Gastrointestinal physiology Flashcards
What are the four basic processes of the GI system?
Motility
Secretion
Digestion
Absorption
What is the purpose of digestive secretion?
Moisten the food so that it is more easily absorbed
Secretions usually contain digestive enzymes which breakdown the food chemically.
Which food/minerals is unabsorbableand what happens when it isn’t absorbed?
Complex carbohydrates
Sometimes food isn’t totally absorbed leading to diarrhoea.
What are the key organs of the GI tract?
Mouth
Oesophagus
Stomach
Small intestine
Large intestine
Rectum
Anus
Also:
Pancreas
Liver
Bile duct
Salivary glands
Why may we think of the lumen of the gut being outside of the body?
Hostile environment:
pH 2.2, gut tissue would be destroyed
Gut microbiome in the large intestine some in the small intestine.
If these bacterium crossed into the body may cause septic shock.
Release of proteases into gut lumen, in the body it would breakdown the protein in the body.
Leaky gut, food that is not broken down is seen as foreign material and if it got into the body, it would mount an immune response.
If acid gets through to the gut what happens?
Peptic ulcers
How many layers in the gut wall and what are they called?
Mucosa (next to lumen)
Submucosa (vasculature and nerves)
Musclaris externa (between the inner and outer layer of the gut is the myenteric nerve plexus)
Serosa (lining secretes mucus to stop it sticking)
What are the different layers within the mucosa (small intestine)?
Lining cells - line the lumen (epithelial cells)
Goblet cells - mucus
Food products travel through this layer to get to the capillaries in the lamina propria.
What is the structure of the lamina propria?
Layer of connective tissue which also contains lymphatic nodules.
What is the structure of the muscularis?
Layer of muscle, responsible for changes in folds. Also a thin layer of nerve plexus.
What is found in the submucosa?
Rich blood supply, branches of lacteals also a layer of muscle
What is beneath the submucosa?
Layer of inner circular muscle which is required for contracting the diameter of the lumen
Layer of outer longitudinal muscle which is required for propelling the food substances forward, controls the length of the gut.
Myenteric nerve plexus is in between.
Function of the serosa?
Wet moist substances preventing sticking. Continuous with the mesentery.
What is the myenteric nerve plexus?
The gut’s own nervous system
Describe the structure of the mucosa layer and how does that link to its function?
The mucosa is different depending where it is in the gut. In the small intestine the mucosa has lots of villi (vascular projections) which increases the surface area of the small intestine, maximising absorption.
Villi are only present in the small intestine.
What does the serosa do to stop it sticking to other organs?
Produces a fluid to keep its layer moist.
Describe how the mucosa in the oesophagus would be different to that of the mucosa in the small intestine.
No absorption occurs in the oesophagus, food is only there for a couple of seconds, therefore to ease the movement of food down the tube you would ideally want a flat mucosa. The cells here would produce mucus for lubrication.
Which type of muscle makes up the gut?
Most of the gut is smooth muscle (involuntary control), skeletal muscle (under voluntary control) makes up swallowing muscles and the external anal sphincter.
What is the role of intrinsic nerve plexus in the gut?
They are nerves controlling motility between the circular and longitudinal muscle.
What is the main extrinsic nerve that controls digestion?
Vagus nerve - parasympathetic (rest and digest)
Where is the palate located?
It forms the roof of the oral cavity separating the mouth from the nasal passage.
What is the function of the Uvula?
Separates the mouth from the nasal passage during swallowing (it is the bit that drops down at the back of the mouth).
Respiratory system in the brain stops breathing in those couple of seconds.
How many teeth do adults have?
32 teeth, many only have 28 due to absence of wisdom teeth.
What is the function of molar teeth?
It crush and grind - grains and seeds for example, needed less now for adult diets.
What is the split of our adult teeth?
Permanent (or adult) teeth of which there are 32 in total, made up of 8 incisors, 4 canines, 8 premolars and 12 molars.
What are the different taste buds found on the tongue?
Sweet
Sour
Bitter - associated with toxicity
Salty
Enomy
How many taste receptors are present per taste bud?
50 taste receptors per taste bud
What are the functions of the tongue?
It forms the floor of the oral cavity
Made of skeletal muscle
Helps aid speech
Role in chewing and swallowing
What is the function of the pharynx?
It is the cavity at the rear of the throat, is the passageway for digestion and respiratory.
How much saliva is produced daily?
1-2 litres (0.5ml a minute - basal)
What is the main secretion in the mouth?
Saliva
Where are the palatine tonsils found?
At the back of the mouth.
What drugs inhibit the salivary glands and hence cause dry mouth?
Main drugs:
Antimuscarinics
Antihistamines
Tricyclic antidepressants
Some diuretics
What are the three main glands that produce saliva?
Parotid glands
Sublingual gland
Submandibular gland
What makes up saliva?
99.5% water
0.5% amylase and bicarbonate (amylase needs a neutral pH)
Mucus to moisten food
Lysozyme (antibacterial)
What are the main functions of the saliva?
Digest carbohydrates
Lubricates food
Antibacterial action
Solvent for taste buds, dissolves chemicals in food, makes them more available
Aids speech
Keeps mouth clean
Explain the control of saliva secretion.
Either a simple reflex where it is picked up by pressure or chemoreceptors in the mouth that food has arrived, signal to salivary centre in the medulla in the brain, stimulating autonomic nervous system.
The complex/conditioned reflex - triggered by smell, sight of food
What drugs are absorbed in the mouth?
Sublingual drugs
Buccal - antiemetics
Describe the process of swallowing.
Food is pushed to the back of the mouth with the tongue after chewing, then swallowing occurs. Skeletal muscle with the tongue which is under voluntary control until the back of the mouth then smooth muscle control (involuntary).
What are the two phases of swallowing and how long do they last?
Oropharyngeal (one second)
Oesophageal (five to nine seconds) time of peristalsis
What are the two sphincters found in the oesophagus?
At the top of the oesophagus there is a sphincter known as the pharynoesphagus sphincter which stops the food from going back up to the pharynx.
The gastroesophageal sphincter prevents acid reflux or food into the oesophagus.
Explain how food is pushed down the oesophagus.
Peristaltic waves push bolus of food down, pressure receptors pick up when food gets stuck and so there is a second peristaltic wave.
What is the three sections of the stomach?
Fundus at the top- stores air
Body in the middle - for storage, salivary amylase in continuing to work, secretions
Antrum at the bottom - contains thicker muscle for mixing (peristaltic waves) and secretions, moving food towards the pylori sphincter
What is the function of the pylori sphincter?
Prevent food entering the small intestine until it is ready.
What are the three main functions of the stomach?
Store food until it is moved to the small intestine
Secretes HCl and other enzymes to aid digestion. Stomach produces pepsin for protein breakdown
Mixing movements produce chyme
What is the product of the stomach?
Chyme
What type of meal stays in the stomach for longer?
Fatty meals, sits there for 6-8 hours. Body says you are still satisfied.
What is the four aspects of the motility?
Filing- receptor relaxation mediated by the Vagus nerve
Storage
Mixing
Emptying - controlled by factors in the duodenum
What are some of the factors that control emptying?
In the stomach:
Amount of chyme, more chyme the longer it takes for emptying.
In the duodenum:
High fatty meals, takes longer for absorption, emsulfication of fat globules
Unneutralisation of stomach acid- hostile environment, must be neutralised by bicarbonate from the pancreas
Hypertonicity - when osmolality rise, one carb molecule but multiple glucose molecules. Osmo receptors signals back and slows down motility.
Distension- too much chyme slows down the emptying of the stomach.
The duodenum factors mentioned above determining gastric emptying rate - what type of signals are used?
Either neural signals via the intrinsic nerve plexus (short-term) or the autonomic nerve plexus (long-term).
Hormonal signals involve the release of hormones from the duodenal mucosa known as enterogastrones (secretin, cholecystokinin)
What other factors determine gastric motility?
Emotions:
Sadness and fear and pain- decrease
Anger and aggressive - increase
Impact on the hormones
What is the two types of gastric mucosa?
Oxyntic mucosa - body
Pyloric gland area - antrum
What are the three types of gastric exocrine secretory cells and what do they secrete?
Mucus cells - mucus
Chief cells - pepsinogen
Parietal cells - HCl and intrinsic factor (Vitamin B12 absorption in terminal ileum) - ATPase pump, hydrogen gets secreted also hydroxide ions from water.
Describe the structure of the mucosa in the stomach.
Flat but with gastric pits lined with mucus cells to prevent acid damaging the mucosa. Hormones are produced here and mucus.
At the bottom chief and parietal cells are present.
What are the functions of HCl?
Activate pepsinogen
Aids breakdown of connective tissue and muscle fibres
Denatures proteins by uncoiling
Kills ingested micro-organisms
How does the cell wall in the stomach prevent stomach acid from getting through?
Cells themselves due to being mucus producing
Between cells, tight junctions
Thicker layer of mucus, physically stops acid getting through
Mucus contains bicarbonate so neutralises it
Why is pepsinogen known as autocatalytic?
HCl cleaves the pepsinogen bond, producing pepsin. Pepsin then works on itself to cleave pepsinogen into more pepsin.
What are the three gastric endocrine cells and what do they produce?
Enterochromaffin like cells - Histamine
G cells - Gastrin
D cells - Somatostatin
What are the stages of gastric acid secretion?
Cephalic phase - increase in HCl and pepsinogen before food actually reaches the stomach in response to stimuli in the head
Gastric phase - food in the stomach increasing acid secretion
Intestinal phase - inhibitory phase - distension, hypertonicity - stopping gastric acid secretion
Explain digestion in the stomach.
Salivary amylase continues in the body of the stomach
Protein digestion by pepsin begins in the antrum
Explain absorption in the stomach.
Alcohol - lipid soluble diffuses slowly through mucosa into blood
Aspirin and some other weak acids drugs - unionised, lipid soluble so absorbed quickly
What is the endocrine function of the pancreas?
Secretion of insulin and glucagon which are vital for blood glucose homeostatsis from Islets of Langerhans.
What is the exocrine function of the pancreas?
Pancreatic enzymes are secreted from acinar cells.
Duct cells secrete aqueous alkaline solution rich in NaHCO3 which neutralise HCl- help enzymes work in a neutral environment.
What is pancreatic exocrine secretion controlled by?
Hormones that have been secreted in the blood.
Secretin stimulates NaHCO3
Cholestykini stimulates acinar cells
Which proteolytic enzymes are produced as a result of exocrine stimulation?
Trypsinogen - converted to active form trypsin by enterokinase (in epithelial cells that line the duodenum)
Chymotrypsinogen – converted to active form chymotrysin (by trypsin)
Procarboxypeptidase – converted to active form carboxypeptidase
Proteolytic to prevent digestion of the pancreatic ducts, released into the duodenum
What is the function of pancreatic amylase?
Converts polysaccrides to diasaccrides amylase
What is the function of pancreatic lipase?
Digest fats
Explain the hormonal control when acid gets into the duodenum.
When acid is present in the duodenum, there is an increase in secretion of secretin from the duodenal mucosa which is carried in the blood to the pancreatic duct cells. This triggers an increase of secretion of aqueous NaHCO3 solution into the duodenal lumen, neutralising the solution.
What happens when protein and fat enter the duodenal lumen?
There is an increase in cholesytekinin secretion from the duodenal mucosa which is then carried in the bloodstream to the pancreatic acinar cells. This leads to an increase in secretion of pancreatic digestive enzymes into the duodenal lumen.
Describe the two gastric endocrine hormones.
Gastrin, released by protein presence and Ach from G cells, stimulates HCl and pepsinogen
Somatostatin, released from D cells, inhibits HCl, pepsinogen and gastrin
Describe the two duodenal endocrine hormones.
Secretin, released by presence of HCl, stimulates NaHCO3 secretion
CCK, released by presence of fat and protein, stimulates pancreatic enzyme release
What is the main function of the liver?
Produce bile salts which emulsify fat globules into smaller droplets, making digestion by lipases more accessible.
Describe the blood supply to the liver.
Hepatic artery- supplies the organ with oxygen and nutrients
Hepatic portal vein - nutrients from gut to the liver (apart from the gut)
Portal vein - metabolites to the heart or whatever system is required
What are the other functions to the liver?
- Metabolic processing of the carbohydrates, proteins
and lipids after absorption
– Detoxifying or degrading body wastes and hormones, drugs, and other foreign compounds
– Synthesizes plasma proteins
– Stores glycogen, fats, iron, copper, and many vitamins
– Activates vitamin D (along with kidneys)
– Removes bacteria and worn-out red blood cells
– Produces acute phase proteins (innate immune system) and hormones, eg hepcidin, thrombopoietin and IGF-1
– Excretes cholesterol and bilirubin through the bile
What is bile produced by?
Hepatocytes and then diverted to the gall bladder for storage in between meals, bile enters duodenum after meals
What is bile compromised of?
Bile salts
Cholesterol
Lecithin
Bilirubin (forms after the breakdown of red blood cells)
What do bile salts do?
Emulsify large fat droplets into micelles, afterwards bile salts are then absorbed into the blood.
What are the three sections of the small intestine?
Duodenum
– Jejunum
– Ileum
All three involved in absorption
What is the mesentary?
It is part of the serosa, produces a fluid keeping parts of the small intestine separate preventing fistular. Allows the small intestine to fit in the abdominal cavity.
Describe the structure of the small intestinal mucosa.
Have fold (plica) in the intestinal wall, within these folds there are thousands of villi that protrude upwards increasing the surface area. Within these villi the epithelial cells that line the surface, have brush border microvilli further increase the surface area.
Describe the process of segmentation in the small intestine.
Forms ring like structures that relax and contract, pushing the food into areas of villi and hence absorption, this mixes and propels the food forwards.
What does the migrating motility complex do?
Sweeps the small intestine clean in between meals.
What initiates segmentation in the small intestine?
Initiated by pacemaker cells in small intestine which produce basic electrical rhythm
What influences circular smooth muscle contraction?
Distension of intestine
Gastrin
Extrinsic nerve activity
What is the function of segmentation?
-Mixing chyme with digestive juices secreted into small intestine lumen
-Exposing all chyme to absorptive surfaces of small intestine mucosa
What does intestinal digestive juices in the small intestine contain?
Alkaline buffers
Brush border enzymes:
Enterokinase
* Disaccharidases - maltase, lactase
* Aminopeptidases
What is the key enzymes involved in digestion in the small intestine?
Pancreatic enzymes - carbohydrate and protein digestion
Brush-border enzymes complete digestion of carbohydrates and protein
Fat is digested entirely within small intestine lumen by pancreatic lipase
How much is absorbed in the small intestine?
Approximately 9 litres a day (most of this is secretions)
Where does the majority of the absorption take place within the small intestine?
Duodenum and Jejunum
How has the small intestine adapted to maximise its absorption function?
The folds, villi and microvilli
How often is the lining of the small and large intestine replaced?
Every 3 days and every 5-7 days
What are the main secretions of the digestive system?
Saliva
Gastric juice
Pancreatic juice
Bile
Intestinal juice
Outline the digestion of carbohydrates and the enzymes involved.
Carbohydrates are broken down into its substituent parts:
Polysaccharide Starch/glycogen which is further broken down to the disaccharide Maltose by salivary and pancreatic amylase. Maltase converts Maltose into the monosaccharides - two glucose molecules which are then absorbable.
Lactose a disaccharide is broken down by lactase to two absorbable units Glucose and Galactose.
Sucrose a disaccharide is broken down by sucrase to two absorbable units Glucose and Fructose.
Describe carbohydrate absorption into an epithelial cell.
The three monosaccharide products of carbohydrate digestion are glucose, fructose and galactose.
Glucose and galactose are absorbed into the epithelial cell by a Na+ dependent secondary active transport SGLT symporter located at the luminal membrane.
Fructose is transported by a GLUT-5 pump into the epithelial cell.
What is cellulose broken down by?
Gut microbiome
How do monosaccharides exit the epithelial cell?
Fructose, Galactose and Glucose exit the cell at the basal membrane via passive facilitated diffusion at GLUT-2.
What are the brush border enzymes?
Lactase, Maltase, Sucrase
Describe the process of fat digestion and absorption.
Large fat, triglyceride droplet is emulsified by bile salts to micelles in the duodenum.
These micelles are then vulnerable to pancreatic lipase attack which converts them to free fatty acids and monoglycerides. Bile salts facilitate their movement into the cell where triglycerides are then made. Triglycerides becomes part of Chylomicrons, responsible for transporting lipids around the body.
Describe the process of protein digestion.
Protein digestion begins in the stomach. Pepsinogen which secreted with the gastric juices under the presence of acid is converted to pepsin (this is auto-catalytic as pepsin will then cleave more pepsinogen). Pepsin breaks down proteins in the stomach to small peptides and amino acids. Once in the small intestine pancreatic proteolytic enzymes take over further breaking down proteins into small peptides and amino acids. The peptidases are then further broken down into amino acids by aminopeptidases and intracellular peptidases.
What are the pancreatic enzymes involved in protein digestion?
Trypsinogen which is converted to trypsin at an alkaline pH. This cleaves different bonds within a protein compared to pepsin, further breaking it down.
How are proteins absorbed?
Amino acids are absorbed into the epithelial cell by means of Na+ and energy dependent secondary active transport via a symporter. Many amino acids are absorbed by transporters specific to them.
Small peptides are absorbed by Na+ or H+ or energy dependent tertiary active transport.
Amino acids exit the cell by passive transporters and diffuses into the blood.
What is the function of the large intestine?
Drying (reabsorption of any chyme) and storage of feces.
What are the four areas of the large intestine?
Colon - extract water and salts
Cecum
Appendix
Rectum
What is the structure of the large intestinal wall?
Does not have an area of longitudinal muscle like the small intestine but does have a mucosa and crypts within the mucosa. Submucosa and circular muscle.
It just has three strips of longitudinal muscle called Taenaei coli which pucker up the large intestine into pouches known as Haustra. When these contracts this facilitates the movement of chyme along the large intestine.
What does the cecum attach to?
The terminal ileum
What are the three areas of the colon?
Ascending colon
Transverse colon
Descending colon
Sigmoid colon - next to the rectum
Describe what type of muscle found in the anus.
Internal anal sphincter which is made of smooth muscle (involuntary)
External anal sphincter which is made of skeletal muscle (voluntary)
What is the main motility mechanism in the large intestine?
Haustra contractions which are initiated by autonomic rhythmicity of colonic smooth muscle, this actively changes the location of the haustra pouches, driving peristalsis forwards.
In addition to mass movement at the end of the day, forcing any left over chyme through.
What is the gastrocolic reflex?
A physiological reflex that controls the motility, or peristalsis, of the gastrointestinal tract following a meal.
Initiated from the stomach to the colon by gastrin and autonomic nerves.
How is defecation initiated?
Stretch receptors in the rectal wall are stimulated by distension, causing the internal anal sphincter to relax allowing faeces to enter the anus.
Voluntary control of skeletal muscle (external anal sphincter) then decides when we release the faeces.
What secretions, digestion and absorption occurs here in the large intestine?
Only mucus secretions, provides lubrication
No digestion but microbiome breaks down insoluble fibres for example to urtitic or small fat acids which are also absorbed.
Water and salt absorption or anything made by bacterium such as Vitamin K
What comprises the faeces?
Should be 2/3 water and then any waste products
What is the role of the gut microbiome?
Promote angiogenesis
Promote the immune system
Biosynthesise Vitamin B12 and K
Metabolises some drugs
Modify the nervous system
Breakdown food components
Resistance to pathogens
Promotion of fat storage
Protects against epithelial cell injury
Bone mass density
What is gastrin secretion stimulated and inhibited by?
Release is stimulated by presence of protein in stomach
Secretion inhibited by accumulation of acid in stomach
Where is gastrin released from?
It is released from G cells in the pyloric antrum of the stomach, duodenum and pancreas.
What are the functions of gastrin?
Acts in several ways to increase secretion of HCl and pepsinogen
Enhances gastric motility, stimulates ileal motility, relaxes ileocecal sphincter, induces mass movements in colon
Helps maintain well-developed, functionally viable digestive tract lining
What stimulates secretin secretion?
Presence of acid in the duodenum
Where is secretin produced?
Secreted from S cells in the duodenum.
What is the functions of secretin?
Inhibits gastric emptying in order to prevent further acid from entering duodenum until acid already present is
neutralized
Inhibits gastric secretion to reduce amount of acid being produced
Stimulates pancreatic duct cells to produce large volume of aqueous NaHCO3 secretion
Stimulates liver to secrete NaCO3 rich bile which assists in neutralization process
Along with CCK, is trophic to exocrine pancreas
What are the functions of Cholecystokinin?
Inhibits gastric motility and secretion
- Stimulates pancreatic acinar cells to increase secretion of pancreatic enzymes
- Causes contraction of gallbladder and relaxation of sphincter of Oddi
- Along with secretin, is trophic to exocrine pancreas
- Implicated in long-term adaptive changes in proportion of pancreatic enzymes in response to prolonged diet changes
- Important regulator of food intake
What type of hormone is Cholecystokinin?
Peptide hormone
Where is Cholecystokinin secreted from?
I-cells in the small intestine
What does GIP stand for?
Glucose dependent insulinotrophic peptide
When is GIP secreted?
Presence of glucose in the digestive tract and causes secretion of insulin.
What are the two types of muscles involved in motility?
Smooth muscle - involved in involuntary control - this is peristalsis driving the food forward through the GI tract
Skeletal muscle - involved in voluntary control chewing, swallowing and defecating
Describe the two types of smooth muscle contraction.
Tonic - constant low pressure, prevent wall from being permanently stretched
Phasic - action potential bursts of contraction
What secretes GI hormones and what secretes digestive juices?
GI hormones are secreted by endocrine glands
Digestive juices are secreted by exocrine glands
Where are endocrine glands found?
Line the mucosa
What makes up digestive juices?
Water
Electrolytes
Specific products for absorption or digestion
What is needed for secretion to occur?
Raw materials and water so a lot of energy is required, most of this is then reabsorbed.
What stimulates secretion of digestive juices?
Neural or hormonal stimulation
Describe the process of autonomic nervous system control in the GI.
Autonomic nervous system control is mediated by pacemaker cells are located between the circular and longitudinal muscle. There is a basal electrical rhythm, but if a threshold is reached determined by either mechanical, neural or hormonal factors, smooth muscle contraction occurs.
Rate of contraction varies
Intensity depends on the action potentials once the threshold is reached.
What are the two components that make up the intrinsic nerve plexus?
Myenteric nerve plexus
Submucosa nerve plexus
What types of neurons are involved in the intrinsic nervous system?
Input neurons, respond to stimuli
Output neurons control motility
Acetylcholine for smooth muscle contraction
Nitrous oxide and vasoactive intestinal peptide relaxation
What are the extrinsic nervous system?
Parasympathetic nervous system stimulates digestion
In particular Vagus Nerve
Increases motility
Increases digestive enzyme secretion
What are the three main types of sensory receptors?
Chemoreceptors
Osmoreceptors - important in duodenum and small intestine, slows down motility
Mechanoreceptors
What happens when the sensory receptors are stimulated?
Neural reflexes and hormone secretion altering motility and secretion in the gut.
