The Digestive System Flashcards
1
Q
What is the digestive system made up of?
A
The digestive system is made up of the alimentary tract or the gastrointestinal tract (GI tract) and accessory organs, salivary glands, liver, pancreas, and gallbladder.
2
Q
What is the GI tract, and what are the different structures found in the GI tract?
A
The GI tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus (the mouth, oesophagus, stomach, small intestine, large intestine, and anus).
3
Q
What are the main accessory organs found in the digestive system?
A
Accessory organs; the salivary glands, liver, pancreas, and gallbladder are the solid organs of the digestive system
4
Q
What are the 4 main functions of the digestive system?
A
Main Functions in the Digestive System:
- Digestion
- Absorption
- Secretion
- Motility
5
Q
What is a definition for “Digestion”?
A
Digestion is the breaking down of the food we eat into simple substances that our bodies can absorb.
6
Q
What is “Digestive Absorption”?
A
Digestive absorption is the passage of the end products of digestion from the gastrointestinal tract into the blood, and lymphatic vessels.
Absorption of this kind can take place either by diffusion or by active transport.
7
Q
What is “Secretion”?
A
Secretion is a process by which substances are produced and discharged from a cell, gland, or organ for a particular function in the organism or for excretion.
8
Q
What is “Motility”?
A
Motility is the ability of an organism to move independently, using metabolic energy.
9
Q
Diagram of the structures of the Digestive System
A
NOTION 1.1
10
Q
What is the function of the following structures:
1. Mouth
2. Oesophagus
3. Stomach
4. Pancreas
5. Liver
6. Gallbladder
7. Small intestine
8. Large intestine
A
- Mouth: Foodstuffs broken down by chewing; saliva added as lubricant
- Oesophagus: Conduit between mouth and stomach
- Stomach: Digestion of proteins; foodstuffs reduced to semi-liquid form; storage; sterilisation
- Pancreas: Digestive enzymes for digestion of fats, carbohydrates and proteins
- Liver: Bile salts for digestion/absorption of fats in small
intestine - Gallbladder: Stores and concentrates bile
- Small intestine: Final stages of chemical digestion and nutrient absorption
- Large Intestine: Water absorption, bacterial fermentation and formation of faeces
11
Q
Does the diameter of the alimentary canal vary?
Where does the alimentary canal start, and where does it end?
How long is the alimentary canal?
Does the structural organisation of the alimentary canal vary?
A
Alimentary Canal:
- Continuous hollow tube (varying diameter)
- Oesophagus => rectum (~ 8 metres long)
- Tube wall has same structural organisation throughout length
12
Q
What are the 4 distinctive layers of the tubes in the GI Tract?
A
Four distinctive layers (tunics):
1. Mucosa (Epithelium, Lamina Propria, Muscularis Mucosae)
2. Submucosa
3. Muscularis Externa (circular muscle layer & longitudinal muscle layer)
4. Serosa/ adventitia
13
Q
Serosa vs Adventitia
A
Inside peritoneal cavity = Known as serosa (Surrounds stomach, small intestine, and large intestine)
Outside of peritoneal cavity = Known as Adventitia (Attaches Oesophagus and rectum to surrounding structures)
14
Q
What is the peritoneum?
A
The peritoneum is the serous membrane forming the lining of the abdominal cavity.
15
Q
Diagram of the 4 layers of the tubes in the GI tract
A
NOTION 1.2
16
Q
What 7 structures can epithelial cells form?
A
- Simple squamous epithelium
- Stratified squamous epithelium
- Simple cuboidal epithelium
- Stratified cuboidal epithelium
- Simple columnar epithelium
- Stratified columnar epithelium
- Pseudostratified columnar epithelium
NOTION 1.3
17
Q
What type of epithelium does the mouth, Oesophagus and anal canal contain?
A
Stratified squamous epithelium
18
Q
What type of epithelium does the stomach, small and large intestine contain?
A
Simple columnar epithelium
19
Q
What are some of the functions of the epithelium, found in the alimentary canal?
A
Functions of epithelium in alimentary canal:
- Barrier separating lumen of alimentary canal from body
- Synthesis and secretion of digestive enzymes, hormones & mucus
- Absorbs products of digestion
20
Q
What is the lamina propria?
A
Lamina propria = loose connective tissue (glands, blood/lymph vessels)
21
Q
What is the muscularis mucosae?
A
Muscularis mucosae = Thin smooth muscle layer
22
Q
What is the structure/ composition of the submucosa?
A
Submucosa:
- Composed of Thick, irregular connective tissues (This allows it to support the mucosa)
- Contains neurones, bloods vessels, lymphatic vessels
- These neurons form extensive network (i.e the Submucosal (Meissner’s) plexus (parasympathetic))
- Submucosal glands can also be found in the Oesophagus and Duodenum
23
Q
Structure of the layers found in the:
- Intestinal mucosa
- Oral mucosa
- Oral mucoperiosteum
A
NOTION 1.4
24
Q
What is the function of alpha amylase?
What is the function of lipases?
What is the function of peptidases?
A
Alpha amylase = Break down of glycogen, starch
Lipases = Break down of lipids, fats
Peptidases = Break down of peptides
25
What is dumping syndrome?
Dumping syndrome is a condition in which food, especially food high in sugar, moves from your stomach into your small bowel too quickly after you eat
26
What are the 2 concentric thick layers found in the smooth muscle of the muscularis externa?
Two concentric thick layers of smooth muscle:
- Inner layer = Circular muscle (constricts lumen)
- Outer layer = Longitudinal muscle (Shortens tube)
NOTION 1.5
27
What plexus is located between the circular and longitudinal layer of the muscularis externa?
Myenteric (Auerbach’s) plexus
28
What plexus is located between the submucosa and muscularis externa?
Submucosal plexus
29
What are the Myenteric plexus and Submucosal plexus known as collectively?
Myenteric plexus + Submucosal plexus = Intramural plexus
30
What is the purpose of the muscularis externa?
It produces motility => Peristalsis, Segmentation
31
What helps make up the enteric nervous system?
Submucosal + Myenteric plexuses = Enteric Nervous System
32
1. What nerve is involved in the parasympathetic control of the alimentary tract?
2. What is an exception?
3. What is the effect of the parasympathetic control on the alimentary tract?
1. Vagus Nerve (X)
2. Except salivation = Facial (VII) and Glossopharyngeal (IX)
3. Stimulates secretion & motility
33
1. What nerve is involved in the sympathetic control of the alimentary tract?
2. What is the effect of the sympathetic control on the alimentary tract?
1. Splanchnic nerve
2. Inhibited secretion & motility (except salivation)
34
Why do we chew?
Why chew?
• Prolong taste experience
• Defence against respiratory failure
35
How is chewing voluntarily controlled?
Via somatic nerves => Skeletal muscles of mouth/ jaw
36
What is the reflex involved in control of chewing?
Contraction of jaw muscles => pressure of food against gums, hard palate and tongue => mechanoreceptors => inhibition of jaw muscles => reduced pressure => contraction => etc...
37
What 3 glands secrete saliva?
Saliva is secreted by 3 pairs of glands:
- Parotid
- Submandibular
- Sublingual
NOTION 1.6
38
What is the composition of saliva?
What is in saliva? (…and why?)
- Water: 99% of secreted fluid. Softens, moistens, dilutes particles. Solvent
- Mucins: Major glycoprotein component. Mucins + water = mucus. Viscous solution - lubricant function
- Alpha-Amylase: Catalyses breakdown of polysaccharide (starch, glycogen) into disaccharide (maltose) + glucose
- Electrolytes: Tonicity/pH
- Lysozyme: Bacteriocidal - cleaves polysaccharid component of bacterial cell wall
39
What controls salivary secretion?
Salivary secretion is controlled by parasympathetic and sympathetic NS. Both have stimulatory effects!
40
What nerve(s) exhibits parasympathetic control on salivary secretion?
What is their ultimate effect?
Parasympathetic
• Cranial nerves VII (facial) & IX (glossopharyngeal)
• Stimulation => profuse watery salivary secretion
41
What is the effect of sympathetic stimulation on salivary secretion?
What is the difference between activating alpha 1 adrenoreceptors in comparison to beta 2 adrenoreceptors?
Sympathetic
• Stimulation => small volume, viscous salivary secretion
• High mucus content (Alpha 1 adrenoceptors)
• High amylase content (Beta 2 adrenoceptors)
42
What is the reflex control of salivary secretion?
Reflex control = Presence of food in mouth => chemoreceptors/pressure receptors (walls of mouth/tongue)
43
What are the 7 stages of swallowing?
1. Oral Phase (Voluntary)
2. Pharyngeal Phase
3. Bolus approaches Oesophagus
4. Food enters Oesophagus
5. Oesophageal Phase
6. Bolus approaches stomach
7. Receptive relaxation of stomach
44
What is involved in the oral phase of swallowing?
Oral Phase (Voluntary) = Bolus pushed to back of mouth by tongue
45
What is involved in the Pharnygeal Phase of swallowing?
Pharyngeal Phase:
- Presence of Bolus = sequence of reflex contractions of pharyngeal muscles
- Coordinated by swallowing centre (medulla)
- Soft Palate reflected backward and upward (closes off nasopharynx)
46
What happens as the Bolus approaches the Oesophagus?
As bolus approaches oesophagus…
• Upper oesophageal sphincter (UOS) relaxes and epiglottis covers opening to larynx (prevents food entering trachea)
47
What happens once the food has entered the Oesophagus?
Once food has entered oesophagus…
• UOS contracts (prevents food reflux)
48
What happens during the Oesophageal Phase of Swallowing?
Oesophageal Phase
- Propulsion of bolus to stomach
- Peristaltic wave sweeps along entire oesophagus
- Propelled to stomach in ~10 secs
49
What happens as the Bolus approaches the stomach?
As bolus nears stomach…
• Lower oesophageal sphincter (LOS) relaxes =>
bolus enters stomach
50
What happens during receptive relaxation of the stomach?
Receptive relaxation of stomach
• Initiated following relaxation of LOS and entry of bolus into stomach
• Vagal reflexes => relaxation of thin, elastic smooth muscle of gastric fundus and body
• Stomach volume: 50ml => 1500ml (no change in pressure)
51
What are the 5 main functions of the Stomach?
Functions of Stomach:
- Temporary store of ingested material
- Dissolve food particles and initiate digestive process
- Control delivery of contents to small intestine
- Sterilise ingested material
- Produce intrinsic factor (Vitamin B12 absorption)
52
Anatomy of the Stomach
Oesophagus => gastroesophageal (cardiac) opening (lower oesophageal (cardiac) sphincter) => cardiac region => fundus => body (greater and lesser curvature) => pyloric region => pyloric opening (pyloric sphincter)
NOTION 2.1
53
Histology of the Stomach
1. Serosa = connective tissue outer layer
2. Muscularis externae = three layers: longitudinal (outer), circular (middle), oblique (inner)
3. Submucosa and mucosa folded (= rugae) when empty => stretch as stomach fills
4. Lumen surface = surface mucus cells => gastric pits => gastric glands => mucus neck, parietal and chief cells
54
Structure of the Stomach Wall
NOTION 2.2
55
What is the function of the fundus of the stomach?
Storage
56
What is the function of the body of the stomach?
What molecules/ hormones are released in the body of the stomach?
Function = Storage
Some molecules/ hormones that are released:
- Mucus
- HCl
- Pepsinogen
- Intrinsic Factor
57
What is the function of the antrum of the stomach?
What hormone is released by the antrum of the stomach?
Function = Mixing/ Grinding
Hormone released = Gastrin
58
Cellular composition of gastric glands
Mucous neck cells = Mucus
Parietal cells = HCl, Intrinsic Factor
Chief Cells = Pepsinogen
NOTION 2.3
59
How is the pH (of <2) maintained in the stomach lumen, while the blood in the body has a pH of >7.4?
NOTION 2.4
60
What is involved in the reaction cascade activated by the following:
- Gastrin
- Histamine
- Prostaglandins
- Acetylcholine
What is the importance of these reactions cascades in the stomach?
Gastrin => Cckb receptor => Ca2+ => PKC => Protein Kinases
Histamine => H2 Receptor => Gs & AC => ATP -> cAMP => PKA = Protein Kinases
Prostaglandins => EP3 Receptor => Gi => Inhibition of AC => Lower Production of cAMP
Acetylcholine => M3 Receptor => Ca2+ => PKC => Protein Kinases
The reaction cascades involving Gastrin, Histamine & ACh activate protein kinases.
Whereas the reaction pathway involving Prostaglandins reduce the activation of Protein Kinases.
This is important because Protein Kinases can provide the ATP for the Hydrogen Proton Pump.
NOTION 2.5
61
What 3 mechanisms help control gastric acid secretion?
Gastric acid secretion is controlled by 3 mechanisms:
- Neurocrine (vagus/ local reflexes)
- Endocrine (Gastrin)
- Paracrine (Histamine)
62
What is involved in the Cephalic Phase, involved in the stimulation of Gastric Acid Secretion?
Activated upon sight, smell or taste of food =>
Increased activation of vagus nerve =>
Acetylcholine is released, which activates Parietal Cells & G Cells also secrete Gastrin, which also activate Parietal Cells.
In addition, the presence of Gastrin/ ACh => Activates ECL Cells => Triggers release of Histamine = Activates Parietal Cells
NOTION 2.6
63
What is involved in the Gastric Phase, involved in the stimulation of Gastric Acid Secretion?
Triggered by distension of stomach (arrival of food) => Vagal/ Enteric Reflexes => ACh => Parietal cells activated
Also triggered by peptides in lumen => G Cells activated => Gastrin released => Parietal cells activated
Finally, the accumulation of Gastrin/ ACh => Activates ECL Cells => Releases histamine => Parietal cells activated
NOTION 2.7
64
What is involved in the Cephalic, Gastric & Intestinal Phase, involved in the inhibition of Gastric Acid Secretion?
Cephalic Phase: Stop eating => Decreased vagal activity
Gastric Phase: Decreased pH (Increased HCl) => Decreased Gastrin
Intestinal Phase I: Acid in duodenum => Enterogastric (splanchnic) reflex & secretin release => Decreased secretion of Gastrin
Intestinal Phase II: Fat/ CHO in duodenum => GIP release => Decreased Gastrin & HCl secretion
NOTION 2.8
65
What are enterogastrones?
What is their function?
Enterogastrones are hormones released from gland cells in duodenal mucosa - secretin, cholecystokinin (CCK), GLP, GIP.
They are released in response to acid, hypertonic solutions, fatty acids or monoglycerides in duodenum.
And act collectively to prevent further acid build up in duodenum.
CCK = Acts by stimulating release of pancreatic juice & bile.
Secretin = Regulation of gastric acid & osmoregulation.
GIP = Gastric Inhibitory Peptide
66
What are 2 strategies by which enterogastrones help prevent further acid build up in the duodenum?
Two strategies:
• Inhibit gastric acid secretion
• Reduce gastric emptying (inhibit motility/contract pyloric sphincter)
67
Where is Pepsinogen secreted?
Pepsinogen (zymogen i.e inactive enzyme) is secreted by Chief Cells in Gastric Glands.
68
What is the function of Pepsinogen?
Pepsinogen is an inactive precursor enzyme produced in the stomach by chief cells, which is converted into the active digestive enzyme pepsin when exposed to the acidic environment of the stomach, playing a crucial role in protein digestion by breaking down protein chains into smaller peptides.
69
What does storage of Pepsinogen help to prevent?
How are Pepsins inactivated?
What mechanism allow control for pepsin secretion?
- Pespsinogen storage prevents cellular digestion
- Pepsins inactivated at neutral pH
- Mechanisms for pepsin control of pepsin secretion parallel HCl secretion
70
Where is mucus produced in the stomach?
It is produced by surface epithelial cells & mucus neck cells.
71
What Cytoprotective roles does mucus play in the stomach?
Cytoprotective Role
- Protects mucosal surface from mechanical injury
- Neutral pH (HCO3) → Protects against gastric acid corrosion and pepsin digestion
72
What is the only essential function of the stomach?
Production of intrinsic factor is the only essential (non-compensated) function of the stomach.
73
Where is intrinsic factor produced in the stomach?
It is produced by parietal cells.
74
What is intrinsic factor required for?
It is required for Vitamin B12 absorption.
75
Where is the intrinsic factor/ B12 complex absorbed?
Intrinsic factor/ B12 complex is absorbed from ileum
76
What is a disease which results from a defect in the production/ action of intrinsic factor?
Defect → Pernicious Anaemia (failure of erythrocyte maturation)
Symptoms of Pernicious Anaemia can take 3 years to develop, since Vitamin B12/ Intrinsic Factor complex can reside in liver for up to 3 years.
77
Where and how are peristaltic waves produced?
Peristaltic waves: Body => Antrum
Body: Thin muscle => weak contraction & no mixing
Antrum: Thick muscle => powerful contraction & mixing
78
What is the effect of contraction of pyloric sphincter at the antrum of the stomach?
Contraction of pyloric sphincter:
1. Only small quantities of gastric content (chyme) entering duodenum
2. Further mixing as antral contents forced back towards body
79
Graphs of Membrane Potential & Smooth Muscle Tension in the stomach
NOTION 2.9
80
What is the peristaltic rhythm generated by?
Peristaltic rhythm (~3/min) generated by pacemaker cells (longitudinal muscle layer).
81
What are slow gastric waves?
How are slow waves conducted?
Is the slow wave depolarisation below or above threshold?
Slow waves - spontaneous depolarisation/ repolarisation
Slow wave rhythm = basic electrical rhythm (BER).
Slow waves conducted through gap junctions along longitudinal muscle layer.
Slow wave depolarisation = sub-threshold. Therefore requires further depolarisation to induce action potentials.
82
What hormone affects motility of the stomach?
What other factors affect motility?
Motility under neural/hormonal control
- Gastrin => increases contraction
- Distension of stomach wall => long/short reflexes => increased contraction
- Fat/acid/amino acid/hypertonicity in duodenum => inhibition of motility
83
What is the effect of NSAIDs on the stomach wall?
NSAIDs inhibit prostaglandins, which increases the secretion of gastric acid. In addition, NSAIDs damage the mucus lining of the stomach wall.
84
What is dumping syndrome?
- Occurs 15-30 minutes after eating
- Involves a large increase in tonicity in the intestine
- Can occur due to: No fluids with meals, or no high carbs
- Some symptoms include:
- Weakness
- Dizziness, vertigo
- Diaphoresis
- Tachycardia
- Abdominal cramping
85
How does neutralisation of acid occur in the duodenum?
Bicarbonate (HCO3) secretion from Brunner’s Gland duct cells (submucosal glands) helps to neutralise the acid. The liver and pancreas can also aid in this neutralisation, through the release of bile and pancreatic juices.
86
How is Duodenal HCO3 secretion controlled?
Acid in duodenum triggers…
✓ Long (vagal) & short (ENS) reflexes => HCO3 secretion
✓ Release of secretin from S cells => HCO3 secretion
• Secretin => HCO3 secretion from pancreas & liver
• Acid neutralisation => inhibits secretin release (negative feedback control)
87
Anatomy of the Pancreas
NOTION PF.1
88
What are the 3 main parts of the pancreas?
3 main parts of the pancreas: Head (Located within curvature of duodenum), body , tail (extends to spleen)
89
What makes up the endocrine portion of the pancreas?
Endocrine portion => pancreatic islets (islets of Langerhans): islet cells produce insulin, glucagon (control [glucose]blood ) and somatostatin (controls secretion of insulin and glucagon).
90
What makes up the exocrine portion of the pancreas?
Exocrine portion => acinar cells => lobules
91
How are the lobules connected in the pancreas? What do these ducts then connect to?
Lobules connected by intercalated ducts => intralobular ducts => interlobular ducts => main pancreatic duct => common bile duct => hepatopancreatic ampulla (Sphincter of Oddi) => duodenum.
Also accessory pancreatic duct => duodenum.
92
What is the exocrine section of the pancreas responsible for? What does its anatomical structure include? What are its main functions?
This section is responsible for the digestive function of pancreas. The anatomical structure includes:
Acini => Ducts => Pancreatic Ducts
The main 2 functions of this section are:
- Secretion of bicarbonate by duct cells
- Secretion of digestive enzymes by acinar cells
93
What do acinar cells contain?
How are these zymogens converted to their active forms?
At what pH does this occur?
Acinar cells contain digestive enzymes stored as inactive zymogen granules. This prevents auto digestion of the pancreas.
Enterokinase (bound to brush border of duodenal enterocytes) converts trypsinogen to trypsin.
Trypsin converts all other zymogens to active forms.
Trypsin works in neutral or slightly alkaline pH.
94
What are the 6 categories of pancreatic enzymes?
1. Proteases: Cleave peptide bonds
2. Nucleases: Hydrolyses DNA/ RNA
3. Elastases: Collagen digestion
4. Phospholipases: Degrade Phospholipids to fatty acids
5. Lipases: Degrade triglycerides to fatty acids & glycerol
6. Alpha - Amylase: Degrade starch to maltose & glucose
95
What stimulates bicarbonate secretion?
Bicarbonate secretion stimulated by secretin.
Also under neural control (vagal/local reflexes) -
triggered by arrival of organic nutrients in duodenum.
96
What stimulates secretion of secretin?
Secretin released in response to acid in duodenum.
97
What stimulates zymogen secretion?
Zymogens secretion stimulated by cholecystokinin (CCK).
Also under neural control (vagal/local reflexes) -
triggered by arrival of organic nutrients in duodenum.
98
What stimulates release of CCK?
CCK released in response to fat/amino acids in duodenum.
99
Summary of control of pancreatic function
NOTION PF.2
100
What is the largest internal organ/ gland of the human body? What % of body weight does it make up?
The liver is the largest internal organ/gland of the human body, accounting for approximately 2-3% of the total body weight of an adult and providing a host of functions necessary for maintaining normal physiological homeostasis.
101
What is the average weight of a liver?
Around 1.5 kg
102
Where is the liver located?
The liver is located in the upper quadrant of the abdomen (tucked against interior surface of diaphragm).
103
How many lobes make up the liver?
2 Major Lobes: Right > Left
2 Minor Lobes: Caudate, Quadrate
104
What blood vessels, lymphatic vessels, ducts and nerves enter/ leave the liver? What do they enter/ exit via?
Entry/exit of blood vessels (hepatic portal vein, hepatic artery), lymphatic vessels, ducts (right/left hepatic ducts) => common hepatic duct, nerves (hepatic nerve plexus) via Porta on inferior surface.
105
What two ducts form the common bile duct? And then what does this common bile duct combine with?
Common hepatic duct joined by cystic duct from gallbladder => common bile duct + pancreatic duct => major duodenal papilla (Sphincter of Oddi)
106
Anatomy of the liver
NOTION LF.1
107
What is a specialised characteristic of hepatocytes?
Hepatocytes are polarised (they contain different membranes, each with different structure & function). For example, hepatocytes have a:
- Apical membrane
- Basal lateral membrane (face the sinusoidal cells)
108
What are the sinusoidal cells?
They form the capillaries of the liver
109
What are the 3 subtypes of Liver Cells?
3 subtypes of Liver Cells:
- Parenchymal cells
- Hepatocytes
- Non Parenchymal cells
- LSEC, HSC, Kupffer Cells & Pit Cells
- Cholangiocytes
- Biliary Epithelial Cells
NOTION LF.2
110
What is activated when there is damage to the liver?
The hepatic stellate cells are activated, and are responsible for the secretion of fibrogenic material, such as collagen. This can lead to fibrosis, or cirrhosis.
111
What are kupffer cells?
They are the “macrophages” of the liver.
112
Where does blood come from, when it enters the sinusoid?
Blood comes from the portal vein, and the hepatic artery, which combine to form the sinusoid. The blood from the portal vein has a much lower concentration of oxygen, in comparison to the blood coming from the hepatic artery.
113
What is a hepatic functional unit?
A hepatic lobule. The structure of a hepatic lobule can be seen in the following diagram:
NOTION LF.3
114
What are the Portal Triad?
Portal Triad:
- Portal Vein
- Bile Duct
- Hepatic artery
115
What is the liver covered by?
Liver covered by connective tissue capsule and visceral peritoneum except for bare area (small area on diaphragmatic surface surrounded by coronary ligament).
116
What happens at the porta?
At porta, connective tissue capsule => branching network of septa into body of liver => support.
Vessels, ducts and nerves follow septa throughout liver.
117
What does the septa do?
The septa divides liver into hexagonal lobules.
118
Where is the portal triad located?
Portal triad (hepatic portal vein, hepatic artery, hepatic duct) located at each corner of hexagonal lobule (also nerves and lymph vessels).
119
Where is the central vein located?
Central veins at centre of each lobule => hepatic veins => inferior vena cava,
120
Where do hepatic cords radiate out from?
What are hepatic cords composed of?
Hepatic cords radiate out from central veins (like spokes of wheel).
Hepatic cords composed of hepatocytes = functional cells of liver.
121
Where is the bile canaliculus found?
Bile canaliculus (cleft-like lumen) lies between cells within each hepatic cord.
122
Where are hepatic sinusoids found?
Spaces between hepatic cords = hepatic sinusoids (liver blood capillaries).
123
What is the direction of movement of blood & bile between the central vein & portal triad?
- Blood moves from portal triad to central vein
- Bile moves from central vein to portal triad
124
What is the composition of blood coming from the hepatic portal vein to the hepatic sinusoids?
O2 depleted/ nutrient rich blood
125
What is the composition of blood coming from the hepatic artery to the hepatic sinusoids?
O2 rich/ nutrient depleted blood
126
What does blood exit the hepatic sinusoids via?
Hepatic sinusoids => Central veins => Hepatic Veins
127
What are the 4 main functions of the hepatocytes?
1. Bile Synthesis
2. Nutrient Storage (Glycogen, fat, vits (B12, A, D, E, K) Cu, Fe)
3. Nutrient interconversion
4. Detoxification
128
What are the main functions of the liver?
Involved in metabolism/ synthesis of:
- Xenobiotics
- Bilirubin
- Carbohydrates
- Lipids
- Amino acids & proteins
- Bile Acids
- Hormones
- Blood Clotting Factors
Involved in the clearance of toxins & pathogens.
Involved in Immune Response Regulation.
129
The liver helps produce & secrete bile. What are the 6 components of bile?
Six components of bile:
1. Bile acids (Synthesised in liver & Solubilise Fat)
2. Lecithin (Synthesised in liver & Solubilise Fat)
3. Cholesterol (Synthesised in liver & Solubilise Fat)
4. Bile Pigments (made from bilirubin i.e from haemoglobin)
5. Toxic metals (Detoxified in the liver)
6. Bicarbonate (Neutralisation of acid chyme)
(1) => (5) secreted by hepatocytes
HCO3 is secreted by duct cells.
130
1. What are bile pigments?
2. What is the predominant bile pigment?
What colour is this?
3. How is bilirubin modified?
4. Where does reabsorbed bilirubin end up?
1. Bile pigments = Breakdown products of haemoglobin from old/damaged erythrocytes.
2. Bilirubin (predominant bile pigment) - extracted from blood by hepatocytes and secreted into bile => yellow bile.
3. Bilirubin modified by bacterial enzymes => brown pigments => brown faeces
4. Reabsorbed bilirubin excreted in urine => yellow urine.
131
Where are bile acids synthesised?
And what are they synthesised from?
What is the production rate per day?
What happens to the bile acids before secretion?
What is the effect of this?
What is the destination of these bile salts?
Synthesised in liver from cholesterol (0.5g/day).
Before secretion, bile acids conjugated with glycine or
taurine => Bile Salts (Increased solubility).
Secreted bile salts recycled via enterohepatic circulation.
Liver => bile duct => duodenum => ileum => hepatic
portal vein => liver etc...
132
What is the structure of the gallbladder?
And where is the gallbladder located?
Saclike structure (8 cm long x 4 cm wide) on inferior surface of liver.
133
What are the 3 layers of the wall of the gallbladder?
Three layers in wall:
• mucosa (folded => rugae => expansion)
• muscularis (smooth muscle) => contraction
• serosa (connective tissue)
134
Where does bile go, once it has left the gallbladder?
Gallbladder => cystic duct => common bile duct
135
What is the sphincter of oddi?
What happens when it contracts?
Sphincter of Oddi
• Controls release of bile and pancreatic juice into duodenum
• When contracted (closed) => bile forced back into gallbladder
• Gallbladder concentrates bile 5-20 times (absorbs Na+ & H2O)
136
What triggers the release of CCK?
What is the effect of CCK, on the sphincter of oddi?
Fat in duodenum => release of CCK
CCK release:
(A) Sphincter of Oddi relaxes
(B) Gallbladder contracts
137
Summary of Actions of Secretin & CCK
NOTION LF.4
138
What is the length of the small intestine?
Total length = ~6 metres (range 4.5 - 9 metres)
139
What are the 3 sections of the small intestine?
3 sections of small intestine:
- Duodenum
- Jejunum
- Ileum
140
What length is the duodenum?
What are its main functions?
Duodenum
• 25 cm long
• Gastric acid neutralisation; Digestion; Iron absorption
141
What length is the Jejunum?
What is its main function?
Jejunum
• 2.5 m long (2/5 total length)
• Nutrient absorption - 95%
142
What length is the ileum?
What are its main functions?
Ileum
• 3.5 m long (3/5 total length)
• NaCl/H2O absorption => chyme dehydration
143
What is the absorptive surface area of the small intestine enhanced by?
Absorptive surface area enhanced by folds, villi, microvilli.
144
Effect of the following on enhancement of surface area:
- Intestine as cylinder
- Circular folds (plicae)
- Villi
- Microvilli
NOTION 4.1
145
Structure of a villus
NOTION 4.2
146
What is lacteal?
Lacteal: “The lymph capillary inside the villi of the small intestine”.
147
What is the function of the villi cells, and the crypt cells?
Villus Cell is responsible for absorbing:
- NaCl
- Monosaccharides
- Amino Acids
- Peptides
- Fats
- Vitamins
- Minerals
- Water
Crypt cell is responsible for secreting Cl & water.
NOTION 4.3
148
What are the 6 principal dietary constituents?
1. Carbohydrate
2. Protein
3. Fat
4. Vitamins
5. Minerals
6. Water
149
What are the different types of carbohydrates?
1. Monosaccharides
2. Disaccharides
3. Oligosaccharides
4. Polysaccharides
150
What are the 3 main hexoses?
1. Glucose (Aldose)
2. Galactose (Aldose)
3. Fructose (Ketose)
NOTION 4.4
151
What are the 3 main disaccharides?
1. Maltose (Glucose-Glucose)
2. Sucrose (Glucose-Fructose)
3. Lactose (Glucose-Galactose)
NOTION 4.5
152
What (Brush Border) enzymes are used to break down:
1. Lactose
2. Sucrose
3. Maltose
1. Lactase
2. Sucrase
3. Maltase
153
Structure of Starch, Glycogen & Cellulose
NOTION 4.6
154
Where is starch found?
What is starch composed of?
What bond links the monomers in starch?
How is starch hydrolysed?
Starch: plant storage form of glucose
• Alpha-amylose: glucose linked in straight chains
• amylopectin: glucose chains highly branched
Glucose monomers linked by Alpha-1,4 glycosidic bonds - hydrolysed by amylases (saliva, pancreas)
155
Where is cellulose found?
What is cellulose composed of?
What bonds join the monomers in cellulose?
How is cellulose digested?
Cellulose: constituent of plant cell walls
• Unbranched, linear chains of glucose monomers linked by Beta-1,4 glycosidic bonds
• Dietary fibre (no enzymatic digestion in vertebrates - require bacteria (cellulase)
156
Where is glycogen found?
What is glycogen composed of?
What bonds are found in glycogen?
Glycogen: animal storage form of glucose
Glucose monomers linked by Alpha-1,4 glycosidic bonds
157
Summary of break down of Starch & Glycogen
NOTION 4.7
158
Basolateral vs Apical Membrane
NOTION 4.8
159
1. What receptor is involved in the transport of glucose/galactose from the lumen of the intestines, into the intestinal cells?
2. And then what receptor transports glucose/galactose into the bloodstream?
1. SGLT1 (Sodium Glucose Transporter 1)
2. GLUT-2 (Glucose Linked Uni Transporter 2)
NOTION 4.9
160
1. What receptor is involved in the transport of fructose from the lumen of the intestines, into the intestinal cells?
2. And then what receptor transports fructose into the bloodstream?
1. GLUT-5 Transporter
2. GLUT-2 Transporter
NOTION 4.10
161
What are enzymes which hydrolyse peptide bonds known as?
Enzymes which hydrolyse peptide bonds and reduce proteins or peptides to amino acids are called proteases or peptidases.
162
Endopeptidases vs exopeptidases
NOTION 4.11
163
What receptor is involved in the transport of amino acids from the lumen of the intestines, into the intestinal cells?
SAAT1 Transporter
NOTION 4.12
164
What receptors are involved in the transport of di/tripeptides from the lumen of the intestines, into the intestinal cells?
PepT1 Transporter (Linked to NHE3)
NOTION 4.13
165
What are almost all ingested fats in the form of?
Triacylglycerol
166
What is the structure of a triacylglycerol?
NOTION FA.1
167
What enzyme is involved in fat digestion?
Where does this take place?
All fat digestion in small intestine by pancreatic lipase.
168
What are triacylglycerols present as within the human body?
Triacylglycerols present as large lipid droplets which are insoluble in water.
169
Is lipase a water soluble or insoluble enzyme?
Lipase = Water soluble enzyme
170
Why is digestion of fat very slow?
Since lipase is water soluble => digestion can only take place at surface of droplet => very slow
171
What is the product of the breakdown of a triacylglycerol?
Triacylglycerol => monoglycerides + 2 fatty acids
172
What does emulsification involve?
Emulsification = dividing large lipid droplets into smaller droplets (~1 mm diameter) => increased surface area and accessibility to lipase action.
173
What are 2 components required for emulsification?
Emulsification requires:
• Mechanical disruption of large lipid droplets into small droplets. Smooth muscle contraction grinds and mixes lumen contents.
• Emulsifying agent - prevents small droplets reforming into large droplets. Bile salts + phospholipids secreted in bile =
Amphiphatic molecules (ie polar (charged) and non-polar portions)
174
How do emulsifying agents aid in the distribution of a fat globule throughout water?
Non-polar portions associate with non-polar interior of lipid droplet leaving polar portions exposed at water surface.
NOTION FA.2
175
How is the reformation of the fat globule prevented?
Polar portions repel other small lipid droplets (also coated with bile salts/phospholipids) => prevent reforming into large droplets.
176
What helps enhance the absorption of lipase digested products?
Absorption enhanced by formation of micelles.
177
What are micelles?
Micelles similar to emulsion droplets but much smaller (4-7µm diameter).
Micelle = bile salt + monoglycerides + fatty acids + phospholipids.
178
Diagram of emulsification & absorption of lipids
NOTION FA.3
179
What is involved in micelle breakdown?
Micelle breakdown => release of small amounts of free fatty acids (FFA) and monoglycerides into solution => diffusion across plasma membrane of absorbing cells.
180
What does the dynamic equilibrium between fatty acids and monoglycerides in solution and in micelles allow?
Dynamic equilibrium between fatty acids and monoglycerides in solution and in micelles - retains most of fat digestion products in solution while constantly replenishing supply of free molecules for absorption.
181
Are micelles absorbed?
No, micelles themselves are not absorbed.
182
What happens to the fatty acids and monoglycerides, after entering epithelial cells?
After entering epithelial cells fatty acids and monoglycerides enter smooth endoplasmic reticulum (sER) where they are reformed into triacylglycerols (by enzymes located within the sER).
183
What happens to the triacylglycerol after being formed in the sER?
Triacylglycerol droplets transported through cell in vesicles formed from sER membrane - processed through Golgi apparatus and exocytosed into extracellular fluid at serosal membrane (basolateral membrane).
184
What are extracellular fat droplets known as?
Extracellular fat droplets = Chylomicrons (also contain phospholipids, cholesterol & fat-soluble vitamins) ~1 µm diameter.
185
Where do chylomicrons travel to, after formation?
Chylomicrons pass into lacteals between endothelial cells (cannot pass through capillary basement membrane).
186
What are the 2 distinct types of movement in the intestine?
Two distinct types of movement:
- Segmentation
- Peristalsis
187
When is segmentation most common?
What is the effect of contraction during segmentation?
What is the benefit of segmentation?
• Most common during meal
• Contraction <=> relaxation of short intestinal segments
• Contraction (few seconds) moves chyme (up & down) into adjacent areas of relaxation
• Relaxed areas then contract and push chyme back
• Provides thorough mixing of contents with digestive enzymes
• Brings chyme into contact with absorbing surface
188
Diagram of Segmentation
NOTION 5.1
189
How are segmentation contractions generated?
• Initiated by depolarisation generated by pacemaker cells in longitudinal muscle layer (cf gastric motility)
• Intestinal basic electrical rhythm (BER) produces oscillations in membrane potential => threshold => action potential => contraction
190
What is the frequency of segmentation determined by?
Magnitude of BER going from intestine -> rectum
• Frequency of segmentation determined by BER
• BER decreases as move down intestine → rectum
191
How is chyme moved towards large intestine?
Segmentation produces slow migration of chyme towards large intestine (more chyme pushed down than up)
192
Effect of the parasympathetic vs sympathetic NS on segmentation contractions.
Effect of autonomic nervous system on BER.
• Parasympathetic NS (vagus) => increased contraction
• Sympathetic NS => decreased contraction
• No effect of autonomic nervous system on BER
193
What happens following absorption of nutrients?
Following absorption of nutrients: segmentation stops, and peristalsis starts.
194
What is the Migrating Motility Complex?
Migrating Motility Complex (MMC)
• Pattern of peristaltic activity travelling down small intestine (starts in gastric antrum)
• As one MMC ends (terminal ileum) another begins
195
What stimulates cessation of MMC?
Arrival of food in stomach => cessation of MMC and initiation of segmentation.
196
What are the 2 main functions of MMC?
MMC acts to:
- Move undigested material into large intestine
- Limit bacterial colonisation of small intestine
197
What hormone is involved in the initiation of MMC?
Motilin
198
Diagram of Peristalsis
NOTION 5.2
199
What is the Law of the Intestine?
What is this mediated by?
If intestinal smooth muscle is distended (eg by bolus of chyme):
• Muscle on oral side of bolus contracts
• Muscle on anal side of bolus relaxes
• Bolus is moved into area of relaxation towards colon
This is mediated by neurones in myenteric plexus.
200
What is involved in the Gastroileal Reflex?
Gastric emptying => Increased segmentation activity in the ileum:
- Opening of ileocaecal valve (sphincter)
- Entry of chyme into large intestine
- Distension of colon
- Reflex contraction of ileocaecal sphincter (prevents backflux into small intestine)
201
Structures flowing from the ileum
Ileum => ileocaecal valve => caecum (blind sac) =>
vermiform (worm-like) appendix – lymph nodes
202
How long is the colon?
Colon = 1.5 - 1.8 metres
203
What are the 4 parts of the colon?
Four parts of the colon:
- Ascending
- Transverse
- Descending
- Sigmoid
204
Muscle layer found in the large intestine
• Circular muscle layer complete but longitudinal muscle layer incomplete
• Three bands – teniae coli (entire length of colon)
• Contractions of teniae coli => pouches (haustra) – puckered appearance
205
Mucosa in the large intestine
Mucosa comprised of simple columnar epithelium – flat.
206
What cells are found in the large intestine, to aid movement of faeces?
Large, straight crypts lined with large number of goblet cells
=> lubrication for movement of faeces
207
1. Structure/ Location of the Rectum
2. Mucosa of the Rectum
3. Muscularis Externa of the Rectum
1. Straight, muscular tube (between end of sigmoid colon and anal canal)
2. Mucosa – simple columnar epithelium
3. Muscularis externa – thick compared to other regions of alimentary canal
208
1. Distance between distal rectum and anus
2. Muscularis externa of the anal canal
3. Skeletal muscle of the anal canal
4. Epithelium of the anal canal
1. 2-3 cm between distal rectum and anus
2. Muscularis thicker than rectum => internal anal sphincter
3. External anal sphincter = skeletal muscle
4. Epithelium: simple columnar => stratified squamous
209
Summary of Structure of Large Intestine
NOTION 5.3
210
What is the function of the Colon?
Are nutrients absorbed in the Colon?
• No important nutrient absorption role in humans
• Actively transports sodium from lumen into blood => osmotic absorption of water => dehydration of chyme => solid faecal pellets
211
What does the long residence time in the colon result in?
Long residence time in colon => bacterial colonisation
212
Weight of the colonic microflora.
How many bacteria are roughly found in the colon?
Colonic microflora = 10^14 bacteria (~1Kg)
213
Function of the bacteria residing in the colon
Bacterial fermentation of undigested carbohydrate
➢ Short chain fatty acids (energy source in ruminants)
➢ Vitamin K (blood clotting)
➢ Gas (flatus) - nitrogen, CO2, hydrogen, methane, hydrogen sulphide
214
Role of sphincters in the closure of the anus
Normally, anus closed by internal anal sphincter (smooth muscle under automatic control) and external anal sphincter (skeletal muscle under voluntary control).
215
What is Mass Movement Contraction?
Following meal…
• Wave of intense contraction (Mass Movement Contraction) - colon → rectum
• Distension of rectal wall produced by mass movement of faecal material into rectum → mechanoreceptors → defaecation reflex → urge to defaecate
216
What is involved in the Defaecation Reflex?
Under parasympathetic control – via pelvic splanchnic nerves (no sympathetic influence)
A = Contraction of rectum
B = Relaxation of internal and contraction of external anal sphincters
C = Increased peristaltic activity in colon
