lower gastrointestinal system physiology

what tissue is the pancreas made of

exocrine and endocrine tissue

where is the pancreas located

behind and below the stomach

what is the endocrine function of the pancreas

• islets of langerhans

• found throughout pancreas

• secrete insulin and glucagon

what is the exocrine function of the pancreas

secretes pancreatic juices containing

• pancreatic enzymes actively secreted by acinar cells that form the acini

• aqueous alkaline solution actively secreted by duct cells that line pancreatic ducts- rich in NaHCO3 for neutralising HCl

what is exocrine secretion of pancreatic enzymes regulated by and released by

• regulated by hormones (enterogasterones)

• released by presence of chyme in the duodenum

what does secretin stimulate

NaHCO3 secretion from pancreatic duct cells

what does CCK stimulate

enzymes from acinar cells

what are proteolytic enzymes

digestive proteins

what are the 3 digestive proteins

• trypsinogen

• chymotrypsinogen

• procarboxypeptidase

what is trypsinogen converted to

converted to the active form trypsin by enterokinases

why is trypsin said to be at the top of the cascade

it is the first one to be activated

what is chymotrypsinogen converted to

converted to the active form chymotrypsin

what is procarboxypeptidase converted to

active form carbooxypeptidase

what do proteolytic enzymes begin as

pro-peptide so they can be converted into the active form

what do pancreatic amylase convert

polysaccharides into the disaccharide amylase

what is special about pancreatic lipase

it is the only enzyme secreted throughout the entire digestive system that can digest fat

what hormonal control occurs when there is acid in the duodenum lumen

• increased secretin release from duodenal mucosa

• secretin travels in the blood

• pancreatic duct cells are stimulated

• increased secretion of aqueous NaHCO3 solution into duodenal lumen

what hormonal control occurs when there is fat and protein products in the duodenal lumen

• increased CCK release from duodenal mucosa

• CCK carried by blood

• pancreatic acinar cells are stimulated

• increased secretion of pancreatic digestive enzymes into duodenal lumen

what is gastrin released by and what does it stimulate

• released by protein presence and Ach from G cells

• stimualtes HCl and pepsinogen

what is somatostatin released by and what does it inhibit

• released from D cells

• inhibits HCl, pepsinogen, and gastrin

what is secretin released by and what does it stimulate

• released by HCl

• stimulates NaHCO3 secretioon

what’s CCK released by and what does it stimulate

• released by presence of fat and protein

• stimulates pancreatic enzyme release

what is the largest and most important metabolic organ in the body

the liver

what is the body’s major biochemical factory

the liver

what is the liver important to

important to digestion- secretes bile salts

what does the hepatic artery carry

oxygen and metabolites

what does the hepatic portal vein carry

absorbed nutrients from the digestive tract

what are the functions of the liver not related to digestion

• metabolic processing of the carbohydrates, proteins, and lipids after absorption

• detoxifying or degrading body wastes and hormones, drugs, and other foreign compounds

• synthesises plasma proteins

• stores glycogen, fats, iron, copper, and many vitamins

• activates vitamin D

• removes bacteria and worn-out red blood cells

• produces acute phase proteins and hormones e.g. hepcidin, thrombopoietin, IGF-1

• excretes cholesterol and bilirubin

what is bile actively secreted by and actively directed to

• actively secreted by lier

• actively diverted to gallbladder between meals

where is bile concentrated and stored in

the gallbladder

what does aqueous alkaline fluid contain

• bile salts- emulsifiers

• cholesterol

• lecithin- emullsifiers

• bilirubin

what does bilirubin break down product from

breaks down product from haem

what happens to bile after a meal

bile enters the duodenum

what are bile salts derivatives of

cholesterol

what do bile salts do

convert large fat globules into a liquid emulsion

what happens to bile salts after participation in fat digestion and absorption

most are reabsorbed into the blood

what is the small intestine the site of

site where most digestion and absorption takes place

what are the 3 segments of the small intestine

• duodenum

• jejunum

• ileum

what are the 2 types of motility in the small intestine

• segmentation

• migrating motility complex

what is migrating motility complex a type of

type of peristalsis

what is the mesentery

• an extension of the serosa

• membrane that keeps the small intestine folds apart

what are the major folds of the small intestine called

pilca circulares

what are the finger-like projections of the small intestine called

villi

what do the columnar epithelial cells have

microvilli

what are microvilli considered as

a brush border

what is segmentation

primary method of motility in the small intestine

what does segmentation consist of

ringlike contractions along length of small intestine

how long do contractions in segmentation take place

within seconds, contracted segments relax and previously relaxed areas contract

what does the action of segmentation cause

mixing and propelling of chyme throughout the small intestine lumen

what is segmentation initiated by

pacemaker cells in the small intestine which produce basic electrical rhythm (BER)

what is circular smooth muscle responsiveness in segmentation influences by

influenced by distension of intestine, gastrin, and extrinsic nerve activity

what are the functions of the small intestine

• mixing chyme with digestive juices secreted into small intestine lumen

• exposing all chyme to absorptive surfaces of small intestine mucosa

what does the migrating motility complex do

sweeps intestines clean between meals

what is secreted by the small intestine

juice that does not contain any digestive enzymes

where do synthesised enzymes in the small intestine act within

act within brush border membrane of epithelial cells

• enterokinase

• disaccharides

• amino peptidases

digestion in the small intestine

• pancreatic enzymes continue carbohydrate and protein digestion

• brush-border enzymes complete digestion of carbohydrates and proteins

• fat is digested entirely within small intestine lumen by pancreatic lipase

how much does the small intestine absorb

almost everything presented to it - 9L/day

where does most absorption in the small intestine take place

duodenum and jejunum

what are adaptations that increase the small intestines surface area for absorption

• inner surface has permanent circular folds

• microscopic finger-like projections called villi

• brush border (microvilli) arise form luminal surface of epithelial cells

how often is the lining of the small intestine replaced

about every 3 days

what do products of fat digestion undergo in the small intestine

undergo transformations that enable them to be passively absorbed- eventually entering the lymph

how is monosaccharide glucose and galactose absorbed into the epithelial cells

• by Na+- and energy-dependent secondary active transport

• symporter SGLT

how does fructose enter the cell

by passive facilitated diffusion via GLUT-2

describe step by step what happens in carbohydrate absorption

1. dietary starch and glycogen are converted into maltose through the action of salivary and pancreatic amylase

2. maltose and dietary lactose and sucrose are converted into their respective monosaccharides by maltase, sucrase, and lactase located in the brush borders of the small intestine epithelial cells

3. glucose and galactose absorbed by Na+- and energy dependent secondary active transport located at the luminal membrane

4. fructose enters the cells by passive facilitated diffusion via GLUT-5

5. glucose, galactose, and fructose exit the cell at the basal membrane by passive facilitated diffusion via GLUT-2

6. they enter the blood by simple diffusion

describe step by step what happens in fat absorption

1. dietary fat composed of triglycerides is emulsified by the action of bile salts into smaller fat droplets. lipid emulsion prevents fat droplets from coalescing and thereby increases SA available for attack by pancreatic lipase

2. lipase hydrolyses triglycerides into monoglycerides and free fatty acids

3. carried into luminal surface of small intestine epithelial cells within micelles, formed by bile salts and other bile constituents

4. when micelle approaches absorptive epithelial surface, monoglycerides and fatty acids leave the micelle and passively diffuse through lipid bilayer of luminal membranes

5. monoglycerides and fatty acids are resynthesises into triglycerides

6. triglycerides aggregate and are coated with a layer of lipoprotein from ER to for chylomicrons

7. chylomicrons extruded through the basal membrane of the cells by exocytosis

8. chylomicrons unable to cross the basement membrane of capillaries so enter the lymphatic vessels

describe step by step what happens in protein absorption

1. dietary and endogenous protein hydrolysed into constituent amino acids by gastric pepsin and pancreatic proteolytic enzymes

2. small peptides converted into their respective amino acids by amino peptidases located in the brush borders of the small intestine epithelial cells

3. amino acids absorbed by Na+- and energy-dependent secondary active transport via a symporter

4. some small peptides absorbed by different type of symporters driven by H+, Na+-, and energy dependent tertiary active transport

5. most absorbed small peptides broken down into their amino acids by intracellular peptidases

6. amino acids exit cell at basal membrane via various passive carriers

7. amino acids enter blood by simple diffusion

what sort of organ is the large intestine

primarily a drying and storage organ

what does the large intestine consist of

• colon

• cecum

• appendix

• rectum

what does the chyme from the small intestine consist of

indigestible food residues, unabsorbed biliary components, and remaining fluid

what does the colon extract

more water and salt form contents

what remains to be eliminated in the colon

feces

what are taeniae coli

longitudinal bands of muscle

what is different about the longitudinal muscle in the large intestine

• only 3 bands

• doesnt have a full layer of longitudinal msucle

what are haustra

• pouches or sacs

• actively change location as a result of contraction of circular smooth muscle layer

what is the main motility of the large intestine

haustra contractions

what are haustra contractions initiated by

autonomous rhythmicity of colonic smooth muscle cells

what are the 4 main parts of the large intestine

• ascending colon

• transverse colon

• descending colon

• sigmoid colon

what are the functions of the large intestine

• mass movements

• gastrocolic reflex

• defecation reflex

describe mass movements of the large intestine

• massive contractions

• moves colonic contents into distal part of large intestine

what is the gastrocolic reflex mediated from

mediated from stomach to colon by gastrin and by autonomic nerves

what is the gastrocolic reflex often followed by

the urge to defecate

when is the defecation reflex initiated

when stretch receptors in rectal wall are stimulated by distension

what does the defecation reflex cause

internal anal sphincter to relax and rectum and sigmoid colon to contract more vigorously

what happens if the external sphincter is also relaxed

defecation occurs

what are the 2 anal sphincters

• internal anal sphincter

• external anal sphincter

what is the muscle of the internal anal sphincter

smooth muscle- involuntary control

what is the muscle of the external anal sphincter

skeletal muscle- voluntary control

what does the large intestine secrete

alkaline mucus for protection and lubrication

what digestion occurs in the large intestine

• no digestion

• as no digestive enzymes

what do colonic microflora digest

cellulose to short chain fatty acids

what is absorbed in the large intestine

salt and water

what is vitamin K synthesised by in the large intestine

synthesised by bacteria

what is the excretion in the large intestine

• approx. 2/3 water

• undigested cellulose, bilirubin, bacteria, and salt

what is gastrin released stimulated by

presence of protein in stomach

what is gastrin secretion inhibited by

accumulation of acid in stomach

what are the functions of gastrin

• acts in several ways to increase secretion of HCl and pepsinogen

• enhances gastric motility, stimulates ideal motility, relaxes ileocecal sphincter, induces mass movements in colon

• helps maintain well-developed, functionally viable digestive tract lining

what is secretin release stimulated by

presence of acid in duodenum

what are the functions of secretin

• inhibits gastric emptying in order to prevent further acid from entering duodenum until acid already present is neutralised

• inhibit gastic 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 neutralisation process

• along with CCK, s trophic to exocrine pancreas

what are the functions of CCK

• inhibits gastric motility and secretion

• stimulates pancreatic acing cells to increase secretion of pancreatic enzymes

• causes contaction of gallbladder and relaxation of sphincter Oddi

• along with secretin, is trophic to exocrine pancreas

• implicated in long-term adaptive changes in proportion of pancreatic enzymes in response to prolongs diet changes

• important regulator of food intake

what is GIP

glucose dependent insulinotrophic peptide