Physiology and Pharmacology of the Small and Large Intestine Flashcards
What is the longest part of the small intestine?
Ileum (3m)
What three substances does the small intestine recieve?
- Chyme from stomach
- Pancreatic juice from pancreas
- Bile from gallbladder
What substances does the small intestine secrete?
Succus entericus
What does the ileocaecal valve open in response to?
Proximal pressure and gastrin
What three parts of the small intestine increased its surface area?
- Circular folds (or Kerckring)
- Villi
- Microvilli
What is gastrin secreted from?
G cells of gastric antrum and duodenum
What is Cholecystokinin (CKK) secreted from?
I cells of duodenum and jejunum
Where is secretin secreted from?
S cells of duodenum
What is secreted from M cells of the duodenum and jejunum?
Motilin
What is an incretin from K cells of duodenum and jejunum?
Glucagon-like Insulinotropic Peptide (GIP aka, gastric inhibitory peptide)
What do incretins act on?
Beta-cells of pancreas to stimulate release of insulin
What is an incretin from L cells of the gut?
Glucagon-like peptide 1 (GLP-1)
What do all secretions of the small intestine: Gastrin, CCK, Secretin, Motilin, GIP, GLP-1 nad ghrelin all act on?
G-protein coupled receptors
Where is Ghrelin secreted from?
Gr cells of the gastric antrum, small intestine and pancrease
What 5 control mechanisms are there for succus entericus?
- Distension/irritation
- Gastrin
- cck
- Secretin
- Parasympathetic nerve activity (all enhance) and sympathetic nerve activity (decrease)
What two substances does succus entericus contain?
Mucus - for lubrication/protection (from goblet cells)
Aqueous salt - for enzymatic digestion (mostly from crypts of Lieberkuhn)
What involves Na+/K+ATPase, Na+/K+/2Cl-cotransporter and chloride channel (CFTR)?
Seretion of succus entericus
What is secreted at the crypts of Lieberkuhn?
Succus entericus
What involves alternating contraction and relaxation of segments of circular muscle?
Segmentation (mixing)
What is segmentation initiated by?
Small intestine pacemaker cells causing the BER which is continuous
What triggers segmentation in the empty ileum?
Gastrin from the stomach (gastroileal reflex)
Which has more segmentation contractions - ileum or duodenum?
Duodenum 12 per minute
Ileum 9 per minute
What enhances strength of segmentation and what decreases it?
Enhanced by parasympathetic activity and decreased by sympathetic activity
In peristalsis, what two activities occur in the interdigestive, or fasting, state?
- A few localised contractions
2. The migrating motor complex (MMC)
When does the MMC occur?
Between meals every 90-120 minutes
What is a strong peristaltic contraction passing the length of the intestine (stomach to ileocaecal valve)?
Migrating motor complex
What does the MMC clear and act as?
Clears small intestine of debris, mucus and sloughed epithelial cells between meals, “housekeeper”
What inhibits the MMC?
Feeding and vagal activity
What triggers the MMC and what two secretions inhibit it?
Motilin stimulates
Suppressed by Gastrin and CCK
What do macrolide antibiotics (erythromycin) mimic the effect of?
Motilin and may cause GI disturbances
What two substances are secreted from the endocrine pancreas and where are they secreted to?
Insulin and glugagon - secreted to blood
What cells of the exocrine pancreas secrete digestive enzymes?
Acinar cells
What cells in the exocrine pancreas secrete aqueous NaHCO3- solution?
Duct cells
What two substances secreted from the exocrine pancreas, make up pancreatic juice and are secreted to the duodenum collectively?
Digestive enzymes
Aqueous NaHCO3-
What enzymes can completely digest food in the absence of all other enzymes?
Pancreatic enzymes
What are the enzymes from acinar cells stored in?
Zymogen granules
What are the three classes of pancreatic enzymes?
- Proteases
- Amylases
- Lipases
Name 3 proteases from the pancreas?
- Trypsinogen
- Chymotrypsinogen
- Procarboxypeptidase A and B
Name an amylase from the pancreas?
Pancreatic amylase
Name a lipase from the pancreas?
Pancreatic lipase
What out of proteases, amylases and lipases are active and what are inactive?
Active = amylases and lipases Inactive = proteases
What pancreatic enzyme joins with enterokinase from the duodenum to make trypsin?
Trypsinogen
What enzyme is released from the duodenum (from mucosal cells) that influences pancreatic enzymes?
Enterokinase
What does trypsin catalyse in the duodenum?
Chymotrypsinogen to chymotrypsin
What does trypsin catalyse other than chymotrypsinogen to chymotrypsin in the duodenum?
Procarboxypeptidase A and B to Carboxypeptidase A and B
What neutralises the acidic chyme entering the duodenum?
Alkaline HCO3-rich fluid from duct cells in the pancreas
What is fluid secretion like in patients with CF?
Reduced (because of CFTR channel)
What are the three phases in the control of pancreatic secretion?
- Cephalic
- Gastric
- Intestina;
What mediates cephalic phase in control of pancreatic secretion?
Vagal stimulation of mainly the acinar cells
How does control of the gastric phase work in control of pancreatic secretion?
Gastric distension evokes a vagovagal reflex resulting in parasympathetic stimulation of acinar and duct cells
What is the cycle of aqueous NaHCO3 solution neutralising acid in duodenal lumen?
- Increased secretion of aqueous NaHCO3 solution into duodenal lumen
- Neutralises acid in duodenal lumen
- Increased secretin release from S cells, secretin carried in blood to pancreatic duct cells
- Cycle repeats
What is the cycle of increasing secretion of digestive enzymes into duodenal lumen to digest fat and protein in duodenal lumen?
- Increased secretion of digestive enzymes into duodenal lumen
- Digestion of fats and protein in duodenal lumen
- Increased CCK release from I cells, CCK carried in blood to pancreatic acinar cells
- Cycle repeats
What is entry of ileocaecal material (indigestible residues, unabsorbed biliary components and unabsorbed fluid), permitted through the ileocaecal sphincter by?
Gastroileal reflex
What three nervous control is the ileocaecal sphincter under?
- Vagus nerve
- Sympathetic nerves
- Enteric neurones
What three substances does the large intestine absorb and secrete?
Absorbs H2O, Na+ and Cl-
Secretes K+, HCO3 and mucus
What does HCO3 secreted from the large intestine do?
Neutralises acid produced by bacterial fermentation
What happens to carbohydrate that is not absorbed by the small intestine?
It is fermented by colonic flora to short chain fatty acids
What are faeces composed of?
Water, solids including cellulose, bacteria, bilirubin and salt
What two functions takes place in the ascending and transverse colon?
Fluid reabsorption and bacterial fermentation
What takes place in the descending colon to the sigmoid colon?
Final drying (desiccation) and storage
What is the term for non-propulsive segemntation?
Haustration
What are haustra?
Saccules caused by contraction of the circular muscle
Where does haustration occur and what does it cause?
Occurs in proximal colon and causes orad movement
In the colon, what is the outer longitudinal muscle layer comprised of?
Three bands, the taenia coli
What two things does the gastrocolic response involve?
Gastrin and extrnisic nerve plexus
What action of the large intestine is triggered by the gastrocolic response?
Mass movement - simultaneous contraction of large sections of the circular muscle of ascending and transverse colon
What response does mass movement in the distal colon, propelling faeces into the rectum cause?
Defaecation reflex
What happens after the rectum has filled with faecal matter?
Activation of rectal stretch receptors
Once rectal stretch receptors have been activated, what two responses occur (both controlled by the pelvic nerve)?
- Activation of afferents to spinal cord
2. Activation of afferents to brain (urge to defaecate)
Once afferents have been activated to the spinal cord, what becomes activated?
Parasympathetic efferents
What do activation of parasympathetic efferents cause via the pelvic nerve?
Contraction of smooth muscle of colon and rectum - internal anal sphincter relaxes
What is the rectospinteric reflex and what condition is it defective in?
Internal anal sphinter relaxation, defective in Hirschprung disease
Once there has been activation of afferents to the brain (urge to defaecate) what occurs?
Altered firing rate in efferents to spinal cord
What nerve does altered firing in efferents to spinal cord utilise to cause contraction or relaxation of skeletal msucle and the external skeletal sphincter?
Pudendal nerve
What do colonic bacteria synthesise?
Vitamin K2 and free fatty acids (from carbohydrate) that are absorbed
What does the presence of hard, dried faeces within the colon result from?
Delay in defaecation and enhanced absorption of H2O
What is one cause of appendicitis?
Hardened, calcified, faecal matter within the appendix (appendicolths)
What are purgatives?
Agents that cause purging, or cleansing of the bowels by promoting evaculation
How do laxatives work?
Increase peristalsis and/or soften faeces causing,or assisting evacuation
What are indigestible polysaccharide polymers that improve stool consistency and are slow acting?
Bulk laxatives
Name a bulk laxative?
Methylcellulose
What do bulk laxatives retain?
Water
Name three stimulatn purgatives?
- Bisacodyl
- Sodium picosulfate
- Senna
What type of laxative is senna?
Anthraquinone
What side effect can stimulant purgatives give?
Abdominal cramping
What along with faecal softening, do stimulant purgatives do?
Increase H2O and electrolyte secretion
Increase peristalsis
What are poorly absorbed solutes that retain water, increase bulk and increase perstalsis?
Osmotic laxatives
Name three osmotic laxatives?
Magnesium sulphate
Sodium citrate
Lactulose
What drugs increase water and electrolyte secretion, increase perstalsis and faecal softening? They also have a detergent like action?
Faecal softeners
Name two faecal softeners?
Docusate sodium - orally
Arachus oil - enema
What are prolonged use of glucocorticoids for acute IBD attacks limited by?
Adrenal suppression
