Download Secretion, Digestion, Absorption - University of Sri Jayewardenepura

Digestive System
Dr. Dinithi Peiris
Dept. of Zoology
University of Sri
Jayewardenepura
1
Definitions
A.  Ingestion = to bring food into the digestive
system
B.  Digestion = to break food down into smaller
molecules that can be absorbed by the body
a)  1. Mechanical – physical, makes use of
chewing, grinding, etc.
b)  2. Chemical - makes use of enzymes
C.  Egestion = elimination of non-digestible
materials
2
Four basic process of GI
Digestion
l  Absorption
l  Motility
l  secretion
l 
3
1
Digestive Organs
TRUE
l  food
passes through
l  Alimentary Canal
• one way passage for
food/nutrients/waste
l  mouth--->
anus
ACCESSORY
l  no food passes
through them
l  secrete enzymes
that aid in
digestion
ex. Liver,
salivary glands
4
Digestive Function:
Integrated Steps
Digestion: the preparation of food by the
animal for absorption
It refers to the chemical breakdown of food into
small organic fragments suitable for absorption by
the digestive epithelium
5
Digestive Function:
Integrated Steps
Mechanical processing is physical
manipulation and distortion that makes
materials easier to swallow and increase
the surface area for enzymatic attack.
Ingestion occurs when materials enter
the digestive tract via the mouth
6
2
Digestive Function:
Integrated Steps
Physical
• small particles
produced from large
ones
• food -> food particles
• change in particle's
physical state
ex. Chewing
Chemical
• food is digested
into components
• food -> nutrients
ex. Starch Δ
monosaccharides
7
Digestive Function:
Integrated Steps
Secretion is the release of water, acids,
enzymes, buffers and salts by the epithelium
of the digestive tract and by glandular organs
Absorption is the movement of organic
substrates, electrolytes, vitamins and water
across the digestive epithelium and into the
interstitial fluid of the digestive tract
8
Digestive Function:
Integrated Steps
Excretion is the elimination of waste
products from the body.
Secreted into the lumen of the tract
Most will leave the body through defecation
9
3
Mouth--chewing
Path of Food
Pharynx--conscious swallowing
Esophagus--transport to stomach
Stomach--mechanical and chemical breakdown
Small Intestines--
chemical digestion and absorption
Large Intestines--
reabsorb water, form feces
Rectum---collect and expel feces
10
Major Components:
Accessory Organs:
Human elementary
MOUTH(ORAL CAVITY)
Canal
PHARYNX
ESOPHAGUS
STOMACH
LIVER
GALLBLADDER
SMALL INTESTINE
PANCREAS
LARGE INTESTINE (COLON)
RECTUM
ANUS
11
Fig.37.6, p. 616
Digestion
12
4
Mouth or the oral cavity
• 
It is the site for mechanical & chemical digestion
• 
Mechanical digestion begins as a person bites with
grinding and chewing with teeth.
• 
Mammalian teeth are specialized for specific
functiono small fragments
13
Mouth or the oral cavity
• 
During this process food is broke down into small
fragments
• 
This will increase the surface area for enzymatic
action
14
Jaw, Cheek & Teeth
l 
Used for storage- rodents and squirrels
l 
Modified placoid scales- sharks
l 
Polyhyodont- permanent replacement of teeth
l 
Diphyodont- two sets of teeth
l 
Monophyodont- one set of teeth
15
5
Mammalian Teeth
l 
Incisors
l 
Canines
•  For cutting
•  Ex: elephant tusks
•  For piercing
•  Ex: walrus tusks
Figure:
Mammalian
teeth
l  Diastema- space without teeth;
specializations.
l 
Premolars & Molars
•  To matriculate food
e.g., no canines
16
Salivary glands
l 
l 
Produce saliva
•  Prevents bacterial
infection
•  Lubrication
•  Contains salivary amylase
Three pairs
•  Parotid: Largest
•  (25%)
•  Submandibular
•  (70%)
•  Sublingual: Smallest (5%)
17
Composition of saliva
Hypo-osmotic solution ( 97-99.5%
water)
l  pH 6.8 – 7.4
l  proteins: amylase, lysozyme, albumins,
globulins, lactase
l  Na, K, phosphate, bicarbonate, chloride
l  Mucin – a glyco protein for lubrication
l 
18
6
Composition of saliva
Hormones : estrogen, progesterone,
prolactin
l  Metabolic wastes: urea, uric acid
l 
l 
Resident bacteria reduce nitrate (NO3-) into
nitrite (NO2-), converted into NO (nitric
oxide), esp. in acidic environment (around
gums, acids produced by bacteria), NO bactericical
19
Functions of saliva
Lysozymes, "
Proxidases"
Urea,"
Uric acid"
Thiocyanate
"
Absorb "
Latent heat"
Carbonic anhydrases"
."
Cleansing" Buffering "
& protec-"
Amylases,"
tion"
Lactase"
Excretory"
Digestion" Lipase"
products"
Thermo"
regulation"
Water "
intake"
Dryness "
Lubricat-"
Help in" Ion & "
speech" swallowing"
by keeping the"
Mouth & tongue"
moist"
Mucins"
20
Control of salivation
•  1-1.5 / day
•  Controlled by the parasympathetic &
sympathetic nervous system
•  Control are of 2 types
•  Unconditional reflexes – present at birth
food in mouth → brain stem
→parasympathicus output → increased
salivation esp. in response to acidic foods
(fruit juices, vinegar) or hot spices
21
7
Control of salivation
•  Conditional reflexes – learned by previous
experiences
sight, smell, hearing of food preparation,
thought of food
This is triggered via cortical stimulation
22
Control of salivation
l 
Effect of sympathetic nervous system
• decreases overall secretion
• change to thick, mucous secretion
• strong sympathetic stimulation → vasoconstriction
of vessels of serving glands → near cessation of
salivation (dry mouth, difficulty swallowing,
talking)
• accumulation
of food particles → enhanced
bacterial growth → halitosis
23
Control of salivation
l 
Salivary glands Infection
• Mumps (myxovirus infection) spread to testes in male
adults → 25% chance of sterility
24
8
Deglutition (swallowing)
l 
l 
l 
Tongue mixes food pieces & secretions
producing a bolus
Pharynx initiate swallowing
Swallowing occurs when the tongue moves the
bolus to back of the mouth
25
Deglutition (swallowing)
Swallowing occurs in three phases
•  Buccal phase (Voluntary)
•  Bolus of food moved by tongue from oral
cavity to pharynx
• Pharyngeal (involuntary)
•  Initiated when the tongue forces bolus back
into pharynx
26
Deglutition (swallowing)
Reflex: soft plates rises to block naso-pharynx
•  Upper esophageal sphincter relaxes,
•  Elevated pharynx opens to the esophagus,
• 
Peristalsis movement pushes food into
esophagus
• Esophageal
Reflex: Epiglottis is tipped posteriorly, larynx
elevated to prevent food from passing into
larynx
27
9
Deglutition (swallowing)
Esophageal Phase
•  P e r is t a ls is ( a lt e r na t e c o nt r a c t io ns a nd
relaxations of smooth muscles forming wave)
continues to mix food with saliva
•  Mucus secreted but no enzyme
•  Cardiac sphincter opens in response to
peristalsis moving food into stomach and closing
to prevent food back up
•  No absorption or digestion in esophagus
28
Deglutition (swallowing)
29
Mechanisms of peristalsis
• Peristalsis is a reflex initiated by presence
bolus in esophagus
of
• C ircular muscle layer contracts behinds
pushing bolus down & relaxes in front
•  Longitudinal smooth muscle layer contracts
(shortening) moving wall to receive bolus
•  Rate 2 – 4 cm per sec
30
10
Mechanisms of peristalsis
31
Stomach
Thick
walled,
saclike
structure
.
l 
l 
Openings
•  Gastroesophageal:
To esophagus
•  Pyloric: To
duodenum
Regions
•  Cardiac
•  Fundus
•  Body
•  Pyloric
32
Digestion in stomach
–  G a s t r i c m u c o s a i s fo l d e d i n t o
invaginations (gastric pits)
–  At the base of gastric pits, Gastric glands
are located
–  Highly convoluted inner surface stretches
during meals
–  50 ml capacity when empty; 2L – 4L
capacity when full
.
33
11
Gastric secretion
• Stomach
secretes 2L of gastric juice/day. Highly
acidic due to presence of HCL
• Gastric mucosa is folded into invaginations (gastric
pits) At the base of gastric pits, Gastric glands are
located
• Mucosal
epithelial cells: Divide at a rate higher
than elsewhere. Entire stomach mucosa is replaced
every 3 days
34
stomach
35
Gastric glands
Six types of gastric glands are present
1. Parietal cells: release HCl and intrinsic factor
(vitamin B 12 absorption)
2.Chief cells: release Pepsinogen, rennin, lipase,
amylase
3. Mucous cells: release mucus (thick
4. Enterochromaffin – like cells: release histamine
5. G cells: release gastrin
6. D cells: secrete somatostatin
36
12
Composition of gastric juice
1. Water (99.5%)
2. HCl (40 – 60 meq / L)
3. Enzymes
4. Gastric mucin
5. Inorganic salts – Na, K, Ca & Mg
6. Intrinsic factor
37
Functions of stomach
l 
l 
l 
l 
Mechanical breakdown of food – smooth muscle
in wall
Mixing of food – via contraction of smooth
muscles
Protein breakdown – pepsin secreted by
epithelial lining
Acidic conditions – for pepsin to work & to kill
bacteria
38
Functions of stomach
l 
Bulk storage of undigested food in the fundus
l 
Absorption of water, ions & some drugs (e.g.
aspirin, alcohol)
l 
Preliminary digestion of proteins
l 
Fat digestion: Fat + Gastric lipase = fatty acid and
glycerol
Inactive pepsinogen + HCL = Active pepsin
Protein + Pepsin (pH – 2) = Peptone (smaller chains)
39
13
Summary
40
PBL
A student complained a stomach ache during
night, Her room-mate suggests that she should
one of the new acid blockers that she has seen
on TV. These drugs are known as histamine
receptor blockers and bind competitively to H2 –
type histamine receptors. Fortunately the roommate has just bought a packet of drugs. Two
hours after taking the drug the student felt better.
41
HCl acid production
42
14
Phases of Gastric Secretion
•  food in mouth & in stomach
G cells
Gastrin
Direct
&
Indirect
43
Phases of Gastric Secretion
Gastrin
Direct
Parietal cells
Indirect
Histamine (ECL)
&
Combine with H2- R on parietal
cells
HCl secretion
44
Phases of Gastric Secretion
• HCl production is stimulated:
Indirectly by gastrin.
Indirectly by ACh.
• ACh and gastrin stimulate release of
histamine.
• Histamine:
Bind to histamine receptors &
stimulates parietal cells to
secrete HCl.
45
15
Control of gastric secretion
•  Cephalic phase: Excitatory. Smell, taste,
thought etc. initiates gastric secretion before
food reaches stomach (30%)
•  Gastric phase: Excitatory. Presence of food
increases gastric secretion. Mixing waves
begin (70%)
•  Intestinal phase: Inhibitory. Presence of food
in duodenum decreases gastric secretion
46
Phases of Gastric Secretion
47
Movements in Stomach
Gastric motility –
• Filling phase
• Mixing phase
• Emptying phase
48
16
Gastric Movements
•  Pyloric sphincter opens and closes in
response to nervous and endocrine control
•  Nervous: Via stretch receptors. When
duodenum is distended, sphincter is closed
•  Endocrine: Secretin and CCK released by SI
inhibit opening. Hormone release controlled
by
- Acidic chyme
- High fat content
49
Protective mechanisms of stomach
•  Parietal and chief cells impermeable to HCl
•  Alkaline mucus contains HC03-.
•  Tight junctions between adjacent epithelial
cells.
•  Rapid rate of cell division (entire
epithelium replaced in 3 days).
•  Prostaglandins inhibit gastric secretions
50
Gastritis, Gastric Ulcers
• breaching
of the mucosal barrier →
inflammation → persistent gastritis →
erosion of underlying tissue → ulcer
• pain, risk of hemorrhage
• triggered
by hyper secretion of HCl, hypo
secretion of mucus, 90% of affected patients
carry Helicobacter pylori
51
17
Gastritis, Gastric Ulcers
l 
l 
l 
l 
Bacteria release cytotoxins and chemotactic
proteins → immune response (suspected of
increasing risk for stomach cancer)
Bacterium also found in >40% of healthy people
Treatment: Antibiotics, H2-antihistamines,
cimetidine, ranitidine (histamine R blocker)
Antacids neutralize HCl - (Al(OH)3, Mg(OH)2
52
Digestion in small intestine
•  Three subdivisions: Duodenum, Jejunum, Ileum
•  Has villi and microvilli to increase absorptive surface
•  Food movement is slow through small intestine (3-5
hrs)
•  Chemical digestion is completed in small intestine.
Most of the absorption is also done here
53
Intestinal juice
Accessory organ
secretion into small
intestine
1. Pancrease
54
18
Pancreas
l 
l  Secretions
Anatomy
Composition of
• Regions: Head, body, tail • pancreatic
juice
• Endocrine
(exocrine)
• Pancreatic islets
produce insulin and
• pH: 8.00 – 9.00
glucagon
• Water (98%)
• Exocrine
• Alkaline slats
• Acini produce digestive
• Enzymes
enzymes
• Mucus
55
Pancreas
l 
Digestive enzymes
•  Trypsinogen, Chymotrypsinogen
•  Pro carboxypeptidase
•  Pancreatic amylase
•  Pancreatic lipases
•  Enzymes that reduce DNA and ribonucleic acid
•  Cholesterol esterase
•  Phospholilpases
56
Bicarbonate Ion Production
57
19
Regulation of Pancreatic Juice
l  HCO3-
& enzymes are secreted by different types
of cells
Acidity in the chyme
HCO3Neural
Hormonal
Parasympathetic nerves
CCK - Cholecytokinin
(Ach)
Act on duct cell via R
Release of HCO ions
58
Regulation of pancreatic juice
Distention, food in the upper intestine
Neural
Hormonal
Entero-pancreatic reflex
CCK - cholecytokinin
(Ach)
Act on aciner cells via R
Release enzyme rich secretion (effect by the diet)
59
Regulation of pancreatic juice
Somatostatin, pancreatic polypeptides IB secretion
3 phases: Cephalic, Gastric, Intestinal
• Cephalic : vagus – direct
secretin releasing peptides
enzymes
secretin
enzymes
60
20
Regulation of pancreatic juice
Gastric : gastrin
distention
enzymes
gastro pancreatic reflex
Intestinal : H+ + fats
AA s, fats
vago-vagal
Secretin
CCK
enzymes
HCO3 fluid
enzyme secretion
enzyme secretion
This is the major phase
61
Pancreatic Secretion Control
62
2. Liver
l 
Bile
•  Produced by liver
•  Stored in gallbladder
• Bile: breaks down fats
=
emulsification
63
21
Bile production and secretion
• The liver produces and secretes 250 – 1500 ml of
bile/day.
• Made in liver cells from cholesterol
cholesterol oxidized in to bile acids
(chenodeoxycholic acid and cholic acid)
conjugated to glycine or taurine (amino acids)
conjugation of bile acids with amino acids
improves its ability to form micelles
64
Bile production and secretion
• Bile pigment (bilirubin) is produced in spleen,
bone marrow, and liver.
• Free bilirubin combines with glucuronic acid and
forms conjugated bilirubin.
• Secreted into bile.
• Converted by bacteria in intestine to urobilinogen.
65
Bile production and
secretion
• Urobilogen is absorbed by intestine and enters
the hepatic vein.
• Recycled, or filtered by kidneys and
excreted in urine.
• Stones:
high cholesterol & low bile salts or
lecithin
ppt of cholesterol
66
22
Other functions of liver
1. Storage:
l 
l 
l 
Excess glucose is converted to insoluble glycogen
(insulin needed) and stored. E.g: After a heavy
meal, blood glucose level rises/after exercise.
When blood glucose level drops, induces liver to
convert glycogen back to glucose which enters
blood.
In addition to glycogen, fat vitamins A,D,E and Cu
& ion are stored
67
Quote for the Day
Smile
It Releases
Endorphins!!!
68
Other functions of Liver
2. 
Deamination of amino acids: Excess amino acids
are brought to liver and converted to urea.
3. 
Detoxification: harmful substances absorbed into
the blood are converted to harmless substances
by liver cells. Alcohol is broken down in the liver
Phagocytosis: Kupffer cells phagocytize worn-out
and dying red and white blood cells, some bacteria
Synthesis: Albumins, fibrinogen, globulins,
heparin, clotting factors, plasma protein, lipo
proteins & cholesterol
4. 
5. 
69
23
Bile storage
l 
l 
l 
Bile stored in the gall
bladder
Muscular sac
Between right +
quadrate liver lobes
70
Bile storage
Gallbladder
• Concentrates bile by removing 90% of water
• Stores bile
• Bile release is stimulated by CCK
• CCK secreted in response to amino acids and
lipids
• Bile is secreted into duodenum
71
•  pH 8 – 8.6 Composition
•  Water (86%)
of bile
•  Mineral salts – Cl, HCO3, P04-, Na+, K+
•  Bile salts – Na taurocholate, Na glycocholate
*Emulsifies fat by turning into droplets
•  Bile pigments – Waste products ; bilirubin (golden
yellow) & biliverdin (green)
•  Mucus
•  Cholesterol & lecithin; phospholipids -excretory
products
72
24
Regulation of bile secretion
l 
l 
Bile salts itself
Hormonal
Acid chyme
mucus cells
secretin + CCK
Secretin:
Acts on duct cells (as in pancrease)
Bicarbonate secretion
73
Regulation of bile secretion
CCK:
Contraction of GB smooth muscles &
relaxation of sphincter
Bile salts
l 
Neural control - insignificant
74
Emulsification of Fat by Bile
75
25
Emulsification of Fat by Bile
76
Enterohepatic
circulation
77
The Wall of the Small Intestine
78
Figure 14.9
Slide 14.10A
26
Intestinal juice
• Secreted about 1.5 L/day
• Help buffer acids from stomach by alkaline
mucus
• Contain secretions from pancreas, liver and
mucus from goblet cells in intestinal wall
• Brush-border: Contains membrane bound
enzymes that complete protein and fat
digestion.
79
Digestion in small intestine
l 
l 
Brush border enzymes include peptidases,
disaccharidases and a protease called
enteropeptidase = enterokinase
Brush border enteropeptidase converts
inactive trypsinogen & chymotripsinogen
into active trypsin
80
l 
Intestinal
Contractions &
Motility
2 major types of contractions
occur in the small intestine:
• Peristalsis:
Insert fig. 18.14
• Slow movement.
• Highly coordinated to
propel content down the
intestine.
• Pressure at the pyloric end
of small intestine is greater
than at the distal end.
81
27
Intestinal Contractions & Motility
• Segmentation:
• Major contractile activity of the small intestine.
• Section of SM contracts in isolation in both oral&
caudal directions
• When smooth muscle relaxes, content flows
back to the segment.
• Back & forth movement of SM mixes chyme
•  No forward movement.
82
Contractions of Intestinal
Smooth Muscles
l 
l 
Occur automatically in
response to endogenous
pacemaker activity.
Rhythm of contractions is
paced by graded
depolarizations called slow
waves.
• Slow waves produced by
interstitial cells of Cajal.
Insert fig. 18.15
• Slow waves spread from 1 smooth muscle cell to
another through nexuses.
83
Contractions of Intestinal
Smooth Muscles
84
28
Contractions of Intestinal
Smooth Muscles
l 
l 
l 
l 
When slow waves above threshold, it triggers APs
by opening of VG Ca2+ channels.
Inward flow of Ca2+:
• Produces the upward depolarization phase.
• Stimulates contraction of smooth muscle.
Repolarization:
• VG K+ channels open.
• Slow waves decrease in amplitude as they
are conducted.
.
85
Contractions of Intestinal
Smooth Muscles
l 
l 
May stimulate contraction in proportion to the
magnitude of depolarization.
Parasympathetic NS, stretch and gastrin increase
the amplitude of slow waves.
• Stimulate APs.
l 
Sympathetic NS decrease APs.
86
Summary of Digestion:
Carbohydrates
Amylase
Maltase
Starch ----------- Maltose ---------- 2 Glucose
Glycogen --------- Maltose ---------- 2 Glucose
Invertase
Sucrose ------------------- Glucose + Fructose
Lactase
Lactose ------------------ Glucose + Galactose
87
29
Summary of Digestion: Proteins
Pepsin
1. Complex Proteins ------------------- Peptones
Trypsin, chymotrypsin,
2. Peptones ----------------------------Higher
Peptides
carboxypeptidase
3. Higher peptides --------------------Dipeptides
dipeptidase
4. Peptides -------------------------- Amino acids
88
Summary of Digestion: Fat &
Nucleic acids:
lingual lipase
Fat ------------------------------ Fatty acid + glycerol
Bile in SI
Fat ----------------------------emulsified fat
pancreatic lipase
Fat --------------------------- Fatty acid + glycerol
deoxyribonuclease
DNA----------------------- sugar, phosphate, bases
RNA -------------------ribonuclease Sugar, phosphate, bases
89
Major Digestive Enzymes
90
Table 14.1
Slide 14.11
30
Absorption in Small Intestine
Duodenum and jejunum:
• Carbohydrates, amino acids, lipids,
iron, and Ca2+.
l  Ileum:
• Bile salts, vitamin B12, electrolytes,
and H20.
l 
91
PBL
Cholera toxin, produced by Vibrio cholerae
causes a an indirect reduction in the activity
of the Na + / K + ATPase in intestinal
epithelial cells. This results in reduced uptake
of
small sugars and amino acids from the
intestine
92
Absorption of monosaccharides
Fructose;
,
Intestinal
epithelium
Glucose Na+ Galactose
GLUT-2
GLUT-5
Fructose
Lumen of
intestine
Glucose
Galactose
Brush border
3Na+ 2K+
Contra-luminal membrane
GLUT-5
ATP
ADP + Pi
to capillaries
3Na+ 2K+
Rate = 1.4 g /
kg body wt
= facilitated diffusion
= Na,K-ATPase
+
= Na -dependent co-transport
93
31
Absorption of proteins
Intestinal epithelial
Epithelial cell layer
Blood
Small peptides
Peptides
Amino acids
Na+
Na+
C plasmic peptidases
Di & tri peptides
• Anti bodies through placenta
H+
• Intestine: (cow, horse, goat)
st
during 1 few day via mothers milk
H+
94
Absorption of Fats
Simple diff.
into blood
Small FA ≤ 10C
cholesterol
>10C
95
Lipid Transport
Lipoproteins-how lipids travel
Lots of lipid=less dense
l  More protein=more dense; would sink
l 
l 
•  Chylomicrons-most triglyceride & least dense
•  VLDL-remnants of chylomicrons repacked
•  LDL-loaded with cholesterol/little protein
•  HDL-carry cholesterol from cells back to liver/
lots of protein
96
32
Health Implications
l 
LDL represents high blood cholesterol
l 
HDL represents the cholesterol being
picked up for breakdown and excretion
l 
High LDL associated with increased risk
of heart attack
l 
LDL and HDL refer to the proportions of
lipid and protein
97
Factors to Improve the LDL to
HDL Ratio
Weight control
Monounsaturated or polyunsaturated,
instead of saturated fat in the diet
l  Soluble fibers
l  Antioxidants
l  Moderate alcohol consumption
l  Physical activity
l 
l 
98
Functions of the large intestine
l 
Reabsorb water and compact material into
feces by mass movement (mixing wave)
l 
Absorb vitamin K produced by colon
bacteria
l 
Store fecal matter prior to defecation
99
33
Functions of the large intestine
l 
No food absorption or enzyme secretion in
LI, only mucus
If chyme moves too slowly, too much water
is reabsorbed= constipation
If chyme moves too fast, too little water
reabsorbed = diarrhea
100
Rectum: defecation reflex
Internal and external anal sphincters
present
1.  Stretching sends signals to CNS
2. Spinal cord sends parasympathetic
impulse to open internal anal sphincter
101
Rectum: defecation reflex
3.External anal sphincter is under
conscious control and may remain close.
With continued rectal distension, reflex
returns sooner and sooner
4. Young children lack voluntary control
102
34