1
Q
pancreatic secretions and biliary secretions
A
they are essential for digestion and are added to the small intestine lumen and mixed with the chyme right after it leaves the stomach
2
Q
what is another major physiological role of the pancreas
A
another major physiological role of the pancreas is to secrete pancreatic juice: a mixture of pancreatic enzymes and an aqueous alkaline secretion
3
Q
pancreatic enzymes
A
- The exocrine part of the pancreas is predominant and consists of grape-like clusters of secretory cells called acini, which connect to ducts that empty into the duodenum.
4
Q
Pancreatic acinar cells secrete three different types of pancreatic enzymes:
A
- Proteolytic Enzymes
- Pancreatic Amylase
- Pancreatic Lipase
5
Q
Proteolytic Enzymes 2
A
- The major proteolytic enzymes secreted are trypsinogen, chymotrypsinogen, and procarboxypeptidase.
- like pepsinogen, all are secreted in their inactive form.
6
Q
Trypsinogen 2
A
- converted to the active trypsin by enterokinase, which is embedded in the luminal border of the cells lining the duodenal mucosa.
- Within the secretory vesicles containing trypsinogen is trypsin inhibitor to protect the pancreas in case trypsinogen is accidentally activated
7
Q
Chymotripsinogen
A
converted to chymotrypsin by trypsin
8
Q
Procarboxypeptidase
A
converted to carboxypeptidase, also by trypsin
9
Q
Pancreatic Amylase 3
A
- Similar to salivary amylase, pancreatic amylase converts polysaccharides into the disaccharide maltose.
- It hydrolyzes starches, glycogen, and most other carbohydrates.
- The exception is cellulose.
10
Q
Pancreatic Lipase 2
A
- This is the only enzyme within the entire digestive system that can digest fats.
- It hydrolyzes triglycerides into monoglycerides and free fatty acids, both of which can be absorbed.
11
Q
control of the pancreatic exocrine secretion 3
A
- During the cephalic phase of digestion, there is a small amount of parasympathetically mediated paracrine secretion and during the gastric phase of digestion, there is a further small increased secretion due to gastrin.
- However, the bulk of pancreatic secretion is under hormonal control.
- The presence of chyme in the duodenum causes the release of secretin and CCK
12
Q
secretin 3
A
- The presence of acid causes the duodenal and jejunal mucosa to release secretin into the blood.
- The secretin then travels to the pancreas where it stimulates the duct cells to increase their secretions of the Na HCO3- rich fluid.
- The amount of secretin released is proportional to the amount of acid that enters the duodenum.
13
Q
CCK 3
A
- the duodenal mucosa releases CCK into the blood in response to fats in the duodenum blood and protein where it travels to the pancreas and stimulates the pancreatic acinar cells to increase digestive enzyme secretion.
- The presence of carbohydrates in the duodenum does not stimulate the release of CCK.
- since all three classes of pancreatic enzymes are stored together, a high protein meal does not preferentially release proteolytic enzymes
14
Q
most important role of the liver in digestion
A
production of bile salts which are important for fat digestion and absorption
15
Q
other functions of the liver 7
A
- Metabolic processing of fats, carbohydrates, and proteins after they have been absorbed
- Detoxification and or degradation of body wastes, hormones, drugs, and other foreign compounds
- Synthesizing plasma proteins
- Storage of glycogen, fats, iron, copper, and vitamins
- Involved in the activation of vitamin D
- Removal of old red blood cells and bacteria
- Excreting cholesterol and bilirubin
16
Q
liver blood flow 2
A
- Each liver cell, or hepatocyte, performs essentially the same tasks.
- In order to perform these tasks, each hepatocyte has to be in contact with both arterial blood and venous blood coming from the digestive tract
17
Q
hepatic portal vein
A
Each minute, about 1 L of venous blood flows from the digestive system, through the hepatic portal vein, into the liver to deliver anything that was absorbed
18
Q
hepatic artery
A
The hepatic artery supplies about another 350 ml of arterial blood to deliver oxygen and blood-borne metabolites
19
Q
lobules 4
A
- The functional unit of the liver are called lobules.
- These are hexagonal arrangements of tissue surrounding a central vein.
- At each of the six outer corners, a branch of the hepatic artery, a branch of the hepatic portal vein, and a bile duct are located.
- The bile ducts converge to form the common bile duct, which transports the bile from the liver to the duodenum
20
Q
sinusoids 2
A
- Blood from both the hepatic artery and portal vein flow from the periphery of the lobule through capillary-like structures called sinusoids.
- The sinusoids run between rows of liver cells towards the central vein
21
Q
what cells line the sinusoids and what do they do
A
Kupfer cells, which are macrophages, that remove old blood cells and bacteria
22
Q
The central vein of all liver lobules 3
A
- The central vein of all liver lobules converge to form the hepatic vein, which carries the blood away from the liver.
- Also located between the hepatic cells are the thin bile-carrying channels called bile canaliculus.
- Hepatocytes are constantly secreting bile into the canaliculus which carries the bile from the centre of the lobule to the periphery and into the bile duct
23
Q
what is each hepatocyte in contact with
A
Each hepatocyte is in contact with a sinusoid on one side and a bile canaliculus on the other side
24
Q
bile 4
A
- Bile is constantly produced, but it does not enter the duodenum unless active digestion is occurring.
- The opening of the bile duct into the duodenum is guarded by the sphincter of Oddi that is closed until bile is needed.
- When closed, secreted bile is diverted back up the common bile duct and into the gallbladder for storage.
- The gallbladder is capable of storing about 50 ml of bile and about 250 ml of bile are produced each day
25
composition of bile 2
- Bile is composed mainly of salts, cholesterol, lecithin, and bilirubin, in an aqueous fluid similar to the pancreatic Na HCO3 secretion.
- Bile salts are derivatives of cholesterol and are important for fat digestion
26
enterohepatic circulation
enterohepatic circulation is when almost all of the bile salts are reabsorbed in the ileum and transported back through the hepatic portal system to the liver, where they are recycled by secretion back into the bile.
27
4 steps of enterohepatic circulation of bile
1. Secreted bile salts consist of 95% old, recycled and 5% newly synthesized bile salts.
2. 95% of bile salts are reabsorbed by terminal ileum
3. Reabsorbed bile salts are recycled by enterohepatic circulation.
4. 5% of bile salts are lost in feces
28
structure of bile salts 5
- Act like detergents → emulsify fats.
- Break large fat globules → many small droplets (<1 mm).
- Increases surface area for pancreatic lipase.
- Structure: lipid-soluble side + negatively charged water-soluble side.
- This reduces surface tension and keeps fat droplets dispersed.
29
fat digestion and absorption 4
- Polar ends repel each other → prevents fat droplets from recombining.
- Keeps fats dispersed as an emulsion.
- Lipase alone cannot access fat because bile salts coat the droplets.
- Colipase (from pancreas) binds lipase + bile salts, anchoring lipase so it can digest triglycerides
30
micelles 4
- The structure of lipid-soluble compounds surrounded by water-soluble bile salts are called micelles.
- bile salts form these clusters with an outer lipophobic shell while maintaining a lipophilic core.
- The cholesterol is found within this core along with any digested fats and other lipid-soluble compounds.
- The micellular formation allows lipid-soluble substances to be transported within the chyme to their site of absorption.
31
bilirubin 2
- Bilirubin is not involved in digestion but instead is a waste product that is secreted in the bile.
- It primarily comes from the degradation of haemoglobin in old red blood cells.
32
how the colour of bilirubin plays a role in the body’s appearance 3
- Bilirubin is yellowish in colour but once it is in the digestive tract, bacterial enzymes, mainly found in the large intestine, modify its colour to the brown colour associated with feces.
- In the absence of bile secretion, the feces would be greyish white.
- Most bilirubin is excreted in the feces but some is reabsorbed in the small intestines where it is eventually excreted in the urine, which is why urine has a yellow colour to it
33
chemical regulation of bile secretion 2
- The most potent chemical signal is bile salts themselves.
- When bile salts are reabsorbed and return to the liver via the enterohepatic circulation, they stimulate the further release of bile.
34
hormonal regulation of bile secretion
Secretin stimulates the secretion of the aqueous alkaline component of bile but does not stimulate the release of bile salts
35
neural regulation of bile secretion
During the cephalic phase, vagal stimulation of the liver promotes the release of bile before food reaches the stomach or intestine
36
bile storage in the gallbladder 2
- Active transport of salt out of the gallbladder, with water osmotically following it, concentrates the bile 5 to 10 times.
- CCK both causes the gallbladder to contract and causes the sphincter of Oddi to relax allowing bile to flow into the duodenum
37
morbidity (condition) of the gallblader 3
- Because the gallbladder concentrates bile, the bile salts can precipitate and form gallstones.
- However, the gallbladder can be removed with no negative effects on digestion.
- Bile is still produced but instead of being stored in the gallbladder, excess bile is stored in the common
bile duct, which dilates
