Digestive System Flashcards
(99 cards)
1
Q
What is the Digestive System also referred to as?
A
Digestive tract
Gastrointestinal tract (GI Tract)
Alimentary canal
2
Q
What is the anatomy of the DS?
A
Oral cavity Pharynx Oesophagus Stomach Small intestine Large Intestine
3
Q
What are the accessory organs and glands of the digestive system?
A
Teeth Tongue Salivary glands Liver Gall bladder Pancreas
4
Q
What is the energy requirement of the human body at rest and how is it obtained?
A
30kcal/kg of body weight per day.
Ingestion of food, usually a blend of different types of complex nutrient macromolecules such as carbohydrates, proteins, lipids.
5
Q
Subunits of complex molecules can be used by our cells for:
A
Energy generation (ATP)
Cell growth
Cell repair
6
Q
Where is ATP produced in our cells?
A
mitochondria
7
Q
Digestive system provides essential nutrient subunits from complex foods through the integration of what processes?
A
ingestion propulsion mechanical digestion chemical digestion secretion absorption
eliminates indigestible waste material by:
excretion/defecation
8
Q
What is ingestion?
A
The process of introducing food and liquid nutrients into the digestive tract via the oral cavity (OC)
9
Q
What is propulsion?
A
The process that moves food through the DS at a rate that permits optimal digestion and absorption.
Includes: Deglutition and Peristalsis
10
Q
What is Deglutition?
A
Swallowing food
11
Q
What is Peristalsis?
A
Successive waves of contraction and relaxation of smooth muscle layers in gut wall.
12
Q
What physiological stimuli initiate peristalsis?
A
Mucosal pinching
Mechanical distention with gas, liquid or solid bolus.
13
Q
Describe mechanical digestion in the oral cavity:
A
Solid foods mechanically processed by chewing (mastication) and shearing action of teeth.
Food mixed with saliva by tongue to form food boluses.
Reducing size of ingested food to increase surface are so chemical digestion is more efficient.
14
Q
Describe chemical digestion in the oral cavity:
A
Saliva contains enzymes to breakdown complex macromolecules.
Amylases for carbohydrates
Lipases for lipids
15
Q
Describe mechanical digestion in the stomach:
A
Muscles in the stomach allow Churning of food
16
Q
Describe the chemical digestion in the stomach:
A
Hydrochloric acid and pepsin in the stomach allow for the digestion of proteins.
17
Q
Describe mechanical digestion in the small intestine:
A
Segmentation movements of gut wall ensure food is mixed well with digestive enzymes
18
Q
What is secreted during the digestion of food to act as an aiding accessory?
A
Saliva - Amylase and Lipases Gastric juice - HCl and Pepsin Bile Alkaline mucous Pancreatic Juice
19
Q
How is the small intestine adapted for nutrient absorption?
A
Villi (projecting structures) allow for larger surface area for more absorption.
Villi are one cell thick - small diffusion distance
Lots of them
20
Q
The gut wall consists of 4 major layers, what are these?
A
- Mucosa
- Submucosa
- Muscularis externa
- Serosa/adventitia
21
Q
The mucosa (mucous membrane) consists of what structurally?
A
- Epithelium - may be simple or stratified
- Lamina propria - highly vascular loose connective (areola) tissue, rich in lymph vessels, lymphoid cells (mast cells), smooth muscle cells, sensory nerve endings, mucous secreting cells)
- Muscularis mucosae
22
Q
What is the function of the mucous membrane?
A
Protective against pathogens and dirt.
23
Q
Where in the small intestines would you expect mucosa to be most developed?
A
Duodenum
24
Q
What are the three parts of the small intestines?
A
Duodenum - Most digestion
Dedunum - Most absorption
Ileum - important absorption
25
What is the structure of Submucosa?
Thick irregular layer of connective tissue
Rich in blood vessels
Contains lymph vessels, glands, lymphoid tissue(MALT)
Contains submucosal plexus
26
What is the submucosal plexus and what is its function?
Nerve fibres braided together to form plexus.
Innervates blood vessels to change diameter
Glands to secrete enzymes
Muscularis mucosa to change diameter
27
What is the structure of the muscularis externa (ME)?
Made up of two muscle layers:
Circular muscle
Longitudinal muscle
28
What is the function of the circular muscle in the muscularis externa?
Narrows lumen through contraction to mix and segment food and form sphincters
29
What is the function of the longitudinal muscle in the muscularis externa?
Shortens tube during contractions to help move food along the digestive tract during peristalsis
30
What is the enteric nervous system (ENS) made up of?
Most external layer
Double membrane layer of epithelial cells
Visceral - stuck to organ
Parietal - closer to cavity
Serosa and adventitia
31
What are serosal membranes and what are their function within the ENS?
Surrounding organs that sit within the peritoneal cavity
| e.g stomach, spleen, liver
32
What are adventitia and their function within the ENS?
Loose connective tissues to hold organs into place.
Outside peritoneal cavity
e.g pancreas, duodenum, ascending and descending colon
33
What is the myenteric plexus?
Plexus of neurons in the longitudinal and circular muscle layers of the muscularis that control the contraction and relaxation of the smooth muscles in the ME.
34
What is the oral cavity comprised of?
```
(or buccal cavity)
The lips - Anterior boundary
The cheeks - Lateral boundary
Palate - Superior boundary
Tongue forms inferior boundary
```
35
What is the cavity lined by?
Stratified squamous epithelium
36
The gingiva (gums) are covered by what?
protective layer of keratinised epithelial cells
37
What are the different parts of the Palate?
Anterior - Hard Palate
| Posterior - Soft Palate
38
What is a common congenital disorder that results in malformation of the hard palate?
Cleft palate
39
When does fusion of the palatine shelves to form the palate occur?
During the 7th week of embryonic development
40
How can cleft lips occur?
Gene mutation
| Defects in mechanisms that coordinate fusion processes
41
What is the anatomy of the soft palate?
Soft palate is a flexible flap of skeletal muscle tissue
Uvula dangles from edge of soft palate for deglutition
attached to the tongue and the pharyngeal wall by muscular pharyngeal arches:
Anterior arch - palatoglossal arch
posterior arch - palatopharyngeal arch
42
What are the functions of the hard palate?
Separate oral and nasal cavities
| Mucosa corrugated to facilitate deglutition to propel down the GI tract.
43
How is the hard palate formed in terms of embryonic origin?
From cranial neural crest cells
44
What is the anatomy of the hard palate?
Palatine shelves on each side of tongue
| Nasal cavity is open to the oral cavity
45
What is the anatomy of the tongue?
Intercalating bundles of intrinsic and extrinsic skeletal muscle fibres
Attached to floor of oral cavity by thin mucosal tissue - the lingual frenulum
Superior surface and lateral edges covered with papillae
46
What is the function of the tongue?
Manipulate food with saliva to form food boluses.
| Lingual frenulum helps for movement
47
What are the three types of papillae in the back of the adult tongue?
1. Filiform- (threadlike) projections arranged in parallel rows in the anterior 2/3rds of the tongue.
Contain receptors for touch
2. Fungiform - (mushroom-like) extensions of the tongues lamina proporia
Interspersed between filiform papillae but more numerous at tip of tongue
Contain gustatory receptors
3. Circumvallate papillae - arranges in upside down V array in posterior surface of back.
All contain gustatory receptors
48
Where in the oral cavity are minor salivary glands located?
```
In the mucous membranes of:
Cheeks - buccal glands
Lips - labial glands
Palate - palatal glands
Tongue - lingual glands
Each produces a small quantity of saliva
```
49
What are the major salivary glands?
Parotid gland
Submandibular gland
Sublingual gland
50
What is the structure and function of the major salivary glands?
Paired compound exocrine glands
Specialised secretory organs that use branching system of ducts to deliver secretory products to OC
Each gland divided into small lobules that contain structural and functional secretory units:
- Acinus
- Intercalated duct
51
What are the protein populations that are secreted by salivary acinar cells?
Mucous secreting acinar cells: secrete mucin
| Serous secreting acinar cells: produce watery secretion containing amylase, proline rich proteins and ions.
52
What type of populations of acinar cells are in each of the major salivary glands?
Parotid gland - Only serous secreting
Submandibular gland - both
Sublingual gland - Only mucin secreting
53
What is the histology of acinar cells?
polarised epithelial cells
| specialised for production of large amounts of proteins due to well developed rough endoplasmic reticulum (RER)
54
Describe the mechanism of saliva secretion:
Acinar cells produce primary isotonic secretion
Sodium-Potassium ATPase pumps in the basolateral membrane of acinar cells pump Sodium ions out of cell to the extracellular fluid.
This creates a Sodium concentration gradient that drives sodium, potassium and chlorine ion entry into acinar cells via secondary active transport through NKCC1 co-transporter
Chlorine ions diffuse out of acinar cells through Chlorine channels in apical membrane
Sodium ions leave via potassiumffd ATPase pumps
Potassium leaves via basolateral potassium ion channels
Saliva is hypotonic as Sodium chloride is reabsorbed without water.
55
Describe neural regulation of saliva:
Controlled by the parasympathetic nervous system
Food stimulates mechanoreceptors on the tongue
Action potentials are sent via sensory afferents to the superior and inferior salivary nuclei in the brainstem between the medulla and the pons.
In the PONS:
Superior salivary nuclei
Parotid gland uses facial nerve to send impulse to submandibular ganglion to secrete saliva
In the MEDULLA:
Inferior salivary nuclei
Otic ganglion uses glossopharyngeal nerve to stimulate secretion of saliva
56
What is the composition of saliva?
```
Sodium ions
Potassium ions
Chlorine ions - for amylases
Hydrogen carbonate ions - buffer acidic foods
Phosphate ions - buffer acidic foods
```
Proteins:
lysozymes
proline-rich proteins
immunoglobulins IgA
57
What are the functions of saliva?
Digestion
Lubrication
Protection
58
What causes dry mouth?
```
Acetylcholine binds to basal surface of acinar cells causing sodium and water to enter lumen during periods such as:
Sleep
Fear
Dehydration
Fatigue
```
59
What is the structure and function of the pharynx?
```
Passageway for food, liquid nutrients and air.
Consists of three distinct regions:
Nasopharynx
Oropharynx
Laryngopharynx
```
60
What is the histology of the gut wall of the pharynx?
Like OC, oropharynx and laryngopharynx are lined with stratified squamous epithelium
lamina proporia has rich supply of mucous secreting glands
external musculature consists of three layers of skeletal muscle involved in deglutition
61
Describe the physiology of deglutition:
Food propulsion from the OC to the pharynx.
Deglutition is initiated voluntarily but proceeds automatically
Complex process involving multiple sets of skeletal muscles that control the activity of the tongue, soft palate, oropharynx, laryngopharynx and oesophagus.
All of these result in deglutition
62
Describe the voluntary phase of deglutition:
Voluntary phase (buccal phase):
begins with ingestion of food
ends after tongue compresses food bolus against hard palate, retracts and propels bolus towards oropharynx
food boluses propelled towards oropharynx stimulate mechanoreceptors on the uvula, palatoglossal arch, palatopharyngeal arch, posterior pharyngeal wall.
63
Describe the involuntary phase of deglutition:
Involuntary phase (pharyngeal-oesophageal phase):
stimulation of mechanoreceptors activates sensory afferent neurons in cranial nerve IX (glossopharyngeal)
action potentials sent from cranial sensory neurons to swallowing centre in nucleus tractus solitarius (NTS) located in medulla and lower pons of brain
sensory input processed in NTS activating adjacent motor nucleus ambiguos (NA)
action potentials then sent to NA via cranial nerves IX and X (Vagus) to skeletal muscles involved in deglutition
Involuntary phase events:
Respiratory centres in CNS inhibited - breathing stops momentarily
Contraction of superior pharyngeal constrictor muscle initiates this phase
Soft palate and uvula rise to close of nasopharynx
Larynx raised and epiglottis folds over opening in larynx to respiratory tract
All routes to pharynx except into GI tract are sealed off
Food moves through pharynx towards oesophagus via peristalsis
64
What is Dysphagia?
Common complication of neurological disorders such as amyotrophic lateral sclerosis (ALS) and strokes.
65
What is ALS?
Amyotrophic lateral sclerosis is characterised by selective death of motor neurons in the brain cortex, brain stem (NA) and somatic nervous system.
Symptoms include:
- impaired speech
- laboured breathing
- dysphagia
- death usually occurs into 2-5 years of disease onset
66
What is the anatomy and function of the oesophagus?
- Muscular tube extending from laryngopharynx to stomach.
- Resting muscle tone of circular muscle layer of proximal oesophagus prevents air entering oesophagus ( this region is termed the upper oesophageal sphincter / UOS )
- the other region in the distal oesophagus is termed the lower oesophageal sphincter / LOS
- LOS normally constantly contracted to prevent reflux
67
What are the secretions of the oesophagus?
Mucosa and submucosa of empty oesophagus form longitudinal folds that flatten when food present. Submucosa contains mucus producing oesophageal glands to lubricate the oesophageal wall for food to move through efficiently
68
Food propulsion in the oesophagus occurs by two types of peristaltic movement, what are these?
1. primary peristaltic waves
| 2. secondary peristaltic waves
69
What are the functions of the stomach?
Temporary storage of ingested food
mechanical digestion of food boluses
chemical digestion of food boluses by acid and enzymes
production of intrinsic factor
70
What is the anatomy of the stomach?
1. Cardia
surrounds lower oesophagus sphincter
has rich supply of mucous secreting glands
mucous protects cardia and oesophageal mucous from gastric acid and enzymes during episodes of acid reflux
2. Fundus
dome - shaped region beneath diaphragm
3. Body (Corpus)
mid portion of stomach
largest part
acts as mixing tank
4. Pyloric antrum
corpus narrows to form pyloric canal which empties into duodenum
pyloric sphincter regulates emptying of chyme into duodenum
5. Rugae
longitudinal folds of mucosa and submucosa of relaxed stomach
6. Muscularis mucosae
extra outer circular smooth muscle layer
7. Muscularis externa
has additional layer of smooth muscle cells for extra constriction to help peristalsis
71
What is chyme?
highly-acidic partially digested food
72
What is the histology of the stomach?
Stomach lined by simple columnar epithelium
Epithelium has many goblet cells that produce HCO3- ions and mucous
mucous surface layer traps bicarbonate rich fluid beneath it as a protective alkaline mucous layer
deep depressions in surface epithelium of gastric mucosa are gastric pits that lead into gastric glands
73
There are gastric glands in the stomach, they contain different cell types, what are these?
Mucous neck cells
parietal (oxyntic) cells
chief (peptic) cells
enteroendocrine cells - release hormones like gastrin directly into lamina propria:
- histamine secreting enterochromaffin (ECL cells)
- somatostatin secreting enteroendocrine cells (D cells)
74
What is gastric juice?
Combined secretions of parietal (HCL) and chief (enzymes) cells.
75
What are parietal cells (PC) and what do they do?
Large triangular secretory cells located in proximal region of each gland in the fundus and corpus of the stomach
secrete HCL and intrinsic factor
innervated by vagus nerve
76
What are chief cells and what do they do?
Located close to base of each gland in the fundus and corpus of the stomach.
secrete zymogen pepsinogen
77
In the distal stomach there is no secretion of HCL, intrinsic factor or pepsinogen in the pyloric antrum. What do the pyloric glands contain?
Endocrine cells
G cells secrete gastrin that stimulate acid secretion and release of histamine from ECL cells.
Promote mucosal growth in corpus and small and large intestines
78
What do D cells secrete?
Somatostatin to inhibit gastrin secretion and block acid secretion from parietal cells.
79
Parietal cells do not produce HCL in their cytoplasm. Describe the mechanism they use to secrete HCL:
Carbonic anhydrase converts CO2 and H2O to carbonic acid - H2CO3
H2CO3 dissociates to HCO3- and H+ ion
HCO3- ions released into ECF by HCO3-/CL- antiporter that exchanges intracellular HCO3- ions for extracellular CL- ions
CL- ions diffuse passively across cell through open chlorine channels in apical membrane of parietal cells into lumen of gastric glands.
H+ ions actively transported into lumen of gastric glands by H+/K+ ATPase pump
exchanges intracellular H+ ions for luman K+ ions (key step in gastric secretion)
80
How is the parietal cell apical membrane specialised for its function of acid secretion?
Tubulovesicles (TVs) present in apical region of parietal cells at rest.
TVs express H+/K+ ATPase pumps, K+ and Cl- ion channels.
When stimulated, TVs fuse with apical membrane and form canaliculi to increase surface area for acid secretion
81
How is the stomachs gastric epithelium protected from acids?
Mucosal barrier made from mucosal gel layer and HCO3- ions
Tight junctions between epithelial cells exclude gastric juice from deeper layers of gastric wall
Apical membrane of deep glandular cells impermeable to HCL
Rapid replacement of damaged epithelial cells by stem cells in the gastric pits.
82
What is the mucosal gel layers composition?
Mucosal gel layer composition:
- mucin
- phospholipids
- electrolytes
- water
83
What is mucin?
Tetrameric protein
Consists of four identical peptides linked by strong disulphide bonds
Viscous and forms mucous gel
Secreted by epithelial goblet cells and mucous neck cells.
84
Neural and hormonal mechanisms control gastric acid secretion (GAS) from parietal cells during digestive periods. What are the three main phases of GAS during a digestive period?
Cephalic
Gastric
Intestinal
85
What happens during the Cephalic phase of GAS?
Prepares stomach for digestive functions.
Sight, smell, taste, thought of food trigger action potentials (APs) in higher brain centres.
APs relay to dorsal motor nuclei (DMN) of vagus nerve (DMNV) in the medulla.
DMN contain cell bodies of parasympathetic, bagal, pre-ganglionic efferent motorneurons.
Their axons run through CNS
APs activate neurons
APs then sent via vagus nerve to stomach and initiate 4 physiological processes that increase GAS.
86
What are the 4 physiological processes that increase GAS during the cephalic phase?
1. in gastric glands of fundus and corpus:
- vagal post-ganglionic motorneurons release acetylcholine (ACh) in the vicinity of muscarinic (M3) ACh receptors in the membrane of parietal cells (PC)
- ACh binds to M3 receptors and stimulates H+ secretion into lumen of gastric glands
2. ACh released from axon collaterals of cholinergic vagal postganglionic neurons diffuses through lamina propria to adjacent ECL cells:
- ECL cells release histamine
- Histamine binds to H2 receptors on membrane of PCs to stimulate H+ secretion into lumen of gastric glands
3. in gastric glands of antrum:
Peptidergic postganglionic vagal efferent motorneurons release gastrin releasing peptide (GRP) near membrane of G cells.
- G cells release gastrin
- Gastrin stimulates acid secretion directly through Gastrin receptors (CCK2R) on membrane of PCs and indirectly through histamine release from ECL cells
4. in both corpus and antrum of stomach:
- vagus nerve inhibits release of somatostatin from D cells
- reduces inhibition on histamine and gastric release by somatostatin
87
What is Zollinger-Ellison Syndrome?
one or more tumours form in the pancreas (gastrinomas)
causes the overproduction of gastrin - hypergastrinemia
chronic hyper-secretion of HCL erodes mucosa of stomach wall leading to gastritis and gastric (peptic) ulcers.
88
What are gastric acid secretagogues?
molecules that stimulate GAS from PCs through 2nd messenger systems: ACh, histamine, gastrin.
89
How do gastric acid secretagogues affect HCL secretion from parietal cells?
HCL secretion is low if only one ligand binds to PCs
When all 3 secretagogues bind:
- volume produced is greater than sum volume produced by each acting alone
POTENTIATION EFFECT
90
What are the treatments of Zollinger- Ellison Syndrome?
Pharmacological:
Ranidine (H2 receptor antagonist)
- H2 receptor antagonists disrupt potentiation effects of secretagogues on GAS from PCs
Omeprazole - blocks H+/K+ aTPase pump blocker (proton pump inhibitor)
Non-pharmacological:
- Vagotomy
91
What happens during the gastric phase of GAS?
Entry of food into stomach triggers 2 primary stimuli of GAS:
Distention of gastric mucosa
Partially digested proteins in gastric gland lumen.
92
Describe distention of gastric mucosa of the corpus and antrum during the gastric phase:
1) stimulates mechanoreceptors in gastric mucosa that then activate a vagal afferent pathway from stomach to DMN
2) DMN integrates the sensory inputs from stomach and transmits APs conveyed through vagal efferent pathway to stomach
3) Collectively, vagal afferent and vagal efferent pathways are termned the vago-vagal reflex
4) the vago-vagal reflex maintains gastric acid secretion through 4 processes that mediate H+ ion secretion from PCs when vagus nerve is activated during cephallic phase.
5) Distention of gastric mucosa activates a local ENS pathway (short myenteric reflex) that releases ACh into vicinity of PCs
6) ACh stimulates acid secretion from PCs.
93
Describe what happens in the gastric phase of GAS in the antrum:
Partially digested proteins (peptone) are potent indirect stimulators of acid secretion from PCs
they directly stimulate G cells to release gastrin
Coffee , caffiene, ingredients in beer and wine stimulate acid secretion via the same mechanism as peptones
94
What is the inhibitory component of GAS during the gastric phase?
Low luminal gastric pH stimulates D cells to secrete somatostatin
Somatostatin inhibits gastrin secretion from G cells
Reducing acid secretion during the gastric phase.
95
What are the two components of the intestinal phase of GAS?
Excitatory component
| Inhibitory component
96
During the intestinal phase of GAS, what happens during the excitatory phase?
Partially digested food in duodenum stimulates intestinal mucosal cells to release a gastrin-like hormone
(intestinal or enteric gastrin)
This stimulates HCL secretion from PCs.
97
During the intestinal phase of GAS, what happens during the inhibitory phase?
Enterogastric reflex is initiated as duodenum expands with chyme containing fats and H+ions
Enterogastric reflex inhibits dorsal motor nuclei in medulla.
Activating sympathetic nerves that induce pyloric sphincter to contract and prevent more chyme entering duodenum
98
What does the enterogastric reflex do?
Reduces gastric activity and protects duodenum from acidity.
99
What else inhibits GAS?
```
Enterogastrones
Secretin
CCK
VIP
GIP
```
