Download The Digestive System

The Digestive System
Overview
Food breakdown
1.
•
CHO, Protein, Fat
2.
Anatomy
3.
Digestive process
¾
Primary function: breakdown & transport
nutrients, H2O, & electrolytes
1. Motility – propulsive or mixing movements
2. Secretion – energy requiring secretion of H2O,
electrolytes, & enzymes, bile salts, or mucus
Lumen
Duct
cells
Exocrine
gland cells
Secretory
product
Figure 16.1
Page 592
Capillary
1
¾
Primary function: breakdown & transport
nutrients, H2O, & electrolytes
3. Digestion – CHO
Carbohydrates
Polysaccharides (starch and glycogens)
Amylase
Maltose
Monosaccharides
(glucose, galactose,
fructose)
Maltase
Disaccharides
Sucrose
Sucrase
Lactose
Lactase
¾
Table 16.1 (1) Page 593
Primary function: breakdown & transport
nutrients, H2O, & electrolytes
3. Digestion – proteins & fats
Fats
Monoglyceride
Triglyceride
Free
fatty acids
Lipase
¾
Primary function: breakdown & transport
nutrients, H2O, & electrolytes
4. Absorption – primarily in small intestines
2
General Digestive Anatomy
1. Digestive Tract
2. Accessory
Digestive Organs
Nasal passages
Mouth
Salivary glands
Pharynx
Trachea
Esophagus
Liver
Stomach
Gallbladder
Pancreas
Duodenum
Descending colon
Transverse colon
Ascending colon
Jejunum
Cecum
lleum
The Digestive Tract (inner layer to outer)
1. Mucosa
•
Mucous membrane
Mucous membrane
9 Exocrine cells (digestive
juices)
9 Endocrine cells (hormones)
9 Epithelial cells (absorption)
•
Degree of folding depends
on location
9 Highest in small intestine
3
The Digestive Tract (inner layer to outer)
2. Submucosa
•
Thick connective tissue
Submucosa
Muscularis externa
9 Blood & nerve innervation
3. Muscularis externa
•
•
Inner circular layer
Outer longitudinal layer
4. Serosa
•
Outer connective tissue
9 Prevents friction
Digestive regulation
Smooth-muscle function
1.
•
Slow-wave action potentials (basal electrical
rhythm)
9 Interstitial cells of Cajal
•
Regulate peristalsis and segmentation
Intrinsic nerve plexuses (enteric nervous
system) – digestive nervous system
2.
•
•
Respond to local stimuli for secretion (digestive
juices & GI hormones)
Both excitatory & inhibitory
Digestive regulation
Extrinsic nerves
3.
•
Modify intrinsic activity and other various digestive
organs (generally sympathetic &/or
parasympathetic)
Gastrointestinal hormones
4.
•
Carried throughout blood influencing other areas of
digestive tract
4
Sensory Receptors
Chemoreceptors
1.
•
Sense changes in chemical components within
lumen
Mechanoreceptors
2.
•
Respond to stretch and tension
Osmoreceptors
3.
•
Changes in osmolarity
External
influence
Local changes in
digestive tract
Receptors in digestive tract
Intrinsic
nerve plexuses
Extrinsic
automatic
nerves
Gastrointestinal
hormones
Smooth muscle
(contraction for motility)
Exocrine gland cells
(secretion of digestive juices)
Figure 16.4
Page 595
Endocrine gland cells
(secretion of gastrointestinal
and pancreatic hormones)
The Digestive Process
5
The Digestion Process
1.
Mouth
• Chewing
• Saliva secretion
¾
3 major salivary glands
• Salivary proteins
1. Amylase (CHO breakdown)
9 Polysaccharides to disaccharides
2. Mucus (lubrication)
3. Lysozyme (antibacterial)
Salivary
Regulation
Cerebral cortex
Salivary center
in medulla
Pressure receptors
and chemoreceptors
in mouth
Unconditioned
reflex
Other inputs
Conditioned
reflex
Autonomic nerves
Salivary glands
Salivary secretions
Figure 16.5 Page 601
The Digestion Process (cont.)
2.
Pharynx & Esophagus
• Swallowing reflex
6
The Digestion Process (cont.)
2.
Pharynx & Esophagus
• Peristalsis
Bolus
Figure
16.7
Page 603
The Digestion Process (cont.)
Stomach
Esophagus
Fundus
Gastroesophageal
sphincter
Body
Pyloric
sphincter
Antrum
Duodenum
Figure
16.8
Page 604
Stomach Functions
1.
Storage
2.
Gastric mixing & mucous secretion
3.
Production of chyme
4.
Secretes hydrochloric acid (HCl)
• Reduces large food particles
• Kills microorganisms ingested in food
5.
Initial stages of protein breakdown
• Pepsinogen forming pepsin
7
Figure 16.9 (1)
Page 605
Basal electrical rhythm
~ 3 per minute
Duodenum
Pyloric sphincter
Direction of
movement
of peristaltic
contraction
Peristaltic
contraction
Gastric emptying
Figure 16.9 (2)
Page 605
Peristaltic
contraction
Gastric mixing
Regulation of Gastric Emptying
Amount of chyme
¾
Neural response
¾
•
•
Intrinsic nerve plexus (short reflex)
Autonomic nerves (long reflex)
Enterogastric
Reflex
Hormonal
¾
•
Enterogastrones (secretin & cholecystoinin –
CCK) released from duodenal mucosa
9 Inhibit antral contractions
8
Regulation of Gastric Emptying
¾
Duodenum
1. Fat ~ can only be processed in small intestine
2. Acid (unneutralized)
9 Excess HCl not buffered by sodium bicarbonate
3. Hypertonicity ~ increased osmolarity due to
abundance of amino acids and glucose
4. Distension
¾
Emotions
Gastric Digestive Juices
~ 2 liters/day
¾
Responsibility of cells lining gastric mucosa
1.
Oxyntic mucosa
• Body
• Fundus
2.
Pyloric gland area (PGA)
• Antrum
Table 16.4
(1) Page 609
Oxyntic
mucosa
Gastric
pit
Mucosa
Stomach
lumen
Pyloric
gland
area
Submucosa
9
In oxyntic mucosa
In pyloric gland area
Surface epithelial cells
Gastric
pit
Mucosa cells
Alkaline mucus
Gastric
gland
Chief cells
pepsiongen
G cells
Gastrin
D cells
Somatostatin
Parietal cells
HCl
Enterochomaffinlike (ECL) cells
Histamine
Chief & Parietal Cell Regulation
¾
Acetylcholine (Ach)
• Stimulates both
• Also stimulates G and ECL cells
¾
Histamine (paracrine)
• Released from ECL cells & increases HCl
secretion
¾
Somatostatin
• Released from D cells
• Provides negative feedback
HCl & Pepsinogen
Plasma
HCl secretion into lumen
Gastric
lumen
Parietal cell
Cellular
metabolism
Chief cell
10
¾
HCl functions to:
• Activate pepsinogen to form pepsin
Autocatalysis
Pepsinogen
Pepsin
Digestion
Protein
Gastric
lumen
HCI
Peptide fragments
• Breakdown of connective tissue
• Denatures proteins
• Kills microorganisms
Gastric Mucosal Barrier
Mucus coating
Impermeable
to HCI
Tight
junction
Submucosa
Cells lining gastric mucosa
(including those lining
gastric pits and glands)
Page 16.12
Page 614
Food leaving the
Stomach…
Mixed with secretions
from pancreas and liver
11
Bile duct
from liver
Duodenum
Page 16.13
Page 616
Stomach
Hormones
(insulin,
glucagon)
Blood
Endocrine portion
of pancreas
(Islets of Langerhans)
Duct cells
secrete aqueous
NaHCO3 solution
Acinar cells
secrete digestive
enzymes
Exocrine portion of panaceas
(Acinar and duct cells)
The glandular portions of
the pancreas are grossly
exaggerated
Pancreas
¾
Exocrine & endocrine tissue
1. Exocrine: secretes enzymes capable of breaking
down CHO, fat, & protein
9 Proteolytic enzymes: protein
9 Pancreatic amylase: CHO
9 Pancreatic lipase: fat
Regulated by secretin
& cholecystokinin
(CCK)
2. Endocrine (hormones): Insulin & glucagon
Hormonal Regulation
Acid in
duodenal
lumen
Fat and protein
products in
duodenal lumen
Secretion release
from duodenal
mucosa
CCK release
from duodenal
mucosa
(Secretin carried Neutralizes
by blood)
Pancreatic duct
cells
Secretion of aqueous
NaHCO3 solution into
duodenal lumen
(CCK carried
by blood)
Digests
Pancreatic acinar
cells
Secretion of
pancreatic digestive
enzymes into
duodenal lumen
12
Liver
¾
Digestive role: secretion of bile salts
• Aid fat digestion & absorption
Hepatic
artery
Hepatic vein
Blood flow –
Hepatic Portal
System
Hepatic
portal vein
Digestive
capillaries
Digestive
tract
Page 16.15
Page 618
Liver
¾
Bile secretion
Bile salts
Cholesterol
Common bile duct
Gallbladder:
Bile storage
Portal
circulation
Colon
Duodenum
Terminal ileum
Page 16.17
Page 620
Small Intestine
¾
Primary site of digestion & absorption
Duodenum
primary
Jejunum
Ileum
13
Segmentation of chyme
¾
Initiated by pacesetter cells
Small intestine – digestion & absorption
Potential for increased surface area
¾
1. Extensive folding
2. Villi & microvilli
9 Increased digestive enzyme release
Pancreatic enzymes
¾
•
Fat reduced to FFA (with help of bile salts)
9 Lipase
•
Proteins to AA
9 Aminopeptidases
•
CHO to di- and monosaccharides
9 Maltase, sucrase, lactase
Lumen
Na+- and
energydependent
secondary
active transport
Facilitated
diffusion
Energy
required
Epithelial cell
of villus
Capillary
14
Lumen
Na+- and
energydependent
absorption
Energy
required
Epithelial cell
of villus
Capillary
Lipid emulsion
Micelles
diffusion
Lumen
Lumen
Micelle
Micelles
Microvillus
Fatty acids,
monoglycerides
Aggregate and
coated with
lipoprotien
Epithelial
cell of villus
Short or
medium
chain
Passive absorption
Basement
membrane
(Exocytosis)
Central lacteal
Capillary
Small intestine – digestion & absorption
Vitamin absorption (passive)
1.
•
•
Water-soluble
Fat-soluble
Iron absorption – regulated
2.
•
Absorbed into epithelial cells
9 Either used immediately for production of RBC or
9 Stored as ferritin
Calcium absorption – regulated
3.
•
Active transport stimulated by Vitamin D
15
Biochemical Balance
Digestive tract
lumen
Stomach
parietal cell
Blood
Pancreatic
duct cell
Intestinal
epithelial cell
Large Intestine
Colon
Cecum
Drying & storage
Rectum
16