Pancreatic juice| Revised| Digestion |

    

Pancreatic Juice

In this topic, we will learn about pancreatic juice. This article is beneficial for students of medical sciences as well as people suffering from indigestion who want to know about human physiology.

Upon completing the article, you will have increased knowledge regarding the subject and use it with great confidence.

Keywords: Pancreatitis |Trypsinogen | Trypsin inhibitors | enterokinase | Proenzymes|  Chymotrypsinogen| isotonic | exocrine | endocrine|

Table of contents

1.	Introduction
2.	Composition
3.	Mechanism of  Secretion
4.	Regulation of secretion
5.	Functions
6.	Clinical
7.	Links

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                                            Introduction

The pancreas is a dual organ. It is a mixed gland -it works as an endocrine and an exocrine organ. The word pancreas comes from two Greek words, pan, meaning all, and creas, meaning flesh. Therefore, the pancreas is all flesh. In an adult, its length is about 12-15 cm. It appears lobulated and salmon-colored in living conditions. 

It is important to remember that the exocrine part of the pancreas synthesizes and secretes digestive enzymes into the duodenum. The exocrine portion of the pancreas has acinus, duct, pancreatic duct cells, blood vessels, nerves, and connective tissues. The exocrine part comprises more than 95% of the pancreatic mass. 

Relation of Pancreas, Duodenum, GB, and liver
This picture is licensed under a creative commons attribution).

   Pancreatic juice is the secretion of the exocrine part of the pancreas. 

                                       Daily secretion   

Daily secretion: 1000-1500 ml/day.

Pancreatic juice is transparent and isotonic with plasma.

Its specific gravity is 1010-1018, and its pH is 7.8-8.7, alkaline due to a high concentration of bicarbonate ions.

Composition of pancreatic Juice :

Water is 99.5%

Inorganic:

The main anions are  HCO₃^-and Cl ^-., Other anions are also in trace amounts. The concentrations of HCO₃^-and Cl ^– anions depend upon the secretion rate of pancreatic juice. When the pancreatic juice secretion increases, the concentration of HCO₃^-  increases, and the Cl ^– concentration decreases and vice versa. So that The total concentration of HCO₃^-and Cl ^– anions remain constant.

  The main cations are  Na⁺  K⁺   Ca⁺⁺  Mg++. Their concentrations did not depend on the rate of secretion of pancreatic juice. The concentrations of  Na⁺  K⁺ are almost identical to those of plasma.

Organic: Enzymes: 1. Pancreatic α- amylase  2. Pancreatic lipase,       3. Pancreatic esterase 4. Pancreatic pro phospholipase A.

Proenzymes: Trypsinogen, Chymotrypsinogen, Ribonuclease, and pro-elastane.

Trypsin inhibitors are also secreted from acinar cells.

Some mucin is present.

Phases of pancreatic juice secretion

Pancreatic secretion  3 phases

1. Cephalic –mediated by the vagus nerve. The neurotransmitter acetylcholine increases acinar cell secretion and is responsible for  25% of the daily secretion of pancreatic juice.

Acidic chyme in the duodenum stimulates acinar secretion and ductal bicarbonate secretion.

2. Gastric phase: the meal in the stomach, especially distention of the stomach, stimulates the secretion of enzymes with the secretion of water and bicarbonate. Is responsible for  10 % of the daily secretion of pancreatic juice.

 

3. Intestinal phase, when chyme enters the duodenum, the intestinal phase starts, which is mediated by hormones and pancreatic vasovagal reflexes. Ductal secretion increases, causing a large amount of water and bicarbonate secretion. Is responsible for  65% of the daily secretion of pancreatic juice. Hormones secretin and CCK. Secretin stimulates water, bicarbonate, and other electrolytes in pancreatic juice.

 Regulation of pancreatic secretion :

The pancreatic exocrine secretion is controlled by

1. Nervous and 2. Humoral.

Nervous: The Vagus nerve exerts a parasympathetic effect. Its stimulation causes enzyme-rich pancreatic juice secretion, mediated by acetylcholine released at the vagus nerve end. Acetylcholine activates phospholipase C to cause increased secretion of pancreatic juice from the acinar cells.

Reflex stimulation also increases the secretion of pancreatic juice by

 (a) conditioned reflex sight and smell of food and

 (b) unconditioned reflex chewing and swallowing. Again, the vagus mediates this reflex.

Humoral:

Secretin:  

 Secretin is the first hormone identified 100 yrs ago. Secretin secretion is stimulated by acidification of the duodenal contents. Secretin receptors are members of the G protein-coupled receptor superfamily located in the basolateral membrane of all pancreatic ductal cells. Secretin stimulation increases :

(1)  Intracellular cyclic adenosine monophosphate activates the HCO3- Cl- anion exchanger in the apical membrane of the pancreatic duct cells.

(2)  Increases activity of carbonic anhydrase enzymes, the excretion of H ion outside the duct cell, and the activity of the CFTR.

S cells in the small intestine's upper part secrete the prosecretin. Gastric HCl and fatty acid salts convert prosecretin to its active form-secretin. Secretin stimulates the pancreatic duct cells to produce alkaline, watery pancreatic juice. An increase in intracellular cAMP mediates the effect. It also stimulates bile secretion and potentiates the effect of (CCK-PZ).

Cholecystokinin pancreozymins( CCK-PZ)

CCK secretion is stimulated by lipids, proteins, and carbohydrates in the duodenum and causes the release of pancreatic enzymes. CCK potentiates the effects of secretin and vice versa.

	secretin	CCK
1. Secreted by	S cells in the duodenum	I cell in the duodenum.
2. stimulated by	The acid in the duodenum	Food –lipid, protein, and carbohydrate.
3. stimulate secretion of	Water and electrolyte	Enzymes

 

Functions:

1. The pancreatic Juice with alkaline bile neutralizes the gastric chyme from the stomach and raises the duodenal contents' pH.

2. The alkaline media of the duodenal contents provide proper media for pancreatic and other enzymes' actions.

3. Digestive functions:  

Pancreatic α- amylase splits the α -1-4 glycosidic bond of starch( boiled or unboiled ) to maltose. However, the salivary α- amylase does not break the α -1-4 glycosidic bond of unboiled starch.

Pancreatic lipase hydrolyses neutral fats to glycerol and fatty acids.

Another type of pancreatic lipase activated by bile salts is 'Bile salt-activated lipase.' which catalyzes the hydrolysis of cholesterol esters, phospholipids, and triglycerides.

Proteolytic enzymes :

Endopeptidases cleave the peptide bonds present in the protein molecule. E.g., chymotrypsin, and trypsin.

Exopeptidases work on the terminal peptide bonds. One end is the free carboxyl group in the peptide bond, and another is the free ammonical group.

Carboxypeptidase A and B -It splits the peptide chain by the stepwise removal of amino acid residue from the free carboxyl group at the end of the polypeptide chain.

  Aminopeptidase: Exopeptidse that cleaves a single amino acid from the amino terminals of the polypeptide chain is called Aminopeptidase.

 Enterokinase secreted by duodenal mucosa converts trypsinogen into the active form of trypsin. Once formed, trypsin will also convert trypsinogen into trypsin–autocatalyst action. Trypsin is a potent proteolytic enzyme that hydrolyses proteins by splitting bonds containing L-lysine and L-arginine in which ԑ amino or guanidino groups are free. Therefore proteins are digested into small polypeptides and some amino acids.

Trypsinogen-----> enterokinase & trypsin----------> trypsin

Chymotrypsinogen is converted into chymotrypsin (active form) by trypsin. It hydrolyses proteins into small polypeptides.

Chymotrypsinogen----->trypsin-------------> chymotrypsin

Enterokinase and trypsin activate Pro- carboxypeptidase A and B in carboxypeptidase A and B, respectively. Carboxypeptidase A and B  split the peptide chain by the stepwise removal of amino acid residue from the free carboxyl group at the end of the polypeptide chain.

Pro carboxypeptidase A and B ---> enterokinase& trypsin--> carboxypeptidase A and B

Proelastage is converted into its active form, elastage, by trypsin and digesting elastin and some other proteins into simple substances.

Proelastage---------> Trypsin ----------->  elastage

Deoxyribonuclease splits nucleic acids of ribose and deoxyribose into nucleotides.

Trypsin inhibitors secreted by acinar cells protect the pancreas from proenzymes.

4. Diagnostic:

The pancreatic juice secretion is more in the case of pancreatic carcinoma. Therefore, it may be a biomarker of PC[pancreatic carcinoma].

Proteins associated with Parkinson's disease –aSyn and PARK7 are increased in Parkinson's disease and in the pancreatic juice. 

Clinical : Pancreatitis: is inflammation of the pancreas. It occurs when digestive enzymes start digesting the pancreas. It may be acute or chronic-both are severe conditions. 

Chronic pancreatitis is pancreatitis that persists for more than three weeks. Pancreatitis may be due to heavy alcohol intake, cystic fibrosis, or high level of calcium or fat in the blood. In addition, it may be due to an autoimmune disorder.

The pancreas stores many digestive enzymes; injuring the pancreas is potentially fatal due to pancreatic juice production, causing pancreatic self-digestion. In addition, many factors cause increased pressure in the pancreatic duct. As a result, the pancreatic duct may rupture, and pancreatic Juice leakage will cause autodigestion.

Pancreatic cancer.

Hashtags# Pancreatitis# Trypsinogen# Trypsin inhibitors# Chymotrypsinogen# enterokinase

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 Internal link:

https://totalphysology20.blogspot.com/2021/03/small intestine  we must  know: 

https://totalphysology20.blogspot.com/2021/04/stomach  we must  know

https://totalphysology20.blogspot.com/2021/05/protein digestion and absorption, we must  know

https://totalphysology20.blogspot.com/2021/05/ Pancreas an introduction- we must  know

 External link:

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https://en.m.wikipedia.org>wiki

https://www.medicalnewstoday.com

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