Gastrointestinal Flashcards
What are the roles of the buccinator and suprahyoid muscles in swallowing?
- Manipulate food when chewing
- Elevate hyoid bone
- In phase 1
What are the roles of the muscles of the palate?
- Contract to cause soft palate to shut of nasopharynx
- Also helps to form bolus
- In phase 2
What are the roles of the muscles of the floor of the mouth?
- Elevate hyoid bone
- In phase 2
What are the roles of the infrahyoid muscles of the neck?
- Depress larynx
- In phase 3
What are the roles of the pharyngeal constrictors?
Contract sequentially to drive bolus down
Name the muscles involved in swallowing, in order of their use
- Buccinator and suprahyoid
- Palate muscles/ floor of mouth muscles
- Infrahyoid/ pharyngeal constrictors
Describe phase one of swallowing. Is it voluntary or involuntary? Name the muscles used
- Voluntary
- Food compressed to roof
- Pushed towards oropharynx
- Buccinator and suprahyoid muscles used
Describe phase two of swallowing. Is it voluntary or involuntary? Name the muscles used
- Involuntary
- Soft palate closes nasopharynx
- Pharynx shortened and widened by elevation of hyoid bone
- Palate muscles, muscles of floor of mouth
Describe phase three of swallowing. Is it voluntary or involuntary? Name the muscles used
- Involuntary
- Sequential contraction of pharyngeal constrictors to drive bolus down
- Return (depression) of hyoid bone.
- Infrahyoid muscles, pharyngeal constrictors
List 4 roles of saliva
- Lubricant for mastication, swallowing and speech
- Immunty (contains antifungals etc)
- Buffer
- Contains solvents necessary for taste
What is the optimum pH of saliva
Around 7.2
What percentage of BMR is used by the liver?
21%
What percentage of BMR is used by the brain?
20%
What sources of energy does the liver use?
Amino acids, fatty acids, alcohol, gluscose
What sources of energy does the brain use?
Glucose and ketone bodies
What sources of energy do muscles use?
Glucose, acetate, triglycerides, branched amino acids
What sources of energy red blood cells use?
Glucose
What are free sugars and what are they associated with?
- Mono and disaccharides
- Associated with increased blood glucose/insulin
What are 2 types of starch?
Rapidly digesting and slowly digesting (RDS and SDS)
What is rapidly digesting starch associated with?
Increased blood glucose and insulin levels
Name 4 functions of the stomach?
- Store and mix food
- Kill microbes
- Secrete proteases/intrinsic factor
- Lubrication
Name 4 key cells of the stomach
- Parietal
- Mucous
- Chief
- Enteroendocrine
Which cells in the stomach secrete acid, and what type of acid?
Parietal cells secrete HCl
Approximately how much acid does the stomach produce per day?
2L
What are the four phases of digestion?
1 - Cephalic
2 - Gastric
3 - Intestinal
Describe what happens in the cephalic phase of digestion
- The sight, smell, taste and chewing of food causes the parasympathetic NS to release acetylcholine.
- This therefore stimulates the release of gastrin and histamine
Describe what happens in the gastric phase of digestion
- Gastric distension, the presence of peptides and the presence of amino acids (all due to food), cause gastrin release.
- This therefore stimulates histamine release.
- When pH is too high in this phase due to the acid secretion from parietal cells, somatostatin release is triggered.
Describe the role of proteins in the stomach in the process of digestion
Proteins from food acts as a buffer to mop up excess H+ ions, meaning the overall pH increases and so the stimulus for samatostatin release is removed.
Describe what happens in the intestinal phase of digestion
- Chyme entering the duodenum, a low pH and the presence of amino acids/fatty acids leads to the release of enterogasterones.
Name 2 enterogasterones and their function
- Secretin: decreases gastrin levels, increases somatostatin levels.
- Cholecystokinin: CCK.
What is the role of acetylcholine in digestion?
- It is a parasympathetic neurotransmitter released by enteric nuerons.
- It acts directly on parietal cells to increase acid secretion.
- It also stimulates the release of histamine and gastrin.
What is the role of gastrin in digestion?
- It is a hormone
- It doesn’t act directly on parietal cells, rather stimulates the release of histamine.
What is the role of histamine in digestion?
- It is a paracrine.
- Acts directly on parietal cells to increase acid production.
What is the role of somatostatin in digestion?
- Acts directly on parietal cells to decrease acid production.
- Also inhibits the production of gastrin and histamine.
- Release is stimulated by a low pH due to excess acid release.
What is the definition of a peptic ulcer?
A breach in the gastric mucosal surface
What are peptic ulcers caused by?
Helicobacter pylori infection, drugs such as NSAIDs or chemical irritants (e.g. alcohol, bile salts)
I.e. due to a increased mucosal attack or reduced mucosal defence.
What are 4 gastric mucosa defences?
- Alkaline mucus
- Tight junctions
- Replacement of damaged cells
- Feedback loops
Which cells in the stomach secrete pepsinogen?
Chief cells
What is pepsinogen?
An inactive form of pepsin
What is pepsionogen mediated by?
ACh
When and how is pepsinogen converted into pepsin?
At pH of less than 2 by HCl
Why is pepsiongen to pepsin conversion an example of positive feedback?
Because after being converted from pepsinogen into pepsin, pepsin then catalyses further conversion of pepsinogen into pepsin.
What is the volume of an empty and full stomach?
- Empty: 50mL
- Full: 1.5L
How can the volume of a full stomach be so much greater than that of an empty stomach?
Because smooth muscle in the body and fundus undergoes receptive relaxation
How is receptive relaxation achieved?
By the parasympathetic nervous system releasing NO and serotonin through afferent input via the Vagus nerve.
Describe the process of peristalsis in the stomach
1 - Small peristaltic wave in the gastric body
2 - Larger wave in the gastric antrum. Pylorus closes as the wave reaches it.
3 - This means only a small amount of chyme passes into the duodenum. The antral contents is pushed back towards the body of the stomach i.e. to undergo mixing.
How is the rhythm set for peristaltic waves and what at?
- Set by pacemaker cells in the muscularis propria (i.e. muscular layer) at 3 per minute.
What increases the strength of peristaltic waves?
- Gastrin
- Gastric distension
What decreases the strength of peristaltic waves?
- Duodenal distension
- Increased duodenal pH
- Increased sympathetic action
- Decreased parasympathetic action
Why and how does duodenal filling occur?
- The capacity of the stomach is greater than that of the duodenum.
- If food moves too fast from the stomach to duodenum, can cause overfilling
How is duodenal emptying prevented?
- Release of enterogastrones
- Increased sympathetic and decreased parasympathetic action
Where are the 4 different types of stomach cells found?
- In gastric glands within gastric pits of the mucosal layer.
What 2 types of enteroendocrine cells are there are what is their function?
- G cells secrete gastrin
- ECL cells secrete histamine
If water input is 9000ml, how much is ingested?
2000ml
Name some ways in which water is secreted from the body?
Saliva, bile, via pancreas.
Where is water reabsorbed? What percentage is reabsorbed?
Jejunum, ileum. colon. 98%
How much water is there across the gastric mucosa?
Only little due the the high osmotic load of HCl
Where and how is Na+ reabsorbed?
- Actively pumped from lumen of ileum and jejunum
- Actively pumped into lumen of colon
Where and how is K+ reabsorbed?
By passive diffusion in the jejunum, ileum and colon
Where and how are Cl- and HCO3- reabsorbed?
- Cl- actively absorbed in exchange for HCO3- in ileum and colon
What are the two types of optical isomers of amino acids and which type are found in proteins?
- D and L
- L in proteins
What is a zwitterion?
A molecule which has 2 different charges within it
Where does protein digestion begin?
Stomach
What is the first part of protein digestion occurs first?
- Pepsins (from pepsinogen) cleave some of the peptide bonds
What are the 2 types of pepsinogen and where are the found?
- Pepsinogen I: In HCl secreting regions
- Pepsinogen II: In pyloric region
What happens in the second part of protein digestion and where does it occur?
- In the small intestines
- Polypeptides are further fragmented by pancreatic proteases
What are the 2 types of pancreatic proteases?
Endopeptidases and exopeptidases
What happens in the final part of protein digestion and where does it occur?
- Intracellular peptides break down some di/tripeptides in the lumen, brush border and within cells.
What happens to the amino acids formed by protein digestion?
Pass into cells via Na 2ndary transporter, then into interstitial fluid by facilitated transporter.
What is a lipid?
A fatty acid
What is a triglyceride/triglycerol?
A glycerol molecule with 3 fatty acids attached
Where does digestion of fats begin?
In the duodenum
What are triglycerides hydrolysed by and how?
- Pancreatic lipase
- With the help of co-lipase
- Hydrolyses bond 1 and 3 quickly and 2 more slowly
What does pancreatic lipase act on specifically?
Emulsified fats
How are fats emulsified?
Broken up into smaller droplets which are surrounded by bile salts to prevent re-association
What happens to the fatty acids and monoglycerides after digestion?
They combine with bile salts and cholesterol to form micelles. This enables them to reach the brush border by diffusion, where they break down.
What happens when fatty acids and monoglycerides reach the cell after digestion and what is the purpose of this?
They are esterified back into triglycerides to maintain the diffusion gradient across the cell membrane.
What happens after monogycerides and fatty acids are esterified back into triglycerides in the cell?
They are packaged into chylomicrons which leave the cell by exocytosis
What are chylomicrons?
Lipoproteins
What happens to chylomicrons after they leave the cell by exocytosis?
They enter lacteals and circulate in the lymphatic system
What happens to chylomicrons after they enter the lymphatic system?
They are delivered to cells where lipoprotein lipase hydrolyses them for use by cells
What are monosaccharides? Give 2 examples.
Single sugars e.g. glucose and fructose
What are oligosaccharides? Give 2 examples.
2 - 10 sugars e.g. disaccharides lactose and sucrose.
What are polysaccharides? Give 2 examples.
Many sugars e.g. starch and glycogen
What types of isomers can sugars form? Which kind are used in metabolism?
D and L isomers. D used in metabolism.
Describe the structure of glycogen and starch
- Glycogen: Polymer of alpha glucose. 1-4 glycosidic bonds with some 1-6 branches.
- Starch: Same as glucose but fewer 1-6 branches.
Where does carbohydrate/starch digestion begin?
In the mouth. Alpha-amylase degrades starch in saliva.
How are starch/carbohydrates digested in small intestine?
Pancreatic alpha-amylase catalyses 1-4 linkages but not 1-6.
How do hexoses and pentoses cross the intestinal mucosa?
Rapidly absorbed across it, into capillaries and to the portal vein
How does glucose cross the mucosal surface?
By the Glucose-Na+/K+ co-transporter, hence the high Na+ on the mucosal surface
How do you calculate BMI?
Weight (kg)/Height^2 (m)
What are the categories for BMI?
18.5 - 25 = Normal
25 - 30 = Overweight
30 - 40 = Obese
40+ = Morbidly obese
Define malabsorption
Defective absorption of nutrients across the GI tract.
List 3 mechanisms of malabsorption
- Defective digestion
- Defective absorption
- Abnormal transport across mucosa
What is the main cause of malabsorption?
Disease of small bowel
List 3 causes of poor weight gain
Insufficient calories, protein or fluid.
Is saliva isotonic, hypertonic or hypotonic and why?
- Hypotonic as striated ducts transport ions out of saliva and ensures water remains in.
What does hypotonic mean?
A solution with a lower osmotic pressure than surrounding fluids. I.e. more water and fewer particles dissolved in it.
What is somatostatin produced by?
D cells
What do endo and exopeptidases do?
Endo: Convert large polypeptides to smaller ones.
Exo: Breakdown dipeptides and amino acids.
What do parietal cells secrete and what is the function of the products it secretes?
- HCl for digestion.
- Intrinsic factor: Aids vitamin B12 absorption.
