Alimentary System Flashcards
Mid GI symptoms
Abdominal pain, steatorrhea (fatty stool), diarrhoea, distension
Upper GI symptoms
Haematemes, black/tar like stool, nausea, vomiting, dysphasia, odynophagia, belching, acid reflux
Lower GI symptoms
Abdominal pain, bleeding, constipation, diarrhoea, incontinence
Hepatobiliary symptoms
Right-upper quadrant pain, biliary colic, jaundice, dark urine, pale stool, ascites
What is the function of the oesophagus?
Conduit for food, drink and swallowed secretions from pharynx to stomach
Describe the structure of the oesophagus, including type of epithelium.
Non-keratinising stratified squamous epithelium Has skeletal muscle at the top and smooth muscle at the bottom. Muscle arrange in circular and longitudinal arrangements.
How does the oesophagus move a bolus of food from the mouth to the stomach? (Include process of swallowing)
Oesophagus moves food down by peristalsis where the muscle above the bolus contracts and that below relaxes pushing food down. Topically active When swallowing starts message goes to brain leading to opening of upper and lower oesophageal sphincter. Once food passes upper the sphincter closes. Lower remains open until food enters the stomach.
What is the function of the stomach? (3 parts)
- Break down food into smaller particles (due to acid and pepsin).
- Hold food and release at a controlled, steady rate into the duodenum.
- Kill parasites and certain bacteria
What is the function of chief cells? What happens to their product?
They secret pepsinogen. Pepsinogen is the inactive form of Pepsin. It is activated when in a low pH environment pepsinogen proteins cleave one another forming the active form
What are the function of parietal cells?
They secrete HCl
What are the three phases involved in acidic control of the stomach? Describe what happens.
- Cephalic phase: thoughts, taste, smell of food activate the vagus nerve (parasympathetic). This increase acetylcholine release and so more acid is secreted from parietal cells
- Gastric phase: - stomach distension detected by stretch receptors lead to more acid - local systems (enteric nervous system) play a part - chemoreceptors detect increase in pH and gastrin is secreted
- Intestinal phase - usually inhibitory. When intestine detects acidic chyme sends protein signals via blood to stomach inhibits acid production - can stimulate production if proteins are not sufficiently broken down
What is gastrin? What does it do?
It’s a protein that is secreted in response to an increase in pH in the stomach. It triggers the parietal cells to release more acid, and histamine release from other cells in the stomach
What are the main organs of the digestive system?
Stomach, large intestine, small intestine, pancreas, gall bladder, liver, oesophagus
What is the function of the small intestine?
To absorb nutrients, salt and water
What type of epithelial are present in the small intestine and what is their function? How are the cells specialised for their function?
Simple columnar - absorption and transport of substances - microvilli making a brush border. On top of brush border a layer of glycocalyx
What is glycocalyx made of and what is its function?
Carbohydrate later protecting the lumen but allowing absorption. Has digestive enzymes mixed in in the small intestines. (In large intestine it doesn’t)
What is the function of goblet cells? How does their abundance change as you pass down the bowel and why?
They secret mucus facilitating movement of material through the bowel and also traps bacteria and particulates. There is increasing abundance of goblet cells as you pass down the bowel because the material becomes more solid as water is drawn out so much mucus is needed to move it along.
Where are paneth cells found and what is their function?
Found in the crypts of small intestine. They contain acidophilic granules containing lysozyme which kills pathogens. Also glycoproteins and zinc secreted which are needed by some enzymes. Help control flora
What is the function of enteroendocrine cells? Where are they found? What is the difference in their abundance between the small intestine and large intestine?
They are found in lower parts of the crypts and are hormone secreting cells. More in the small intestine than in the large. This is because hormone regulated processes from small intestine are more complex and require more careful control.
Describe the rapid turnover of enterocytes in the small intestine and why this is advantageous in some circumstances?
The rapid turnover is achieved because the enterocytes have a very short life span. Those at the tips of the villus die quickly and are sloughed off by contents. They are then incorporated into the chyme in the lumen of the small intestine. New enterocytes are formed in the crypts where the stem cells rapidly divide and the cells move up the villus in a manner similar to an escalator. The rapid turnover means that agents interfering with cell function or metabolism have reduced effects and lesions are short lived. E.g. Cholera toxin is survivable so long as the patient remains hydrate because the cells effected die after a few days and the disease is therefore short lived.
What are the differences and/or identifying features of each section of the small intestine (duodenum, jejunum and ileum)?
Duodenum has brunner’s gland (secrete alkaline fluid neutralising acid chyme) Jejunum characterise by numerous, large folds in sub-mucosa = plicae ciculares. These are taller, thinner and more frequent than in other sections of the small intestine. Ileum has lots of peyer’s patches to prevent bacteria moving from large intestine into the small intestine
How is motility achieved in the small intestine (the different types of movement)?
Segmentation: mixes contents and occurs by stationary contraction of circular muscles at intervals.
Non organised Peristalsis: involves sequential contraction of adjacent rings of smooth muscle propelling chyme along around 10cm
How does the digestion of proteins occur?
Pancreatic proteases are secreted as precursors and activated in the duodenum by enterokinase. Trypsin activates other proteases. Brush border also contains some proteases. Di and tri peptides and single AA absorbed into enterocytes by facilitated diffusion and secondary active transport. Di and tri peptides are broken down by cytoplasmic proteases. Note: pancreas secrets enzymes with a trypsin inhibitor to prevent auto digestion of the bile duct and ensuring enzymes are only activated in the small intestine
How are carbohydrates digested and absorbed?
Pancreatic amylase digests complex carbohydrates into disaccharides and olgiosaccharides. This are converted into monosaccharides by brush border enzymes such as Maltese, lactase and sucrase. Absorption occurs via facilitated diffusion (e.g. Fructose via carrier glut-5) or secondary active transport (e..g glucose coupled with na+ via SGLT-1).
What is the portal triad and in what direction do blood and bile drain?
Portal triad = branch of hepatic portal vein, branch of hepatic artery and bile duct Blood drains from triad to central vein, bile drains towards triad
How is bile secreted?
Bile is secreted from the apical surface of hepatocytes and drains via the canalicular network to bile ductiles.
How is the blood supplied to the liver? How much of the cardiac output does the liver use?
Dual supply. 20% comes from hepatic artery and 80% from hepatic portal vein. Uses 25 % of cardiac output
What is the function of hepatocytes? How are they specialised to fulfil their function?
Perform most major functions of the liver. The apical part is bound to bile canaliculi. - protein synthesis - metabolism of amino acids - carbohydrate metabolism - lipid metabolism - drug metabolism/detoxification
What do hepatic stellate cells do?
They are involved in vitamin A storage and reside in the space of disse. In disease they are major pathway for fibrogenesis. They are activated and form fibroblasts.
What do kupffer cells do? Where are they found?
They are sinusoid and phagocytose RBC (also use lysosomal activity to break them down)
Cholangiocyte function?
Modification of bile: this occurs via coordinated transport of various ions and solutes and water
What is the function of the liver?
Digestion, bile posy thesis, energy metabolism, degradation and detoxification
Describe the role of the liver in blood glucose metabolism?
Stores glycogen which can be broken down into glucose and used to raise levels.
Cori-cycle: lactate produced in skeletal muscle travels to liver were it is converted back into pyruvate. The pyruvate returns to skeletal muscle so that it can be used in respiration. Amino acids and deamination forming pyruvate which can form glucose Triglycerides are broken down to form glucose
Describe the livers role in protein metabolism?
Synthesis 90% of plasma proteins which carry out important roles in blood clotting, binding/carrier proteins for hormones and help maintain blood osmotic pressure. Metabolism of amino acids results in production of toxic ammonia. This is converted into inert urea which is excreted. Transamination: exchange of amino acid group on one acid with a ketone group on the other acid. For many glutamic acid is an intermediate particularly for those without a suitable precursor. Deamination: conversion of amino acids to corresponding keto acid by removal of amine groups as ammonia and replacing it was ketone group. Occurs primarily on glutamic acid.
Describe fat metabolism in the liver.
Excess glucose and amino acids can be converted into fat as storage when the liver glycogen store is full it’s converted into fat.
The liver synthesis lipoproteins, cholesterol and phospholipids. The liver convert 2 acetyl-coA into acetoacetic acid for transport to other tissues as a energy supple. Once there it’s converted back into acetyl-coA Fatty acid via beta oxidation creating acetyl coA which enters the TCA cycle in liver.
Ketone bodies can be made in liver from fatty acids. They can travel in blood and be an energy supply (e.g used by brain) Lipoproteins are formed and cholesterol ester transfer protein shuffles cholesterol from HDL to LDL.
Describe formation of bile acids. What are the components?
Bile salts, cholesterol, bilirubin, HCO3- and water. Cholesterol is converted into colic acid and chenodeoxycholic acid.
They are then carboxylated and hydroxylated to increase water solubility. They are conjugated with tauricholic and glycocholic acid to form bile acids, this forms primary bile acids.
Secondary bile acids are de-conjugated and de-hydroxylated primary bile salts. This is done by GI bacteria.
Describe how bile salts emulsify fats in the intestine. Describe how bile salts are recycled.
Bile salts emulsify fats into small drops in an aqueous medium. The emulsified fats are broken down from triglycerides into fatty acids by lipases. The glycerol is then wrapped in bile salts to form micelles. The micelles then comes into contact with mucosal wall. Then recycled to move more lipids into enterocytes. Finally the bile salts are actively reabsorbed in terminal ileum. It passes via the hepatic portal vein back to the liver were they can be desecrate again
Describe lipid digestion
- Secretion if bile (bladder) and lipases (pancreas)
- Emulsification: bile salts suspend fats in lipid droplets (increase surface area for digestion)
- Enzymatic hydrolysis of ester linkages: triglycerides are converted into 2 fatty acids and monoglycerides.
- Solubilisation of lipolysis products in bile salt micelles
On entering enterocytes the monoglycerides and free fatty acids are processed in 2 pathways:
- Monoglyceride acylation (major)
- Phosphatidic acid pathway (minor)
Triglycerides combine with proteins, cholesterol, phospholipids and trace carbohydrate to form chylomicrons. Chylomicrons pass through basement membranes and enter lacteals.
What does the larder function?
Stores fat soluble proteins (A,D,E and K and B12). Also stored ferritin for RBC production. Stores fat and glycogen.
How is the liver involved in vitamin D?
Ca2+ metabolism: UV light converts cholesterol to vitamin D precursor. The precursor needs to be hydroxylated twice. The first of which occurs in the liver and the second in the kidney.
What is the function of the large intestine?
Reabsorption of electrolytes and water and eliminating of undigested food
What are the layers surrounding the lumen (in particular muscle) in the small intestine? What are the differences in the large intestine?
From lumen to the outside: Mucosa (epithelial cells, goblet cells, cells in crypts etc) ➡️ submucosa ➡️ circular muscle ➡️ longitudinal muscle ➡️ serosa In the large intestine the longitudinal muscle is concentrated into 3 bands : taenia coli which are essential for motility and formation of haustra (pouch of intestine)
How is the large intestine supplied with blood? Which area is most susceptible to ischaemia?
Proximal transverse colon is supplied by middle colic artery. Transverse colon is perfumed by inferior mesenteric artery Region between the 2 is sensitive to ischaemia.
How does the colon absorb water and electrolytes? Overview
Na+ and cl- ions are absorbed by exchange mechanisms and ion channels. The water then follows by osmosis and k+ moves out into the lumen.
Describe movements of large intestine.
Colonic contractions is a kneading prices but minimally propulsive. In proximal colon anti-propulsive patterns dominate to retain chyme and try and remove as much water and electrolytes. In transverse and descending: localised segmental contractions of circular muscle causes mixing Short propulsive movements every 30 minutes and more frequently post meal 1-3 days there is a mass movement such as a peristaltic wave. Properly move to around 1/3 to 3/4 of the length of the colon
How is the large intestine innervated?
Parasympathetic: innervated ascending colon and most of the transverse colon by vagus nerve. Distal is by pelvic nerves. Sympathetic: innervated via the lower thoracic and upper lumbar spinal cord
Describe the process of defecation
Defection is controlled by reflex and voluntary. Reflex opens the internal sphincter but the external is under conscious control. Peristaltic waves move the faeces down and out of the anus. The last bit is called the social rectum and can distinguish between solid, gas and liquid.
What functions do the intestinal flora do?
Synthesis and excrete vitamins Prevent colonisation by pathogens through competition Antagonise other bacteria through production of substances that inhibit/kill non-indigenous species. Stimulate cross-reactivity antibodies preventing invasion Help break down some fibre.
Describe the endocrine function of the pancreas.
The pancreas has areas called islets that are responsible for its endocrine functions. It is largely involved in blood glucose control Alpha cells = glucagon, beta cells = insulin, delta cells = somatostatin
Describe the exocrine function of the pancreas? How are the product secreted. What are the function of the secretions?
The pancreatic juices are secreted via the bile ducts. Duct cells secrete high volume, watery solution with a very high bicarbonate concentration. The bicarbonate neutralises the acidic chyme that enters the duodenum. The solution also washes down the enzymatic solution prevent blockages in the ducts. Acinar cells produce zymogen granules (pro-enzymes) which contain enzymes which are involved in digestion. This is a viscous solution.
How is bicarbonate produced in the pancreas?
Bicarbonate is formed with carbon dioxide and water is converted into protons and bicarbonate by carbonic anhydride, the bicarbonate is pumped into the lumen via cl-/HCO3- exchange and the protons release into blood via na+/h+ exchange,
How is bicarbonate secretions from pancreas controlled ?
When chemoreceptors in the duodenum sense the acidic chyme. The cells secrete secretin which moves via the blood to the pancreas. This stimulates bicarbonate release (increase cAMP in pancreatic cells and bicarbonate moves out). This stops when neutralisation of chyme occurs the stimulus for secretin release is stopped. So bicarbonate release is reduced. The duodenum also secreted CCK which stimulates enzyme release hut it has a potentiating effect for secretin rapidly increasing bicarbonate release. This means neutralisation occurs more quickly and duodenum is not exposed to damaging acidic pH.
