Gastrointestinal Tract Physiology Flashcards
1
Q
What are the 4 digestive processes?
A
- Motility
- Secretion
- Digestion
- Absorption
2
Q
What is motility and what is the function?
A
Contraction of smooth muscle in the digestive tract
- Mixing: promote digestion & facilitate absorption
- Moving the contents in the GIT
3
Q
Secretion (Basic digestive processes)
A
Both exocrine (enzymes, HCl) & endocrine (hormonal) secretions
Mucus: along entire digestive tract
Saliva, acid, enzymes, bile, bicarbonate, hormones, etc
4
Q
What is digestion (basic digestive processes)?
A
Chemical break down of complex macromolecules into smaller absorbable molecules
- Food that we consume are large and X cross plasma mbn of the intestinal epithelial cells
5
Q
What is absorption (basic digestive processes)?
A
Digested material transferred from GIT lumen into blood/lymph
6
Q
Basic digestive processes (motility, secretion, digestion, absorption) in the oral cavity & oesophagus
A
M: Swallowing, chewing
S: Saliva (salivary glands), lipase
D: Carbs, fats (minimal)
A: None
7
Q
Basic digestive processes (motility, secretion, digestion, absorption) in the stomach
A
M: Peristaltic mixing & propulsion
S: HCl (parietal cells), pepsinogen & gastric lipase (chief cells), mucus & HCO3- (surface mucous cells), gastrin (G cells), histamine (ECL cells)
D: Protein, fats
A: Lipid-soluble substances such as alcohol & aspirin
8
Q
Basic digestive processes (motility, secretion, digestion, absorption) in the small intestine
A
M: Mixing & Propulsion primarily by segmentation
S: HCO3- & enzymes (pancreas), bile (liver), mucus (goblet cells), hormones: CCK, secretin, GIP, others
D: Carbs, fats, polypeptides, nucleic acids
A: Peptides → AT; amino acids, glucose & fructose → secondary AT; fats → simple diffusion; water → osmosis, ions; minerals & vitamins → AT
9
Q
Basic digestive processes (motility, secretion, digestion, absorption) in the large intestine
A
M: Segmental mixing; mass movement for propulsion
S: Mucus (goblet cells)
D: None (except by bacteria)
A: ions, water, minerals, vitamins, small organic molecules prod. by bacteria
10
Q
What regulates digestive function?
A
- Autonomous smooth muscle function
- Intrinsic nerve plexuses
- Extrinsic autonomic nerves
- GI hormones
11
Q
Short reflex pathway in the regulation of digestive function
A
Local changes in digestive tract detected by receptors in digestive tract = short reflex to intrinsic nerve plexuses (submucosal plexus & myenteric plexus) =
- smooth muscle (contraction for motility)
- Exocrine gland cells (secrete digestive juices)
- Endocrine gland cells (secrete gastrointestinal & pancreatic hormones
12
Q
Hormonal pathway in the regulation of digestive function
A
Local changes in digestive tract detected by receptors in digestive tract = gastrointestinal hormones =
- smooth muscle (contraction for motility)
- Exocrine gland cells (secrete digestive juices)
- Endocrine gland cells (secrete gastrointestinal & pancreatic hormones
13
Q
Long reflex pathway in the regulation of digestive function
A
- Local changes in digestive tract detected by receptors in digestive tract = stimulates extrinsic autonomic nerves
- External infleunces act directly on extrinsic autonomic nerves
=
- smooth muscle (contraction for motility)
- Exocrine gland cells (secrete digestive juices)
- Endocrine gland cells (secrete gastrointestinal & pancreatic hormones
14
Q
What are the receptors in the digestive tract?
A
- Chemoreceptors
- Mechanoreceptors
- Osmoreceptors
15
Q
What is the oral cavity?
A
Entrance to digestive tract
- Muscular lips help procure, guide, contain food in mouth
16
Q
What are the functions of the oral cavity?
A
Mastication
Initiation of swallowing (deglutition)
Salivary secretion
Taste
MIST
17
Q
What is mastication?
A
Involves mechanical action of slicing, tearing, grinding, mixing of ingested food by teeth
18
Q
What are the functions of mastication?
A
- Breaks food into smaller pieces = facilitate swallowing & inc. SA of food particles
- Mixes food with saliva
- Expose food to taste buds (inc. secretions to prepare for arrival of food)
19
Q
Composition of saliva
A
Water (99.5%)
Electrolytes, proteins, mucus (0.5%)
20
Q
Functions of saliva
A
- Initiates digestion of dietary starch by salivary amylase
- Initiates digestion of some lipids by lingual lipase (activity in adults are vv MINIMAL)
- Some antibacterial effect (lysozyme & salivary IgA)
- Dissolving molecules (taste buds)
- Helps in swallowing, speech, oral health (contains bicarbonate buffer = neutralise acid released by bacteria)
21
Q
Pharynx
A
Cavity at rear of throat
Common passageway for digestive & respiratory tract
22
Q
Swallowing is an _____-or-______ reflex
A
Swallowing is an all-or-none reflex
23
Q
Oropharyngeal stage of swallowing
A
Initiated by tongue voluntarily pushing bolus of food to pharynx
- Food bolus voluntarily pressed by tongue against roof of mouth & backwards towards pharynx
- Activation of pharyngeal pressure receptors = medulla (swallowing centre) initiates reflexes to prevent food entry into resp. passages
- Uvula contracts = blocks nasal passages from pharynx
- Laryngeal muscles contract = close glottis at top of trachea (tightly aligning vocal cords)
24
Q
Oesophageal stage of swallowing
A
Primary peristaltic wave sweeps from the beginning to end of oesophagus, forcing the bolus ahead of it toward the stomach
25
Functions of the stomach
1. Stores food (& releases contents into s. intestine at controlled rate)
2. Begins protein digestion (HCl & enzymes)
3. Mixing movement produces chyme
J-shaped chamber
26
Gastric storage
- Body of stomach
- Receptive relaxation (bc of rugae) = expansion of stomach cavity with little change in intragastric pressure
27
Gastric mixing
- Mainly in pyloric antrum = peristaltic contractions are stronger & more vigorous
- Retropulsion = churning action breaks the food into smaller pieces producing chyme
28
Gastric emptying
Controlled propulsion of chyme into the duodenum w each peristaltic wave
29
Steps in gastric emptying
1. A peristaltic contraction originates in the upper fundus & sweeps down toward pyloric sphincter
2. The contraction becomes more vigorous as it reaches the thick-muscled antrum (bc muscle walls thicker)
3. Strong antral peristaltic contraction propels the chyme forward
4. A small portion of chyme is pushed through the partially open sphincter into the duodenum. Stronger antral contraction = more chyme emptied with each contractile wave
30
What does the upper fundus usually contain?
air
31
Steps in gastric mixing
5. When the peristaltic contraction reaches the pyloric sphincter, the sphincter is tightly closed & no further emptying takes place
6. When chyme that was being propelled forward hits the closed sphincter, it is tossed back into the antrum. Mixing of chyme = when chyme is propelled forward & tossed back into the antrum with each peristaltic contraction (retropulsion)
32
What is gastric emptying largely controlled by?
Factors in the duodenum
33
What are the factors regulating gastric motility & emptying?
Within stomach
- Vol. of chyme
- Degree of fluidity
Within duodenum
- Presence of fat, acid, hypertonicity or distension
Outside the digestive system
- Emotion
- Intense Pain
34
What is the mode of regulation and effects of volume of chyme in the stomach?
(Factors regulating gastric motility & emptying)
Mode of regulation: Distension has direct effect on gastric smooth muscle excitability + acting through intrinsic plexuses, vagus nerve, gastrin
Effects on gastric motility & emptying:
Inc. vol. stimulates motility & emptying
35
What is the mode of regulation and effects of degree of fluidity within stomach?
(Factors regulating gastric motility & emptying)
Mode of regulation: Direct effect --> contents must be in fluid form to be evacuated
Effects on gastric motility & emptying: Inc. fluidity allows more rapid emptying
(less fluid = slower emptying bc food particles still big therefore dec. fluidity)
36
What is the mode of regulation and effects of presence of fat, acid, hypertonicity, or distension in duodenum?
(Factors regulating gastric motility & emptying)
Mode of regulation: Initiates enterogastric reflex/trigers the release of enterogastrones (secretin, cholecystokinin: CCK)
Effects on gastric motility & emptying: Inhibit further gastric motility & emptying until duodenum has coped with factors already present
37
What is the mode of regulation and effects of emotion?
(Factors regulating gastric motility & emptying)
Mode of regulation: Alters autonomic balance
Effects on gastric motility & emptying: stimulates/inhibits motility & emptying
38
What is the mode of regulation and effects of intense pain?
(Factors regulating gastric motility & emptying)
Mode of regulation: Inc. sympathetic activity
Effects on gastric motility & emptying: inhibits motility & emptying
39
What is vomiting?
AKA emesis
- Contraction of respiratory muscles & abdominal muscles
- Stomach, oesophagus & sphincters relaxed
- NOT a function of retroperistalsis in stomach
40
Where is the vomiting center?
Medulla of brainstem
41
Causes of vomiting
- Throat stimulation
- Irritation of stomach
- Elevated intracranial pressure
etc
42
What do mucous cells secrete and what is the function of the secretory product?
(Gastric glands)
Secrete: Alkaline mucus
Function: protects mucosa against mechanical, pepsin, and acid injury
43
What is the stimuli for secretion of alkaline mucus by mucous cells?
Mechanical stimulation by contents
44
What do chief cells secrete and what is the function of the secretory product?
(Gastric glands)
Secrete: Pepsinogen
Function: When activated, begins protein digestion
45
What is the stimuli for secretion of pepsinogen by chief cells?
ACh, gastrin
46
What do parietal cells secrete and what is the function of the secretory product?
(Gastric glands)
Secrete:
- Hydrochloric acid
- Intrinsic factor
Functions:
- HCl: Activates pepsinogen, breaks down connective tissue, denature proteins, kills micoorgs.
- Intrinsic factor: facilitates absorption of Vit. B12
47
State the exocrine cells in the stomach
- Mucous cells
- Chief cells
- Parietal cells
48
What is the stimuli for secretion of HCl and intrinsic factor by parietal cells?
ACh, gastrin, histamine
49
What do Enterochromaffin-like (ECL) cells secrete and what is the function of the secretory product?
(Gastric glands)
Secrete: Histamine
Function: Stimulates parietal cells (helps to inc. acid production by stimulating parietal cells)
50
What is the stimuli for secretion of histamine by ECL cells?
ACh, gastrin
51
What do G cells secrete and what is the function of the secretory product?
(Gastric glands)
Secrete: Gastrin
Function: Stimulates parietal, chief, and ECL cells
52
What is the stimuli for secretion of gastrin by G cells?
Protein products, ACh
53
What do D cells secrete and what is the function of the secretory product?
(Gastric glands)
Secrete: Somatostatin
Function: Inhibits parietal, G, and ECL cells
54
What is the stimuli for secretion of histamine by G cells?
Acid
55
Steps for HCl production
1. CO2 + H2O <-> HCO3- + H+
- Catalyzed by carbonic anhydrase
2. H+-K+ ATPase pump (proton pump)
- Pumps H+ out of the cell by AT
- For every H+ pumped out of cell, one K+ enters the cell
3. Luminal K+ channel allows K+ to passively leak back
4. Cl-HCO3- antiporter
- Accumulation of HCO3 in cell drives tpt of Cl- into cell
- For every HCO3- that exits cell, one Cl- is brought into the cell by secondary AT
5. Cl- channel
- Cl- diffuses out into lumen
56
How does HCl form pepsin?
HCl activates/cleaves pepsinogen into pepsin
- Pepsin can result in activation of pepsinogen (once pepsin forms)
- Pepsin breakdown/cleaves protein (polypeptide) into peptide fragments
57
What are the phases in gastric secretion control?
1. Cephalic phase
2. Gastric phase
3. Intestinal phase
58
What happens in the cephalic phase of gastric secretion?
Prepares the stomach for arrival of food
- Short phase (minutes)
- Inc. production of gastric juice & release of gastrin
59
What happens in the gastric phase of gastric secretion?
Increased secretion (started in previous phase)
Initiates digestion of proteins
- Long phase (3-4 hr)
- Release of histamine + local effect
- Sustained inc. production of gastric juice & inc. motility (mixing)
60
What happens in the intestinal phase of gastric secretion?
Control rate of gastric emptying
- long duration
61
What is the cephalic phase directed by?
(Gastric secretion)
CNS
- via the viagus nerve & submucosal plexus
62
How is the cephalic phase initiated?
(Gastric secretion)
1. Sight/smell/taste/thought of food stimulates the CNS
2. Vagus nerve carries sensory info & synapses with the submucosal plexus
3. Submucosal plexus stimulates mucous cells (= mucus), chief cells (= pepsinogen), parietal cells (= HCl), G cells (= gastrin)
63
How is the gastric phase initiated?
(Gastric secretion)
1. Food in the stomach causes distension
2. Stretch receptors & chemoreceptors detect and stimulate the submucosal & myenteric plexuses
3. Submucosal & myenteric plexuses stimulates mucous cells (= mucus), chief cells (= pepsinogen), parietal cells (= HCl), G cells (= gastrin)
Note: G cells also stimulated to release gastrin by presence of partly digested peptides
64
How is the intestinal phase initiated?
(Gastric secretion)
1. Distension of duodenum = detected by deuodenal stretch & chemoreceptors
2. The receptors triggers enterogastric reflex which stimulates the myenteric plexus
3. Signals to the stomach to dec. peristalsis (dec gastric contractions) + dec. gastrin production
4. The presence of lipids & carbs in the duodenum stimulates CCK release + dec. pH stimulates secretin
5. These GI hormones are carried by blood stream to the chief cells & parietal cells
6. Inhibits the function of the stomach + also signals peristalsis to dec.
7. Feedback inhibition of pepsinogen & HCl production & gastric motility
65
What is the effect of gastrin on gastric function?
Inc. HCl
Inc. Pepsinogen
Inc. gastric motility
Overall inc. gastric function
66
What is the effect of secretin, gastric inhibitory peptide (GIP) & Cholecystokinin (CCK)?
Dec. gastric motility & secretion
67
What are the functions of the liver?
1. Detoxifying/Degrading (body waste, hormones, drugs)
2. Produce proteins (Plasma proteins: clotting factor, carrier tpt proteins, etc)
3. Process nutrients (metabolic processing)
4. Store nutrients (glycogen, fats, vitamins, etc)
5. Vitamin D (activates)
6. Remove bacteria & worn-out RBCs
7. Bile salts (secretion)
8. Cholesterol bilirubin
9. Hormone secretion (e.g. Thrombopoietin)
Dumb Poor People Store Valuable Rings in Blue Cardboard Houses
68
Where does the liver receive blood from?
Hepatic artery
Hepatic portal vein
69
What blood does the hepatic artery deliver to the liver?
Arterial blood
- Provides liver's O2 supply
- Carries blood-borne metabolites for hepatic processing
70
What blood does the hepatic portal vein carry to the liver?
Venous blood draining from the digestive tract
- Carried to liver for processing (detox) & storage of newly absorbed nutrients
71
The blood leaves the liver via which vasculature?
Hepatic vein
72
What does bile contain?
Bile salts
Cholesterol
Lecithin
Bilirubin
etc
DOES NOT CONTAIN DIGESTIVE ENZYMES
73
Where is bile secreted from?
Continuously secreted by the liver (in the duodenum)
- diverted to the gallbladder b/w meals
- bile is concentrated in the gallbladder (therefore higher chance of formation of gallstones)
74
Bile salts are recycled through the _____________ _____________
Bile salts are recycled through the enterohepatic circulation
75
What are bile salts?
Derivatives of cholesterol
76
What is enterohepatic circulation?
Recycling of bile salts b/w the small intestine & liver
77
How is bile redirected to the gallbladder in between meals?
- In between meals, the sphincter of Oddi (aka hepatopancreatic sphincter) is closed
- Bile travelling down the common bile duct towards the duodenum is unable to enter the duodenum (bc sphincter is closed)
- Bile is redirected back up to the gallbladder
78
What percentage of bile salts are reabsorbed by the terminal ileum?
What percentage of bile salts are lost in faeces?
95% reabsorbed
(recycled by blood)
5% lost
79
Secreted bile salts consist of ____ % of old, recycled bile salts and ____ % newly synthesized bile salts
Secreted bile salts consist of 95 % of old, recycled bile salts and 5 % newly synthesized bile salts
80
What are the functions of bile?
1. Aid fat digestion & absorption
2. Excretion of water insoluble substances
81
What is the detergent action of bile salts?
Bile salts can convert large fat globules --> lipid emulsion
Lipid emulsion: many small fat droplets (suspended in aqueous chyme)
82
Formation of micelles by bile salts
Bile salts & lecithin aggregate in small clusters with their fat-soluble parts huddled tgt in the middle to form a hydrophobic ("water-fearing") core
Basically, allows water & oil to mix tgt
83
What water insoluble substances are excreted?
E.g. cholesterol & bilirubin
Bilirubin is a waste product & has no role in digestion
84
Effect of cholecystokinin (CCK) on gallbladder
1. CCK secretion is stimulated by presence of fats in the duodenum
2. Inc in CCK secretion = inc in plasma CCK
3. CCK promotes gallbladder contraction (= emptying) & relaxation of sphincter of Oddi (hepatopancreatic sphincter)
4. Inc. bile flow into duodenum
85
What are the most common liver disorders?
Hepatitis
Cirrhosis
86
What is the pancreas?
- Mixture of exocrine and endocrine tissue
- Elongated gland --> lies behind & below stomach
87
What is the exocrine function of the pancreas?
- Acinar cells secrete pancreatic enzymes (works well in slightly alkaline pH)
- Duct cells secrete aqueous alkaline solution (neutralises acidic chyme)
88
What are the pancreatic enzymes?
- Pancreatic amylase
- Pancreatic lipase
- Pancreatic proteolytic enzymes (Proteases)
89
What does pancreatic amylase do?
Breaks down complex carbohydrates --> starch & glycogen
- Secreted in active form
90
What does pancreatic lipase do?
Main enzyme for fat digestion in GIT
- Hydrolyses dietary triglycerides into monoglycerides & free fatty acids
- Secreted in active form
91
What does pancreatic proteolytic enzymes (proteases) do?
Secreted as inactive form:
- trypsinogen
- chymotrypsinogen,
- procarboxypeptidase
Trypsinogen activated by enteropeptidase to trypsin
- Trypsin then activates other enzymes
Chymotrypsinogen --> Chymotrypsin
Procarboxypeptidase --> carboxypeptidase
- Trypsin is also autocatalytic (can activate trypsinogen)
92
What are the pancreatic proteolytic enzymes (proteases)?
Trypsin
Chymotrypsin
Carboxypeptidase
93
What is the aqueous alkaline solution secreted by the pancreas?
- High conc. of bicarbonate (HCO3-) used to produce the secretion
- Neutralizes acidic chyme (bc. pancreatic enzymes optimal in neutral/slightly alkaline env.)
94
What is pancreatic insufficiency?
- Deficient of pancreatic enzymes
- Dietary fat digestion seriously impaired (bc. fats mostly digested by pancreatic enzymes)
95
How is the pancreatic aqueous NaHCO3- secretion stimulated?
1. Acid in duodenal lumen causes inc. in release of secretin from duodenal mucosa
2. Secretin carried by blood to the pancreatic duct cells
3. Stimulate the pancreatic duct cells to inc. secretion of aqueous NaHCO3 solution into duodenal lumen
4. Aqueous NaHCO3 solution neutralizes the acid in duodenal lumen
96
How is the pancreatic digestive enzymes secretion stimulated?
1. Fat and protein products in duodenal lumen cause inc. CCK release from duodenal mucosa
2. CCK carried by blood to the pancreatic acinar cells
3. Stimulate the pancreatic acinar cells to inc. secretion of pancreatic digestive enzymes into duodenal lumen
4. Pancreatic digestive enzymes digests fat & protein
97
List all the functions of CCK
1. Inhibits gastric motility & secretion
2. Stimulates pancreatic enzyme secretion
3. Stimulates contraction of gallbladder & relaxation of hepatopancreatic sphincter
4. Inc. production of bile
(Stimulated by chyme in duodenum)
98
List all the functions of gastrin
1. Inc. secretion of HCl & pepsinogen
2. Enhances gastric motility
(Stimulated by protein in stomach)
99
List all the functions of secretin
1. Inhibits gastric emptying & gastric secretion
2. Stimulates pancreas to produce HCO3-
3. Stimulates secretion of bile
(Stimulated by acid in duodenum)
100
State the functions of gastric inhibitory peptide (GIP)
1. Promotes metabolic processing of nutrients once they are absorbed
2. Stimulates release of insulin from pancreas
101
State the functions of vasoactive intestinal peptide (VIP)
1. Stimulate secretion of intestinal glands
2. Dilation of intestinal capillaries = facilitates nutrient absorption
3. Inhibits acid production in stomach
102
Motility of the small intestines
1. Peristalsis
2. Segmentation contractions
103
What is peristalsis?
- Waves of muscular contractions
- Moves bolus along the length of digestive tract
- Oral to anal direction
104
What is essential for peristalsis?
Myenteric plexus
105
Steps in peristalsis
1. Contraction of circular muscles behind the bolus
2. Contraction of longitudinal muscles ahead of bolus
3. Contraction in circular muscle layer forces bolus forward
106
What is the function of segmentation contraction?
- Mixes chyme with digestive juices
- Exposes chyme to all absorptive surfaces
107
What is segmentation contraction?
Oscillating, ringlike contractions every few cm
(mixes & slow propels chyme)
108
Which part of the small intestine has a higher rate of segmentation contraction?
Higher rate at the start of the small intestine
- 12 contractions/min
Lower rate at the end: terminal ileum
- 9 contractions/min
109
Peristalsis vs segmentation contraction
Peristalsis:
- 1 ring contract & move a certain distance
- Moves along FAST
- Can occur simultaneously
Segmentation contraction
- Multiple rings (9-12) contract at the same time (alternates b/w contracting & relaxing BUT DOES NOT contract & move)
- Propels slowly
- Can occur simultaneously
110
Where is the myenteric plexus located ?
B/w longitudinal & circular muscle layers
- Part of the enteric plexus
111
What does the myenteric plexus control and what stimulates it?
Controls mainly gastrointestinal movements
Stimulated by receptors in digestive tract (stretch receptors, chemoreceptors)
112
What is the migrating motility complex?
Peristaltic movement
- sweeps the intestine clean b/w meals
113
What is the function of the ileocecal juncture?
Prevents contamination of the small intestine by bacteria in the colon (basically prevents the bacteria from entering the small intestine)
- consists of ileocecal valve & sphincter
114
Where are enterocytes located?
they are localized at the brush border of the small intestines
115
What enzymes do enterocytes secrete into the small intestines?
1. Enteropeptidase
2. Disaccharidases
3. Aminopeptidases
116
What is enteropeptidase?
- small intestine brush border enzyme
- needed for activation of trypsinogen --> trypsin
117
What are disaccharidases?
- Maltase
- Lactase
- Sucrose-isomaltase
- Breaks down disaccharides --> monosaccharides
118
What are aminopeptidases?
Breaks down peptide --> a.a.
119
List the monosaccharides
1. Fructose
2. Glucose
3. Galactose
120
List the disaccharides
1. Sucrose
2. Maltose
3. Lactose
121
List the polysaccharides
1. Starch (plant source)
2. Glycogen (animal source)
Long chains of glucose
- Broken down by amylase
122
What is protein?
1 (more than 50 aa.a.) or more polypeptide chain (with 3D folding)
123
What are polypeptides?
Long chains of amino acids (>10)
124
What are peptides?
Short chain of a.a. monomers linked by peptide bonds
125
State the
1. Enzymes for digesting nutrient
2. Source of enzymes
3. Site of action of enzymes
4. Action of enzymes
5. Absorbable units of nutrients
For carbohydrate (amylase)
1. Amylase
2. Salivary glands; Exocrine pancreas
3. Mouth & (mostly) body of stomach; Small-intestine lumen
4. Hydrolyzes polysaccharides to disaccharides & α-limit dextrins
5. Monosaccharides, esp. glucose
126
State the
1. Enzymes for digesting nutrient
2. Source of enzymes
3. Site of action of enzymes
4. Action of enzymes
5. Absorbable units of nutrients
For carbohydrate (Disaccharidases: maltase, sucrase-isomaltase, lactase)
1. Disaccharidases: maltase, sucrase-isomaltase, lactase
2. Small intestine epithelial cells
3. Small-intestine brush border
4. Hydrolyze disaccharides to monosaccharides
5. Monosaccharides, esp. glucose
127
State the
1. Enzymes for digesting nutrient
2. Source of enzymes
3. Site of action of enzymes
4. Action of enzymes
5. Absorbable units of nutrients
For protein (pepsin)
1. Pepsin
2. Stomach chief cells
3. Stomach antrum
4. Hydrolyzes protein to peptide fragments
5. Amino acids & a few small peptides
128
State the
1. Enzymes for digesting nutrient
2. Source of enzymes
3. Site of action of enzymes
4. Action of enzymes
5. Absorbable units of nutrients
For protein (trypsin, chymotrypsin, carboxypeptidase)
1. Trypsin, chymotrypsin, carboxypeptidase
2. Exocrine pancreas
3. Small-intestine lumen
4. Attach different peptide fragments
5. A.a. & a few small peptides
129
State the
1. Enzymes for digesting nutrient
2. Source of enzymes
3. Site of action of enzymes
4. Action of enzymes
5. Absorbable units of nutrients
For protein (Aminopeptidases)
1. Aminopeptidases
2. Small-intestine epithelial cells
3. Small-intestine brush border
4. Hydrolyze peptide fragments to amino acids
5. A.a. & a few small peptides
130
State the
1. Enzymes for digesting nutrient
2. Source of enzymes
3. Site of action of enzymes
4. Action of enzymes
5. Absorbable units of nutrients
For fat (lipase)
1. Lipase
2. Exocrine pancreas
3. Small-intestine lumen
4. Hydrolyzes triglycerides to fatty acids & monoglycerides
5. Fatty acids & monoglycerides
131
State the
1. Enzymes for digesting nutrient
2. Source of enzymes
3. Site of action of enzymes
4. Action of enzymes
5. Absorbable units of nutrients
For fat (bile salts)
1. Bile salts (not an enzyme!)
2. Liver
3. Small-intestine lumen
4. Emulsify large fat globules for attack by pancreatic lipase
5. Fatty acids & monoglycerides
132
What is the adaptation for absorption of the small intestine?
Inc. SA of small intestine - Inc. rate of absorption
- Circular fold in small intestine
- Villus
- Microvilli (brush border)
133
Which nutrients are absorbed via simple diffusion?
Monoglycerides & fatty acids
etc
134
Which nutrients are absorbed via facilitated diffusion?
Fructose, a.a.
Water-soluble vitamins
etc
135
What is facilitated diffusion?
Requires carrier protein
but NOT energy dependent
136
What nutrient is absorbed via active transport?
Sodium
137
What nutrients are absorbed via secondary active transport?
Glucose, a.a. & peptides
etc
138
What is secondary active transport?
When primary AT is working, it creates a differential concentration gradient of molecules
- uses this to pull another molecule in/out of cell
- secondary AT is dependent on energy of primary AT
139
Steps in facilitated diffusion
1. Carrier protein takes conformation in which solute binding site is exposed to region of higher conc. (basically conformation site on area with higher conc.)
2. Solute molecule binds to carrier proteins
3. Carrier protein changes conformation so that binding site is exposed to region of lower conc.
4. Transported solute is released & carrier protein returns to conformation in step 1
140
Steps in the sodium potassium pump
1. Sodium-potassium pump has 3 high-affinity sites for Na+ & 2 low-affinity sites for K+ when exposed to ICF
2. When 3 Na+ from ICF (where Na+ conc. is low) bind to pump, it splits ATP into ADP + phosphate; phosphate group binds to pump
3. Phosphorylation causes pump to change conformation so the Na+ binding sites are exposed to opp. side of mbn and the 3Na+ are released to ECF (where Na+ conc. is high) + affinity of Na+ binding sites dec.
4. Conformation change also exposes pump's binding sites for K+ to ECF + infinity of K+ inc.
5. When 2K+ from ECF (where K+ conc. is low) = binds to pump & releases phosphate group. Dephosphorylation causes pump to revert to original conformation
6. Two K+ released to ICF (where K+ conc. is high) as affinity of K+ binding sites dec. during change in shape + affinity of Na+ binding sites inc.
141
What is a symport?
Membrane protein that moves two or more different molecules across a cell mbn in the same direction
142
What is an antiport?
Membrane protein that moves two or more different molecules across a cell mbn in the opposite direction
143
What are the absorbable units of lactose?
Galactose + Glucose
- Broken down by lactase
144
What are the absorbable units of maltose?
Glucose + Glucose
- Broken down by maltase
145
What are the absorbable units of sucrose?
Glucose + Fructose
- Broken down by sucrase-isomaltase
146
What are the steps in carbohydrate absorption?
1. Starch & glycogen converted into disaccharide maltose by salivary & pancreatic amylase
2. Maltose & lactose & sucrose converted to their respective monosaccharides (by disaccharides located in brush borders)
3. Glucose + galactose absorbed into epithelial cells by Na+ & secondary AT located in luminal mbn
4. Fructose enters cell by facilitated diffusion via GLUT-5. Glucose, galactose, fructose exit cell at basal mbn by facilitated diffusion via GLUT-2
6. Monosaccharides enter the blood by simple diffusion
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Steps in protein absorption
1. Proteins hydrolyzed into a.a. & few small peptide fragments by gastric pepsin & pancreatic proteolytic enzymes
2. Small peptide fragments converted into a.a. by aminopeptidases (in brush borders)
3. a.a. are absorbed into epithelial cells by Na+/amino acid symport (secondary AT)
4. Small peptides absorbed by H+/small peptides symport (tertiary AT)
5. Most absorbed small peptides broken down into a.a. by intracellular peptidases
6. a.a. exit cell at basal mbn via passive carriers
7. a.a. enters blood by simple diffusion
148
Steps in fat digestion
1. Fat emulsified by bile salts into a suspension of smaller fat droplets
2. Lipase hydrolyzes triglycerides into monoglycerides & free fatty acids
3. Water-insoluble products carried to luminal surface of small-intestine epithelial cells within micelles
4. When a micelle approaches the absorptive epithelial surface, monoglycerides & fatty acids leave the micelle & passively diffuse through the lipid bilayer of luminal mbns
149
Steps in fat absorption
1. Monoglycerides & fatty acids are resynthesized into triglycerides inside epithelial cells
2. Triglycerides aggregate & are coated with a layer of lipoprotein from the endoplasmic reticulum = form water-soluble chylomicrons
3. Chylomicrons extruded through the basal mbn of cells by exocytosis
4. Chylomicrons unable to cross the basement mbn of capillaries so enter lymphatic vessels = central lacteals (will enter bld eventually through lymphatic system)
150
How is water reabsorbed?
Passive absorption
- Active reabsorption of electrolytes & nutrients create osmotic gradient
- Na+ - K+ ATPase creates a localised concentrated area of high osmotic pressure
151
What happens to water in the duodenum?
Flow into the lumen due to hyperosmotic chyme
- Chyme in duodenum is hypersomatic (a lot of solutes) = retains water well
152
What happens to water in jejunum & ileum?
Water is absorbed
- Nutrient molecules get absorbed first, followed by water
153
Which vitamins are water soluble?
- Vitamin B complex (all vit B except B12)
- Vitamin C
etc
Mostly by carrier mediated mechanisms (transporters)
154
Which vitamins are fat-soluble vitamins?
Vitamin A, D, E, K
etc
Carried in micelles & absorbed with other lipids by simple diffusion
155
How is Vitamin B12 absorbed?
- Needs receptor to bind with B12 and gastric intrinsic factor = forms a complex
- Then absorbed by receptor-mediated endocytosis
156
Which form of iron is absorbed best?
1. Heme iron - absorbed best
2. Fe2+ (ferrous iron)
3. Fe3+ (ferric ion) - absorbed...
157
How does vitamin C inc. iron absorption?
Vitamin C reduces Fe3+ to Fe2+
158
What is required for effective absorption of calcium?
Activation of Vitamin D in the kidney/liver
159
How much of daily ingested iron is absorbed?
Slightly <10%
women > men
160
In the small intestine, what is _________ will be _________.
In the small intestine, what is secreted will be absorbed
E.g. secrete 9.5L, absorb around 9L
Extensive absorption by small intestine keeps pace with secretion
161
Whatever that is absorbed into the digestive capillaries must pass through what first?
Must pass through the hepatic biochemical factory
Except lipid --> bypass this as they enter the lymphatic system
162
What has to be maintained among the stomach, pancreas, small intestine?
Biochemical balance
- Body usually X experience a net gain/loss of acid/base during digestion
163
What is diarrhoea?
Passage of a highly fluid faecal matter, often with inc. frequency of defaecation
164
What does diarrhoea result in?
1. Dehydraton
2. Loss of nutrients (X time for absorption)
3. Metabolic acidosis = lose a lot of alkaline fluids from intestine
165
What are the causes of diarrhoea?
1. Excessive small intestinal motility
2. Excess osmotically active particle in lumen (hold onto water = too much fluid in intestine)
3. Toxins from Vibrio cholera etc = food poisoning
166
Which anal sphincter can be voluntarily controlled?
External anal sphincter
167
Types of anal sphincter
1. Internal anal sphincter = smooth muscle = X be voluntarily controlled
2. External anal sphincter = skeletal muscle = Can be voluntarily controlled
168
Main purpose of the large intestine
Primarily a drying & storage organ
- forms & store faeces
Receives indigestible food, unabsorbable & remaining fluid components
169
What does the large intestine secrete?
Mucus & some ions
170
What does the large intestine digest?
Nothing
BUT bacteria present digest food (that we can't digest) & produce Vit K, Vit B12 & folate, etc
171
What does the large intestine absorb?
Water & ions
172
What does the haustra do (large intestine)?
Contractions of the haustra slowly shuffle the colonic contents back & forth
- Initiated by the rhythmic contractions of the colonic smooth muscles
- Slow & nonpropulsive
173
How is faeces propelled long distances in the large intestine?
Mass movement propels into the distal part of the large intestines (to the signmoid colon & rectum)
- Large segments of the asc. & transverse colon contract simult.
174
What is the purpose of haustra contractions?
For absorptive & storage function
X propel forward
175
What is the defecation reflex initiated by?
Distension of the rectum
- Stimulates stretch receptors in the rectal wall
176
What is the defecation reflex?
Muscle contraction with relaxation of both the internal & external anal sphincter muscle
[can be blocked - e.g. you don't go toilet; after some time urge to defecate subsides
177
What helps defecation?
Voluntary straining movements increases intraabdominal pressure
- Contraction of abdominal muscles (bc cannot control large intestine muscles directly)
- Forcible expiration against closed glottis
178
What are the two positive feedback loops in the defecation reflex?
1. Short intrinsic myenteric defecation reflex
2. Parasympathetic defecation reflex (long reflex)
Both works together
179
What is the short intrinsic myenteric defecation reflex?
Stretch receptors stimulate the plexus to initiate inc. local peristaltic contractions in sigmoid colon & rectum
- mediated by myenteric plexus
- Impt for movement in rectum
180
What is the parasympathetic defecation reflex (long reflex)?
Stimulate mass movements in descending and sigmoid colon
- Involves the spinal cord
- Impt for movement in desc. & sigmoid colon
181
Steps in the defecation reflex
1. Faeces move into rectum causing distension, stimulating stretch receptors
2. The first loop = short reflex that triggers a series of peristaltic contractions in the rectum = moves faeces toward anus. Internal anal sphincter relaxes
3. The long reflex = spinal reflex coordinated by sacral parasympathetic system = stimulates mass movements that push faeces towards rectum from desc. colon & sigmoid colon. Further relaxes internal anal sphincter
4. Voluntary releaxation of the ext. anal sphincter allows defecation
182
What is constipation?
- When faeces become too dry
- Abdominal discomfort, dull headache, loss of appetite, nausea, etc
183
What causes constipation?
- Ignoring urge to defecate = keep absorbing water
- Decreased colonic motility
- Obstruction (e.g. tumour)
- Impairment of defecation reflex (neuro pathways)
184
Causes of intestinal gases (flatus)?
- Swallowed air
- Bacterial fermentation (Contains beneficial bacteria = microbiota & microbiome)
185
What does colonic secretion consist of?
Alkaline (NaHCO3) mucus solution = protective function
186
How is water & salt absorbed by the large intestine?
Once salt & water are absorbed --> luminal contents converted to faeces
- Some absorption takes place within colon but majority in small intestine
187
How are water and salts absorbed by the large intestine?
- Na+ actively absorbed
- Cl- follow down electrical gradient
- Water follows osmotically
