The alimentary tract Flashcards
(138 cards)
1
Q
What are the 5 parts/regions of the stomach?
A
The cardia, the fundus, the body, the antrum and the pylorus
2
Q
Which parts of the stomach can relax to accommodate for food that is yet to be digested?
A
The fundus and the body
3
Q
What’s the function of the antral region of the stomach?
A
The antrum is responsible for the mixing and grinding of food with gastric secretions
4
Q
What’s the function of the colon and rectum?
A
Storage of digestive residues and faeces
5
Q
What do gastric juices consist of?
A
Mucus, pepsinogen, intrinsic factor and lipase
6
Q
What’s the function of the mucus in gastric juices?
A
The mucus lubricates the stomach and colon
7
Q
What’s the function of the lipase in gastric juices?
A
Lipase digests triglycerides into fatty acids and glycerol
8
Q
What’s the function of the intrinsic factor in gastric juices?
A
The intrinsic factor helps with vitamin B12 absorption
9
Q
What’s the function of the pepsin derived from pepsinogen in gastric juices?
A
Pepsin is important for protein digestion
10
Q
Give an example of a paracrine secretion and its function
A
Somatostatin inhibits gastrin release in the stomach
11
Q
Give 4 examples of exocrine secretions
A
Gastrin
Secretin
Pancreozymin-cholecystokinin
Insulin
12
Q
What’s the function of gastrin and what releases it?
A
Gastrin is released from G cells in the pyloric antrum of the stomach, the duodenum and the pancreas. It acts to stimulate gastric acid secretion by parietal cells
13
Q
What’s the function of secretin and where’s it produced?
A
Secretin is a peptide hormone secreted from S cells of the duodenum and jejunum. Secretin is a hormone that regulates water homeostasis throughout the body, as well as regulating secretions in the stomach, pancreas and liver. Secretin inhibits gastric acid secretion and gastric contractions. It stimulates bicarbonate-rich solution secretion from the gallbladder and duct cells of the pancreas
14
Q
What’s the function of cholecystokinin and where does it act?
A
CCK, also known as pancreozymin, is a peptide hormone released from I cells of the duodenum and jejunum in response to partially digested lipids and proteins in the duodenum. It contributes, together with gastrin and secretin, to the gut hormone triad. It inhibits gastric emptying and stimulates acinar cells of the pancreas to release a juice rich in pancreatic digestive enzymes that catalyse the breakdown of fats, proteins and carbohydrates. It also plays a role in satiety by slowing gastric emptying. CCK directly and indirectly (via the DMVN) causes contraction of the gallbladder and relaxation of the Sphincter of Oddi, therefore causing bile secretion into the duodenum.
15
Q
Where’s insulin produced and what does it do?
A
Insulin is secreted by beta cells in the islets of Langerhans of the pancreas. It stimulates glycogenesis, increases the number of GLUT4 transporters in adipose and muscle tissue, and inhibits glycogenolysis and gluconeogenesis
16
Q
Where does nutrient absorption mainly happen?
A
In the small intestine
17
Q
Where does fluid absorption mainly happen?
A
The colon and small intestine
18
Q
Name 4 mechanisms for defence in the gut
A
- Sight, smell and taste
- Stomach acid that can kill most bacteria
- Vomit reflex
- Aggregations of lymphoid tissue called Peyer’s patches, which can mount an response to food-borne antigens
19
Q
What are the 4 types of control of the alimentary tract?
A
Endocrine
Paracrine
Neurocrine
Metabolic
20
Q
Name the 2 neurotransmitters responsible for stimulating contraction of gut muscle
A
Ach and SubP/SubK
21
Q
Name the 2 neurotransmitters responsible for stimulating relaxation of gut muscle
A
VIP and NO
22
Q
What controls gastric smooth muscle activity?
A
Contractie cholinergic neurons and relaxant NANC neurons in the myenteric plexus between the circular and longitudinal smooth muscle layers.
Gastrin and CCK and serotonin and motilin also affect smooth muscle
23
Q
What provides parasympathetic innervation to the gut muscle and mucosa?
A
The vagus nerve
24
Q
What provides sympathetic innervation to the gut muscle and mucosa?
A
The splanchnic nerves
25
What are the 3 types of relaxation of the fundus?
Receptive relaxation
Adaptive relaxation
Feedback relaxation
26
Upon arrival of a peristaltic wave, what happens to the pyloric sphincter?
It closes, but opens when chyme is repulsed through it
27
What receptors does gastrin bind to, and what happens when it does?
Gastrin is released from G cells and acts on CCKB receptors on target cells, triggering the release of histamine, as well as inducing insertion of K+/H+ ATPase pumps into the apical membrane of HCl secreting cells, causing gastric acid release into the stomach
28
What activates the pepsinogen released from chief cells to become pepsin?
High acidity alters the tertiary structure of pepsinogen, so it becomes pepsin
29
What is the NTS?
The nucleus tracts solitarius is the main site of termination of vagal afferent fibres. It's a series of purely sensory nuclei in the medulla of the brainstem
30
What is the DMVN?
The dorsal motor vagal nucleus is the main site of origin of vagal efferents supplying the gut
31
What does the vagovagal reflex control?
The vagovagal reflex controls contraction of the gastrointestinal smooth muscle layers in response to distention of the tract by food
32
When is the vagovagal reflex activated?
During receptive relaxation of the stomach in response to swallowing of food
33
What are the 2 nerve fibres intrinsic to the gut and their functions?
The myenteric plexus (Auerbach's plexus) is responsible for motor function.
The submucosal plexus (Meissner's plexus) is responsible for intestinal secretions.
34
What type(s) of autonomic input does Auerbach's plexus have?
Auerbach's plexus has both sympathetic and parasympathetic input
35
What type(s) of autonomic input does Meissner's plexus have?
Meissner's plexus only has parasympathetic fibres
36
What's the effect of cholinergic action in Auerbach's plexus?
Increase in gastric motility and secretion
37
What's the effect of adrenergic action in Auerbach's plexus?
Decrease in gastric motility and secretion
38
Where does Meissner's plexus provide secretomotor innervation?
To the mucosa nearest the lumen of the gut
39
What reflex causes the urge to go to the bathroom after a large meal?
The colo-colonic reflex of gut motility
40
What's the rate of emptying of the stomach dependent on?
The material's ability to be absorbed
41
What cells secrete pancreozymin?
Enteroendocrine cells in the duodenum
42
What are the 3 digestive functions of the stomach?
Accommodation and storage
Mechanical and enzymatic breakdown
Low delivery of chyme to the duodenum
43
What 2 mechanisms transport digesta from the gastric reservoir into the antral pump?
Tonic contractions that originate from the fundus
| Peristaltic waves in the region of the gastric corpus
44
Only chyme of what size can pass through the pyloric sphincter out of the stomach?
2mm
45
What does the descending inhibitory reflex elicited by middle antrum contraction do?
The descending inhibitory reflex causes pyloric relaxation (VIP/NO)
46
What does the ascending excitatory reflex cause?
Pyloric contractions and increase in tone, preventing duodeno-gastric reflux
47
What is gastric emptying regulated by?
Negative feedback systems
48
What is antral over-distension regulated by?
The vago-vagal reflex
49
Describe the emptying of liquids from the stomach
Liquids rapidly disperse, emptying without lag time. The rate of emptying is influenced by nutrient control
50
Describe the emptying of solids from the stomach
There are 2 phases- lag time and the linear phase. The duration of lag time is related to the size of the particle. Large particles are triturated to smaller ones
51
Describe the emptying of fatty foods from the stomach
Fatty foods liquefy at body temperature and float on top of the liquid layer, therefore emptying slowly. Fats are potent inhibitors of gastric motor events and gastric emptying
52
Describe the emptying of indigestible solids from the stomach
Indigestible solids are not emptied in the immediate post-prandial period. MMC activity facilitates peristalsis for movement of indigestible substances through the GI tract
53
What is the migrating motor complex?
A distinct pattern of electromechanical activity during fasting, triggering peristaltic waves that facilitate transport of indigestible entities from the stomach to the colon
54
What's the basal electric rhythm (BER)?
The spontaneous depolarisation and repolarisation of pacemaker cells in the smooth muscle of the stomach, small intestine and large intestine, which is spread through gap junctions in the smooth muscle of the GI tract
55
What's the name of the pacemaker cells in the GI tract?
Interstitial cells of Cajal
56
Which hormones mediate a decrease in fundic motor activity?
```
Gastrin-releasing peptide
Pancreozymin
Secretin
VIP
Somatostatin
Glucagon
Duodenal distension, duodenal acid
```
57
What hormone increases fundic motor activity?
Motilin
58
What 3 hormones stimulate contractions in the intestines?
CCK, gastrin and motilin
59
What inhibits contractions in the small intestine?
Secretin
60
What hormone causes the cyclic bursts of gastroduodenal contractions during fasting?
Motilin
61
What 2 substances decrease motility?
VIP and glucagon
62
What 2 hormones increase motility?
Serotonin and insulin
63
What is segmentation?
A process that originates in the pacemaker cells (ICC's). It involves divisions and subdivisions of chyme, bringing chyme in contact with the intestinal walls. Segmentation causes the slow migration of chyme towards the ileum
64
What are propulsive contractions?
The contractions responsible for peristalsis
65
What are the 3 phases of motor activity in the GI tract?
Phase 1 is quiescence- a quiet period
Phase 2 is irregular propulsive contractions
Phase 3 is a burst of uninterrupted phasic contractions, also known as a peristaltic rush
66
Define peristalsis
A propagating contraction of successive sections of circular gastric smooth muscle, preceded by a dilation, that propel the gastric contents through the tract
67
What cells coordinate contractions of GI smooth muscle?
ICC's of the enteric nervous system
68
What is nausea?
An unpleasant sensation that triggers aversion
69
What is emesis?
A physical act in which the contents of the upper GI tract are expelled via the mouth, usually associated with a sensation of relief
70
What is aversion?
A strong dislike or disinclination
71
What is retching?
Making the sound and movement of vomiting
72
How is vomiting a defence mechanism for the body?
Emesis can potentially prevent ingestion of toxins by expelling the harmful agents before they have chance to be absorbed
73
What is the chemoreceptor trigger zone?
Part of the area postrema in the medulla oblongata which receives inputs from blood-borne drugs or hormones and communicates with other structures in the vomiting centre to initiate vomiting
74
What are the 5 neurotransmitters implicated in the control of nausea and vomiting?
Acetylcholine, dopamine, histamine, substance P and serotonin
75
What receptors does histamine bind to to control nausea and vomiting?
H1 receptors
76
What receptors does serotonin bind to to control nausea and vomiting?
5-HT3 receptors
77
What significance does the anatomical position of the CTZ have?
The CTZ is located on the floor of the 4th ventricle , outside the blood-brain-barrier, so large polar molecules, such as emetic toxins, can diffuse through to and reach the CTZ quite easily
78
What coordinates our anti-poison defences?
The NTS (nucleus tractus solitarius), found in the medulla of the brainstem
79
What 4 things does the NTS receive different warning inputs from?
Visceral afferents-triggered by irritants, toxins, inflammation or distension
Area postrema- triggered by toxins in the blood
Vestibular system- triggered by toxins from the blood that disrupt receptors
Higher centres- triggered by things that have made you sick in the past, or other people nearby being sick
80
Name the 5 events of nausea
```
Reduced mixing and peristalsis
Proximal stomach relaxation
Giant retrograde contraction
Retching
Emesis
```
81
Describe the mechanical process of retching
Co-ordinated contractions of the abdominal muscles and the diaphragm cause waves of high pressure in the abdomen and compression of the stomach, but anti-reflux barriers remain intact to prevent expulsion
82
What's the difference between visceral and somatic pain?
Somatic pain is localised precisely, while visceral pain is referred to the body surface and so is very imprecise
83
What does the submucosal Meissner plexus regulate?
The digestive glands
84
What does the myenteric Auerbach plexus regulate?
Gut motility
85
What's the lamina propria?
A thin layer of connective tissue that forms part of the mucosa lining the GI tract, respiratory tract and urogenital tract
86
Why are all carbohydrates always broken down into monosaccharides during digestion?
Carbohydrates can only be absorbed as monosaccharides
87
What enzymes reduce complex carbohydrates to disaccharides?
Amylases
88
What is a brush border?
A microvilli-covered surface of simple cuboidal or simple columnar epithelia found in the small intestine, kidney and large intestine
89
What's the importance of the brush border to carbohydrate digestion in the small intestine?
The brush borders of the intestinal lining have enzymes anchored into their apical plasma membrane as integral membrane proteins for digestion of disaccharides into monosaccharides to be absorbed by transporters
90
How are glucose and galactose absorbed in from the lumen of the small intestine?
They're rapidly absorbed by a secondary active transport process
91
How can galactose act as a competitive inhibitor of glucose transport?
Galactose can bind to the luminal glucose carrier system and be actively transported out of the lumen.
92
How is an Na+ gradient established between the small intestinal lumen and the epithelial cells lining it?
Na+-K+ pumps actively transport cytoplasmic NA+ out through the basolateral membrane
93
What's the name of the glucose symporter that uses this Na+ concentration gradient to move glucose out of the lumen?
Glucose symporter SGLT1
94
How does glucose symporter SGLT1 work?
Glucose symporter SGLT1 co-transports 1 glucose molecule into the cell for every 2 Na+ ions it imports into the cell
95
Does fructose affect glucose or galactose absorption?
No
96
How is fructose absorbed?
Fructose isn't actively transported by the enterocytes, but is absorbed by a carrier-mediated, facilitated diffusion system of GLUT5 transporters, which is passive
97
What is a passive method of transport of glucose across the brush border membrane?
Up-regulation of GLUT2 glucose carrier proteins in the brush border membrane for facilitated diffusion
98
What enzyme breaks down polypeptides?
Pepsin
99
What are the 3 brush border peptidases?
Enteropeptidases, amino-polypeptidases and dipeptidases
100
What are cytoplasmic peptidases particularly active in breaking down?
Dipeptides and tripeptides
101
What are the 2 mechanisms of protein absorption?
Amino acids are transported on a sodium-coupled carrier system similar to that for glucose. There are separate carriers for different AAs.
Some di- and tri-peptides are transported on a carrier system using an inwardly directed H+ gradient
102
What are the 4 different dietary lipids?
Triglycerides, phospholipids, cholesterol and fat soluble vitamins
103
What digests TGs in the mouth?
Salivary lipase
104
Where are most dietary lipases digested?
The small intestine
105
What does lipase action require?
Emulsification of TGs by bile salts, which dissolve TGs in water.
106
How do bile acids work in lipid assimilation?
On exposure to a large aggregate of triglyceride, the hydrophobic portions of bile acids intercalate into the lipid, and this coating with bile acids aids in breakdown of the large aggregates into small and smaller droplets
107
What enzyme binds to the surface of the small emulsion particles and hydrolyses triglycerides into glycerol and fatty acids?
Pancreatic lipase
108
What's the name of the obesity treatment drug that inhibits pancreatic lipase to reduce digestion and absorption of fat?
Orlistat
109
What are micelles?
Small aggregates (4-8nm diameter) of mixed lipids and bile acids suspended within the ingesta
110
How are micelles absorbed?
Miscelles bump into the brush border of small intestinal enterocytes, and the lipids, including monoglyceride and fat acids, are taken up into the epithelial cells by simple diffusion
111
What happens to fatty acids and glycerol once in the enterocytes?
They're transported into the endoplasmic reticulum, where they're used to synthesise triglyceride. Triglyceride is then packaged with cholesterol, lipoproteins and other lipids in the Golgi into particles called chylomicrons.
112
What happens to the chylomicrons once they're formed?
They're extruded from the Golgi into exocytotic vesicles, which are transported to the basolateral aspect of the enterocyte. The vesicles fuse with the plasma membrane and undergo exocytosis into central lacteals, the lymphatic vessels that penetrate into each villus
113
What does lymph containing large numbers of chylomicrons look like?
Milky
114
Define steatorrhoea
The excretion of abnormal quantities of fat with the faeces owing to reduced absorption of fat by the intestine
115
What is the biliary tree?
A system of ducts to transport bile out of the liver into the small intestine
116
What ligament divides the liver into 2 primary lobes?
The falciform ligament
117
What's the blood supply system to the liver?
Around 75% of blood supply comes from the portal vein, coming straight from the GI tract, and 25% of the supply is from the hepatic artery
118
What cells make up 60% of the liver?
Hepatocytes
119
What cells make up 30% of the liver?
Kupffer cells
120
What's the function of hepatocytes?
To perform most metabolic functions like removing toxic substances, including alcohol from the blood, before the blood exits the lobule via the central vein
121
What is the function of Kupffer cells?
Phagocytic activity to remove aged and damaged red blood cells, bacteria, viruses and immune complexes
122
What's the functional unit of the liver?
Hepatic lobules
123
In what structure within the liver are Kupffer cells found?
In sinusoids- small, irregularly shaped blood vessels found in certain organs
124
What is bile?
A complex fluid of water, electrolytes and a mix of organic molecules: bile acids, cholesterol, bilirubin and phospholipids.
125
What cells secrete bile?
Bile is secreted by hepatocytes and by epithelial cells lining bile ducts
126
What hormone stimulates bile secretion and what triggers the release of this hormone?
Cholecystokinin, released due to the presence of fat in the duodenum, stimulates contractions of the gallbladder and common bile duct to release bile into the gut
127
What hormone increases bile outflow into the intestines?
Secretin increases bile secretion. Secretin is released when acid is detected in the duodenum
128
What causes gallstone formation?
Abnormal conditions caused by an imbalance in the chemical composition of bile within the gallbladder
129
What are the 2 types of gallstones?
Cholesterol stones and pigment stones
130
What are the 3 functions of bile?
Bile and pancreatic juices neutralise gastric juice as it enters the small intestine, aiding digestive enzyme activity. Bile is essential for fat digestion and absorption via emulsification. Bile eliminates waste products from the blood, particularly bilirubin and cholesterol.
131
What is bilirubin and what forms it?
Bilirubin is a useless, toxic, yellow pigment formed from the breakdown of haemoglobin
132
What is jaundice?
Jaundice is a condition caused by excessive quantities of either free or conjugated bilirubin in the ECF, which leads to a yellow discolouration of the skin, sclera and mucous membranes. This occurs when plasma bilirubin concentration is above 1.5mg/dL
133
What are the 3 types of jaundice?
Pre-hepatic jaundice, hepatic jaundice and post-hepatic jaundice
134
How does pre-hepatic jaundice happen?
Increased haemolysis can cause excess bilirubin and the liver has no capacity to process or conjugate it. Unconjugated bilirubin cannot be excreted in urine and remains in circulation
135
How does hepatic jaundice happen?
Problems with hepatocytes such as damage from cirrhosis, drugs or viral infections result in an increase in unconjugated and conjugated serum bilirubin
136
How does post-hepatic jaundice happen?
Passage of conjugated bilirubin into the duodenum is blocked and it leaks into the circulation and urine, making urine very dark
137
Name 3 sources of fatty acids for metabolism
Dietary triglycerides
Triglycerides stored in adipocytes
Triglycerides synthesised in the liver
138
What are the 3 phases of drug and hormone metabolism in the liver?
Phase 1- Oxidation. Primarily by P450 enzymes, to make substrate into polar compounds
Phase 2- Conjugation. To make compounds more water soluble
Phase 3- Elimination. The conjugate substance is eliminated into blood or bile using ATPase pumps
