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Number 324
The Pancreas The pancreas is a 15cm long gland positioned in the ‘U’ bend of the duodenum. It is connected to the lumen (space) inside the duodenum by the pancreatic duct. The liver and pancreas share part of the pancreatic duct (Fig. 1).
Fig. 1 Position and structure of the pancreas Gall blader stores bile Liver
Bile duct carries bile from liver
Pancreas
Network of ducts ducts in the pancreas pancreas
Opening of pancreatic duct into duodenum
Common duct shared by liver and pancreas
Duodenum - the fi r st st part of the small intestine
Glands secrete enzymes or hormones. There are two types of gland: Exocrine glands e.g. salivary glands pour their secretions down a tube or duct directly into the target area. Endocrine glands e.g. adrenal glands are ductless and pour their hormones directly into the blood.
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The pancreas is unusual because it works as both an exocrine and an endocrine gland. Fig.2 shows the dierences between exocrine and endocrine glands.
Fig. 2 Exocrine and endocrine glands
blood cap illaries surround the secretory cells
duct
secretion poured into the blood
secretion pou ring out of gland Exocrine gland
cells lining the gland create the s ecretion
1
Endocrine gland
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Fig. 3 Endocrine and exocrine functions of the pancreas Pancreas
Pancreas
Endocrine
Exocrine
secrete glucagon
acinar cells Carbohydrases Nucleases (nucleic acids)
Alpha cells
Lipases Beta cells
Alkaline pancreatic juice (HCO3-)
secretes insulin
Role of the Pancreas
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The pancreas produces enzymes which play a role in digestion. This is how it acts as an exocrine gland (Fig.4). It also produces two hormones which regulate the blood sugar levels. This is its endocrine function.
Pancreatic amylase. This digests starch into maltose.
Pancreatic juice is alkaline because it also contains bicarbonate ions. These ions create the correct alkaline conditions in the duodenum. The alkali neutralizes the acidic gastric juice from the stomach creating the optimum pH for the enzymes to work. The secretion of pancreatic juice is regulated by hormones. These hormones are secreted by the walls of the duodenum when food enters from the stomach. Two hormones are produced. These are: Secretin This is secreted by the walls of the duodenum. The acidic contents from the stomach entering the small intestine stimulates the duodenal cells to produce secretin . Like all hormones, secretin is released into the blood. It is transported to the exocrine cells in the pancreas. Secretin stimulates the acinar cells to release their secretions into the pancreatic duct.
Only about 5% of the pancreatic tissue are endocrine cells. These cells are found in small clusters called islets of Langerhans. The islets are scattered throughout the pancreas. Islets can only be recognised by staining the tissues and examining them under a microscope.
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Fig. 4 Exocrine cells in the pancreas pancreatic exocrine cells cells
bile duct
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pancreatic duct
Proteases
islets of Langerhans scattered through the exocrine cells
Cholecystokinin The duodenal cells secrete another hormone called cholecystokinin or CCK . This is produced in response to the arrival of partially digested food. CCK acts on the exocrine cells of the pancreas and stimulates the secretion of a juice which is rich in the pancreatic hormones.
The Pancreas as an Endocrine Organ The cells of the islets of Langerhans synthesise the hormones which regulate blood glucose levels. There are two dierent type of cell in the islets. The alpha cells which secrete glucagon The beta cells which secrete insulin
duodenum
The Pancreas in Digestion
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Acinar cells surrounding the network of ducts in the pancreatic tissues secrete pancreatic juice. This alkaline liquid contains digestive enzymes. The ducts eventually join to form the main pancreatic duct which opens into the duodenum. Pancreatic enzymes complete the digestion of the food passing through the duodenum. The enzymes found in pancreatic juice are: Trypsin, chymotrypsin and carboxypeptidase These are proteases. These complete the digestion of proteins and small chains of polypeptides into amino acids. Lipase The enzyme lipase digests fats, breaking them down into fatty acids and glycerol.
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Alpha and beta cells can only be distinguish by careful staining. The beta cells form dark crystalline deposits whilst the glucagon-producing glucagon-producing alpha cells are much less visible.
Bloods sugar regulation
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The concentration of glucose in the blood needs to be kept at a relatively constant level. This is between 80 – 120 mg of glucose per 100cm 3 of blood. After a meal the blood glucose level rises and needs to be brought back to the optimum concentration. This is achieved through the action of insulin.
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Stored glycogen is broken down into glucose. This is called glycogenolysis. Glucose is created from other compounds. This is gluconeogenesis. • Fatty acids are metabolised to release energy instead of glucose. • These eects of glucagon raise the blood sugar levels.
If the blood glucose level drops, the cells may run short of glucose for respiration. A rise in the blood glucose levels is brought about by the hormone glucagon. Insulin and glucagon work antagonistically i.e. they have opposite eects to each other.
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Insulin is a small protein hormone. When the blood ows through the pancreatic islets, the alpha and beta cells monitor the glucose levels. If the blood glucose levels are too high, there are two outcomes: The beta cells start secreting insulin. The alpha cells stop producing glucagon.
The interplay between the insulin and glucagon help to maintain the blood glucose levels within normal parameters. This is an example of a homeostatic mechanism. It is controlled by a negative feedback system.
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For example, when blood glucose levels are high, insulin is produced. The insulin lowers the blood sugar level. The lowered blood sugar levels switch o the insulin.
Insulin has many target cells but the main ones are the liver and muscles. The role of insulin is to lower the blood glucose level. This is achieved by: Stimulating cells to increase their uptake of glucose Increasing the rate of respiration using glucose as a respiratory substrate Stimulating the rate at which glucose is converted to insoluble glycogen. The glycogen acts as a store of glucose. •
High blood glucose
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Pancreatic beta cells
The eects of insulin cause the blood sugar levels to fall. This drop is detected by the alpha and beta cells in the islets. The lowered blood sugar level has two eects: The beta cells stop secreting insulin The alpha cells start secreting glucagon. Glucagon is transported to the liver which is its target organ. (Muscle cells do not respond to glucagon.) In the liver, glucagon has the following eects:
Insulin secreted
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low blood glucose levels switch o the insulin production
Target Target cells liver and muscles
Low blood glucose
Use the diagram below to explain why regulation of blood glucose levels is an example of negative feedback. If blood glucose levels fall……………………………………
Fig. 5 Insulin
Body cells take up more glucose Liver takes up glucose and strores it as glycogen
Beta cells of pancreas release inslulin into the blood
Stimulus Blood glucose level rises
Blood glucose level declines
Homeostasis Blood gluose level (70 - 110 mg/100 mL)
Blood glucose level rises
Stimulus Blood glucose level falls Alpha cells of pancreas release glucagon into the blood
Liver converts glycogen back to glucose into glucose into the blood Glucagon
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Controlling insulin secretion Fig. 6 Shows how high levels of glucose molecules stimulate the release of insulin from a beta cell. 1. Glucose Glucose enters enters the the beta cell of of the islet. islet. 3. ATP levels levels rise rise and this causes causes the potassium ion (K +) channels to close.
ATP
protein carrier in cell surface membrane
4. K ions accumulate in the cell. This changes the cell membrane potential to - 30mv. +
ATP
ATP AT P ATP
ATP
2. The glucos glucosee has has phosp phosphat hatee added (phosphorylated) and the glucose phosphate is metabolised to ATP.
pottassium ions K +
5. The calcium calcium ion channe channels ls are voltage voltage gated. gated. The change in the membrane potential causes these channels to open. Calcium ions flood into the cell.
vesicles of insulin 6. Calciu Calcium m ions ions cause cause the vesicles of insulin to move to the cell surface membrane. The vesicles fuse with the membrane and the contents spill out by exocytosis.
calcium ions Ca2
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Exam Hints:- Check, using the Specication, whether you need to know the terms glycogenolysis (conversion of glycogen to glucose) and gluconeogenesis (production of glucose from noncarbohydrate sources e.g. amino acids) . If you do, don’t mix them up! Don’t confuse either with glycolysis. Remember, it isn’t the glucagon itself that converts glycogen into glucose, it’s breakdown is catalysed by the enzyme glycogen phosphorylase.
Type 2 diabetes or non-insulin dependent diabetes This condition develops later in life but is often associated with obesity.
Type 2 diabetics secrete insulin but the target cells are unaected by it. This is because the specific insulin receptors on the target cells do not respond to insulin in older people. In some cases the beta cells also fail to secrete insulin. Treatment involves: Controlling the diet and reducing the intake of carbohydrates. o o Slowing the absorption of carbohydrates using drugs Administering insulin injections if necessary. o
Diseases of the Pancreas Diabetes mellitus Also known as sugar diabetes, this disease has two forms: Type 1 or insulin dependent diabetes • This develops when the pancreas fails to secrete insulin. It can develop at any age but often occurs in younger people.
There is a genetic link to this type of diabetes. It is an example of an auto-immune disease where the body’s immune system attacks the healthy beta cells.
Symptoms of both types of diabetes include weight loss and extreme thirst. This is because the blood glucose level remains very high after a meal. The kidney cannot reabsorb all this excess glucose. Consequently, glucose is lost in the urine. Water and salts are also lost with the excreted glucose.
Pancreatitis Type 1 diabetes is treated by regular monitoring of the blood sugar levels and injections of insulin.
In pancreatitis, the enzyme trypsin, which is normally secreted in an inactive form, becomes active inside the cells of the pancreas and starts digesting the tissue. The damaged cells then release trypsin and amylase into the blood and their presence may be used in the detection of the early stages of pancreatitis.
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324 The Pancreas
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Practice Questions 1. The pancreas acts both as an exocrine and an endocrine gland. Describe its: (i) endocrine function (3) (ii) exocrine function (3) 2. Suggest why some of the enzyme-producing cells of the pancreas contain many: (i) ribosomes (1) (ii) mitochondria (2)
levels. (6) 3. Describe the role of the pancreas in regulating blood glucose levels. 4. Explain how measurements of the levels of pancreatic enzymes such as amylase in the blood and faeces may be used to identify pancreatic disease. disease. (2)
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Acknowledgements: Acknowledgements: This Factsheet was researched and written by Margaret Royal. Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU. Bio Factsheets may be copied free of charge by teaching sta or students, provided that their school is a registered subscriber. No part of these Factsheets may be repro reproduced, duced, stored stored in a retrieval system, system, or transmitted, in any other other form or by any other means, without without the prior permission of 1351-5136 the publisher. ISSN 1351-5136
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