GI Tract Physiology W11 Flashcards
What are the three salivary glands and their roles? Order LARGEST- SMALLEST
Parotid: serous (hydrate)
Sublingual: mixed, serous and mucous (lubricate)
Submandibular: serous (hydrate)
How is digestive secretion regulated (overall)?
Sympathetic: inhibits (noradrenaline, adrenaline)
Parasympathetic: stimulates (ACTH)
What are the three important digestive enzymes that hydrate and lubricate food in the mouth? (2 not found in stomach)
Lingual lipase- digestion of triglycerides- make fatty food taste good to activate metabolism.
Lingual amylase- digestion of polysaccharides (starch)- tastes sweeter with more chewing.
Lysozyme- digestion of bacteria in food- reduces bacterial load into small intestine. Also found in stomach
What do gastric glands secrete?
Acid- for protein digestion
Pepsinogen- for protein digestion
Lipase- lipid digestion
Mucous- protection
Lysozyme
What cells release acid and pepsinogen?
Acid: parietal cells
Pepsinogen: chief cells
What is the role of mucous in gastric digestion?
Buffer to stop acid getting into the cells.
What the first two phases of gastric secretion and their regulation?
Cephalic phase: neural- cerebral cortex stimulate taste and smell vagus nerve. Loss of appetite = no stimulatory impulses to parasympathetic centres. AND hormonal stretch + chemical composition
Gastric phase: Stomach distension activates stretch receptors - stimulates gastrin release (hormone control) -> stimulates parietal cells to release histamine = acid secretion.
Describe the intestinal secretion phase and its regulation?
Intestinal phase: Once food in intestine, hormonal control via CCK,secretin & VIP- inhibits acid secretion and gastric emptying, stimulates pancreatic secretions- enzymes and bicarb, stimulates gall bladder contractions.
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Neural stretch/local response (distension of duodenum) - increase secretion of enzymes + bicarb into S.I
Describe the ways in which acid is neutralised for the small intestine
Pyloric sphincter: Relatively steady flow from stomach to duodenum ie. Inhibits acid secretion and gastric emptying rate.
Duodenum Brunner’s glands - release bicarbonate neutralising chyme from stomach
Pancreatic secretion: release bicarb and digestive enzymes
Gall bladder secretion: bile - lipid breakdown and bicarb
Small intestine secretion mostly comes from the pancreas and the liver. Particularly from the pancreas and gall bladder.
Where are these hormones released in the small intestine specifically?
Pancreas and gall bladder join at a opening called the hepatopancreatic duct -> major duodenum papilla (on duodenum).
How does ‘local hormone release’ regulate intestinal secretion?
Hormone gastrin stimulates stomach acid secretion in response to food intake. The hormone somatostatin stops the release of stomach acid.
As well as secretions in the intestine regulated by hormones CCK, VIP and secretin that inhibit acid secretion and slow down gastric emptying.
What is gall stone?
Bile that contains too much cholesterol or not enough bile salts
When is swallowing voluntary vs involuntary?
Voluntary until the pharynx (involuntary/autonomic)
What does the term deglutition mean?
Swallowing-epiglottis closes so doesn’t go into lungs
Describe the autonomic response of swallowing
Generated by stretch mechano and chemo receptors present in pharynx. Detect lump - swallowing reflex
Close epiglottis - so food doesn’t go into trachea
Lump of food pushed back and oropharynx relaxes = Peristalsis (muscle contraction that move food through oesophagus).
What is peristalsis and the process of it from the oesophagus?
Circular and longitudinal muscle contraction that propels food through oesophagus to the stomach. These peristaltic waves start in the oesophagus (1/3 is skeletal muscle) requires cranial nerves for control CNS. Latter 2/3 are under smooth muscle, autonomic local control. Process is bidirectional.
What is primary peristalsis?
First response from swallowing food.
It starts in pharynx and involves skeletal muscle - regulated by vagus nerve.
In the oesophagus circular muscle relaxes and stretches- strong wave-like motions of the smooth muscle contracts and moves balls of swallowed food down to bottom of oesophagus.
The longitudinal muscle contracts and shortens and moves the food down further.
What is secondary peristalsis?
Stimulated by local reflex- food leftover in oesophagus, secondary contraction that pushes it to the stomach via stretch response.
Stretch causes contraction- can’t be reversed.
End of peristalsis - relaxes LES sphincter to let food enter stomach.
What causes GERD?
Inappropriate relaxation of LES (cardiac sphincter). Usually closed to hold acidic contents in stomach. Should only open when food is passing from oesophagus into the stomach.
Disordered peristalsis- if sphincter is open, pushes food back toward mouth instead of stomach.
Acid in mouth can erode the teeth.
Why don’t you get GERD normally?
Normally this LES valve closes tightly after food enters your stomach. If it relaxes when it shouldn’t, your stomach contents rise back up into the esophagus.
What is the difference between Cardiac Sphincter and Pyloric Sphincter?
Cardiac = lower oesophageal sphincter= seals from oesophagus and stomach. Physiological sphincter (increased tone of muscle)
Pyloric = seals from duodenum- separates stomach from small intestine. Anatomical sphincter (circular muscle band)
How does nutrients pass from small intestine to body?
Every vein that comes out of the stomach, intestines goes into the hepatic portal vein- therefore goes to liver first. Liver is biochemical processing centre. Stores excess glucose, takes amino acids and converts to ones you need.
Blood from superior mesenteric to hepatic portal vein.
Why is it called heart burn?
Because the lower oesophageal sphincter, cardiac sphincter, is next to the heart.
When does the pyloric sphincter open?
When there is food in the stomach and stimulus from mixing
What is gastric motility?
Pylorus regulates flow.
Food is churned into a liquid mixture called chyme that moves into the small intestine where peristalsis continues. Pylorus is shut.
Autonomic motility starts in the A
Is called B
Involves contractions of C
Involves relaxation of D
A = Oesophagus
B = Peristalsis
C = Longitudinal and circular smooth muscle
D = Sphincters (also smooth muscle)
What are the main sites of digestion prior to the intestine?
Carbohydrates = polysaccharides digested by amylase in mouth
Proteins = digested by acid peptides by pepsin in stomach
Lipids = triglycerides digested by lingual and gastric lipase.
How is a slow, constant flow of materials to the intestine developed after a large meal? Large molecules into small ones- then to liver.
Periodic relaxation of pyloric sphincter in stomach.
The main site for digestion is the duodenum.
There are proteins (trypsin, chymotrypsin, elastase, carboxypeptidases, amino peptidases on brush border) all inactive zymogens till trypsin (from pancreas) is activated when released into Lumen of GIT.
What happens when digestive enzymes are activated early (like in the pancreas before the lumen of GIT)?
Breaks down your own tissue- can lead to pancreatitis.
Amino acids can only be absorbed as single amino acids, or di/tri peptides. What digestive enzymes digest protein peptides vs amino acids?
Protein Peptides = trypsin, chymotrypsin, elastase
Amino acids = carboxypeptidases, amino peptidases
What are the primary and secondary effects of GIT?
Primary: digestion and absorption
Secondary: immune function and endocrine regulation
How does the absorption of amino acids and carbs differ to fats?
Amino acids have to be made into di and tri peptides- active transport
Only monosaccharides of carbs are absorbed - secondary active process linked to sodium.
Lipids = triglycerides are non-polar need bile to solubilise them. Enzyme breaks down to fatty acids, and monoglyceride. Passively absorbed - Packaged into chylomicrons droplets. Go into lymphatic system THEN they end up in blood.
Give an example of paracellular absorption AND where does this occur
Passive ion gradient transport- electrolytes bring water with them and increase H20 absorption.
In the jejunum - ileum where 90% of H20 absorption occurs.
Describe the absorptive state, just after a high glucose meal.
What are the three major places glucose goes from GIT?
High blood glucose stimulates insulin release.
Tells major tissues (adipose and skeletal muscle) to take up glucose.
Glucose goes to liver where it is converted to glycogen. = glycogenesis, fatty acids = stored as triglycerides.
Goes to all tissue to make ATP.
Go to adipose tissue converted to trigylcerides.
Goes to muscle used for aerobic metabolism.
What occurs a while after a meal (post absorptive state)? Why does blood glucose level always drop?
Fatty Acids used in tissues (switch to ketones) and glucose replacement for tissues and brain from stored glycogen in liver, goes back to glucose used for brain.
Muscle can breakdown glycogen to make lactate, sent to liver to make glucose.
You can’t make glucose from FAs
What is made in the liver as a glucose replacement?
Ketones.
Triglycerides -> into ketones. Only happens when body has been in post-absorptive state for a long time.
What are fatty acids stored as in the absorptive state?
Triglycerides
Describe how metabolism varies during the day?
Depends on when the last meal was, and how much glucose is available to the cells.
Just after a meal = absorptive state. Where glucose is used and stored.
A while after a meal= post-absorptive state. Glucose is conserved and FAs, ketones are the main source of energy. Glycogen converted to glucose in the liver.
What makes lipid digestion slow?
Lipids aren’t soluble in water, but digestive lipases are hydrophilic- therefore can only work on the surface of lipid droplet. This is why digestion of lipids is more efficient in the small intestine.
What digestive enzyme is on the brush border of the epithelial cells to digest peptides/amino acids?
Aminopeptidases. The rest are pancreatic.
Where are most of the enzymes found that breakdown carbs?
Brush border of cell epithelium in jejunum. Break down to monosaccharides.
What are the lipases associated with digestion of lipids?
Why does pancreatic lipase have an advantage?
Lingual lipase
Gastric lipase
Pancreatic lipase- works with bile (released from gall bladder) that help break fat droplets down so that there is more surface area for the lipases to work on. To break up triglycerides to monoglycerides and fatty acids.
What is the most active site for digestion within the small intestine?
Duodenum and jejunum.
Which enzymes are involved in carbohydrate digestion and where are these located/secreted from?
Lingual and pancreatic Amylase
Enterocyte= lactase, sucrase, maltase, isomatase
What cells secrete mucus?
What are the epithelial cells that sit on the small intestine?
Goblet cells
Columnar epithelium- not good tight junctions, mostly absorptive. Form intestinal crypts.
What is the role of villi in intestinal epithelium?
Increases surface area to absorb nutrients
How and where are carbs absorbed?
Mostly in duodenum and jejunum - active transport.
A sodium-glucose transporter- moves sodium out of chyme in intestine, against conc gradient into the cell.
Basolateral = passive transport.
Describe the absorption of lipids
Passive transport on enterocyte- conc of lipids is higher in intestine that enterocyte. Basolateral membrane = active transport
What are the three main ways the liver processes nutrients?
- Regulates outflow of nutrients
- Processing/storage
- Removes bacteria
Why doesn’t the liver pass fat nutrients to body?
Because fats go through lymphatics not blood.
Lipid droplets are absorbed by endocytosis and packed into chylomicrons.
Lacteals then transport these chylomicrons around the body.
Describe the difference in transport of amino acids, glucose and lipids (post-small intestine)
AA & glucose travel in blood to the mesenteric veins into the hepatic portal vein on to the liver.
Lipids are transported by chylomicrons into lymphatic ducts, which drain into the innominate vein -> subcalvian vein to go to the liver.
What sphincter regulates the rate of gastric emptying?
And what muscles mix and propel the food into duodenum?
Pyloric sphincter- as it gets wider.
Longitudinal layer, circular layer and oblique layer
In regards to motility in the stomach, what is the role of the pylorus?
Regulates flow:
Churning = peristalsis, pylorus shut
Emptying = peristalsis, pylorus open- pylorus pump into duodenum.
Gastric emptying is peristalsis with pylorus OPEN.
What is it regulated by: stimulated, and inhibited
Stimulated (increases)= gastrin (acid secretion) from stomach and waves peristalsis, relaxes pyloric sphincter.
Inhibited (decreases) = CCK hormones from intestine, inhibits secreting acid and peristalsis and tightens pyloric sphincter.
Also food volume increases stretch of stomach- contraction gets greater.
Why does fat slow gastric motility?
CCK (intestinal acid), when you have high fats, more CCK released which is difficult to digest- therefore takes more time to move.
What is small intestine motility regulated by?
Local electrical responses by nervous signals from enters system, and hormones, caused by distension - so you get CCK to slow it down so food can mix more.
What is the absorptive process of crypts?
Absorbing both monosaccharides and amino acids via active transport. Fatty acids by passive transport.
Connected to lymphatic system by lacteals- lipids. Go into lymph
Capillaries by Amino acids, sugars. Go into blood
Describe the regulation of salivary secretion
and what does it require?
Parasympathetic system of cranial nerves mostly responsible AND sympathetic system of T1-T3.
Controlled by higher brain function: cephalic phase, local receptors.
Requires blood vessel dilation. Because water in saliva has come from blood
What is the purpose of the brush border in small intestine?
The brush border is made up of microvilli on the apical surface of the villi. The brush border enzymes are digestive enzymes within the microvilli, and complete the digestion of food molecules.
What activates the secretion of digestive juices into the lumen or hormones into the blood?
Sensors: mechano and chemo receptors located in the GIT walls, respond to the stretching by food into the lumen. The sensors also respond to concentration and pH changes.
