GI Flashcards
Describe the overall processes of the GI tract
Initial physical disruption → Ingestion and transport to storage → Initial chemic disruption and creation of suspension (chyme) → Disinfection → Controlled release of Chyme → Dilution and neutralisation → Completion of chemical breakdown → Absorption of nutrients and electrolytes → Final absorption of water and electrolytes → Producing faeces for controlled excretion.
What are the basic functions of the Mouth and oesophagus?
o Mastication
o Saliva - protects mouth
lubricates food
starts digestion of sugars
o Swallowing
o Formation of bolus
o Rapid oesophageal transportWhat are the basic functions of the stomach?
o Storage - Relaxes to accommodate food
o Initial disruption - contract rhythmically to mix and disrupt. Secretes acid and proteolytic enzymes
o Delivers Chyme slowly into the Duodenum
What are the basic functions of the duodenum?
Dilution and neutralisation of Chyme o Water drawn in from ECF. Stomach impermeable, Duodenum permeable. o Alkali (bile) added from Liver and Pancreas o Enzymes added from pancreas and intestine
What are the basic functions of the small intestines?
Absorption of nutrients and electrolytes
o Fluid passes very slowly through the small intestine
o Large surface area
o Epithelial cells absorb molecules, some actively some passive - Often coupled to Na+ absorption
o Pass into hepatic portal circulation
Absorbs the majority of water (1.5L vs. 0.15L large intestine)
What are the basic functions of the large intestines?
o Final absorption of water (0.15)
o Very slow transit
o Faeces form and accumulate in the descending and sigmoid colon
What are the basic functions of the rectum?
o Faeces propelled periodically into rectum
o Urge to defecate
o Controlled relaxation of sphincters and expulsion of faeces
What are the 4 layers of tissue of the GI tract?
Mucosa - Epithelial lining and thin layer of smooth muscle
Submucosa - Fibroelastic tissue with vessels, nerves, leucocytes and fat cells
Muscularis Externa - Inner circular and outer longitudinal layer of smooth muscle with the myenteric plexus lying in between the layers.
Serosa/Adventitia - Thin outer covering of connective tissue
Describe the fluid balance of the gut
1kg of food and about a litre of liquids. The food is mixed with 1.5L of Saliva and about 2.5L of gastric secretions to form chyme.
The small intestine then absorbs about 12.5L of the fluid, and the large intestine absorbs about 1.35L.
What is the enteric nervous system?
Subdivision of the autonomic nervous system that directly controls the GI system. It is made up of two nerve plexuses in the wall of the gut that may act independently of the CNS (short reflex pathway). This activity may be modified by both branches of the ANS (long reflex pathway).
Parasympathetic control however is the most significant. It coordinates both secretion and motility using a range of neurotransmitters, not just Ach as you may expect (parasympathetic).
Describe the role of hormones and other peptides which affect the motility and secretion in the gut
Endocrine cells in the walls of the gut release a dozen or more peptide hormones. These include both hormones with endocrine action and paracrine action. The hormones comprise two structurally related groups – the Gastrin group and the Secretin group. These hormones are released from one part of the gut to affect the secretions or the motility of other parts.
Define Dysphagia
Difficultly swallowing.
May be caused by problems with the oesophagus, e.g. musculature, obstruction by tumour or neurological, e.g. a stroke.
Tumours of the oesophagus, high up are Squamous Cell Carcinoma, lower down are Adenocarcinomas.
Define Acid Reflux
Sphincter between the oesophagus and the stomach is weak, acid refluxes into the oesophagus and causes irritation and pain (heartburn).
Define Barrett’s Oesophagus
Metaplasia of the lower oesophageal squamous epithelium to gastric columnar. This is to protect against acid reflux.
Define Oesophageal Varices
Portal venous system is overloaded due to cirrhosis, blood is diverted to the oesophagus through connecting vessels. This leads to the dilation of sub mucosal veins in the lower part of the oesophagus.
Define Peptic Ulceration
Area of damage to the inner mucosa of the stomach or duodenum, usually due to irritation from gastric acid.
Define Pancreatitis
Inflamed pancreas, causes considerable pain. Characterised by the release of amylases into the blood stream.
Define Jaundice
Liver cannot excrete bilirubin, which accumulates in the blood. If build up of bilirubin is due to excess haemoglobin breakdown it is Pre-hepatic Jaundice. If build up of bilirubin is due to bile duct obstruction and back up of bile causing liver damage it is Post-hepatic or Obstructive Jaundice
Define Gallstones
Precipitation of bile acids and cholesterol in the gallbladder forms gall stones. Often asymptomatic, but may move within the gall bladder causing painful Biliary Colic, or move to obstruct biliary outflow. Tumours of the pancreas may also obstruct outflow.
Define Malabsorption
conditions that affect how well the intestines can absorb things.
Define Appendicitis
Inflammation of the appendix, presents as a sharp pain in the side at the same level as T10, which then localises to the right lower quadrant.
Define Peritonitis
Inflammation of the peritoneum.
Define Inflammatory Bowel Disease
Group of inflammatory conditions of the colon and small intestines
Ulceritive colitis and Crohn’s disease
Define Acute blockage of small intestines
Present with Pain (in their back), vomiting and bloating.
Define Haemorrhoids
Vascular structures in the anal canal that aid with stool control. When they become swollen and inflamed they are painful, itchy and blood may be present in stool.
Define Prolapse
Literally means ‘to fall out of place’. Prolapse is a condition where organs fall down or slip out of place. E.g. the rectum can prolapse.
Define Diverticula
Pressure is too high in the colon, producing an abnormal ‘outpouching’ to form a hollow. The sigmoid colon is the area most prone as the blood supply causes an area of weakness (?)
Define Meckels’ Diverticulum
A pouch in the lower part of the small intestine, a vestigial remnant of the yolk sac. It can produce ectopic gastric mucosa that may then produce gastric acid, causing irritation.
Define Colo-Rectal Cancer
The large intestine is a common site of malignancies, and colo-rectal cancer is a major cause of mortality.
Describe the anatomy of the oral cavity, its content and their functions
The mouth is the entrance of the GI tract. Disrupts foodstuff and mix them with saliva to form boluses
Teeth - incisors (cut) and molars (crush) using force created by masseters (supplied by branch of trigeminal nerve)
Tongue - Collection of 8 muscles that work to manipulate food for mastication and form it into a bolus. Aids in swallowing by pushing bolus to the back of the mouth.
Describe structure of oropharynx and its function
The oropharynx lies behind the oral cavity, and forms the portion of the pharynx below the nasopharynx but above the laryngopharynx.
It extends from the uvula, which is the end of the palate, to the level of the hyoid bone.
Because both food and air pass through the oropharynx, a flap of tissue called the epiglottis closes over the glottis to prevent aspiration.
Describe the structure and function of the oesophagus
Muscular tube
Passes food from the pharynx to the stomach. Continuous with the lower part of the laryngopharynx.
Layers, from inside to out are:
◦ Mucosa - composed of non-keratinized stratified squamous epithelium, lamina propria and a layer of smooth muscle (Muscularis Mucosa)
◦Submucosa - containing the mucous secreting glands
◦Mucularis externa - Upper third of oesophagus is striated, skeletal muscle under conscious control for swallowing. The lower two thirds are smooth muscle under autonomic control (peristalsis).
What is the function of saliva?
1.Lubricates and wets food
2.Starts the digestion of carbohydrates (Amylase)
3.Protects oral environment
◦Keeps mucosa moist
◦Washes teeth
◦Maintains alkaline environment - neutralise acid produced by bacteria
◦High Ca2+ concentration
How much saliva is produced everyday?
1.5L
What is the name given when not enough saliva is produced?
Zerostomia
Still able to eat provided food is moist, but teeth and mucosa degrade very quickly.
What are the components of saliva?
◦Water - Hypotonic solution
◦Electrolytes - Na+, Cl-, usually at a lower concentration than plasma. Ca2+, K+, I- (iodide) usually at a higher concentration than plasma
◦Alkali - HCO3- at a higher concentration than plasma
◦Bacteriostats
◦Mucus - (Mixture of mucopolysaccharides)
◦Enzymes - Salivary amylase
What are the 3 types of salivary glands?
◦Parotid Glands
◦Sub-lingual glands
◦Sub-mandibular glands
Describe the parotid gland secretion
◾Watery secretion, rich in enzymes but little mucus
◾Serous saliva
◾25% of volume secreted
Describe the sub-lingual gland secretion
◾Viscous secretion, no enzymes but lots of mucus
◾Mucus saliva
◾5% of volume secreted
Describe the sub-mandibular gland secretions
◾All components of saliva (mixed serous and mucus)
◾Mixture of serous and mucus acini leading to a common duct
◾70% of volume secreted
Explain the mechanism of serous saliva secretion
Saliva is a hypotonic solution, but there is no cellular mechanism to secrete water. Therefore more concentrated solution is secreted, and solute is then reabsorbed from it to leave the final hypotonic solution.
Acinar Cells secrete an isotonic fluid containing enzymes. Duct Cells then remove Na+ and Cl- and add HCO3-. The gaps between duct cells are tight, so water does not follow the resulting osmotic gradient and so saliva remains hypotonic.
At a low flow rate, the duct cells have the opportunity to remove most Na+, so saliva is very hypotonic. However, the rate at which duct cells can modify saliva is limited, so at a high flow rate a smaller fraction is removed and the saliva becomes less hypotonic. However, the stimulus for secretion (see below) promotes HCO3- secretion, so saliva becomes more alkaline.
Explain the mechanism of acinar secretion
Cl- ions are actively secreted from Acinar cells into the lumen of the duct. Water and other ions (Na+) will then follow passively.
Explain the mechanism of ductal modification
The action of the Na/K-ATPase Antiporter in the Basolateral membrane of duct cells lowers the [Na+] inside the cell. This means there is a concentration gradient, where [Na+] is high in the duct lumen and low in the duct cells. Na+ diffuses passively back into the Duct cells.
The action of the Na/K-ATPase Antiporter also increases the [K+] concentration inside the cell. The resulting concentration gradient drives the expulsion of Cl- from the duct cells into the ECF. Again, a concentration gradient is set up between the duct lumen and cells, with [Cl-] low inside and [Cl-] high outside. This gradient drives the expulsion of HCO3- into the duct lumen.
Which part of the nervous system controls saliva secretion?
Autonomic nervous system
How does the parasympathetic nervous system control saliva secretion?
Parasympathetic stimulation increases the production of primary secretion (Acinar cells) and increases the addition of HCO3- (Duct cells)
◦Parotid Gland
◾Glossopharyngeal Nerve (9th cranial nerve)
◾ Otic ganglion
◦Submandibular and Sublingual Glands
◾ Facial Nerve (7th cranial nerve)
◾Submandibular ganglion
◦Muscarinic receptors
◾Blocked by atropine like drugs
◦Co-transmitters stimulate extra blood flow
Outflow is mediated by: ◦From centres in the medulla ◦Afferent information from: ◾Mouth and tongue - Taste receptors, especially acid ◾Nose ◾Conditioned reflexes - Pavlov's dogs
How does the sympathetic nervous system control saliva secretion?
Sympathetic stimulation reduces the blood flow to the salivary glands, limiting salivary flow and producing the typical dry mouth of anxiety.
◦Superior cervical ganglion
The rate of ductal recovery of Na+ is also increased by the release of aldosterone from the adrenal cortex. (increase ENaC, increase Na/K/ATPase), making saliva even more hypotonic.
What are the 3 phases of swallowing?
1.Voluntary Phase - Tongue moves the bolus back onto the pharynx
2.Pharyngeal Phase
◦Afferent information from pressure receptors in the palate and anterior pharynx reaches the swallowing centre in the brain stem.
◦A set of movements is triggered: ◾Inhibition of breathing ◾Raising of the larynx ◾Closure of the glottis ◾Opening of the upper oesophageal ‘sphincter’
3.Oesophageal Phase
◦The muscle in the upper third of the oesophagus is voluntary striated muscle under somatic control
◦The muscle of the lower two thirds is smooth muscle under control of the parasympathetic nervous system.
◦Peristaltic waves sweep down the oesophagus, propelling the bolus to the stomach in ~9 seconds, coordinated by extrinsic nerves from swallowing centre of brain.
◦Lower oesophageal ‘sphincter’ opens and bolus enters stomach
Define Odynophagia
The symptom of pain whilst swallowing
Define Achalasia
Dysphagia may result as a consequence of a primary oesophageal disorder, for example motility problems of the smooth muscle preventing peristalsis
What can be the causes for dysphagia?
Consequence of a primary oesophageal disorder, for example motility problems of the smooth muscle preventing peristalsis.
Dysphagia may also result as a secondary consequence of another issue, E.g. obstruction or compression of the oesophagus due to a tumour.
What are the two types of dysphagia?
◦Dysphagia for Solids
◾Oesophageal Dysphagia
◾Investigate with a barium swallow/endoscopy
◦Dysphagia for liquids
◾Oropharyngeal Dysphagia
◾Investigate with a flexible endoscopy evaluation of swallowing. This will allow you to view the entire trachea/oesophagus.
◾Most commonly due to a stroke
Describe the anatomical mechanisms that prevent gastro-oesophageal reflux
Lower oesophageal sphincter (one way valve)
This coupled with the angle of His
The crus of the diaphragm helps with the sphincteric action.
Outline some of the clinical consequences of free gastro-oesophageal reflux
Barrett’s Oesophagus - metaplasia from non-keratinised stratified squamous epithelia to columnar epithelium and goblet cells. This is in an attempt to better resist the harmful contents of the stomach.
Barrett’s oesophagus is strongly associated with adenocarcinoma, a particularly lethal cancer.
Gastro-oesophageal Reflux Disease (GORD)
The reflux of the stomach’s contents into the oesophagus and pharynx causes several symptoms, including a cough, hoarseness and asthma.
All of the symptoms result from the acidic contents of the stomach refluxing back out.
Describe areas of potential weakness in the abdominal wall
inguinal canal, femoral ring and umbilicus
Can lead to inguinal, femoral and umbilical hernias respectively
Describe the structure of the inguinal canal
Oblique passage that extends in a downward and medial direction.
Begins at the deep (internal) inguinal ring and continues for approximately 4cm, ending at the superficial (external) inguinal ring.
Lies in between the muscles of the anterior abdominal wall and runs parallel and superior to the medial half of the inguinal ligament (the inguinal ligament is the inferior border of the aponeurosis of the external oblique muscle, attached between the ASIS and the pubic tubercle).
What passes through the inguinal canal?
The spermatic cord in men and the round ligament of the uterus in women passes through the canal. Additionally, in both sexes the ilioinguinal nerve passes through part of the canal.
What are potential hernia complications?
Strangulation
Incarceration
What is Strangulation of a hernia?
Constriction of blood vessels, preventing the flow of blood to tissue
What is incarceration of a hernia?
Hernia cannot be reduced, or pushed back into place, at least not without very much external effort.
What is an inguinal hernia?
Protrusion of the abdominal cavity contents through the inguinal canal.
What are the borders of Hesselbach’s triangle?
◦Inferiorly – Medial half of the inguinal ligament
◦Medially – Lower lateral border of rectus abdominis
◦Laterally – Inferior epigastric artery
Describe a direct inguinal hernia
Protrudes into the inguinal canal through a weakened area in the transversalis fascia near the medial inguinal fossa within an anatomical region known as the Inguinal / Hesselbach’s triangle
Describe an indirect inguinal hernia
Protrudes through the deep inguinal ring, within the diverging arms of the transversalis fascial sling. Most indirect inguinal hernias are the result of the failure of embryonic closure of the deep inguinal ring after the testicle has passed through it.
Describe an epigastric hernia
Occur in the epigastric region, in the midline between the xiphoid process and the umbilicus, through the linea alba.
The primary risk factors are obesity and pregnancy.
Describe an umbilical hernia
Occur through the umbilical ring. They are usually small and result from increased intra-abdominal pressure in the presence of weakness and incomplete closure of the anterior abdominal wall after ligation of the umbilical cord at birth.
Acquired umbilical hernias occur in adults, most commonly in women and obese people.
Describe femoral hernias
Protrusion of abdominal viscera into the femoral canal, occurring through the femoral ring. A femoral hernia appears as a mass, often tender, in the femoral triangle.
Femoral Hernias are bounded by the femoral vein laterally and the lacunar ligament medially. The hernia compresses the contents of the femoral canal (loose connective tissue, fat and lymphatics) and distends the wall of the canal.
Initially femoral hernias are small, as they are contained within the canal, but they can enlarge by passing inferiorly through the saphenous opening into the subcutaneous tissue of the thigh.
In which sex are femoral hernias more common and why
more common in females as they have wider pelvis.
What can lead to strangulation of femoral hernias?
Strangulation of femoral hernias may occur because of the sharp, rigid boundaries of the femoral ring.
Describe the functions of the stomach
◦Stores Food
◦Disinfects Food
◦Breaks food down into Chyme - chemical and physical disruption
What are the components of gastric secretions?
Hydrochloric Acid (HCl) Proteolytic Enzymes (Pepsin) Mucus HCO3- Gastrin
Where does HCl get released from and what is its function?
Parietal (Oxyntic) Cells
Acid keeps luminal pH
Where does pepsin get released from and what is its function?
Chief Cells
Non-specifically breaks down proteins into peptides
Where does mucus get released from and what is its function?
Neck Cells (Surface cells)
Sticky, so is not removed easily from the stomach lining and basic, due to amine groups on proteins
Where does HCO3 get released from and what is its function?
Neck Cells (Surface cells)
Secreted by surface cells into the mucus, provides a buffer for H+ ions
Where does gastrin get released from and what is its function?
G-Cells
Binds to surface receptor on parietal cell, stimulating acid and intrinsic factor
What are gastric pits and what do they contain?
Indentations in the stomach mucosa that are the openings to gastric glands.
Neck cells
What do gastric glands contain?
Parietal
Chief
G-Cells
smooth muscle cells
Explain the mechanism of stomach secretion
Most body fluids are slightly alkaline, so to secrete H+ ions they need to be created in large quantities. This takes place in the mitochondria of parietal cells by splitting water into H+ and OH- ions.
The generated OH- ions combine with CO2 from metabolism to form HCO3-, which is exported to the blood.
For every mol of H+ secreted into the stomach, 1 mol of HCO3- enters the blood.
Parietal cells have lots of mitochondria, allowing them to produce H+ at a high rate. However, these produced ions cannot accumulate in cells. To overcome this problem, parietal cells have invaginations in their cells walls called canaliculi.
Canaliculi have proton pumps, which expel H+ from parietal cells up a high concentration gradient. As the concentration gradient is high, this is a very energy intensive process.
The proton pumps in canaliculi are a key target for drug action, as if inhibited they will reduce the amount of acid in the stomach.
Explain the control of gastric acid secretion
Parietal cells are stimulated by Acetylcholine, Gastrin and Histamine.
Acetylcholine - Ach is released from postganglionic parasympathetic neurones, stimulated by gastric distension as food arrives. It acts on muscarinic receptors on parietal cells.
Gastrin is released from endocrine cells in the stomach, G-Cells. It binds to surface receptors on parietal cells.
Gastrin secretion is stimulated by the presence of peptides and Ach from intrinsic neurones. It is inhibited by low pH in the stomach.
Histamine is released from Mast Cells and diffuses locally to bind H2 surface receptors on parietal cells. Acid secretion is then stimulated via c-amp. Gastrin and Ach stimulate mast cells, so Histamine works as an amplifier.
Explain the 3 phases of gastric secretion
Cephalic Phase
Gastric Phase
Intestinal Phase
Explain the cephalic phase
The sight and smell of food, and the act of swallowing, activates the parasympathetic nervous system, which stimulates the release of Ach. This stimulates parietal cells directly and via histamine ( increase Acid).
Explain the gastric phase
Once food reaches the stomach, it causes distension, further stimulating Ach release, and subsequently parietal cells (increase Acid).
The arrival of food will buffer the small amount of stomach acid in the stomach in between meals, causing luminal pH to rise. This disinhibits Gastrin (increase Acid).
Acid and enzymes will then act on proteins to produce peptides, further stimulating Gastrin release as the pH falls and the initial disinhibition is removed (increase Acid).
Explain the intestinal phase
Once chyme leaves the stomach in significant quantities, it stimulates the release of the hormones Cholecystokinin and Gastric Inhibitory Polypeptide from the intestines that antagonise Gastrin (decrease Acid). The small amount of acid left in the stomach is no longer being buffered by food, and the low pH inhibits Gastrin (decrease Acid).
Why are pain from ulcers worse at night?
The low pH of the stomach between meals can aggravate ulcers. Because of this, pain from ulcers is particularly bad at night.
How can gastric acid secretion be reduced by drugs?
Reduced by inhibition of:
◦Histamine at H2 Receptors - E.g. Cimetidine
Removes the amplification of Gastrin/Ach signal
◦Proton Pump Inhibitors (PPIs) - E.g. Omeprazole
Prevents H+ ions being pumped into parietal cell canaliculi
Describe the function of the stomach defences
Protect against stomach acids
Neck cells secrete mucus to protect the mucosa.
Mucus is Sticky, so is not easily removed from the stomach lining. It is also Basic, due to Amine groups on the proteins.
H+ ions slowly diffuse in and react with the basic groups on mucus and with HCO3- that is secreted by surface epithelial cells.
HCO3- stays close to the surface cells. This means the pH at the surface cells is well above 6.
Mucus and HCO3- secretion from neck cells and surface cells respectively is stimulated by prostaglandins, which are promoted by most factors that stimulate acid secretion.
What substances are able to breach the stomach lining?
◦Alcohol - Dissolves the mucus, allowing the acid to attack the stomach
◦H. Pylori - Surface cells become infected, inhibiting mucus/HCO3- production
◦NSAIDS
◾Inhibit prostaglandins, therefore reducing defences
◾Some, like aspirin are converted to a non-ionised form by stomach acid, allowing them to pass through the mucus layer into cells before they re-ionise.
What happens stomach defences are breached?
Results in peptic ulcers
What are the two types of motility in the stomach?
Receptive Relaxation
Rhythmic Contractions
What happens during receptive relaxation?
As food travels down the oesophagus, a neural reflex carried out by the vagus nerve triggers relaxation of the muscle in the stomach’s wall, so pressure does not increase. This means that pressure in the stomach does not increase as it fills limiting reflux and allowing us to consume large meals
What happens during rhythmic contractions?
The stomach has longitudinal and circular muscle that is driven by a pacemaker in the cardiac region. The pacemaker fires ~3 times a minute, causing regular, accelerating peristaltic contractions from the Cardia to the Pylorus.
