Gastrointestinal Physiology Flashcards
1
Q
Where does the digestive system start?
A
The digestive system begins in the mouth where the process of nutrient digestion begins.
2
Q
What is the function of the oesophagus?
A
From the mouth, the digestive system continues into the oesophagus.
Which acts as a transport mechanism for food and fluid to enter the stomach.
3
Q
What are the 4 regions of the stomach?
A
The cardiac region
The fundus
The body
The pyloric region.
4
Q
What is the function of the stomach?
A
The stomach acts as a reservoir for food and fluid, breaking down solid material, allowing further digestion to occur and ensuring regulated movement of nutrients into the small intestine.
5
Q
What does the small intestine consist of?
A
The duodenum
The ileum
The jejunum
6
Q
What is the main function of the small intestine?
A
Absorption of nutrients.
7
Q
What does the large intestine consist of?
A
The caecum and the colon. The colon is separated into the ascending, transverse, descending and sigmoidal sections of the colon.
8
Q
What is the main function of the large intestine?
A
Transport, with a limited amount of absorption (mainly water.)
9
Q
What are the rectum and anus responsible for?
A
Elimination of waste products.
10
Q
What are some other associated organs of the digestive system?
A
The liver, gall bladder and pancreas.
These are involved in the digestion of nutrients and fats.
11
Q
All areas of the digestive tract have 4 layers:
What are they?
A
- The serosa
- The muscularis
- The submucosa
- The mucosa
12
Q
What is the mucosal layer of the digestive system?
A
It is the innermost layer and therefore closest to the lumen (the inside space) of the tract.
13
Q
The mucosal layer consists of a layer of what?
A
Epithelial cells and loose connective tissue (called the lamina propria.)
A layer of smooth muscle cells, called the muscularis mucosa is also present.
14
Q
What does the submucosal layer consists of?
A
Dense connective tissue.
This provides support to the mucosal layer and anchors the muscularis layer.
15
Q
What is the submucosal layer also home to?
A
Submucosal plexus which forms part of the intramural plexus and is strongly involved in regulation of function.
16
Q
What is the cross section of the digestive system showing the serosa, muscularis, submucosa and mucosa?
A
Figure 8.1 PG 117
17
Q
What does the muscularis layer consist of ?
A
Mainly of layers of circular and longitudinal smooth muscle fibres.
It also contains the myenteric plexus, which forms the other part of the intramural plexus.
18
Q
What is the serosa?
A
The serosa consists of connective tissue and is the outermost layer of the digestive system wall.
19
Q
The digestive system is rich in what?
A
Nerve and blood supply.
20
Q
The regulation of gastrointestinal function is a complicated system,
What does it involve?
A
- The central nervous system
- The enteric nervous system
- Gut hormones acting by circulating or paracrine mechanisms.
21
Q
How does the smooth muscle play a role in regulation of gastrointestinal function?
A
It involves control of smooth muscle contraction and relaxation which affects motility, blood flow and other aspects of gastro intestinal function.
22
Q
How does sympathetic nervous activity work? regarding regulation gastrointestinal function
A
It usually uses noradrenaline as a neurotransmitter and results in the relaxation of smooth muscle, a reduction in blood flow and contraction of sphincters.
23
Q
How does parasympathetic nervous activity work? regarding regulation gastrointestinal function
A
Parasympathetic nervous activity usually uses acetylcholine as a neurotransmitter and results in the contraction of the smooth muscle, an increase in blood flow and relaxation of sphincters.
24
Q
What does the enteric nervous system allow?
A
Changes in gastrointestinal function to occur in response to changes in the local environment.
25
The enteric nervous system consists of 2 enteric plexuses
-The myenteric plexus
-The submucosal plexus
26
What are the roles of neurons in the gastrointestinal regulation?
- Sensory neurones detect changes in the local environment such as temperature , osmolality and motility.
- Motor neurons within the plexus respond to these changes by altering the function of smooth muscle and other factors that affect gastrointestinal function.
- Interneurons are responsible for collating information from sensory neurones and communicating to motor neurones.
27
What is the main neurotransmitter used by enteric nerves?
Acetylcholine
28
What actually is the gastrointestinal system?
It is the largest endocrine organ in the body and produces a large number of hormones that act as both autocrine and paracrine agents.
29
Release of a number of hormones result in numerous effects on other body systems examples include
- Ghrelin
- Peptide YY
Ghrelin is secreted mainly from the fundus of the stomach and acts on the hypothalamus leading to an increase in appetite.
It also acts with growth hormone releasing hormone to increase secretion of growth hormone from the anterior pituitary
Peptide YY is a hormone secreted from the ileum and the colon which results in a reduction in appetite.
30
Release of a number of hormones result in numerous effects on other body systems examples include
Secretin
Gastrin
Motilin
Secretin released from proximal small intestine and acts on pancreatic ductal cells to increase bicarbonate secretion.
Gastrin secreted from G - cells in stomach and duodenum. acts on parietal cells to increase hydrochloric acid secretion.
Motilin secreted from M - cells in small intestine and acts on motilin receptors in the body of the stomach resulting in increased smooth muscle.
31
What is the migrating myoelectric complex?
Wave of smooth muscle contractions throughout the stomach and intestines that occurs when an individual has fasted.
Its purpose is to enhance nutrient absorption as well as removing bacteria from intestines.
Food ingestion interrupts it.
32
What is the process of mastication?
During this process, food is broken into smaller pieces, which assists in the digestive process by providing a greater surface area for enzymes to act on.
33
What is the main enzyme involved in nutrient digestion in the mouth?
Salivary amylase.
34
What is the oesophagus guarded by?
The upper oesophagus sphincter. Sphincters usually allow one directional movement of material.
35
Where is the lower oesophageal sphincter located?
The lower oesophageal sphincter is located in the lower part of the oesophagus at the entrance to the cardiac region of the stomach.
36
What is the lower oesophageal sphincter?
It is the centre of a variety of disorders including oesophageal achalasia and reflux oesophagitis
37
What is oesophageal achalasia?
Progressive disorder which results from an inability to relax the lower oesophageal sphincter.
This may be due to problems with the enteric nervous system.
As a result, ingested food and fluid remains in the oesophagus.
38
What is reflux oesophagitis?
Results from reduction in tone, or transient relaxations in the lower oesophageal sphincter. As a result, reflux of material from the stomach is possible which may lead to erosion of mucosa causing chest pain.
39
What are physical and chemical mechanisms of the stomach?
Physical mechanisms are the result of smooth muscle contraction and consequent movement of food.
Chemical mechanisms are largely due to secretion of hydrochloric acid and enzymes such as pepsinogen.
40
What are parietal cells?
Parietal cells are responsible for the secretion of hydrochloric acid and intrinsic factor (A glycoprotein)
These cells are mainly found in the body of the stomach.
41
Where is the location of key cells in the stomach?
Fig 8.3 PG 120
42
What are chief cells?
Chief cells are responsible for the secretion of pepsinogen (an enzyme involved in the digestion of proteins).
These cells are mainly found in the body of the stomach and are activated in response to increases in secretion of hydrochloric acid and gastrin.
43
What are G - cells?
G - cells are responsible for the secretion of gastrin and are mainly found in the pyloric region of the stomach.
They are activated in response to vagus nerve activity, and the secretion of gastrin results in increased secretion of hydrochloric acid from parietal cells and pepsinogen from chief cells.
44
Where are mucus and bicarbonate secreted from?
All cells in all regions of the stomach
45
What are rugae?
Rugae, which are folds in an organ, can be seen in all areas of the stomach. This ensures the stomach can expand in response to food and fluid ingestion.
46
What follows the ingestion and swallowing of food?
The smooth muscle of the body of the stomach relaxes (receptive relaxation.) This allows an increase in stomach volume without a resultant increase in intragastric pressure.
47
What is gastric emptying?
Term given to the process of movement of food (or chyme) or fluid from the stomach into the small intestine.
48
In order for gastric emptying to occur what must happen?
Intragastric pressure has to increase while the pyloric sphincter and duodenal smooth muscle have to relax.
49
What is the pyloric sphincter?
The sphincter that guards the entrance from the stomach to the duodenum.
50
How is an increase in intragastric pressure achieved?
Through a contraction of stomach smooth muscle. This is the result of increases in the CNS activity and the effects of numerous circulating hormones.
Relaxation of the pyloric sphincter is achieved through changes in vagal nerve activity.
51
Why is the gastric emptying rate of solids longer than that of liquids?
Because of the digestion phase which is required for solids but not in liquids and also because of the reduced friction of liquids across the pyloric sphincter.
52
What are factors that affect gastric emptying rate?
Volume of chyme present,
Nutrients,
pH,
osmolality
exercise
53
What are proton pumps?
Proton pumps are a key component of the mechanism off hydrochloric acid secretion therefore they have to become embedded in the cell membrane before secretion can occur.
The movement of proton pumps into the cell membrane is regulated by the actions of gastrin, acetylcholine and histamine.
54
What does histamine do?
Histamine acts on histamine receptors to increase the cytosolic level of cyclic AMP which in turn activates phosphokinase A.
55
Acetylcholine and gastrin act on muscarinic and cholecystokinin B (CCK2) receptors resulting in an increase in what?
Cytosolic calcium concentration. This activates phosphokinase C.
56
How are hydrogen ions and bicarbonate formed?
How does this result in hydrochloric acid?
From the reaction of water and carbon dioxide.
Bicarbonate is shuttled out of the cell in exchange for chloride. Hydrogen ions are pumped out of the cell in exchange for potassium and chloride diffuses into the lumen, resulting in hydrochloric acid.
57
What leads to ulceration?
Epithelial layer of the stomach can withstand large amounts of hydrochloric acid however, reduced barrier function or excess secretion may lead to ulceration.
58
How long is the duodenum?
Approximately 30cm long
59
How long is the jejunum?
Approximately 2.5m long
60
How long is the ileum?
Approximately 3.5m long
61
What is the main purpose of the small intestine?
Digestion and absorption of nutrients with the vast majority occurring in the duodenum and jejunum.
62
Where does the duodenum receive pancreatic juice and bile from?
The Ampulla Of Vater as well as chyme from the stomach.
63
What marks the end of the ileum?
Ileocecal valve
64
The small intestine contains numerous folds
The mucosal layer of the wall projects into the villi
What is the villi?
Each villus contains a layer of epithelial cells that also project from the brush border membrane. This leads to a very large surface area for absorption to occur.
65
At the lower end of several clustered villi is a Crypt of Lieberkühn
What is the Crypt of Lieberkühn?
Where new epithelial cells are produced which replace cells shed from the villus tip.
The dead cells are moved into the lumen which provides brush border enzymes that are involved in the digestive process.
66
What is a lacteal?
Each villus also contains a lacteal (a lymphatic capillary) and an extensive capillary network.
Long chain fat is absorbed into the lacteal and into the lymphatic system.
67
Where are carbohydrates and proteins absorbed?
Into the capillary network and are transported to the liver before blood is returned to the heart.
68
What are the duodenal glands?
The duodenum also contains duodenal glands which are responsible for secreting mucus.
Bicarbonate within the mucus counteracts the acidity of chyme from the stomach, ensuring protection of epithelial cells in the duodenum.
69
What does the epithelial cells of the villus secrete?
The epithelial cells of the villus secrete a number of ions, including sodium and chloride into the intestinal lumen.
The increase in osmolality within the lumen leads to secretion of water into the small intestine.
This water is reabsorbed further down the intestinal tract.
70
How does water move in the intestine?
Movement of water within the intestine is as a result of concentration gradients.
If the contents of the intestinal lumen are less concentrated than the blood, water will move from the lumen into the blood.
If the contents of the lumen are more concentrated than the blood, water will move into the lumen.
71
What occurs following movement into the duodenum?
Following movement into the duodenum, chyme is moved along the intestinal tract by peristaltic contractions.
These are caused by pacemaker potentials that are exhibited in some smooth muscle cells within the small intestine.
72
What can increase or decrease smooth muscle contraction?
Activity of the enteric nervous system or CNS or the action of a number of hormones can increase or decrease the rate of smooth muscle contraction.
This results in increased or decreased intestinal motility.
73
What is the function of the ileocecal valve?
The ileum and the caecum are separated by the ileocecal valve. This is not a true sphincter but exists to prevent backflow from the large intestine to the small intestine.
74
Why does the large intestine have limited capacity for nutrient absorption?
It has no folds or villi.
75
Where are sodium and chloride reabsorbed?
In the large intestine with some resultant increase in water absorption.
Potassium and bicarbonate are secreted into the large intestinal lumen.
76
Smooth muscle contractions occur at a slower rate in the large intestine.
What does this result in?
Slower transit through this section of the digestive system.
77
What is the internal and external anal sphincter composed of?
The internal anal sphincter is composed of smooth muscle.
However the external anal sphincter is composed of skeletal muscle and thus is under voluntary control.
78
What is the purpose of very large numbers of bacteria colonizing the large intestine?
Some bacterial strains metabolize the contents of the intestinal lumen, which can lead to absorption of some - short chained fatty acids and vitamin K.
79
What does metabolism of some undigested compounds lead to?
Metabolism of some undigested compounds can lead to excess gas production and cause flatulence or diarrhoea.
80
What follows the movement of faeces from the large intestine into the rectum?
The increase in diameter causes contraction of smooth muscle in the rectum and relaxation of the internal anal sphincter.
The increase in pressure would eventually result in the relaxation of the external anal sphincter and defecation.
However this is avoided due to the voluntary nature of the skeletal muscle.
81
How are carbohydrates typically absorbed?
Carbohydrates are typically absorbed as monosaccharides.
The monosaccharides that are found in the human diet are glucose, fructose and galactose.
They all have the chemical formula C6 H12 O6 and are isomers of each other.
82
What are disaccharides?
Disaccharides are 2 monosaccharides bound together following a dehydration synthesis reaction. e.g. sucrose which consists of a glucose and a fructose molecule.
83
What are polysaccharides?
Multiple monosaccharides such as starch or cellulose formed as a result of multiple dehydration synthesis reactions.
84
How are disaccharides and polysaccharides digested?
They have to be broken down into their monosaccharides via a hydrolysis reaction so they can be absorbed into the small intestine.
85
How does the digestion of polysaccharides begin?
In the mouth with salivary amylase and the process is continued by amylases secreted from the pancreas.
86
How are disaccharides hydrolysed?
By enzymes secreted from the brush border membrane in the small intestine.
Sucrase hydrolyses sucrose and lactase hydrolyses lactose.
Lactose is the main sugar found in milk and consists of a glucose molecule and a galactose molecule.
87
What is lactose intolerance?
Lactose intolerance is a relatively common condition that results from a reduced ability to produce lactase.
Consequently when lactose is ingested, it is difficult to hydrolyse the disaccharide into glucose and galactose for absorption.
Lactose is then moved into the colon where bacteria metabolize the sugar, causing an increase in gas. The increase in osmolality within the colon leads to water movement.
As a result main symptom includes diarrhoea and flatulence.
Mange this condition by removal of dairy products.
88
What happens once disaccharides and polysaccharides are within the small intestine and have been hydrolysed to monosaccharides?
Once disaccharides and polysaccharides are within the small intestine and have been hydrolysed to monosaccharides there are 4 main barriers to absorption.
There is a layer of mucus prior to the apical membrane of the enterocyte.
89
What happens once carbohydrate crosses the apical membrane?
The enterocyte can use the carbohydrate as an energy source before it has to cross the basolateral membrane.
90
What does crossing membranes require?
The use of transporters.
These transporters can be active (require ATP) or passive (does not require ATP)
91
How is glucose and galactose transported across the apical membrane?
Glucose and galactose are actively transported across the apical membrane by sodium linked glucose transporter 1.
(It transports glucose while co - transporting sodium)
92
How is glucose actively transported across the basolateral membrane?
Glucose actively transported across the basolateral membrane by glucose transporter 2 (GLUT2)
It appears to have a limited or maximum capacity for absorption and its activity is altered by insulin secretion and amino acids.
93
How is fructose absorbed?
Fructose is passively absorbed by a transporter which is thought to be glucose transporter 5 (GLUT5) on the apical membrane and by GLUT2 on the basolateral membrane
94
What is the majority of dietary fats in the form of?
Triglycerides which consist of 1 glycerol molecule and 3 fatty acids.
They have to be absorbed seperately.
95
Where is the process of fat absorption?
In the mouth with lipases secreted from the tongue beginning the process of breakdown of triglycerides.
This is continued in the stomach as a result of emulsification and the action of lipases from the tongue and the stomach.
The process continues in the small intestine with the secretion of bile from the liver and the packaging of fat into acid - coated droplets.
This leads to a relatively large surface area and more efficient breakdown.
96
What is the job of pancreatic lipase?
Pancreatic lipase hydrolyses the fatty acids from triglycerides in preparation for absorption.
This takes place in the upper part of the jejunum.
97
What is the job of phospholipase?
Phospholipase, secreted from the pancreas, hydrolyses the fatty acids from phospholipids in preparation for absorption.
98
What happens to cholesterol from the diet and that which is secreted from bile?
Cholesterol from the diet and that which is secreted from bile has to be non - esterified to be absorbed.
About 50% of the cholesterol in the intestine is absorbed and the rest excreted.
99
How is esterified cholesterol broken ?
By cholesterol esterase into cholesterol.
100
What happens to free fatty acids, monoglycerol and cholesterol?
Free fatty acids, monoglycerol and cholesterol are packaged into mixed micelles which are absorbed into the enterocyte via diffusion.
101
Where are long chained triglycerides, cholesterol esters and phospholipids re - formed?
In the endoplasmic reticulum and the golgi apparatus of the enterocyte.
102
What are apolipoproteins?
Apolipoproteins are added to triglycerides, cholesterol esters and phospholipids to form chylomicrons, which are absorbed into lymphatic vessels.
103
Where are apolipoproteins C and E released from?
Circulating high density lipoprotein, which results in activation of lipoprotein lipase and the breakdown of triglycerides from the chylomicron and storage of fat.
104
What happens to the remaining chylomicron remnant?
It is transported to the liver where it is repackaged into very low density lipoprotein.
105
What happens when the very low density lipoprotein is released in the blood?
It is converted to intermediate density lipoprotein as a result of activation of lipoprotein lipase and resultant storage of triglyceride.
106
How is intermediate density lipoprotein converted to low density lipoprotein?
Through further storage of triglyceride.
107
What does high density lipoprotein do?
Transports cholesterol to the liver where it is eliminated.
108
What are proteins?
Proteins are chains of amino acids that can be of any length.
They do not necessarily have to be hydrolysed to a single amino acid to be absorbed.
Digestion and absorption of proteins occurs largely in the duodenum and jejunum.
109
Where is pepsinogen secreted from?
Chief cells in the stomach and converted to its active form of pepsin.
This begins the process of digesting proteins containing long chains of amino acids into shorter chains.
110
How is the process of digesting proteins continued in the small intestine?
The process is continued in the small intestine via the action of trypsin and chymotrypsin which are enzymes found within the pancreatic juice.
Enzymes from the brush border membrane also assist in this process.
The end result is a single or very short chains of amino acid.
111
How are short chains of amino acids transported?
Short chains of amino acids are are actively transported with hydrogen ions.
Once inside the cytosol of the cell, they are hydrolysed to their constituent single amino acid.
112
How does co transport of sodium occur?
With single amino acid absorption across the apical membrane.
113
What is the role of the gall bladder?
Stores bile secreted from the liver.
114
What is the locatiohn of the pancreas?
The pancreas lies behind the stomach and close to the duodenum. It has a distinctive shape which can be separated into the head (which is nearest to the duodenum) and the tail (which extends towards the spleen.)
115
How is the pancreas characterized?
The pancreas is characterized by a series of ducts, which ultimately drain into the duodenum via the duct of Wirsung (also known as the pancreatic duct) and the ampulla of Vater.
116
What does the exocrine portion of the pancreas secrete?
Pancreatic juice. This is a combination of ductal bicarbonate and digestive enzymes, such as pancreatic lipase (which assists in the digestion of lipids) and nucleases (which assist in the breakdown of DNA and RNA.)
These enzymes would not act properly without the secretion of bicarbonate as the acidity of chyme is too high.
117
Where is the liver located?
Right upper quadrant of the abdomen and therefore is situated above the duodenum and near to the pancreas.
118
What is the liver concerned with?
The liver is concerned with the production of bile. Bile contains bicarbonate (for neutralization of acidity of chyme), cholesterol, phospholipids and bile salts.
It plays an important role in the emulsification of lipids for absorption.
119
Where does the liver secrete bile?
The liver secretes bile into a duct system.
The right and left hepatic ducts form the common hepatic duct.
Bile can enter the cystic duct which enters the gall bladder.
120
What is responsible for the movement of bile?
Smooth muscle contractions are responsible for moving the bile into the common bile duct.
121
How is the movement of bile and pancreatic juice from the ampulla of vater into the duodenum controlled?
The movement of bile and pancreatic juice from the ampulla of vater into the duodenum is controlled by the sphincter of Oddi.
Relaxation of this sphincter, and consequent movement of bile and pancreatic juice into the duodenum also occurs in response to secretion of cholecystokinin (CCK)
