Transcript 252Ch24

BIOLOGY 252
Human Anatomy & Physiology
Chapter 24
The Digestive System:
Lecture Notes
The Digestive System
• Structure
– Gross Anatomy
– Histology
• Function
– Mechanical
– Chemical
Overview of GI tract Functions
• Mouth - bite, chew, moisten,
swallow
• Through mastication, food is
mixed with saliva and shaped into
a bolus
• Pharynx and esophagus transport
• Stomach - mechanical disruption;
absorption of water & alcohol
• Small intestine - mechanical &
chemical digestion & absorption
• Large intestine - absorb
electrolytes & remaining vitamins
(B & K)
• Rectum and anus – defecation
Functions of the Digestive System
• Ingestion: taking food into the mouth / mastication
• Secretion: release of water, acid, buffers, and enzymes
into the lumen of the GI tract
• Mixing and propulsion: churning and propulsion of food
through the GI tract
• Digestion: mechanical and chemical breakdown of food.
The molecules must be small enough to enter cells lining
the GI tract
• Absorption: the passage of these small molecules
through the epithelial cells lining the lumen of the GI tract
and then into the blood and lymph
• Defecation: the elimination of wastes (feces) from the GI
tract
Layers of the GI Tract
1. Mucosal layer
2. Submucosal
layer
3. Muscularis
layer
4. Serosa layer
Manual: p. 5.4 - 5.5
Mucosa
• Epithelium
– stratified squamous (in mouth, esophagus & anus) =
tough
– simple columnar in the rest
• secretes enzymes and absorbs nutrients
• specialized cells (goblet cells) secrete mucous onto cell surfaces
• enteroendocrine cells - secrete hormones controlling organ function
• Lamina propria
– thin layer of loose connective tissue
– contains blood vessels and lymphatic tissue / absorption
• Muscularis mucosae - thin layer of smooth muscle
- runs both longitudinally and circularly
- Causes folds in mucosa which increases surface area for absorption –
plica circularis
Submucosa
• Loose connective tissue
– containing blood vessels, glands and lymphatic tissue
Muscularis
• Skeletal muscle - voluntary control
– in mouth, pharynx , upper esophagus and anus
– control over swallowing and defecation
• Smooth muscle - involuntary control
– inner circular fibers & outer longitudinal fibers
– mixes, crushes & propels food along by peristalsis
Serosa
• An example of a serous membrane
• Also known as the visceral peritoneum
• Covers all organs and walls of cavities not
open to the outside of the body
• Consists of connective tissue covered with
simple squamous epithelium
• Secretes slippery serous fluid which
reduces friction between contacting
surfaces
• manual 5.2
Peritoneum
• Peritoneum
– visceral layer covers organs
– parietal layer lines the walls
of body cavity
• Peritoneal cavity
– potential space containing a
small amount of serous fluid
• Retroperitoneal
– some organs lie on the
posterior abdominal wall and
are covered by peritoneum
only on their anterior
surfaces. Such organs,
including the kidneys and
pancreas, are said to be
retroperitoneal
manual p. 5.2
Parts of the Peritoneum
•
•
•
•
•
Mesentery
Mesocolon
Lesser omentum
Greater omentum
Peritonitis =
inflammation
–
–
–
–
trauma
rupture of GI tract
appendicitis
perforated ulcer
Salivary Glands
•
•
•
•
Parotid below your ear and over the masseter
Submandibular is under lower edge of mandible
Sublingual is deep to the tongue in floor of mouth
All have ducts that empty into the oral cavity
• manual 5.1
Composition and Functions of Saliva
• Wet food for easier swallowing
• Dissolves food for tasting
• Bicarbonate ions buffer acidic foods
– Bulemia - vomiting hurts the enamel on your teeth
• Chemical digestion of starch begins with enzyme
(salivary amylase)
• Enzyme (lysozyme) - helps destroy bacteria
• Protects mouth from infection with its rinsing action
– produce 1 to 1.5 L/day
Digestion in the Mouth
• Mechanical digestion (mastication or chewing)
• breaks into pieces
• mixes with saliva so it forms a bolus
• Chemical digestion
– amylase
• begins starch digestion at pH of 6.5 or 7.0 found in mouth
• manual 6.4
– lingual lipase
• secreted by glands in tongue
• begins breakdown of triglycerides into fatty acids and glycerol
Esophagus
• Collapsed muscular
tube
• In front of vertebrae
• Posterior to trachea
• Posterior to the heart
• Pierces the diaphragm
at hiatus
– hiatal hernia or
diaphragmatic hernia
manual 5.5
Heart Location
Anterior surface
of heart
• Heart is located in the mediastinum
– area from the sternum to the vertebral column and
between the lungs
Histology of the Esophagus
• Mucosa - stratified squamous
• Submucosa - large mucous glands
• Muscularis - upper 1/3 is skeletal, middle is
mixed, lower 1/3 is smooth
– upper & lower esophageal sphincters are prominent
circular muscles
• Adventitia - connective tissue blending with
surrounding connective tissue - no peritoneum
manual 5.5
Anatomy of Stomach
• Which side is it on?
• Size when empty?
– large sausage
– stretches due to rugae
• Parts of stomach
–
–
–
–
Cardia
Fundus - air in x-ray
Body
Pylorus - starts to narrow as approaches pyloric sphincter
• Empties as small squirts of chyme leave the stomach
through the pyloric valve
- manual 5.1
Layers of the Stomach
Copyright © 2015 John Wiley &
Sons, Inc. All rights reserved.
Physiology - Mechanical Digestion
• Gentle mixing waves
– every 15 to 25 seconds
– mixes bolus with 2 L/day of gastric juice to turn it
into chyme (a thin liquid)
• More vigorous waves
– travel from body of stomach to pyloric region
• Intense waves near the pylorus
– open it and squirt out 1 - 2 teaspoons of chyme
with each wave
Physiology - Chemical Digestion
• Protein digestion begins
– HCl denatures (unfolds) protein molecules
• Fat digestion continues
– gastric lipase splits the triglycerides in milk fat
• HCl kills microbes in food
• Mucous cells protect stomach walls from being
digested with 1- 3 mm thick layer of mucous
• Stomach pH very acidic ~ 2.0
• manual 6.1
Absorption of Nutrients by the Stomach
•
•
•
•
Water, especially if it is cold
Electrolytes – limited Na+ and K+
Some drugs, especially aspirin & alcohol
Fat content in the stomach slows the passage
of alcohol to the intestine where absorption is
more rapid
Anatomy of the Pancreas
• 5“ (13 cm) long by 1"
(2.54 cm) thick
• Head close to curve in Cshaped duodenum
• Main duct joins common
bile duct from liver
• Opens 4 in. (10 cm)
below pyloric sphincter
- manual 5.2
Histology of the Pancreas
• Acini - dark clusters
– 99% of gland
– produce pancreatic juice
• Islets of Langerhans
• (pancreatic islets)
– 1% of gland
– pale staining cells
– produce hormones
– manual 5.6
Composition and Functions of
Pancreatic Juice
• 1.5 L/day at pH of 7.1 to 8.2
• Contains water, enzymes & sodium
bicarbonate
• Digestive enzymes
– pancreatic amylase/starch, pancreatic lipase/fats,
proteases/proteins
– Ribonuclease - to digest nucleic acids
– manual 6.1
Anatomy of the Liver and Gallbladder
• Liver
– weighs 3 lbs.
(1.3 kg)
– below diaphragm
– right lobe larger
– gallbladder on right
lobe
– size causes right
kidney to be lower
than left
– manual 5.2
• Gallbladder
– fundus, body &
neck
– manual 5.2
Bile Production
• One liter of bile/day is secreted by the liver
– Yellow - green in color & pH 7.6 to 8.6
• Components
– water & cholesterol
– bile salts - Na & K salts of bile acids
– manual 6.3
Liver Functions
• Turn proteins into glucose
• Turn triglycerides into glucose
• Turn excess glucose into glycogen &
store in the liver
• Turn glycogen back into glucose as
needed
• The other vital functions include:
processing of drugs and hormones;
storage of vitamins and minerals; and
activation of vitamin D.
manual p 5.7
Figure 2.1
Anatomy of the Small Intestine
• 20 feet long (6m) - 1 inch (2.5cm) in
diameter
• Large surface area for majority of
absorption
• 3 parts
– Duodenum - 10 in (25 cm)
– Jejunum - 8 ft (2.4 m)
– Ileum - 12 ft (3.6 m)
• ends at ileocecal valve
• manual 5.1
Mechanical Digestion in Small Intestine
• Weak peristalsis in
comparison to the
stomach - chyme
remains for 3 to 5
hours
• Segmentation - local
mixing of chyme with
intestinal juices sloshing back & forth
Histology of Small Intestine
Structure of the Small Intestine
Copyright © 2015 John Wiley &
Sons, Inc. All rights reserved.
Histology of the Small Intestine
• Structures that increase surface area
– plica circularis
• permanent ½ inch (1 cm) tall folds that contain part of
submucosal layer
– villi
• 1 millimeter tall
• Contains vascular capillaries and lacteals(lymphatic
capillaries)
– microvilli
• cell surface feature known as brush border
• manual 5.1
Functions of Microvilli
• Digestion and Absorption
• Digestive enzymes found at cell surface
on microvilli
• Digestion occurs at cell surfaces
• Significant cell division within intestinal
glands produces new cells that move up
• Once out of the way - rupturing and
releasing their digestive enzymes &
proteins
• Chemotherapy
• manual 5.5
Digestion of Carbohydrates
• Mouth - salivary amylase
• Esophagus & stomach - nothing
happens
• Duodenum - pancreatic amylase
• Brush border enzymes (maltase,
sucrase & lactase) act on disaccharides
– produces monosaccharides - fructose,
glucose (dextrose) & galactose
– lactose intolerance (no enzyme; bacteria
ferment sugar) - gas & diarrhea
– manual 6.4
Digestion of Proteins
• Stomach
– HCl denatures or unfolds proteins
– pepsin turns proteins into peptides
• Pancreas
– digestive enzymes - split peptide bonds
between different amino acids
– brush border enzymes - aminopeptidase or
dipeptidase - split off amino acid at amino
end of molecule or split dipeptide
– manual 6.1
Digestion of Lipids
• Mouth - lingual lipase
• Small intestine
– emulsification by bile
– pancreatic lipase - splits into both short
chain and long chain fatty acids &
monoglyceride
– no enzymes in brush border
– manual 6.3
Small Intestine
• Absorption is the passage of nutrients from digested food in
the gastrointestinal tract into the blood or lymph. It occurs
mostly in the small intestine by means of simple diffusion,
facilitated diffusion, osmosis, and active transport.
• Monosaccharides, amino acids, and short-chain fatty acids
pass into the blood capillaries.
• Long-chain fatty acids and monoglycerides are absorbed as
part of micelles, resynthesized to triglycerides, and
transported in chylomicrons to the lacteal of a villus.
• The small intestine also absorbs water, electrolytes, and
vitamins.
Copyright © 2015 John Wiley & Sons, Inc. All rights reserved.
Where will the absorbed nutrients go?
Absorption of Water
• 9 liters of fluid released into
GI tract each day
• Small intestine reabsorbs 8
liters
• Large intestine reabsorbs
90% of that last liter
• Absorption is by osmosis
through cell walls into
vascular capillaries inside
villi
Anatomy of Large Intestine
•
•
•
•
•
5 feet (1.5 m) long by 2.5 inches (6.25 cm) in diameter
Ascending & descending colon are retroperitoneal
Cecum & appendix
Rectum = last 8 inches (20 cm) of GI tract anterior to the sacrum & coccyx
Anal canal = last 1 inch (2.5 cm) of GI tract
– internal sphincter - smooth muscle & involuntary
– external sphincter - skeletal muscle & voluntary control
– manual 5.2
Structure of the Large Intestine
Copyright © 2015 John Wiley &
Sons, Inc. All rights reserved.
Mechanical Digestion in Large Intestine
• Smooth muscle - mechanical digestion
• Peristaltic waves (3 to 12 contractions/minute)
– haustral churning - relaxed pouches are filled
from below by muscular contractions (elevator)
– gastroilial reflex - when stomach is full, gastrin
hormone relaxes ileocecal sphincter so small
intestine will empty and make room
– gastrocolic reflex - when stomach fills, a strong
peristaltic wave moves contents of transverse
colon into rectum
Chemical Digestion in Large Intestine
• No enzymes are secreted only mucous
• Bacteria ferment
– undigested carbohydrates into carbon
dioxide & methane gas
– undigested proteins into simpler
substances (indoles) - odor
– turns bilirubin into simpler substances that
produce color
Absorption & Feces Formation in
the Large Intestine
• Some electrolytes - Na+ and Cl• After 3 to 10 hours, 90% of H20 has been
removed from chyme
• Feces are semisolid by time reaches
transverse colon
• Feces = dead epithelial cells, undigested
food such as cellulose, bacteria (live &
dead)
Energy Balance Equation:
• Calories In - Calories Out
Cal. In = food we eat
Cal. Out = RMR
Excreted
TEM
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