small intestine: Lacteals - passive
No smooth muscles, so depend on external source of squeezing - muscularis mucosae
Small intestine: Muscularis mucosae - smooth muscle fibres
Some extend into lamina propria core of villus
When contract –> squeezing motion –> milk lacteals
Small intestine: Mucosa - columnar absorptive cells (enterocytes)
Absorb small molecules resulting from digestion
Small intestine: Mucosa - Where are the microvillus projections located
Columnar absorptive cells (enterocytes)
Small intestine: Mucosa - goblet cells
Embedded among enterocytes
Secrete mucus - provides lubrication to help with movement of substances, absorption and to mix things
Eventually die so need to be replaced
Small intestine: Mucosa - undifferentiated cells
Stem cells dividing to generate new epithelium / diff cells to replace dying cells
Small intestine: Mucosa - paneth cells
Secrete bactericidal enzyme lysozyme - localised immune control
Phagocytic
Apical zymogen granules
Small intestine: Mucosa - Enteroendocrine cells - stimuli
Stimulated by acid from chyme and amino acid fragments (since pepsin has initiated breakdown of proteins)
Small intestine: Mucosa - Enteroendocrine cells - where is secretin secreted
Secreted into capillaries of lamina propria
Small intestine: Mucosa - Enteroendocrine cells - what does secretin stimulate
Release of pancreatic juice and bile
Small intestine: Mucosa - epithelium
Short-lived
Cell division occurs deep in glands and migrates
Entire journey lasts ~5 days
Large intestine - functions
Absorption of salts and water
Conversion of chyme into feces; bacteria ferment remaining carbohydrates
Bacteria produce vitamins (B and K) which are absorbed
Secretion of mucus to lubricate feces
Defecation
Large intestine - main function
Absorption of water
Large intestine: Parts
Caecum –> asc colon –> transverse colon –> desc colon –> sigmoidal colon –> rectum –> anus
Large intestine: Mucous secretion
As colon is reabsorbing water, contents of lumen become more compact/dehydrated
Mucous is secreted to provide lubrication to make sure there’s enough flow
Large intestine: Appendix
Not a vital organ
Enriched with lymphatic tissue - monitors activity in LI
Vermiform - worm-shaped
Large intestine: Appendix - inflammation
Appendicitis - acute condition
Could eventually burst –> content would go into peritoneum and infect surrounding organs in abdominal cavity
Large intestine: Ileocecal valve
Controls intermittent flow of chyme from ileum into caecum
Large intestine: Caecum
A dilated pouch
Bacteria (not enzymes) are responsible for digestion
Large intestine: What do faeces contain
Bacteria 30%
Undigested dietary fibre 30%
Cells shed from intestinal lining
Mucous
Large intestine: External muscle
Outer longitudinal muscle is thickened in three strips = teniae coli, which contract to pull intestinal tube into sac-like pockets (haustra coli)
Large intestine: External muscle - Haustra coli
Changes shape and position
Allows diff segments of colon to process things at diff rates
Large intestine: Mucosa - villi
No villi, but many intestinal glands
Large intestine: Mucosa - no of goblet cells
From asc –> transverse –> desc colon, have an increasing no of goblet cells because content becomes more dehydrated so need more lubrication
Large intestine: Mucosa - goblet cells
Secrete mucus for lubrication
Die so need to be replaced by undifferentiated cells
Large intestine: Mucosa - does it contain paneth cells
No, have WBCs instead
Large intestine: Mucosa - WBCs
Help control environment since lots of bacteria in large intestine
Large intestine: Mucosa - Entire epithelium is replaced every…
4-5 days
Large intestine: Mucosa - lymphocytes (WBCs)
Mostly lymphocytes
Lots in lamina propria - large bacterial content of colon
Large intestine: Mucosa - columnar absorptive cells (enterocytes)
Similar to those in small intestine
Absorb mostly water
Rectum: Anal canal
Last 2cm of rectum = anal canal
Rectum: Anal canal - sphincters
Closed by 2 sphincters;
Inner = involuntary smooth-muscle sphincter
Outer = partial-voluntary skeletal-muscle sphincter
Rectum: Urge to defecate
Felt when rectum fills ~25% of capacity
Stretching of rectal wall initiates reflex contraction of teniae coli –> increases pressure in rectum –> internal anal sphincter relaxes/opens (reflex)
Rectum: External sphincter
If time is convenient, it relaxes voluntarily
If not, it’s maintained in contracted state and defecation reflex subsides
Rectum: Children
Takes some time to learn how to control external sphincter
Rectum - epithelium
At anal region (rectum and anus), epithelium changes from glandular secretory epithelium to stratified squamous to protect from friction
Small intestine: Venules in submucosa are tributaries of…
Hepatic portal vein
Stomach - villi
No villi
Stomach - goblet cells
No goblet cells
Gastric juice - pH
Very low pH
Simple columnar epithelium
Specialised for absorption
Tooth is fixed in socket by the…
Periodontal membrane
