Gastrointestinal System Flashcards
What is the primary function of the GI system?
Bring nutrients into the internal environment so that they can be used.
What are specialised functions of the GI system?
Motility, secretion, digestion, absorption.
What are the accessory organs of the digestive system?
Teeth and tongue, salivary glands, liver, gallbladder and pancreas.
What are the major organs of the digestive tract?
Oral cavity, pharynx, oesophagus, stomach, small intestine and large intestine.
How is epithelia classified?
Shape, number of layers and specialisations.
Where is stratified squamous found?
Mouth/oral cavity and esophagus.
Where is simple columnar found?
Stomach, small and large intestine.
Where is stratified squamous found?
Anus.
What are goblet cells?
Columnar and goblet shaped cell found between columnar epithelium of the small intestine, full of mucous granules.
What are the two types of multicellular glands?
Simple and compound.
What are the four layers of the gut tube?
- Mucosa 2. Submucosa 3. Muscularis
- Adventitia
What are the layers of the mucosa?
Epithelium, lamina propria (FCT) and muscularis mucosae.
What does the submucosa contain?
Glands and blood vessels.
What does the muscularis consist of?
Smooth muscle. With two layers, the inner circular and outer longitudinal.
What does the adventitia consist of?
FCT
What is the peritoneum?
Serous membrane, consisting of a parietal layer (lines body wall) and visceral layer (lines the organs).
What is the mesentery?
Double layer of visceral peritoneum that connects organ to body wall.
What is the Omenta?
Double layer of visceral peritoneum that connects an organs to another organ.
What is the role of the mesentery?
Anchors organs in place.
Describe the mouth and oral cavity?
Where food is ingested. Requires stratified squamous epithelium for protection against abrasion. This is where digestion begins, hence why it contains enzymes. Food goes through fauces, into oropharynx, then into oesophagus.
What are the three pairs of salivary glands in the oral cavity?
Parotid, sublingual and submandibular.
What do Parotid glands secrete?
Serous fluid with amylase.
What do sublingual glands secrete?
Mucous only.
What do submandibular glands secrete?
Mucous and serous.
What is the total volume of salivary secretions per day?
1L
What type of cell secretes enzymes?
Acinar cells.
What do duct cells secrete?
Bicarbonate
What is the Esophagus?
Long tube thats located posterior to the trachea and extends from the pharynx to the stomach.
What is the role of the epiglottis?
Ensures that food enters the esophagus and not the trachea.
What gives the esophagus the capacity to expand?
Highly folded submucosa and mucosa.
How does the Esophagus change as you go down?
First third is skeletal muscle, middle third is a mix of skeletal and smooth and the last third is smooth muscle.
What do the esophagus need mucous for?
Lubrication and protection.
Does the esophagus have goblet cells?
No, instead has glands with ducts to surface.
Describe the stomach?
J shaped organ, located at the base of the esophagus.
What structure prevents reflux?
Lower esophageal sphincter (LES).
What are the four main regions of the stomach?
Cardia, fundus, body and pylorus.
What structure connects the stomach to the liver?
Lesser omentum.
What structure connects the stomach to the transverse colon?
Greater omentum.
What are the three layers of the gut tube?
Oblique, circular and longitudinal.
What are rugae?
Temporary folds that allow for the expansion of the stomach. Important for storage.
What are the cells of the gastric glands?
Parietal cells, G cells and Chief cells.
What is the role of parietal cells?
Secretes acid and intrinsic factor.
What is the role of G cells?
Secrete hormones (gastrin).
What is the role of chief cells?
Secrete pepsinogen (an active precursor of pepsin).
What makes chief cells special?
Chief cells produce enzymes and contain abundant rough endoplasmic reticulum, apical zymogen granules and a basal nucleus.
What makes Parietal cells special?
Parietal cells produce acid and have abundant mitochondria, central nucleus and a folded structure. They also pump H+ ions.
What is the role of the Pyloric Sphincter (PS)?
Controlled release of digested material (chyme) into the small intestine.
What is the pancreas?
Head in C-shaped duodenum, tail to spleen. Located posterior to the stomach.
What are the endocrine and exocrine functions of the GI system?
Endocrine: Pancreatic islet alpha cells secrete glucagon and islet beta cells secrete insulin.
Exocrine: Acinar cells secrete digestive enzymes and duct cells secrete bicarbonate.
What are the three main regions of the small intestine?
Duodenum, jejunum and ileum.
Whats delivered to the small intestine?
Acidic chyme, therefore is lined with epithelia, mucous secreting cells and duct cells to neutralise pH.
How is the small intestine modified for digestion and absorption?
Plicae circulares which slow the movement to increase time for absorption.
Villi which contains lymph lacteals for fat digestion and a capillary network for protein and carb digestion.
What are four ways to increase the small intestine surface area?
- Long tube.
- Plicae Circulares (permanent folds in the submucosa).
- Fold the mucosa to form villi.
- Fold plasma membrane to make microvilli.
Describe the microvilli brush border?
Contains glycoproteins involved in contact digestion in order to absorb nutrients and protect body from intestinal bacteria.
What type of junctions are between epithelial cells?
Tight junctions where only small molecules can diffuse.
What are enterocytes?
Absorptive cells.
What are goblet cells?
Secrete mucous for protection.
What do paneth cells contain?
granules and antibacterial enzymes.
What is the role of endocrine cells?
Secrete hormones.
What are the three main parts of the large intestine?
Cecum, Colon and Rectum
What is the colon further divided up into?
Ascending, transverse, descending and sigmoid.
What is the role of the ileocecal valve?
Regulates the passage of material into the cecum.
What is the appendix?
Reserve of gut bacteria for storage in case of an emergency.
What are the gross structures of the large intestine?
Teniae Coli, Haustra and Omental appendices.
What is teniae coli?
Bands of longitudinal smooth muscle.
What is haustra?
Series of pouches in the wall of the colon. Puckers the intestine to help with expansion and elongation.
What are the sacs of fat in the large intestine called?
Omental appendices.
The teniae coli are a modification of the muscularis layer, what is the purpose of this?
Gives a stronger contraction of the large intestine.
Does the small or large intestine have more villi?
Small - the large intestine lacks villi because it doesn’t need a big surface area, therefore can lose the plicae circulares.
What is the function of mucosa of the colon?
Water and salt absorption?
At what part of the rectum does the epithelium change?
The anal columns!
What type of epithelium is the rectum?
Simple columnar.
What type of epithelium is the anus?
Stratified squamous.
Explain the defecation reflex?
Movement of faeces into the rectum stimulates stretch receptors which initiates stronger reflexes to push the poo. It takes a conscious decision to defecate by relaxation of the external sphincter.
Where is the liver located?
Superior right quadrant of abdominopelvic cavity.
What is the function of the liver?
Produce bile which is then stored in the gall bladder.
Where does the liver receive blood from?
Liver receives ~25% of the cardiac output. 1/3 of blood supply for the hepatic artery and the rest is venous blood from the hepatic portal vein.
The liver is made up of functional units called what?
Lobules.
Rows of _____ produce bile?
Hepatocytes.
Explain the brief process of the liver?
Blood flows towards the central vein.
It is then processed by hepatocytes, which produce bile.
Bile is then secreted into canaliculi which travels to the bile duct.
Explain venous drainage in the liver?
The central veins drain into the hepatic vein which then drains into the inferior vena cava.
What are bile ducts?
A series of thin tubes that transport bile from the liver to the small intestine.
What are the four layers of the basic gut tube?
Mucosa, Submucosa, Muscularis and the Adventitia.
What is the overall function of the GI tract?
Obtain nutrients required for growth and energy needs and replace fluid and salts lost in urine and faeces, sweating and breathing.
CNS and hormones regulate what two processes?
Motility and Secretion.
Where are the receptors found?
In the wall of the GI tract.
What do the receptors respond to?
Stretch, food in the lumen.
Change in composition.
pH, osmolarity, amino acids, sugars and fats.
What are the effectors of the GI tract?
Smooth muscle and glands.
What are the functions of the CNS?
Co-ordinates activity over long distances.
Stimulates motility and secretion (PNS).
Inhibits motility and secretion (SNS).
Modulates activity of the Enteric nervous system.
What are the functions of the Enteric nervous system?
Regulates secretion of submucosal plexus.
Regulates motility of the myenteric plexus.
Involved in peristalsis and segmentation.
And is fully self contained.
Explain the process of external (CNS) regulation of GI functions?
Integration of responses to external stimuli such as senses and emotions and internal stimuli, the GI lumen contents, which lead to reflexes that generally inhibit GI function (noradrenaline) and reflexes that generally stimulate GI function (acetylcholine).
Explain the process of local (ENS) regulation of Gi functions?
Stretch and chemical receptors detect conditions in GI tract which stimulate ENS response which occurs via short LOCAL reflex pathways and acts on GI tracts smooth muscle and epithelia/glands.
What are the critical hormones in GI regulation?
Gastrin, GIP (gastrin inhibitory peptide), secretin and Cholecystokinin (CCK).
What are tonic contractions?
Sustained contractions for minutes to hours, for example sphincters.
What are phasic contractions?
Waves of contraction and relaxation over second, for example peristalsis.
What region of the Gi tract has the greatest frequency of contraction?
Duodenum - 12 per min.
What motility pattern occur when the body is fasting?
Migrating motor complex.
What motility patterns occur when the body is feeding?
Storage, propulsion/movement and mixing.
What is the migrating motor complex?
Serves the role of a housekeeper in the small intestine, occurs 4 hours after every meal and repeats every 2h until you eat again.
Describe the motility pattern, storage?
Occurs in the stomach and the colon, where relaxation of smooth muscles allows volume to increase without change in pressure.
Describe the motility pattern, propulsion?
Movement through the GI tract via peristalsis.
Describe the motility pattern, mixing?
Retropulsion in the stomach, and segmentation in the small and large intestine.
What is peristalsis?
When the longitudinal layer contracts and relaxes in front of the food and chyme is pushed along the tube.
What is segmentation?
Where the inner layer pinches to mix the chyme up.
What motility patterns occur in the mouth?
Chewing and swallowing.
Where does storage take place in the stomach?
The fundus and the body of the stomach.
What functions take place in the antrum?
Mechanical digestion (retropulsion) and mixing.
What controls the delivery to the duodenum?
Pyloric sphincter.
What are the two parts to the storage motility function?
Receptive relaxation and gastric accommodation.
What is retropulsion?
Combination of peristalsis and closure of the pyloric sphincter, also involves mixing with acid, pepsin to help breakdown proteins.
What regulates gastric emptying?
Feedback from the duodenum.
What are the factors that effect gastric emptying?
Size of meal.
Composition of meal (fluids or solids).
Whether meal includes fats (fats slow to digest).
What occurs within the small intestine?
Mixing with secretions from the pancreas, biliary system and intestine. Controlled movement/propulsion to expose products to absorptive surfaces.
Explain the motility of the large intestine?
Storage of faeces, large period of inactivity, segmentation and propulsion which occurs 1-2 times daily following a meal. This also drives faeces into the rectum and initiates defecation.
Define secretion?
Movement of solutes and water from the body to the lumen.
Define absorption?
Movement of solutes and water from the lumen into the body.
What are main secretions of the GI system?
Salivary, Gastric, Pancreatic, Biliary and Intestinal.
How many pairs of salivary glands do we have?
3 pairs.
What is the composition of a salivary secretion?
Mucus, dilute solution of NaHCO3/NaCl and digestive enzymes.
What are the functions of salivary secretion?
Not essential but aids talking, chewing and swallowing. Good for hygiene and digestion.
What regulates salivary secretions?
Nervous system, in terms of smell and sight of food and also the autonomic nervous system which stimulates secretion.
When does gastric secretion occur slowly?
Between meals.
When is gastric secretion superimposed on basal rate?
When eating.
What is gastric secretion composed of?
Mucous and bicarbonate from surface epithelial cells, intrinsic factor from parietal cells and pepsinogen from chief cells.
What are the functions of gastric secretion?
Protection.
Absorption of B12.
Starts digestion of proteins.
Dilutes food.
Creates optimum pH.
What is the driving force for secretion of HCl by parietal cells?
Source of H+.
How is gastric secretion regulated?
Co-ordinated with eating and arrival of food. There are three phases, cephalic phase (head), gastric phase (stomach) and intestinal phase (intestine).
What is the cephalic phase?
Preparation for the arrival of food. Stimulated by thought, smell and sight of food. Stimulates cell secretion and secretion of gastrin hormone.
What is the gastric phase?
Ensures sufficient secretion to handle ingested food. Stretch and distortion of stomach wall, products and elevated pH cause response of the nervous and hormonal regulation.
What is the intestinal phase?
Control of the delivery to small intestine. Chyme, lipids and carbohydrates arrive at the duodenum which stimulates nervous and hormonal regulation. GIP, CCK and secretin are released as well as the enterogastric reflex.
What are the two components of pancreatic secretion?
Enzymes and alkaline fluid.
Secretion of digestive enzymes by acinar cells is stimulated by what?
The hormone cholecystokinin (CCK). This triggers the luminal chemical digestion of food.
Explain the activation of proteolytic enzymes?
Proteolytic enzymes are secreted as inactive precursors and are activated in the duodenum. Firstly enterokinase converts trypsinogen to trypsin and the trypsin then converts the other enzymes into there active form.
Where is HCO3 rich fluid produced?
By duct cells in the pancreas.
What are the three types of biliary secretion?
Bile salts, HCO3 rich fluid and bile pigments.
What regulates the initial delivery of bile.
Hormone cholescytsokinin and secretin.
What is the purpose of enterohepatic circulation?
Bile is metabolically expensive (energy consuming) therefore it is recycled.
What does the small intestine secrete?
Mucus, isomotic fluid and digestive enzymes.
What does the large intestine secrete?
Mucus.
Main nutrients that undergo chemical digestion?
Carbohydrates, Proteins and Lipids.
Carbohydrates
Large complex chain of monosaccharides.
Protein
- Not a major source of energy.
- Required for amino acids.
- Long chains of amino acids linked by peptide bond.
Lipids
Mainly triglycerides.
Fatty acids vary in chain length.
Not essential.
Why do we need chemical digestion?
To reduce the size of nutrients to allow them to be absorbed.
Digestive enzymes.
- Are extracellular.
- Are organic catalysts.
- Have optimal pH.
What are the two stages of chemical digestion?
Luminal digestion and Contact digestion
Luminal digestion
Initial digestion involving enzymes secreted into lumen.
Contact digestion
Occurs in the small intestine and completes digestion before absorption and involves enzymes produced by enterocytes and attached to brush boarder of enterocytes.
Chemical digestion of carbohydrates.
- Salivary and pancreatic amylase breaks down polysaccharides
- Disaccharides converted to monosaccharides in small intestine by sucrase, lactase, maltase etc and enzymes bound to brush boarder.
Chemical digestion of proteins.
- Trypsin and chymotrypsin attack the peptide bonds, enzymes work to convert proteins into polypeptides.
- Contact digestion converts polypeptides into individual amino acids.
Chemical digestion of lipids (fats).
Occurs in lumen of small intestine, where pancreatic lipase in the main digestive enzyme. Lipid digestion becomes a problem because insoluble in water therefore requires complex process.
Several stages of chemical digestion of fats.
Emulsification - Motility breaks up lipid droplets in stomach and small intestine.
Stabilisation - Bile salts break down even further in small intestine.
Hydrolysis - Lipase converts triglycerides to monoglycerides and fatty acids.
Formation of micelles - Products are kept in solution through formation of micelles.
Absorption
Is the net passage of substances from the GI lumen across the lining of the intestine into the interstitial fluid and then into blood or lymph.
Sites of absorption
Mouth, esophagus and stomach - minimal.
Small intestine - main site for nutrients, water and sodium.
Large intestine - small amount of water and sodium.
Factors affecting absorption.
- Motility
- Surface area
- Transport across epithelium.
- Removal from interstitial fluid.
Anatomical adaptations which maximise surface area:
Length of intestine, Circular folds (plicae circulares), villi and microvilli.
Paracellular
Between cells.
Transcellular
Across the cell membranes, through the cytoplasm.
Mechanism for water absorption.
Osmosis.
Mechanism for sodium absorption.
Passive movement via paracellular pathway
OR
Active transport via the cells.
Mechanism for carbohydrate absorption .
Passive or active transport.
Mechanism for absorption of amino acids.
Passive via paracellular pathway
OR
Active via co-transport with Na+
Mechanism for absorption of small peptides.
Active via co-transport with H+.
Bile salt absorption.
bile salts in micelles are absorbed via active transport in the ileum and via passive transport in the colon.
Vitamin absorption.
Fat soluble vitamins are absorbed with fats, while water soluble are Na+ dependent.
Elimination
We do not absorb everything! Expulsion of residues of digestion. Faeces form in large intestine and are transferred to the rectum via peristaltic waves. Elimination from body by defecation reflex.
