Oesophagus and stomach Flashcards
What is key to remember about the G.I tract
Largely looks the same- same basic structure- the epithelium changes- but is always there
What is meant by digestion
Digestion is the process of breaking down macromolecules to allow absorption
What is meant by absorption
Absorption is the process of moving nutrients and water across a membrane
Describe the teeth
Adult humans have 32 teeth in total. 8 incisors 4 canines 8 premolars 12 molars The incisors and canines are used to bite and rip food into a manageable size for the mouth to handle. The premolars and molars are responsible for grinding it down into smaller pieces.
Describe the muscles in the oral cavity
Several muscles control the position of the jawbone (the mandible) and can widen or close the mandibular joint, as well as swing it side to side, or a combination of both. This will be explored more in year 2 but for now I just want to introduce you to the massater muscle. This is the largest jaw muscle, responsible for our biting action. If you clench your teeth you should feel it lateral to your clenched molars on both sides.
Describe the salivary glands
Small pieces of food are mixed with saliva, an aqueous secretion with digestive enzymes. Specifically, lingual lipase and salivary amylase, which for the first stage of fat and carbohydrate digestion respectively.
The mixing of food and saliva creates a bolus that can be swallowed into the stomach once it is the right size and consistency.
What are the two types of muscles found in the tongue
Intrinsic muscles - responsible for fine motor control of the tongue, such as enunciating specific sounds, or for moving food from one part of the mouth to another.
Extrinsic muscles - responsible for gross movements of the tongue, such as in, out, up and down. This is especially useful to assist mechanical digestion, by pounding the food bolus against the hard palate at the roof of the mouth.
What are the functions of the tongue
Key functions of the tongue include: speech, eating and taste (salt, sweet, bitter, sour, umami).
What are the components of the GI system
See diagram!
What 3 glands are present in the oral cavity
Parotid gland
Sublingual gland
Submandibular gland
Outline a basic plan of the gut wall
Mucosa- towards the lumen
Epithelium- type changes
Lamina propria (loose connective tissue)
muscularis mucosae- thin layer of muscle
Submucosa- connective tissue (contains nerve plexus and blood vessels)
Muscularis- smooth muscle (nerve plexus)- muscular structure for peristalsis
Serosa/epithelium- connective tissue (may have epithelium- holds gut to the body wall
What may be present in the mucosa and sub-mucosa
Mucosa- secretory glands (pancreas), GALT
Sub-mucosa- submucosal glands
Why are the structures in the submucosa important
Blood vessels- substrate for the gut- it should remove things from it
Why is mucus important in the alimentary system
Lubrication to allow the food bolus to pass down the G.I tract easily
Where can the oral cavity lead to and what are the consequences of this
Because the oral cavity leads to two different places (the lungs and the stomach) it is essential that we get food and air getting to their correct destinations.
What is the importance of the epiglottis and upper oesophageal sphincter
This is achieved by the epiglottis and the upper oesophageal sphincter. At rest, the oesophageal sphincter is tonically active and the epiglottis is in the upright position. These structures move during swallowing, with the epiglottis coming down to cover the entrance to the trachea as the bolus of food moves through the pharynx, and the upper oesophageal sphincter relaxes allowing it to flow into the oesophagus.
We need to be selective of what enters the lungs and what enters the G.I tract
Describe the differences between the oesophagus and trachea at rest
Trachea is patent- held open by cartilaginous rings
oesophagus is collapsed at rest
Describe the physiological anatomy of the oesophagus
§ Starts at C5 (throat) and ends at T10.
§ Structures located nearby include: trachea, aorta and diaphragm.
§ Trachea and pericardium are anterior.
§ Descending aorta is posterior and thoracic duct crosses posterior to oesophagus between T7 and T4.
Projects through thorax- pops through diaphragm at T10.
What can damage to the oesophagus cause
Damage to nearby structures (aorta, trachea, recurrent laryngeal nerve and vagus nerve)
its passage through the thorax places it in close proximity to key structures such as the heart, lungs and major blood vessels.
If the tube is ruptured it can cause major problems that are usually associated with a poor prognosis.
What is the key function of the oesophagus
Function: Conduit for food, drink and swallowed secretions from pharynx to stomach
No absorption takes place
Epithelium is crucial to its function
Describe the epithelium of the oesophagus
§ Non-keratinizing stratified squamous.
o Withstands ‘wear & tear’ in extremes of temperatures and textures.
o Lubricates with mucus glands.
Describe the oesophagus when there is no food in it
No food- oesophagus shuts off at both ends- the sphincters are tonically active at rest
The thorax has a negative pressure while the gut is slightly above atmospheric pressure so the sphincters act to restrict the escape of air/food/liquid from the stomach as it has a tendency to rise.
Positive pressure in stomach and negative pressure in oesophagus
Describe the differences between the upper and lower oesophageal sphincters
§ Upper oesophageal sphincter:
o Comprised of skeletal muscle only – this is controlled involuntary which is strange for skeletal muscle (detects eating food)
o Activated by the swallowing centre of the brain.
§ Lower oesophageal sphincter:
o Comprised of (skeletal muscle and) smooth muscle.
o Activated by the swallowing centre of the brain ( although to a lesser extent than the upper)
What are the two muscles in the upper oesophageal sphincter
Constrictor pharyngeal medius - which has commonality with the circular muscle layer of the GI tract
Constrictor pharynges inferior - which has commonality with the longitudinal muscle layer
What is the middle third portion of the oesophagus made up of
The middle third is a mixture of the two as the proportion of skeletal muscle tapers along its length.
What is key to remember about the muscles in the oesophagus
Despite the involvement of skeletal muscle, it is not under voluntary control.
Describe the components of the lower oesophageal sphincter
The internal component - which is built into the circular smooth muscle of the oesophageal wall
The external component - which is formed by the right crus of the diaphragm (voluntary control)
Describe peristalsis
Local effect responding to a food bolus
Nerve stimulus (food) causes circular muscle above the bolus to contract while causing the circular muscle below the bolus to relax (like a tube of toothpaste)
Independent of gravity
§ Longitudinal and circular layers of muscle exist.
Describe peristalsis in response to a big bolus
Contraction may not be large enough
May need a second peristaltic wave to ensure food passes down to the stomach (signal sent to brain)
Describe the epithelial change at the gastro-oesophageal junction
This lining exists all the way to the lower oesophageal sphincter, where the epithelial cells are arranged as simple columnar epithelia. This is to address the different conditions these epithelia need to tolerate (i.e. resistance against strong stomach acid). This epithelial change occurs within the sphincter along a jagged line called the z-line.
columnar are better adapted for secretion and absorption
Describe the role of the gastro-oesophageal junction in preventing acid reflux
Gastrooesophageal junction: where the oesophagus meets the stomach, just inferior of the diaphragm (at lower sphincter); lining changes from stratified squamous to simple columnar in order to resist stomach acid - function is to separate acidic contents from vulnerable oesophageal tissue
Reflux: is prevented by the diaphragm and fold of stomach (when empty)
the lower oesophageal sphincter is less well defined
Why are pregnant women more likely to suffer from heartburn
Foetus pushes stomach up- more acid reflux
Why is it important that the stomach lining is ridged (rugae)
Gastric folds - rugae
o To allow for large changes in volume of the stomach.
Reduced in size when empty (compressible- folds)
Can increase in size and become smooth when food enters
Like when you crush a water bottle.
Describe the phases of swallowing
Stage 0: The oral phase. Chewing and saliva help to prepare the bolus for swallowing. Both oesophageal sphincters are constricted.
Stage 1: The Pharyngeal phase. As the food bolus moves to the back of the pharynx the pharyngeal musculature helps to guide it towards the oesophagus. Both oesophageal sphincters open.
Stage 2: Upper oesophageal phase. The upper sphincter closes, and superior rings of circular muscle contract as inferior rings dilate. Sequential contractions of longitudinal muscle help guide the food down the gullet.
Stage 3: Lower oesophageal phase. As food passes through the lower sphincter that too closes, and the peristaltic wave continues to push food into the stomach.
Describe the differences in the opening times of the upper and lower oesophageal sphincters
Open at the same time
Lower open for longer- until food has entered oesophagus until it leaves
What are the key functions of the stomach
Digestion of macronutrients: this can be chemical (acid and enzymes) and mechanical (mixing and churning)
Storage reservoir for food: until downstream organs are ready to receive the stomach contents
Immunological protection: Strong acid helps to destroy ingested pathogens
What does the epithelium of the stomach do
Invaginates into glands
gastric pits which contain specialist exocrine and endocrine cells.
What are the regions of the stomach
From an anterior perspective, the stomach can be split into five anatomical regions, which each contain different cells in different quantities. Gastric acid is secreted from the fundus and body in high volume, up to 2 L per day.
Describe the different regions of the stomach
Cardia & Pyloric Region:Mucus only
Body & Fundus:Mucus, HCl, pepsinogenAntrum:Gastrin
What is the difference between the pyloric canal and pyloric antrum
pyloric antrum- gastrin
pyloric canal- mucus
How does the wall of the stomach differ
The wall of the stomach is structurally similar to other parts of the digestive tract, with the exception that the stomach has an extra oblique layer of smooth muscle inside the circular layer, which aids in performance of complex grinding motions (mechanical digestion).
Describe the stomach in the empty state
In the empty state, the stomach is contracted and its mucosa and submucosa are thrown up into folds called rugae. Following consumption of food and fluids, as the volume of the stomach increases, the rugae are stretched and become flat.
The stomach only has a minor role in absorption, so the presence of rugae (instead of villi) allows the stomach to undertake its reservoir function.
When functioning correctly what should the lower oesophageal sphincter allow
One-way traffic of food
Describe the change in histology between the LOS and stomach
The z-line is a visible threshold between epithelia of the oesophagus (stratified squamous cells) and the stomach (simple columnar cells). The oesophageal epithelium is light pink in colour, and functional as a wear and tear lining. The stomach lining is a bright red and more resistant to low pH.
The mucous gel lining of the stomach provides considerable protection against the corrosive acid
Describe the antrum
Endocrine element - gastrin- in glands
lining/pit- HCO3-/mucus cells- to neutralise acid- protect stomach lining
Describe the body and fundus
pit- same as antrum
neck- mucus secreting cells
gland- pepsinogen and hcl (pinker staining)
Describe the cardia and pyloric regions
mucus only
Describe how much acid the stomach makes
2L/day
150mM H+
3 mill x that in blood
What is the role of HCO3- in the mucus
Mucins = gel
Coating
HCO3- trapped
in mucus gel
pH at;
Epithelial surface = 6-7
Lumen = 1-2
What is the stomach lined with
The stomach is lined with millions of gastric pits, which are deep pores within the stomach mucosa, which lead to multiple gastric glands, which house the functional secretory cells of stomach. These pits contain a mixture of cells responsible for secreting gastric juice, which is a cocktail of hydrochloric acid, enzymes and enzyme zymogens. Cells in these glands also secrete mucous, paracrine signalling molecules and hormones.
Describe peristalsis in the stomach
About 20% of the contractions in the stomach are peristaltic waves
Waves of peristalsis begin as gentle muscular contractions near the lower esophageal sphincter and continue down the stomach toward the pyloric sphincter
As the contractile waves near the distal end of the stomach, they become much stronger and more forceful; this results in thoroughly mixed chyme before it passes through the pyloric sphincter
As the peristaltic wave passes through the partly opened pyloric sphincter, it causes the chyme to move through it in a back and forth fashion. This serves to break up the larger materials left in the chyme
vagus nerve stimulation essential
Describe segmentation in the stomach
About 80% of the contractions in the stomch are segmentation contractions-relatively weak contractions that thoroughly mix ingested food with stomach secretions to form chyme
The more fluid part of the chyme is pushed toward the pyloric sphincter.
The more solid part is pushed back toward the body of the stomach
The neural mechanisms that stimulate stomach secretions also increase stomach motility. The major stimulus is distension (stretching) of the stomach wall
Increased stomach motility increases stomach emptying
Less coordinated than peristalsis
stretching activates enteric nervous system
Describe the role of mucus cells
Mucous cells are high in number and secrete a bicarbonate-rich mucous which helps to protect the stomach lining. The acidic environment in the lumen of the stomach is very low (pH 2-3), however the presence of mucous keeps the pH next to the lining much closer to 7. The mucus lining also helps to protect the stomach lining from active lipase and proteases, which may interfere with the lipid bilayer and its membranous transporter
What do chief cells produce
These cells produce a protease zymogen (pepsinogen) and a lipase (gastric lipase). Pepsinogen is activated to pepsin in the presence of HCl in the gastric lumen; it is secreted as a precursor to prevent it auto digesting the chief cells. Pepsin then breaks dietary proteins into smaller peptide chains. Gastric lipase is an enzyme that digests fats by removing a fatty acid from a triglyceride molecule.
Describe the characteristics of chief cells
The chief cells produce pepsinogen and enzymes in high volume and store them in granules until they are stimulated to exocytose them into the stomach lumen.
Protein-secreting epithelial cell
Abundant RER
Golgi packaging and modifying for export
Masses of apical secretion granules
Lots of organelles that help in protein production and modification
Describe the parietal cell when resting
Many mitochondria (requires lots of ATP)
Cytoplasmic tubulovesicles (contain H+/K+ ATPase)
Internal canaliculi (extend to apical surface)
Describe the parietal cell when secreting
Tubulovesicles fuse with membrane and microvilli project into canaliculi
All canaliculi combine and project into lumen of stomach to release their contents
This increases the surface area for acid secretion
What is the function of the parietal cells
The strong HCl has a number of useful functions: 1) to kill ingested pathogens; 2) activate protease zymogens; 3) alter protein structure to help digestion
Parietal cells also secrete intrinsic factor, a glycoprotein essential for the absorption of vitamin B12. Deficiency in this substance will lead to pernicious anaemia.
Describe the exocrine function of the parietal cells
Carbon dioxide diffuses down its concentration gradient from the blood into the parietal cell and, in the presence of carbonic anhydrase, is combined with water to form carbonic acid, which dissociates into bicarbonate and a proton. The bicarbonate is exchanged with a chloride ion in the interstitial space. This causes chloride ions to move down their concentration gradient into the stomach lumen via chloride channels. To pump the protons into the lumen the cell needs to move potassium from the interstitial space to the lumen. This is achieved via a sodium potassium exchanger in the basolateral membrane, and chloride channels in the apical membrane. Now, potassium can be pumped into the cell in exchange for protons. The potassium then re-enters the lumen through the potassium channels and the secreted proton combines with chloride to form hydrochloric acid
What doe the canaliculi express
H+/K+ ATPase
Why do parietal cells need lots of mitochondria
These cells are rich in mitochondria to provide energy for membrane transport.
Describe the actions of pepsinogen
Autocatalsyed to pepsin in acidic environment of gastric lumen
Breaks down proteins into smaller protein fragments
Proteins are the most difficult things to digest- only occurs in stomach
Describe the actions of gastrin
Pyloric antrum
Local peptide hormones – Gastrin
Stimulate histamine release from chromaffin cells (lamina propia)
gastrin comes back to stomach via blood to stimulate acid production
Describe the cephalic phase of gastrin secretion
Afferents: Sight, smell, taste and thought of food.
Efferents: Vagus nerve (neurons secreting ACh) stimulate secretion from mucous cells, chief cells, parietal cells and G-cells via the submucosal plexus. Gatsrin (from G-cells) and histamine (from ECL cells) also stimulate parietal cell secretion.
Effects: Small secretion for a few minutes
Describe the gastric phase
Afferents: Distension of stomach and chemoreception of nutrients and a reduced pH.
Efferents: Vagus nerve (neurons secreting ACh) stimulate secretion from mucous cells, chief cells, parietal cells and G-cells via the submucosal plexus and increase motility (mixing waves) via the myenteric plexus. Gatsrin (from G-cells) and histamine (from ECL cells) also stimulate parietal cell secretion.
Effect: 3-4 hours of gastric activity (secretion of acid, enzymes and hormones) and mechanical digestion.
How is the ECL cell stimulates
Ach from vagus nerve (or gastrin) causes histamine release
binds to histamine receptor on parietal cell, ATP – cAMP- Ca2+
Ach can bind to muscarinic receptors- depolarisation
gastrin can bind to gastrin receptors
all Ca2+ influx
What is produced in the gastric phase
more acid, more mucus, more pepsin
What happens in the intestinal phase
Intestine response to chemical constituents (lipids) and pH (<2) of chime
signal to brain to switch stomach off (centrally mediated by Ach)
Produces entero-gastric reflex
cells in small intestine produce hormones (enterogastrines) which inhibit the stomach
Describe the intestinal phase
Afferents: Duodenal stretch and chemodetection of reduced pH and duodenal distension
Efferents: I-cells secrete cholecystokinin (CCK) and S-cells secrete secretin into the blood. This decreases parietal cell secretion and inhibits gastric motility and emptying. Stretch receptors input into the enteric nervous system, which reduces activation of the stomach.
Effect: Gastric emptying slows down to allow downstream organs to deal with current contents.
Describe the excitatory intestinal phase
Depends on protein conc- if high- signals that the stomach has not broken down proteins enough
Stimulates gastrin secretion- switching on acid production- stomach needs to work harder.
Describe omeprazole and ranitidine
Omeprazole: inhibits K+/H+-ATPase of parietal cells to prevent acid formation (proton pump inhibitors)
Ranitidine: inhibits histamine binding to receptors on parietal cells (histamine receptor antagonist)
Describe G cells
These are enteroendocrine cells found at the bottom of the gastric pits. G-cells release the hormone gastrin into the bloodstream in response to vagus nerve stimulation, the presence of peptides in the stomach, and stomach distension.
Gastrin travels through the blood to receptor cells in the stomach where it stimulates gastric secretion and motility. Stimulation of smooth muscles by gastrin leads to stronger contractions of the stomach and the opening of the pyloric sphincter to move food into the duodenum.
Gastrin also binds to receptors on cells in the pancreas and gallbladder where it increases the secretion of pancreatic juice and bile.
Descirbe entero-chromaffin like cells
These cells are a type of neuroendocrine cell found deep in the gastric glands, usually in the vicinity of parietal cells. They secrete histamine which stimulates the secretion of acid from the parietal cells.
Describe G cells
These enteroendocrine cells secrete somatostatin, which has a generally inhibitory effect on gastrointestinal function. Within the gastric gland somatostatin inhibits ECL production of histamine and parietal cell activity, both of which inhibit the secretion of hydrochloric acid.
Describe gastric stem cells
These pluripotent cells are capable of differentiating into all of the different cells of the stomach, under the influence of different factors. The development pathway varies for different parts of the stomach
