Digestion and Secretion: Small intestine Flashcards
Where do secretions into the small intestine come from?
Pancreas
Liver
Intestine itself
How much of secretions of the Liver is made per day? From where?
Biliary system
0.5l
How much of secretions of the pancreas is made per day?
1.5l
How much of secretions of the small intestine is made per day?
1l
What is the pH of pancreas secretion?
7.8- 8.4
What is liver secretion (biliary system) composed of?
bile acids
cholesterol
phospholipids
What is the pH of liver )secretion?
7.6
What are thesecretions of liver also used for?
Excretion of drugs from body
What is pancreas secretion composed of? why?
Salts e.g sodium bicarbonate - because so alkaline
enzymes -enzymes to complete the process of carbohydrate digestion fat digestion, and protein digestion in the small intestine
What is small intestine secretion composed of?
mucus
enteropeptidase
salt and water
What are the secretions of liver also used for?
Excretion of drugs from body
What is the pancreas known for?
Champion bicarbonate secreting organ
What are the enzymes in the pancreatic juice
Proteases — digest proteins
Lipases — digest fats
alpha-amylase — digests carbohydrates
What is the name given to cells in the pancreas
Acinar cells
What is important about proteases secreted from the pancreas
Inactive form so it is transported tot he small intestine without digestion at the pancreas
What is important about alpha-amylases secreted from the pancreas
On the apical membrane brush border of the villus epithelial cells
responsible for the final steps of carbohydrate digestion
What is the pancreatic juice made of?
HCO3- : rich in bicarbonate
NaCl
Water
What is important about the composition of the pancreatic juice
Creates the right environment for enzymes to work
rich in bicarbonate- neutralize the acid that is delivered into the duodenum from the stomach
What is important about biacarbonate in pancreatic juice
helps to buffer pH
neutralize the acid that is delivered into the duodenum from the stomach
essential for mucus to have the right physical chemical properties
What happens if there are problems with the exocrine pancreas
Short life expectancy
What is the difference between exocrine and endocrine pancreas
Exocrine- digestion and removal of drugs
Endocrine- Insulin and glucagon
What is the organisation of the pancreas
Main collecting duct in the middle makes the “stem of the leaf”
Main collecting duct connected to lobules by interlobular duct (makes the “ veiny branches of the middle bit of the leaf”)
Intralobular ducts within groups of lobules
Zoom onto lobule:
150- 200 acinar cells form acinus
Acinus connected to intralobular duct by intercalated duct
What are lobules
Secretory unit
What are acinar cells
Specialised pancreas epithelial cells
What is the function of acinar cells
Produce enzymes needed in the final stage of digestion
What is the structure of acinar cells
Zymogen granules near apical membrane(facing the intercalated duct)
Rich rough endoplasmic reticulum at the base of the cell
What are zymogen granules
Store for enzymes that acinar cells releases by exocytosis
What is the acinar cells specialised for?
Protein production
What are the cells in the pancreas?
Acinar
Ductal epithelial cells
How pancreatic juice is produced?
Primary secretion
Acinar cells deliver enzymes into intercalated duct
Secretion of isotonic sodium chloride and water:
- transcellular chloride movements
-Paracellular chloride movements
- chloride drives paracellular movements of sodium and water
At duct-lining epithelial cells Modification of secretion to produce the bicarbonate rich pancreatic juice:
-transcellular absorption of chloride by the lining epithelial cells and the transcellular secretion of bicarbonate
-Paracellular secretion of water and sodium
What does ‘leaky’ epithelia mean?
both transcellular and paracellular movement can occur
What is ‘leaky’ epithelia referring to?
Position of acinar and duct lining epithelial cells
What happens with saliva at the salivary glands
Same as pancreatic juice but:
instead of bicarbonate there is alpha- amylase at the acinar cells
“Leaky” epithelia at the level of the acinar cells but “tight epithelial at the level of the duct-lining epithelial cells- transcellular movement no paracellular movement
Sodium and chloride ion reabsorption
Potassium and bicarbonate secretion
sodium chloride secretion in the acinar cells - basolateral membrane
1- Sodium-Pottassium ATPase using ATP hydrolysis to pump sodium out in exchange for potassium in concentration gradient across the basal lateral membrane
2- used by the sodium potassium two chloride (NKCC) cotransporter to drive the accumulation of chloride ions inside the cell
One sodium one potassium, and two chlorides are transported together
secondary active transport that is dependent upon the sodium potassium ATPase
3-Basolateral membrane potassium channel- membrane pore for potassium travel down favorable electrochemical gradient out of cell-provides an extra source of extracellular potassium for the co-transporter to use
sodium chloride and water secretion in the acinar cells - apical membrane
Exit on the apical membrane is passive-
ion channel selective for chloride- chloride moves passively down a favorable electrochemical gradient from inside the cell into the lumen of the duct
-Negative charge draws sodium across epithelium into lumen
Sodium chloride creates an osmotic gradient and that’s gonna draw water across the epithelium
How are apical chloride channels regulated
an increase in intracellular calcium concentration by:
- acetylcholine interacting with m3 receptors
- hormonal- CCK produced by I cells
What does an increase in intracellular calcium concentration in the acinar cells mean?
activation at the basolateral membrane potassium channels
activation of opening the gated apical passive chloride channel
How does the basolateral potassium channel act as a drive for chloride secretion?
Hyperpolarises makes basolateral membrane more negative
What’s the difference in saliva and pancreatic sodium chloride secretion in terms of regulating?
Only neuronal in saliva
Neuronal and hormonal- pancreas
Activation of pancreatic proteases
Inactive form of enzymes- Trypsinogen Chymotrypsinogen Precarboxypeptidase Enteropeptidase fixed on brush border of the epithelial cell in the duodenum and jejunum at the tip pf the villi This enzyme converts: Trypsinogen → Trypsin Active trypsin then acts on inactive enzymes to make them active: Trypsinogen → Trypsin Chymotrypsinogen → Chymotrypsin Precarboxypeptidase → carboxypeptidases
Where is can enteropeptidase be located?
In the vili on the epethilal cells of duodenum and jejunum
Dislodged in the lumen by bile salts
What is contained in the zymogens other than enxymes? Function?
Trypsin inhibitor
prevent the premature activation of trypsin
What are some components of bile? Percentages?
Bile acid (65%)
Cholesterol(4%)
Phospholipids (20%)
Bile pigments(0.3%)
Which components of the bile is responsible for emulsifying lipids
Bile acid
Which components of the bile is responsible for improving digestion of lipids
Bile acid
Which components of the bile is responsible for forming mixed micelles
Bile acid
Cholesterol
Phospholipids
What is bile acid responsible for
emulsifying lipids
improving digestion of lipids
What is the bile acid, cholesterol and phospholipid responsible for?
form mixed micelles to aid absorption of fatty acids
What are micelles
Substance that carries products of fat digestion to the membrane of the villus epithelial cells where they’re gonna be absorbed
Where is the bile pigment from?
result of the breakdown products of hemoglobin (bilirubin)
What secretes bile?
Hepatocytes
Where is bile stored?
gall bladder
What is the enterohepatic circulation
bile from the gall bladder in fed state will be reabsorbed in jejunum
At illeum active reabsorption of bile acids
delivered into the splanchnic circulation
splanchnic circulation via the portal vein is going to return bile acids and bile salts to the liver
What mechanism is the recycling of bile acids
homeostatic
Why is recycling bile acids important
synthesis of bile acids is metabolically really expensive
small amount is synthesised recycled many times depending on foodstuff
efficient
Similarities and differences in structure if lining if large and small intestine?
Microvilli and Crypts- Large
Micro villi and Villi and crypts- Small
Small intestine longer, Higher area of surface
Small- Nutrient absorption Large-No
Small- No Large - Active K+ secretion
Both- HCO3- secretion and active Na+ absorption
Crypts:
Large- mucous secretion
Small- salt and water secretion
Short chain fatty acids absorbed by large
What is the fluid balance in the GI tract?
2L/day in mouth
Inflow (secretion)- all from small intestine Salivary glands Stomach Bile duct Pancreas Small intestine
Outflow Absorption
From small intestine 6.5L/day max 10
large intestine 1.9L/day max 15-20 L/day
Excretion from anus 0.1L/day
