GI system Flashcards
Primary Function of the GI system
- Bring nutrients into the internal environment of the body so that they can be used
- Extracts the necessary nutrients, fluids & salts from the food & water we ingest & uses it for energy & growth, & replaces losses that occur in the excreta & across the body surfaces
- Food → digestion → absorption → waste
The movement of which regions of the GI system are not caused by the contraction of just smooth muscle?
- Mouth: skeletal muscle
- Oesophagus: skeletal and smooth muscle
- External anal sphincter: skeletal muscle
4 actions of the GI system
- Motility
- Secretion
- Digestion
- Absorption
Structures of the GI System
- Gut tube
- Accessory organs
1) Gut tube:
- Oesophagus
- Stomach
- Small intestine (Duodenum, jejunum & ileum)
- Large intestine (cecum, ascending colon, transverse colon, descending colon, sigmoid colon)
- Rectum
- Anal canal
2) Accessory organs
- Salivary glands
- Pancreas
- Liver & gallbladder
- Appendix
GI system: Motility
- Patterns
- Where
- Control
- Contributes to
- Perstalsis (along GI tract)
- Segmentation (mixing w/in GI tract)
- Sphincters for control
Contributes to digestion & absorption
2 Types of gastric cells
- What they produce
- Identifiable by?
- Chief cells
- produce pepsinogen
- Lots of RER - Parietal cells
- produce acid and intrinsic factor
- Lots of mitochondria
Types of digestion and main action
- Mechanical - motility
- Chemical - secretion of fluid & enzymes
Physiological process of the GI system - Absorption
- Where does it occur?
- What is it aided by?
Transport from the GI lumen into the body
Occurs in:
- Small intestine (nutrients, salt & water)
- Large intestine (salt & water)
Aided by
- Motility
- Secretion
The peritoneum - Parietal peritoneum & Visceral peritoneum:
All organs in the abdominal & upper pelvic cavity are covered in peritoneum: it’s moist & slippery
1) Parietal peritoneum
- Lines the abdominal & pelvic cavities (peritoneal cavity)
2) Visceral peritoneum: Covers the external surfaces of most abdominal organs (incl. intestinal tract)
The peritoneum - Mesentery
What defines the mesentery?
What type of epithelium?
What does it secrete?
What is it’s function?
3) Mesentery: double layer of peritoneal membrane; where they join back together after covering the organ
- The peritoneal membrane: a serous membrane
- Simple squamous epithelium w/ underlying thin layer of connective tissue
- epithelium: secretes serous fluid = moist, slippery
Function
- to give mobility to the viscera
- to prevent friction
- continual movement along the gut tube (the gut tube is long, cannot get tangled/blocked - supported by mesenteries);
- Forms a mesentery of the GI tract (provides rich blood supply)
Definitions of the GI system
- Parietal & visceral peritoneum
- Mesentery
- Omentum
- Retroperitoneum
Parietal & visceral peritoneum - Single layer of peritoneal membrane Mesentery - Double of peritoneal membrane - Epithelium outermost - From body wall to organ Omentum - Double layer of peritoneal membrane - Epithelium outermost - From organ to organ Retroperitoneum - Behind the peritoneum - Organs become retroperitoneal when they lose their mesentery/have peritoneum on their anterior side only
Functions (5) of Peritoneum
- What does it form?
- What does the mesenteries do?
- What does the Omentum & mesentery store?
- What does this prevent?
- It forms a complete or partial covering for abdominal organs
- It forms the smooth lining which enables the abdominal organs to move over each other w/out friction
- The mesenteries of the peritoneum hold the abdominal organs in position
- Omentum & mesentery serve as store house for fat
- the fats of peritoneum prevents infections being carried to abdominal organs
Arterial supply to abdominal viscera features
Branch off abdominal aorta 1) Celiac trunk 2) Superior mesentric (midgut) 3) Inferior mesentric (hindgut) Supply the following regions - Early in development; supplied by 3 branches of aorta: foregut (celiac trunk), midgut (superior m. artery), hindgut (inferior m. artery)
Arterial supply to abdominal viscera - Celiac trunk
Branches to the structures that are derived from the foregut - Common hepatic → liver → Duodenum → Pyloric stomach → pancreas - Left gastric → Lower oesophagus → Stomach → Liver - Splenic → Spleen → Stomach → Pancreas
Arterial supply to abdominal viscera - Superior mesentric
Branches to the structures that are derived from the midgut - Intestinal arteries → Ileum → Jejunum - Ileocolic artery → Ileum → Cecum → Appendix - Colic arteries → Ascending colon → Transverse colon
Arterial supply to abdominal viscera - Inferior mesentric
Branches to the structures that are derived from the hindgut - Left colic artery → Descending colon - Sigmoid arteries → Sigmoid colon - Superior rectal artery → Rectum → Anal canal
Hepatic portal vein features (2)
- A large number & wide distribution of veins feed into the hepatic portal vein ( inferior mes. vein → splenic vein → HPV & superior mes. vein → HPV)
- Drainage via the hepatic portal circulation is nutrient rich
Hepatic portal circulation features (3)
- Capillaries → veins → (capillaries → veins - w/in the liver) → inferior vena cava
- There is a second capillary bed covering the liver
- Blood from GI tract → Liver via HPV
Structure of GI tract (Oesophagus → anus)
Layers?
4 layers
- Mucosa (innermost)
- Submucosa
- Muscularis
- Adventitia (outermost)
What is mucosa composed of?
- Epithelium (mucous secreting)
- Lamina propria (LFCT)
- Muscularis mucosa ( thin layer smooth muscle)
What is Submucosa composed of?
- Smooth muscle
What is Adventitia composed of?
- Fibrous connective tissue
what are the additional internal structures of the GI tract
- Glands (in submucosa)
- Gland ducts (in mucosa)
- Lamina propria (lymph nodes)
What is the epithelial tissue - location
- Simple squamous: peritoneum/peritoneal membrane
- Simple cuboidal: lining ducts
- Simple columnar: lining small intestine
- Stratified squamous: lining oesophagus & anal canal (hardwearing protects against abrasion)
Glandular epithelium structure
What type of epithelium?
- single duct (gland)
- greater than or equal to 2 (gland)
- Unicellular (eg goblet cells small intestine)
- multicellular
- Apical mucous granules
- Basal nucleus
- Columnar
- Goblet shape
- Simple = single duct (gland)
- Compound = >/= 2 duct (gland)
Stratified squamous epithelium: Mouth → esophagus
- Function
- Muscle transition
- For protection from abrasion
- Muscle transition from skeletal (voluntary control) to smooth (involuntary control) in esophagus
Simple columnar epithelium:
- functions
- Layers in: SI, LI, stomach
- Basic tube modified to carry out regional specific functions (secretion, digestion, absorption)
- Smooth muscle: small & large intestine, 2 layers - inner & outer longitudinal: stomach 3 layers (additional oblique layer); large intestine (3 bands of longitudinal muscle)
Structure & function of stratified squamous epithelium: anal canal
- For protection from abrasion
- Internal & external anal sphincter muscles
Mouth features (3)
- Wear & tear; stratified squamous epithelium
- Mechanical digestion (food ingested, digested begins)
- Through fauces; into oropharynx, into oesophagus
Salivary Glands - Parotid salivary glands
- What does it secrete?
- Location?
- How much saliva does it secrete?
- Serous only
- Largest salivary glands; located anterior & inferior to the ear
- Secrete 25-30% of total saliva
Salivary Glands - Submandibular salivary glands
- What does it secrete?
- How much saliva does it secrete?
- Location?
- Mixed (serous & mucous)
- Produce majority of saliva (60-70%)
- Submandibular duct opens through a papilla in the floor of the mouth next to the lingual frenulum
Salivary Glands - Sublingual salivary glands (under tongue)
- What does it secrete?
- How much saliva does it secrete?
- Location?
- Mucous mainly
- Contribute 3-5% of total saliva
- Contain multiple, tiny sublingual ducts that open onto the inferior surface of the oral cavity
Salivary Glands Functions
- What does it produce & secrete?
- Why does it secrete this?
- What type of digestion occurs?
- What enzyme is involved?
- Produce & secrete saliva into the oral cavity
→ Moistens ingested materials to become a slick bolus
→ Moistens, cleanses & lubricates the structures of the oral cavity
→ Begins chemical digestion of carbohydrates w? amylase
→ Antibacterial action w/ lysosome
→ Dissolves food so taste receptors on tongue can be stimulated
Oesophagus: Mucosa layer
- Epithelium
- Protective stratified squamous
Oesophagus: Muscularis layer
- Function
- Muscle transition
- Move food bolus; transitions b/w skeletal & smooth muscle
Oesophagus: Mucous secreting glands
- Function
- Ducts to surface epithelium; protective
- Need mucous to coat the lining of the oesophagus
Oesophagus: Lamina propria composition
- LFCT
Stomach features (3)
- Where?
- Structure?
- What is the large, apron-like fold of visceral peritoneum?
- Posterior to liver, anterior to pancreas & spleen
- Omenta (greater & lesser)
- Greater omentum: large, apron-like fold of visceral peritoneum that hangs down from the stomach over the small intestines & doubles back up to the transverse colon
Stomach functions
1) Fat deposition
2) Immune contribution
3) Infection & wound isolation (it may also physically limit the spread of intraperitoneal infections)
Key anatomical regions of the stomach
- Superior → inferior
Fundus → cardia → body → pylorus
Stomach structural features important for function
Function? Structure? - What allows the stomach to become larger? - What muscle is used for motility? - What breaks down proteins?
Function
1) Storage
2) Mechanical digestion
3) Chemical digestion
Structure
1) Rugae ( unfold → allow stomach to become larger w/out stretching; mucosa & submucosa layer) & sphincters (controls secretion of material, holds food in stomach)
2) Oblique muscles (motility; smooth muscles)
3) Epithelial cells (break down proteins)
Mucosal layer: what forms Gastric pits & glands?
- Infolding of columnar epithelium
Mucosa of the stomach wall - Gastric glands
- What cells are there?
- What do these cells secrete?
- Mucous neck cell; secrete mucous (protection)
- Chief cell; secrete enzymes pepsinogen (digestive)
- Parietal cell; acid HCl & intrinsic factor (produce)
- Endocrine cell; secrete hormones
Chief cell (CC) features (3)
- Rough ER
- Granules - opical
- Secreted = inactive // lumen = active
Parietal cell (PC) (pumps ions)(H+) features (3)
- Lots of mitochondria (energy)
- Large SA
- Folded → microvilli
Endocrine cells features (3)
- What stimulates digestive function?
- How?
- What stimulates appetite?
Hormones secreted
- Gastrin → stimulates digestive function, acts on PC to increase HCl
- Ghrelin → Stimulates appetite
- Nervous control (neuro-transmitters, receptors)
Enteric Nervous System (ENT) features (2)
- What is b/w oblique & circular muscle layers?
- What happens when stomach is filled?
- Neuronal cell bodies & nerve fibres b/w oblique & circular muscle layers (of stomach) → myenteric plexus
- Receptors: stomach filled → mechanical digestion
Location & arrangement of Small intestine (SI)
- What is it framed by?
- What is it inferior to?
- What is it covered by?
- Framed by large intestine
- Lower section of abdominopelivic cavity (inferior to stomach & duodenum & transverse colon)
- Covered by greater omentum
SI:
- Sections and their specific function (if any)
1) Pyloric sphincter
- Controls the release of chyme from the stomach
- Thick, circular smooth muscle
2) Duodenum
- Protection from acid: mucous secreting cells in epithelium & alkaline mucous secreting cells in epithelium & alkaline mucous secreting glands in submucosa
- Neutralize pH: bicarbonate (also pancreas)
3) Jejunum
4) Ileum
SI wall: 4 layers (incl. specialisations)
Modifications to assist in digestion & absorption
- Mucosa - villi w/ microvilli
- Submucosa - plicae circulares
- Muscularis - 2 layers (circular & longitudinal)
- Adventitia
Plicae circulares
- Which layer(s)
- Specialisation to epithelium?
- Inner surface of SI: submucosa & overlying mucosa
- Each plicae circulare (fold) is covered by villi: increase SA for absorption; dont distend/stretch → permanent
Villi
- Covered w/?
- Covered w/ epitheelial columnar cells (enterocytes), which contain goblet cells → protection against abrasion
Microvilli features (3)
- What cells do they cover?
- Cover apical surface of the columnar cells on each villus
- Increase SA for absorption
- Glycocalyx = glycoprotein coat on microvillus → hold brush border enzymes for contact digestion
Segmentation & Muscularis layer features (5)
- What type of contractions?
- How do these contractions work?
- What is this movement for?
- What does segmentation do to chyme?
- What does peristalsis do to chyme?
- Segmentation = localised contractions of circular muscle of the muscularis layer of the alimentary canal
- These contractions isolate small sections of the intestine: moving their contents back & forth whilst continuously subdividing & mixing the contents
- Back & forth movement in lumen = mixes food w/ digestive juices & facilitates absorption
- Can slow progression of chyme, allowing time for digestion & absorption
- Peristalsis; movement along tube (cannot slow chyme)
2 Types of neural control and their function
- Submucosal plexus - regulation of secretion
- Myenteric plexus - regulation of motility
ENS vs. CNS
- Function
- Type of neural reflexes
- Enteric nervous system (ENS) → Primary neural system controlling GI function → Independent - short, local GI reflexes - Central nervous system → Modulates activity of ENS → Long neural reflexes
What do the endocrine cells of the GI tract secrete?
hormones
Accessory Organs:
- What are they
- What do they produce/release/store?
- Function of secretions?
- Liver
→ Produces bile salts, which emulsify lipids, aiding their digestion & absorption - Gallbladder
→ Stores, concentrates & releases bile in response to hormonal signals - Pancreas (exocrine)
→ Produces digestive enzymes & bicarbonate to help neutralize acidic chyme
Pancreatic duct
- Where does it lead to?
- What sphincter does it contain? Incl. its structure and function
- What duct is formed by the union of this duct and the common bile duct?
- Duodenum
- Sphincter of oddi (ring of smooth muscle): controls the release of digestive enzymes & bicarbonate from pancreas
- Hepatopancreatic duct (ampulla of vater duet)
What is the function of the endocrine hormones produced by the pancreas?
regulate blood sugar & pancreatic secretions
Large intestine:
- Diameter/length compared to SI
- Functions
- Wider in diameter but shorter in length than SI
Functions: - Stores faeces until defecation
- Absorbs water & ions
- compacts undigestible & wastes & solidifies them into faeces
LI attachment to abdominal wall
- What is the transverse colon attached to?
- What anchors the transverse colon and what is it anchored to?
- What is against the abdominal wall?
- Transverse colon attached to greater omentum
- Transverse mesocolon anchors the transverse colon to the back wall
- Ascending & descending colon are retroperia (against the abdominal wall)
Sections of the LI and the accessory organ attached
Cecum → Ascending colon → hepatic flexure → transverse colon → splenic flexure → descending colon → sigmoid colon → rectum → anal canal.
(+ appendix)
Distinguishing features (7) of LI
- What valve does it contain? b/w which sections?
- What type of folds?
- Specialsed structures (3)
- Sphincter(s)
- What covers its anterior surface?
- ileo-caecal valve (b/w ileum & cecum)
- Semilunar folds
- Haustra (sacs formed by contraction of Tania coli)
- Taeniae coli (outer longitudinal muscle is condensed into 3 tape-like strips)
- Epiploicae appendices (pouches for fat storage - adipose tissue)
- External anal & sphincter muscle
- visceral peritoneum
Wall of the LI structure
- What does it lack compared to SI?
- Layers and what they’re composed of (if relevant)
- LACKS plicae circulares & villi (of the SI)
- Layers (deep → superficial)
1) Mucosa
→ deep mucosal invaginations & numerous goblet cells
→ epithelium, lamina propria (FCT), muscularis mucosae
2) Submucosa
3) Muscularis
→ Inner: circular
→ Thin outer: longitudinal (taenia coli)
4) Adventitia
Mucosa transition LI –> anal canal
Transitions from simple columnar (containing goblet cells) to stratified epithelium at colorectal zone to anal zone.
Control of anal sphincters
1) Sensory nerve fibres (sigmoid colon)
2) Voluntary motor nerve to external anal sphincter
3) Involuntary motor nerve parasympathetic division (internal anal sphincter - month muscle)
Defecation: process/muscle responses
1) Rectum minimal stretch, minimal pressure: internal contracted, external relaxed
2) rectum becomes stretched: internal relaxed, external contracted
3) Conscious decision to defecate: internal & external relaxed; pressure from contraction of GI & abdominal muscles (rectum)
Function of LI
- What does it digest?
- What does it absorb?
- How does it propel faeces?
- How is faeces removed from body?
→ Digestion - Some remaining food residues are digested by enteric bacteria (which also produce vitamin K & some B vitamins)
→ Absorption - Absorbs most of the remaining water, electrolytes & vitamins produced by bacteria
→ Propulsion - Propels faeces toward rectum by haustal churning & mass movements
→ Defecation - Reflex triggered by rectal distension, eliminates faeces from body
What do the following vessels carry to/from the liver (via the portal triad):
- Hepatic artery
- Hepatic portal vein
- Bile (hepatic) duct
- Hepatic artery: oxygenated blood TO liver from aorta
- Hepatic portal vein: deoxygenated, nutrient-rich blood TO liver from digestive organs
- Bile duct: bile FROM liver to gallbladder
SI: Alkaline mucous
- Secreted from? Ducted in from?
- Rich in?
- Functions
From intestinal glands & bicarbonate-rich juice, ducted in from the pancreas, help neutralise acidic chyme & provide the proper environment for enzymatic activity
SI: Bile
- From where?
- Stored and concentrated where?
- Releases into where? in response to?
- Functions
- From liver
- Stored and conc. in gallbladder
- Released into duodenum in response to hormonal signals
- Emulsifies fats & enhances fat digestion & absorption
What completes the digestion of all classes of food?
- Digestive enzymes from pancreas and
- Brush border enzymes on microvilli membranes
What happens in the SI?
- Breakdown of?
- Digestion of?
- Absorption of?
- Breakdown products of carbohydrate, proteins & fats &
- nucleic acid digestion
- vitamin, electrolyte & water absorption
what are the functions of motility
- Movement?
- Mechanical digestion?
- Mixing?
1) movement at a controlled rate
- propulsion (‘conveyor belt’)
- storage (timing the motility)
2) Mechanical digestion
- ingestion/physical breakdown
- increases surface area (↑ chemical digestion)
3) Mixing
- food
- secretions
- enzymes
4) Exposure to absorptive surfaces
- contact (for nutrients to enter body tissues)
How does the GI tract change the SA of its muscular mucosa layers for motility patterns?
- Circular muscle - ↓ diameter
- Longitudinal muscle - shortening
Which parts of the GI tract are not solely under involuntary control? Which muscle type(s) is/are present?
→ Mouth - skeletal
→ Esophagus - both
→ Anus - skeletal
What is the basis of spontaneous activity in the GI tract?
- How does it result in contractions?
- Basic electrical rhythm (BER) or slow wave
1) Spontaneous variations in membrane potential (pacemakers)
2) Produce action potentials (if/when threshold reached)
3) Result in contractions
The basis of GI motility - Regulation of motility
- What determines the frequency of contractions?
- What is the strength of contraction regulated by?
- How do you achieve this?
- BER determines the frequency of contractions
- Strength of contraction regulated by neutrons & hormonal reflexes
- Achieve this by either depolarising or hyperpolarizing the resting membrane potential
- Why does the ENS coordinate muscles?
- What is the ENS modified by?
- to generate motility patterns
- stretch/nerves/hormones/local feedback loops
What is the fasting motility pattern:
- What complex?
- Where is it coordinated?
- How many hours after a meal?
- How many activities?
→ Migrating motor complex (MMC)
- Stomach → SI → LI
- 4-5 hours after a meal (repeated every ~2 hours)
- 3 periods: inactivity, intermittent activity, intense activity
Feeding motility patterns: storage
- Where?
- Relaxation of which muscle?
- What type of reflex?
- Mainly stomach
- Relaxation of smooth muscle (distention)
- Neural reflex
Feeding motility patterns - Peristalsis
- Where does it occur?
- What is it controlled by?
- What does it do?
- Esophagus, stomach, S & L intestine
- controlled by ENS
- Propulsion of bolus (proximal squeeze, distal relax - like moving toothpaste down an empty tube)
Feeding motility patterns - Segmentation
- Where does it occur?
- What does it do?
- Mainly S intestine, some in L intestine
- Mixing (contraction/relaxation of circular muscle - like squishing empty toothpaste tube) & increasing exposure to absorptive surfaces
Function of chewing
- ↓ size of food
- Mixing with saliva
- Taste
How is chewing controlled?
- Muscle type
- What do we control?
→ skeletal muscle → Reflex control of - Strength - Occlusion - Frequency - Side of chewing
Swallowing (deglutition) function
- Rapid transfer of material from mouth to stomach
How is swallowing controlled?
→ Initiated voluntarily
→ Proceeds reflexly (involuntarily)
→ Co-ordination of multiple muscles
→ Need to protect airway
Swallowing - oesophagus features (2)
- What aids in swallowing? (motility pattern and internal structures)
- Peristalsis (can occur w/ or against gravity)
- Sphincters
Fasting (MMC): effect on stomach
Shrinks to small vol.
Gastric motility - Eating
→ What do the motility patterns allow?
- Storage
- Mixing
- Mechanical digestion
- Controlled delivery to intestine
Gastric motility - Eating (delivery of food)
→ Storage (in proximal stomach)
- What type of relaxation?
- What is it initiated by?
- Relaxation of what?
→ Receptive relaxation
- ↑ in stomach vol. w/out ↑ in pressure
- Initiated by swallowing (nervous reflex)
→ Relaxation of proximal gastric smooth muscle:
- ↓ thickness; highly distensible
Gastric motility - Eating (delivery of food)
→ Peristalsis
- Where is it initiated?
- What happens after first 60 min?
- ” ‘ 60 - 300 min?
- What is present?
→ Peristaltic waves:
- Initiated on greater curvature
- spreads to antrum
- ~3/min
- First 60 min = gentle activity
- 60-300 min = more intense activity
- Presence of products of digestion (AAs)
- Gastrin production (stimulates contraction)
What are the roles of peristalsis in the stomach?
1) Propulsion
2) Retropulsion (mixing & mechanical digestion) - when combined with the action of the pyloric sphincter
Gastric motility - Eating (delivery of food)
→ Controlled delivery of chyme to duodenum
- How is gastric emptying regulated?
- What determines the rate of emptying?
- What controls the stomach?
→ Regulation of gastric emptying through regulation of gastric motility
→ Compositions of food modifies rate of emptying
- Fats slow gastric emptying
→ Duodenum controls stomach
- Hormonal (GIP)
- Neural (enterogastric reflex)
Small Intestine Motility - Overall function
- Chemical digestion of food
- Absorption of nutrients, salts & water
Small Intestine Motility - Function of motility
- Mixing (chyme, chemicals, enzymes)
- Exposure to absorptive surfaces → contact
- Propulsion
Small Intestine Motility - Motility patterns
- Fasting (MMC)
- Feeding (mainly segmentation, peristalsis)
Large Intestine (colon) Motility - Functions
- Temporary storage of faeces
- Regulation of the salt & water content
Large Intestine (colon) Motility - Motility patterns (18-24h)
- Whats happening?
- Inactivity?
- Segmentation?
- How many mass movements per day?
- What causes defaecation?
→ Inactivity → Segmentation - Mixing & slow propulsion → Mass movements - 3-4 times per day - Force faeces into rectum → Defaecation - Reflex responses/voluntary control - Brain/ brain stem/ spinal cord
- What are the exocrine secretions?
- Are they excreted or reabsorbed?
Reabsorbed into body
- Enzymes
- Mucus
- Electrolyte (serous) solution
Functions of exocrine secretions
Composition fits function ∴ variation
- Digest food
- Dilute food
- Optimal pH
- Protection & lubrication
Salivary secretion feature
- Modified as it moves down the duct
- Saliva composition
- Function of components
- Amount secreted per day?
1.5L/day → Mucus - Lubrication → Dilute solution of NaHCO3/NaCl - Dilution - Optimum pH → Enzymes - α-amylase - Lipase
3 functions of saliva
→ Lubrication - Chewing - Swallowing → Hygiene - Irrigation → Digestion - Dissolves food (taste) - α-amylase
Salivary secretion regulation:
- Nervous
- Autonomic
→ Nervous - Thought, smell, sight of food - Presence of food in mouth, chewing → Autonomic nervous system - Parasympathetic - Acetylcholine; copious volumes - Sympathetic Adrenaline; small vol. of viscous fluid
Gastric secretion:
- How much is secreted per day?
- Relative rate b/w meals?
- Composition
- 2-3L/ day
- b/w meals = slow rate
- Mainly mucus
Gastric secretion - Eating/food in stomach
- Relative rate b/w meals?
- Composition
- much faster rate
- NaCl/H2O
- Acid
- Mucus
- HCO3
- Pepsinogen
- Intrinsic factor
Gastric secretion - Function of MUCUS
Protection (abrasion, acid) (coating)
Gastric secretion - Function of INTRINSIC FACTOR
Absorption of Vitamin B12 in small intestine
Gastric secretion - PEPSIN
- What’s it secreted as?
- How is it converted?
- Function?
- Secreted as pepsinogen (inactive)
- Converted to active form pepsin by acid
- Starts digestion of proteins
Gastric secretion - GASTRIC ACID
- What does it activate?
- Function?
- Other features?
- Denatures protein
- Activates pepsinogen to pepsin
- Protection
- Denatures protein
- Optimum pH for pepsin
- Dilutes food
Gastric secretion - Vol. & secretion
→ Regulation features
- What is it coordinated by?
- What are the phases?
→ Coordinated w/ eating & arrival of food → 3 phases - Cephalic (head; thinking) - Gastric (stomach) - Intestinal (intestine)
Different cells = Different secretions
- what are these cells and what do they secrete?
→ Surface cells - Mucus/HCO3 Cells in the gland → chief cells - Pepsinogen → Parietal cells - Acid - IF
Gastric secretion - CELIAC PHASE
- Stimuli?
- Parasympathetic NS:?
- What does it stimulate?
- Preparation (head controls secretion)
- Stimuli: Though, smell, sight & chewing action/taste
- Parasympathetic NS: modifies activity of ENS - stimulates parietal cells & gastrin production
Gastric secretion - GASTRIC PHASE
- Stimuli:
- Response:
- What does it stimulate?
- Ensures sufficient secretion to handle the ingested food
- Stimuli: stretch/distension & products of digestion
- Response: local reflex - ENS, & external reflex-parasympathetic NS
- Both responses stimulate parietal cell & gastrin production
Gastric secretion - INTESTINAL PHASE
- Stimuli:
- Response:
- What is its response?
- Controls amount of acid delivered to SI
- Stimuli: duodenum distension, & arrival of acid and products of digestion into duodenum
- Response: GIP, CCK & secretin (inhibit secretion), & enterogastric reflex (vagus, inhibits secretion)
Pancreatic secretion
- How much is secreted per day?
- What is secreted? From where?
→ 1 1.5 / day → 2 Components: 1) Enzymes - Acinar cells 2) Alkaline fluid (HCO3) - Produced by ducts
Pancreatic secretion - enzymes (digestive)
- What are the types?
- What do they target?
- Secretion stimulated by?
Types → Lipolytic - Lipase, phospholipase - target lipids → Amylytic - amylase - targets carbohydrates → Proteolytic - trypsin, chymotrypsin, carboxypeptidase - targets proteins → Nucleolytic - ribonuclease - deoxyribonuclease Stimulated by: - Cholecystokinin (CCK)
Pancreatic secretion - enzymes (digestive)
- Function
- Chemically digest food material
- Pancreas most important source
Pancreatic secretion - alkaline/HCO3 rich fluid
- Produced by?
- Function?
- Stimulated by?
- Duct cells produce isosmotic HCO3 rich solution
- Neutralise chyme (acidic from stomach)
- Creates optimum pH (for pancreatic & intestinal enzymes)
- Stimulated by secretin
Pancreatic secretion
- Basal rate during fasting
- What type of regulation during meal?
- Hormones secreted?
- Slow basal rate during fasting
- Hormonal regulation during meal
- CCK & Secretin
Pancreatic secretion - CCK
- What is it produced by?
- Stimuli
- What does it stimulate?
- Produced by duodenal endocrine cells in response to digestive products in lumen (AAS, fats, carbohydrates)
- Stimulates enzyme secretion (acinar cells)
Pancreatic secretion - Secretin
- Where is it produced?
- Stimulus
- What does it stimulate?
- Produced by duodenal endocrine cells
- in response to ↑ [H+] in lumen
- Stimulates HCO3- secretion (duct cells)
Biliary secretion (0.5L/day) - Composition
- Excretory products
- Products associated w/ digestion
→ Excretory products - Bile pigments (waste products) - Cholesterol (excreted by liver) → Products associated w/ digestion - Bile salts & lecithin - HCO3 rich fluid (secreted by duct cells)
Biliary secretion:
- Components and their function
→ Bile salts & lecithin - Fat digestion & absorption → HCO3 rich fluid - Neutralizes acid chyme → Bile pigments - Excretion
Biliary secretion - Regulation
- Where is bile produced and stored?
- What stimulates release?
- Where is bile delivered to?
- What does bile contain?
- What is its function?
- Constantly produced by the liver and stored in gallbladder
- Release stimulated by CCK
- Delivered to SI
- Contains cholesterol, waste products, alkaline fluid, bile salts and lecithin
- Assists with lipid digestion
Intestinal secretion
- How much? (both small and large)
- What is secreted? (from each)
→ Small (approx 1.5/day)
- Mucus (lubrication)
- NaHCO3 sol. (neutralise acid, dilute food)
→ Large
- Mucus
- SecretionChemical digestion
- function
- Why its necessary
↓ nutrient size
- Ingest nutrients as large complex molecules, but can only absorb them as small molecules
Chemical digestion
- What are involved in the chemical digestion of food?
- Secreted by or attached to?
1) Enzymes secreted by
- Salivary glands
- Chief cells of stomach
- Acinar cells of pancreas
2) Enzymes attached to enterocytes of SI (brush-border enzymes)
What are carbohydrates ingested as?
- Starch & glycogen (mainly)
- Cellulose (can’t be digested)
- Disaccharides
- Monosaccharides (limited amount)
Carbohydrates chemical digestion
- What are the two steps in this process?
1) Luminal digestion
2) Contact digestion
Carbohydrates chemical digestion: Luminal digestion (initial)
- Which enzymes are involved?
Initial digestion involving enzymes secreted into lumen
- Amylase (salivary glands)
- Pepsin (stomach)
- Trypsin, chymotrypsin, lipase, amylase (SI, from pancreas)
Carbohydrates chemical digestion: Contact digestion (2nd)
- Where does this occur?
- Completed by? Where are these produced?
- Where are they located?
- In SI
- Completed by enzymes produced by enterocytes
- attached to brush border
Carbohydrates chemical digestion: Summary
Polysacch → hydrolysis (salivary amylase & pancreatic amylase) → disaccharide → hydrolysis (brush border enzymes) → monosacch.
Protein digestion:
- Ingested as?
- Same chemical digestion as?
- Ingested as polypeptides but can only be absorbed as AAs
- same 2 step process as carbohydrates
Luminal digestion of proteins:
- What occurs?
- What enzymes is involved?
- Proteins into smaller polypeptide units &processes
- trypsin
Contact digestion of proteins:
- What is broken down? Into what?
- Polypeptides into AAs
Lipid digestion:
- ingested as?
- can only be digested as?
- why is the chemical digestion process different to proteins and carbohydrates?
- Ingested mainly as triglycerides
- can only be digested in the form of micelles
- different process bc insoluble in water
Emulsification of lipids
- What does it do
- Purpose
- Where does it occur
Large fat droplets into smaller fat droplets
- ↑ SA for digestion and disperses droplets
- Occurs in stomach (retropulsion) & SI (segmentation)
Stabilisation of lipids
- Why?
- What is involved?
- Where does it occur?
- stops them from settling again and allow formation of smaller droplets (↑ SA)
- Lecithin & bile salts
- Occurs in SI
Hydrolysis of lipids
- Converts what to what?
- What causes hydrolysis?
- Where are they secreted?
- What is their function?
- Where does it occur?
Triglycerides to monoglycerides & fatty acids
- Colipase (cofactor, anchors lipase to droplet surface)
- lipase (cause the hydrolysis)
- both secreted by pancreas
- Occurs in lumen of SI (no contact digestion)
Purpose of the formation of micelles?
Allows insoluble droplets to stay in water
Lipids digestion summary: 4 stages
Large fat droplet → emulsification (small droplet) → Hydrolysis (lipase) → fatty acids, monoglycerides & glycerol
sites of absorption and relative amounts
→ Mouth, esophagus & stomach
- minimal absorption (lipid-soluble substances)
→ Small intestine
- Main site of absorption (90% water & sodium, all nutrients)
→ Large intestine
- 9% of water & sodium
What are the factors affecting absorption?
- Motility
- Available surface area
- Transport
- Removal of substances from the interstitial space
How does motility affect absorption?
- Need correct rate of propulsion for digestion & absorption (otherwise abnormal bowel movement, i.e constipation, etc)
- Exposure to absorptive surfaces
How does SA affect absorption?
- Rate of absorption proportional to SA
- Anatomical adaptions (villi, microvilli) maximise SA
What are the 2 pathways for solute absorption?
Two pathways:
→ Paracellular pathway
- Solutes move b/w the cells, do not cross the cell membrane
- Relatively non-selective (can cross of small enough)
- Passive (requires a gradient)
- Only barrier is tight junctions b/w cells
→ Cellular pathway (selective)
- Solutes cross 2 cell membranes
- Insoluble substances need transport proteins
Purpose of large blood flow in interstitial space?
Removes substances from interstitial space to maintain gradient (HOMG)
Absorption of water
- Process?
- Gradient set up by?
→ Osmosis
- Gradient set up by absorption of salts & nutrients
Absorption of Sodium (Na+)
- Passive
- Active
→ Passive movement via paracellular pathway
→ Active transport via the cells (requires transporters):
- Na+ transport alone
- Na+ absorption coupled to glucose or AAs
Absorption of Carbohydrates
- Passive
- Active
→ Passive - Monosaccharides - Diffuse down gradients via paracellular pathway → Active - Cotransport w/ Na+ - Monosaccharides
Absorption of Proteins
- Passive
- Active
→ Passive - Products of digestion (AAs) - Diffuse down gradient via paracellular pathway → Active - Cotransport w/ NA+ - AAs coupled to Na+ - Similar process to glucose
Absorption of Fat
→ Products of digestion are lipid soluble
- Can diffuse into cell
- Delivered to brush border via micelles (fatty acids & monoglycerides)
- In epithelial cell: Synthesised into triglycerides, packaged into chylomicrons which exit the cell via exocytosis & enter lacteals
Absorption of Bile salt
- Proportion reabsorbed?
- Where does it occur?
- Where does active transport occur?
- Where does passive absorption occur?
→ Majority is reabsorbed
- Occurs in distal portions of SI to promote fat absorption
- Active transport in terminal ileum
- Passive absorption in jejunum
Absorption of Vitamins
- Fat soluble
- Water soluble
→ Fat soluble (ADEK): absorbed w/ fats
→ Water soluble
- Na+ dependent absorption (eg Vitamin C)
- Vitamin B12 (intrinsic factor)
Absorption of Vitamin B12
- What does it involve?
Where is this secreted? - How does it allow absorption?
- Active or passive absorption?
→ Involves intrinsic factor (secreted by stomach): binds to vitamin B12
→ Receptors in ileum bind IF/B12 complex
→ Vitamin B12 actively absorbed
What regulates conditions in intestinal lumen?
- vol.
- composition
Neural regulation - ENS features
- 1* neural system controlling, GI function
- Independent
- Short, local GI reflexes
Neural regulation - CNS features
- Modulates activity of ENS
- Long neural reflexes (bc long nerves → long travel distance)
Neural regulation - ENS components
1) Submucosal plexus: regulation of secretion
2) Myenteric plexus: regulation of motility
- Smooth muscle
- Neuronal cell bodies
What are local reflexes (ENS) initiated by?
- Distensions
- Acidity of chyme
- Osmolarity of chyme
- Presence of products of digestion
Local reflexes (ENS) - mediated by which receptors in GI mucosa? What do they detect/monitor?
1) Mechanoreceptors - Deistension
2) Chemoreceptors - Chemical composition of lumen
3) Osmoreceptors - Osmolarity
Neural regulation - CNS role & components
- Type of regulation?
- Subdivisions and what they normally do
→ Extrinsic bidirectional regulation (sensory → effector)
→ 2 subdivisions of ANS
1) Parasympathetic NS
- In general stimulates motility & secretion
2) Sympathetic NS:
- In general inhibits motility & secretion
What are the connections of ENS neurons
- Mucosa:
- Muscularis mucosae:
- Submucosal plexus:
- Circular muscle:
- Myenteric plexus:
- Longitudinal muscle:
- Mucosa: endocrine cells, receptors, secretory cells
- Muscularis mucosae: neurons passing through
- Submucosal plexus: blood vessels & neurons
- Circular muscle: neurons
- Myenteric plexus: neurons
- Longitudinal muscle: neurons; lead to
- Sensory, parasympathetic & sympathetic
Endocrine cells
- Paracrine action
- True endocrine action
→ Paracrine action
- Substance acts on cells in immediate vicinity of release
- Provides regional control in response to local conditions
→ True endocrine action
- Hormones released into circulation to target distant cells
What are the GI hormones
Peptide hormones (quick response)
- Gastrin
- Secretin
- CCK
Gastrin features
- Where is it produced?
- What does it target?
- What does it stimulate?
- Produced in the stomach
- Targets parietal cells/gastric muscle
→ Stimulates secretion/motility
GIP festures
- Where is it produced?
- What does it target?
- What does it inhibit?
- Produced in intestine
- Targets G cells/gastric muscle to inhibit gastric secretion/motility
Secretin features
- Where is it produced?
- What does it stimulate
- What does it inhibit?
- Produced in duodenum
- Stimulates enzyme secretion & inhibits effects of gastrin
CCK features
- Where is it produced?
- What does it stimulate?
- What does it inhibit?
- Produced in SI
- Stimulates HCO3 secretion & inhibits H+ secretion
GI hormones function
Regulate multiple sites to coordinate GI motility & secretin - may stimulate some tissues & inhibit others
Integrated response - Meal
1) meal stimulates GI tract - responds in defined phases
→ Cephalic - activation
→ Gastric - Storage, initial digestion
→Intestinal - Digest & absorb (&feedback)
2) Functions co-ordinated to optimise digestion & absorption
- Motility
- Secretion
3) Neural & hormonal
Smooth muscle in the GI tract:
- What is the basal electrical rhythm (BER)?
- What does it cause?
- The resting membrane potential of the GI tract smooth muscle
- Constant fluctuation of the membrane potential above and below the threshold potential
