Transcript Chapter 24 - Anatomy Freaks

Chapter 24
Digestive System
24-1
Digestive System Anatomy
• Digestive tract: also called alimentary tract or
canal
– GI tract: technically refers to stomach and
intestines
• Accessory organs
– Primarily glands, secrete fluids into tract
• Regions
– Mouth or oral cavity with salivary glands
and tonsils
– Pharynx (throat) with tubular mucous glands
– Esophagus with tubular mucous glands
– Stomach with many different kinds of glands
that are tubular
– Small intestine (duodenum, ileum,
jejunum) with liver, gallbladder and
pancreas as major accessory organs
– Large intestine including cecum, colon,
rectum and anal canal with mucous glands
– Anus
24-2
Functions
1. Ingestion: introduction of food into
stomach
2. Mastication: chewing. Chemical digestion
requires large surface area so breaking
down large particles mechanically
facilitates chemical digestion.
3. Propulsion (movement of food-----24-36 hours oral end to anal end)
– Deglutition: swallowing (oral cavity -> esophagus) (bolus = mass of food or liquid)
– Peristalsis: moves material through digestive tract . A wave of circular
smooth muscle relaxation moves ahead of the bolus of food or chyme allowing
the digestive tract to expand. Then a wave of contraction of the circular
smooth muscles behind the bolus of food or chyme (ingested food & stomach
secretions) propels it through the digestive tract.
• Mass movements in large intestine (contractions that extend for larger parts of
digestive tract)
24-3
4. Mixing: Segmental contractions ( mixing contractions that occur
in small intestine.
-Some contractions do not propel food from one end of digestive
tract to the other but, rather, move it back & forth within
digestive tract to mix it with digestive secretions & help break it
into smaller pieces)
24-4
Functions, cont.
5. Secretion: lubricate, liquefy, digest
– Mucus: secreted along entire digestive tract, lubricates food and
lining, coats lining and protects from mechanical digestion, from
acid and from digestive enzymes.
– Water: liquefaction makes food easier to digest and absorb
– Bile: emulsifies fats
– Enzymes: chemical digestion
6. Digestion: Mechanical and chemical
7. Absorption: Movement from tract into circulation or
lymph
8. Elimination: Waste products removed from body; feces.
Defecation
24-5
Digestive Tract Histology:
The Tunics
•Mucosa. Innermost layer,
consisting of mucous
epithelium (stratified squamous
in mouth, oropharynx,
esophagus and anal canal),
simple columnar epithelium in
the rest of the tract.
– Loose connective
tissue: lamina propria
– Muscularis mucosae:
smooth muscle
•Submucosa. Thick C.T. layer
with nerves, blood vessels,
small glands. Parasympathetic
submucosal plexus.
24-6
Digestive Tract Histology:
The Tunics
• Muscularis: 2 or 3 layers of smooth
muscle, two of which are circular
and longitudinal. Exception:
esophagus where the upper 1/3 is
striated & stomach. This layer also
contains the myenteric plexus. The
myenteric and submucosal plexi
together are called the enteric or
intramural plexus. Important in
control of movement and secretion
• Serosa or adventitia: Connective
tissue. Where serosa is present,
called visceral peritoneum. Where
adventitia is present, connective
tissue blends with connective tissue
of surrounding structures
24-7
Digestive System Regulation
• Nervous regulation
– Local: enteric nervous system
• Types of neurons: sensory,
motor, interneurons
• Coordinates peristalsis and
regulates local reflexes
– General: coordination with the
CNS. May initiate reflexes
because of sight, smell, or taste
of food. Parasympathetic
primarily (through vagas nerve).
Sympathetic input inhibits
muscle contraction, secretion,
and decrease of blood flow to
the digestive tract.
• Chemical regulation
– Production of hormones to be
discussed later
• Gastrin, secretin
– Production of paracrine
chemicals like histamine
• Help local reflexes in ENS
control the conditions of the
internal environment of the
digestive tract such as pH
levels
24-8
Peritoneum and Mesenteries
• Peritoneum
– Visceral: Covers organs
– Parietal: Covers interior surface of body
wall
– Retroperitoneal: Certain organs covered
by peritoneum on only one surface and
are considered behind the peritoneum; (lie
against abdominal wall) e.g., kidneys,
pancreas, duodenum
• Mesenteries: two layers of peritoneum with
thin layer of loose C.T. between
– Routes by which vessels and nerves pass
from body wall to organs
–Greater omentum: connects greater curvature of the stomach to the
transverse colon (extends inferiorly from stomach over surface of small intestine).
–Lesser omentum: connects lesser curvature of the stomach and the proximal
part of the duodenum to the liver and diaphragm.
–Transverse mesocolon, sigmoid mesocolon, mesoappendix (mesentery
refers to serous membranes attached to abdominal organs).
•Ligaments
24-9
–Coronary: between liver and diaphragm
–Falciform: between liver and anterior abdominal wall
Oral Cavity
• Bounded by lips
anteriorly, fauces
(opening into pharynx)
posteriorly
– Vestibule: space between
lip/cheeks and alveolar
processes with teeth
– Oral cavity proper: medial
to alveolar processes
– Lined with moist stratified
squamous epithelium
24-10
Lips and Cheeks
• Both structures important in mastication and
speech
• Lips (labia): orbicularis oris muscle within.
Keratinized stratified squamous exterior is thin
and color of blood in dermis gives a red/pink
color.
– Labial frenula (mucous folds) extend from alveolar
processes of maxilla and mandible to the upper and
lower lips, respectively.
– Many facial muscles act to move lips
• Cheeks: lateral walls of oral cavity
– Buccinator muscle
– Buccal fat pad
24-11
Palate and Palatine Tonsils
• Palate
– Hard palate: anterior, supported by maxilla
and palatine bone
– Soft palate: posterior, consists of skeletal
muscle and connective tissue
– Uvula: projects from posterior of soft palate
• Palatine tonsils: lateral walls of fauces
24-12
Tongue
• Muscular with free anterior surface and attached posterior
surface. Covered with moist stratified squamous
epithelium.
– Intrinsic muscles: change shape
– Extrinsic muscles: protrude or retract tongue, move
side to side
• Lingual frenulum attaches tongue inferiorly to floor of
oral cavity
• Terminal sulcus: groove divides tongue into anterior 2/3;
posterior 1/3
• Anterior part: papillae, some of which have taste buds
• Posterior part: no papillae and a few scattered taste buds.
Lymphoid tissue embedded in posterior surface: lingual
tonsil
• Moves food in mouth, participates in speech and
swallowing
24-13
Teeth
• Two sets
– Primary, deciduous,
milk: Lost during
childhood
– Permanent or
secondary: Adult (32)
• Types
– Incisors, canines,
premolars and molars
24-14
Teeth
• Involved in mastication and speech
• Anatomic crown: enamel-covered part
of tooth; clinical crown is section of
tooth above gum line
• Neck: enameled part of tooth below
gum line
• Enamel: outermost layer of anatomical
crown. Non-living; acellular.
Protective.
•Dentin: living, cellular, calcified tissue. In the root, dentin is covered by
cellular bone-like structure that helps hold tooth in the socket.
•Pulp cavity filled with blood vessels, nerves, and connective tissue
•Periodontal ligaments: hold tooth in socket.
•Gingiva: dense, fibrous C.T. covered by stratified squamous epithelium.
24-15
Mastication
• Chewing: incisors and canines bite or cut off food;
molar-type teeth grind food
• Muscles involved: masseter, temporalis, medial and
lateral pterygoids.
– Elevate mandible (close jaw): temporalis, masseter, medial
pterygoids
– Depress mandible (open jaw): lateral pterygoids
– Protraction (moving in anterior direction) and lateral and medial
excursion (lateral = moves mandible to either right or left of midline-------medial = returns mandible to neutral position): pterygoids and
masseter
– Retraction (moves structure back to anatomical position)- temporalis
• Mastication reflex: medulla oblongata, but
descending pathways from cerebrum provide
conscious control. Controls basic movements
involved in chewing
24-16
Salivary Glands
• Three pairs of multicellular
glands
– Parotid: largest. Serous. Just
anterior to the ear. Parotid
duct crosses over masseter,
penetrates buccinator, and
enters the oral cavity adjacent
to the 2nd upper molar
–Submandibular: mixed, but more serous than mucous. Posterior
half of inferior border of mandible. Duct enters oral cavity on either
side of lingual frenulum
–Sublingual: smallest. Mixed, but primarily mucous. Each has 1012 ducts that enter the floor of the oral cavity.
•Lingual glands. Small, coiled tubular glands on surface of tongue.
24-17
Saliva
• Compound alveolar salivary
glands. Produce saliva
– Prevents bacterial
infection
– Lubrication
– Contains salivary amylase
that breaks down starch
into disaccharides maltose
and isomaltose (gives starch
sweet taste in mouth).
– Helps to form bolus for
swallowing
– Parasympathetic input
causes salivary production
24-18
Pharynx and Esophagus
• Pharynx
– Posterior walls of
oropharynx and
laryngopharynx
contains group of
muscles called
pharyngeal constrictors
that contribute to
swallowing
• Esophagus
– Transports food from
pharynx to stomach
– Passes through esophageal
hiatus (opening) of
diaphragm and ends at
stomach
• Hiatal hernia: widening of
hiatus (causes ulcers, acid reflux)
– Sphincters
• Upper. Striated
• Lower. Smooth
– Mucosa is moist stratified
squamous epithelium.
Produces thick layer of
mucus.
24-19
Swallowing (Deglutition)
• Three phases
– Voluntary: bolus of food moved by tongue from oral
cavity to pharynx.
– Pharyngeal: reflex. Controlled by swallowing center in
medulla oblongata. Soft palate elevates, upper
esophageal sphincter relaxes, elevated pharynx opens
the esophagus, food pushed into esophagus by
pharyngeal constrictors’ successive contraction from
superior to inferior. Epiglottis is tipped posteriorly due
to pressure of the bolus, larynx elevated to prevent food
from passing into larynx.
– Esophageal: reflex. Stretching of esophagus causes
enteric NS to initiate peristalsis of muscles in the
esophagus.
24-20
Three Phases of Swallowing
24-21
Stomach Anatomy
• Openings
– Gastroesophageal (cardiac): to
esophagus
– Pyloric: to duodenum
• Parts
–
–
–
–
Cardiac
Fundus
Body
Pyloric: antrum and canal
•Greater and lesser curvatures:
attachment sites for omenta
•Sphincters
–Cardiac (lower esophageal)
–Pyloric
24-22
Stomach Histology
• Layers
– Serosa or visceral
peritoneum
– Muscularis: three layers
• Outer longitudinal
• Middle circular
• Inner oblique (Having a
•Rugae: folds in stomach
when empty. Mucosa and
submucosa.
slanting or sloping direction)
– Submucosa
– Mucosa
24-23
Stomach Histology
•
•
Gastric pits: openings for gastric glands. Lined with simple columnar epithelium
Cells of gastric pits
– Surface mucus: mucus that protects stomach lining from acid and digestive
enzymes
– Mucous neck: mucus
– Parietal: hydrochloric acid and intrinsic factor
– Chief: pepsinogen
– Endocrine: regulatory hormones
• Enterochromaffin-like cells: secretes histamine that stimulates acid secretion
• Gastrin-containing cells: secrete gastrin (a hormone that stimulates acid secretion)
• Somatostatin-containing cells: secrete somatostatin that inhibits gastrin and
insulin secretion
24-24
Secretions of the Stomach
• Chyme: ingested food plus stomach secretions
• Mucus: surface and neck mucous cells
– Viscous and alkaline
– Protects from acidic chyme and enzyme pepsin
– Irritation of stomach mucosa causes greater mucus
• Intrinsic factor: parietal cells. Binds with vitamin B12 and helps
it to be absorbed in the ileum. B12 necessary for DNA synthesis
and RBC production (lack of B12 absorption leads to pernicious anemia)
• HCl: parietal cells
– Kills bacteria (found in ingested food)
– Stops carbohydrate digestion by inactivating salivary amylase
– Denatures proteins
– Helps convert pepsinogen to pepsin (optimal activity at pH 3 or
less)
• Pepsinogen: packaged in zymogen granules released by
exocytosis. Pepsin catalyzes breaking of covalent bonds in
proteins (breaks them into smaller peptide chains)
24-25
Hydrochloric Acid Production
24-26
Cephalic Phase
• The taste or smell of
food, tactile sensations of
food in the mouth, or
even thoughts of food
stimulate the medulla
oblongata.
• Parasympathetic action
potentials are carried by
the vagus nerves to the
stomach, where enteric
plexus
neurons are activated.
• Postganglionic neurons stimulate secretion by parietal and chief cells
(HCl and pepsin) and stimulate the secretion of the hormone gastrin
and histamine.
• Gastrin is carried through the circulation back to the stomach where it
and histamine stimulate further secretion of HCl and pepsin.
24-27
Gastric Phase
• Distention of the stomach activates a parasympathetic reflex.
Action potentials are carried by the vagus nerves to
the medulla oblongata.
• Medulla oblongata stimulates further secretions of the stomach.
• Distention also stimulates local reflexes that amplify stomach
secretions.
24-28
Intestinal Phase
1.
2.
3.
4.
Chyme in the duodenum with a pH less than 2 or containing lipids
inhibits gastric secretions by three mechanisms
Sensory input to the medulla from the duodenum inhibits the motor
input from the medulla to the stomach. Stops secretion of pepsin and
HCl.
Local reflexes inhibit gastric secretion
Secretin, and cholecystokinin produced by the duodenum decrease
gastric secretions in the stomach.
24-29
Movements in Stomach
• Combination of
mixing waves (80%)
and peristaltic waves
(20%)
• Both esophageal and
pyloric sphincters are
closed.
24-30
Small Intestine
• Site of greatest
amount of digestion
and absorption of
nutrients and water
• Divisions
– Duodenum- first 25
cm beyond the pyloric
sphincter.
– Jejunum- 2.5 m
– Ileum- 3.5 m. Peyer’s
patches or lymph
nodules
24-31
Duodenum
• Curves to the left;
head of pancreas in the
curve
• Major and minor
duodenal papillae:
openings to ducts from
liver and/or pancreas.
24-32
Modifications to Increase Surface Area
• Increase surface area
600 fold
– Plicae circulares
(circular folds)
– Villi that contain
capillaries and lacteals.
Folds of the mucosa
– Microvilli: folds of cell
membranes of
absorptive cells
24-33
Mucosa and Submucosa of the
Duodenum
• Cells and glands of the mucosa
– Absorptive cells: cells with microvilli, produce digestive
enzymes and absorb digested food
– Goblet cells: produce protective mucus
– Endocrine cells: produce regulatory hormones (Secretin,
and cholecystokinin)
– Granular cells (paneth cells): may help protect from
bacteria (contain lysozymes)
• Intestinal glands (crypts of Lieberkühn): tubular
glands in mucosa at bases of villi [secrete sucrase ,maltase,
trypsin, chymotrypsin, and pepsin (endopeptidases and exopeptidases) ]
• Duodenal glands (Brunner’s glands): tubular
mucous glands of the submucosa. Open into intestinal
glands [produce a mucus-rich alkaline secretion (containing bicarbonate)24-34
Jejunum and Ileum
• Gradual decrease in diameter, thickness of
intestinal wall, number of circular fold, and
number of villi the farther away from the stomach
• Major site of nutrient absorption
• Peyer’s patches: lymphatic nodules numerous in
mucosa and submucosa
• Ileocecal junction: where ileum meets large
intestine. Ileocecal sphincter (ring of smooth muscle)
and ileocecal valve (one-way valve)
24-35
Small Intestine Secretions
• Fluid primarily composed of water, electrolytes
and mucus.
• Mucus
– Protects against digestive enzymes and stomach
acids
• Digestive enzymes: bound to the membranes of
the absorptive cells
– Disaccharidases: Break down disaccharides to
monosaccharides
– Peptidases: Hydrolyze peptide bonds
– Nucleases: Break down nucleic acids
• Duodenal glands
– Stimulated by vagus nerve, secretin, chemical
or tactile irritation of duodenal mucosa
24-36
Movement in Small Intestine
•
•
•
•
Mixing and propulsion over short distances
Segmental contractions mix
Peristalsis propels
Ileocecal sphincter remains slightly contracted
until peristaltic waves reach it; it relaxes, allowing
chyme to move into cecum
• Cecal distention causes local reflex and ileocecal
valve constricts
– Prevents more chyme from entering cecum
– Increases digestion and absorption in small intestine by
slowing progress of chyme
– Prevents backflow
24-37
• Lobes
Liver
– Major: Left and right
– Minor: Caudate and quadrate
• Porta: on inferior surface.
Vessels, ducts, nerves, exit/enter
liver
– Hepatic portal vein, hepatic artery,
hepatic nerve plexus enter
– Lymphatic vessels, two hepatic ducts
exit
•Ducts
–Right and left hepatics (which transport
bile out of liver) unite to form
–Common hepatic
–Cystic: from gallbladder
–Common bile: union of cystic duct and
common hepatic duct (common bile joins
the pancreatic duct at the hepatopancreatic
ampulla------ampulla empties into duodenum
at major duodenum papilla)
24-38
Liver, Gallbladder, Pancreas and Ducts
24-39
Histology of the Liver
• Connective tissue septa branch
from the porta into the interior
– Divides liver into lobules
– Nerves, vessels and ducts follow
the septa
• Lobules: portal triad at each
corner
– Three vessels: hepatic portal vein,
hepatic artery, hepatic duct
– Central vein in center of lobule
• Central veins unite to form hepatic
veins that exit liver and empty into
inferior vena cava
24-40
Liver Histology
• Hepatic cords: radiate out
from central vein. Composed
of hepatocytes
• Hepatic sinusoids: between
cords, lined with endothelial
cells and hepatic phagocytic
(Kupffer) cells
• Bile canaliculus: between
cells within cords
• Hepatocyte functions
–
–
–
–
–
–
Bile production
Storage
Interconversion of nutrients
Detoxification
Phagocytosis
Synthesis of blood components
24-41
Functions of the Liver
• Bile production: 600-1000 mL/day. Bile salts, bilirubin (bile
pigment that results from breakdown of hemoglobin), cholesterol, fats,
fat-soluble hormones, lecithin
– Neutralizes and dilutes stomach acid (neutralizes chyme so that
pancreatic enzymes can function)
– Bile salts emulsify fats. Most are reabsorbed in the ileum.
(90% bile salts reabsorbed in the ileum & carried back to liver)
– Secretin (from the duodenum) stimulates bile secretions,
increasing water and bicarbonate ion content of the bile
• Storage
– Glycogen, fat, vitamins (A, B12, D, E, and K), copper and
iron. Hepatic portal blood comes to liver from small
intestine (nutrients are stored and secreted back into circulation when
needed)
• Synthesis
– Blood proteins: Albumins, fibrinogen, globulins, heparin,
clotting factors (liver produces its own new compounds)
24-42
Functions of the Liver
• Nutrient interconversion
– Amino acids to energy producing compounds (ex: person on a
excessively high protein diet and low fat & carb diet----------an
oversupply of amino acids & an undersupply of lipids & carbs are
delivered to the liver. The hepatocytes break down the amino acids and
cycle them through metabolic pathways so they can be used to produce
adenosine triphosphate, lipids, and glucose)
– Hydroxylation of vitamin D. Vitamin D then travels to kidney where it is
hydroxylated again into its active form
– Hepatocytes also transform substances that cannot be used by most cells
into usable sunstances. (ex: ingested fats combined with choline
{nutrient in B vitamin family} & phosphorous in liver to produce
phospholipids, which are imp. for cell membranes)
• Detoxification
– Hepatocytes remove ammonia (by-product of amino acid metabolism)
which is toxic & not readily removed by kidneys. Hepatocytes convert it
to urea which is less toxic and easily eliminated by kidneys.
• Phagocytosis
– Kupffer cells phagocytize worn-out and dying red and white blood cells,
some bacteria
24-43
Blood and Bile Flow
Through the Liver
24-44
Gallbladder
• Sac lined with mucosa folded into rugae, inner muscularis,
outer serosa
• Bile arrives constantly from liver is stored and concentrated
• Stimulated by cholecystokinin (from the intestine) and vagal
stimulation
• Bile exits through cystic duct then into common bile duct
• Gallstones: precipitated cholesterol (occurs when excess
cholesterol in bile due to high-cholesterol diet and not enough bile salts to
keep it in solution)
– Can block cystic duct
– If gallstone moves far down the duct, it can block
pancreatic duct, resulting in pancreatitis.
– Can occur because of drastic dieting (as the body metabolizes
fat during prolonged fasting and rapid weight loss—such as “crash
diets”—the liver secretes extra cholesterol into bile, which can cause
gallstones.)
24-45
Control of Bile Secretion and Release
24-46
Pancreas
• Pancreas both endocrine and exocrine
• Head, body and tail
• Endocrine: pancreatic islets. Produce
insulin, glucagon, and somatostatin
• Exocrine: groups acini (grape-like
cluster) form lobules separated by
septa.
• Intercalated ducts lead to
intralobular ducts lead to
interlobular ducts lead to the
pancreatic duct.
• Pancreatic duct joins common bile
duct and enters duodenum at the
hepatopancreatic ampulla
controlled by the hepatopancreatic
ampullar sphincter
24-47
Pancreatic Secretions: Pancreatic Juice
• Aqueous. Produced by columnar epithelium lining smaller ducts. Na+, K+, HCO3, water. Bicarbonate lowers pH inhibiting pepsin and providing proper pH for
enzymes
• Enzymatic portion: (without the enzymes produced by pancreas, lipids, proteins, & carbs not
adequately digested)
–
–
–
–
–
–
Trypsinogen- active form is trypsin--------proteolytic enzyme
Chymotrypsinogen- active form is chymotrypsin--------proteolytic enzyme
Procarboxypeptidase- active form is carboxypeptidase-------proteolytic enzyme
Pancreatic amylase- continues digestion of starch.
Pancreatic lipases- lipid digesting enzyme
Deoxyribonucleases and ribonucleases- reduce DNA & RNA to their nucleotide
• Interaction of duodenal and pancreatic enzymes
– Enterokinase is a proteolytic enzyme from the duodenal mucosa and it
activates trypsinogen to trypsin.
– Trypsin activates chymotrypsinogen to chymotrypsin.
– Trypsin activates procarboxypeptidase to carboxypeptidase.
24-48
Bicarbonate Ion Production in Pancreas
24-49
Control of Pancreatic Secretion
24-50
Large Intestine
• Extends from ileocecal
junction to anus
• Consists of cecum, colon,
rectum, anal canal
• Movements sluggish (18-24
hours); chyme converted to
feces.
• Absorption of water and salts,
secretion of mucus, extensive
action of microorganisms are
involved in the formation of
feces.
• 1500 mL chyme enter the
cecum, 90% of volume
reabsorbed yielding 80-150
mL of feces
24-51
Anatomy of Large Intestine
• Cecum
– Blind sac, vermiform appendix
attached. Appendix walls contain
numerous lymph nodules
• Colon
– Ascending, transverse,
descending, sigmoid
– Circular muscle layer complete;
longitudinal incomplete (three
bands called teniae coli).
Contractions of teniae form
pouches called haustra.
Small fat-filled pouches called
epiploic appendages
– Mucosa has numerous straight
tubular glands called crypts.
Goblet cells predominate, but
there are also absorptive and
granular cells as in the small
intestine
24-52
Anatomy of Large Intestine
• Rectum
– Straight muscular tube,
thick muscular tunic
• Anal canal- superior epithelium
is simple columnar; inferior
epithelium is stratified
squamous
– Internal anal sphincter
(smooth muscle)
– External anal sphincter
(skeletal muscle)
– Hemorrhoids: Vein
enlargement or
inflammation
24-53
Secretions of Large Intestine
• Mucus provides protection
– Parasympathetic stimulation increases rate of goblet cell
secretion
• Pumps: bacteria produce acid and the following
remove acid from the epithelial cells that line the large
intestine
– Exchange of bicarbonate ions for chloride ions
– Exchange of sodium ions for hydrogen ions
• Bacterial actions produce gases (flatus) from particular
kinds of carbohydrates found in legumes and in
artificial sugars like sorbitol
• Bacteria produce vitamin K which is then absorbed
• Feces consists of water, undigested food (cellulose),
microorganisms, sloughed-off epithelial cells
24-54
Movement in Large Intestine
• Mass movements (strong contractions)
– Common after meals
– Integrated by the enteric plexus
• Local reflexes instigated by the
presence of food in the stomach and
duodenum
– Gastrocolic: initiated by
stomach
– Duodenocolic: initiated by
duodenum
• Defecation
– Defecation reflex: distension of
the rectal wall by feces
– Parasympathetic stimulation
– Usually accompanied by
voluntary movements to expel
feces. Abdominal cavity pressure
caused by inspiration and by
contraction of muscles of
abdominal wall.
24-55
Digestion, Absorption, Transport
• Digestion
– Breakdown of food molecules for absorption into
circulation
• Mechanical: breaks large food particles to small
• Chemical: breaking of covalent bonds by digestive
enzymes
• Absorption and transport
– Molecules are moved out of digestive tract and
into circulation for distribution throughout body
24-56
24-57
Carbohydrates: Hydrolyzed into Monosaccharides
• Glucose is transported to cells requiring energy; insulin influences rate
of transport
24-58
Transport of Lipids Across Intestinal
Epithelium
24-59
Lipids
•
•
•
•
Include triglycerides, phospholipids, steroids, fatsoluble vitamins
Bile salts surround fatty acid and glycerol to
form micelles
Chylomicrons are 90% triglyceride, 5%
cholesterol, 4% phospholipid, 1% protein.
Chylomicrons enter blood stream and travel to
adipose tissue. In blood, triglycerides converted
back into fatty acids and glycerol where they are
transported into the adipose cells, then converted
back into triglycerides.
24-60
Lipoproteins
• All lipids carried in the blood are done so in combination
with protein to make them soluble in plasma.
• Cholesterol: 15% ingested; 85% manufactured in liver and
intestinal mucosa
• Lipids are lower density than water; proteins are higher
density than water
• Chylomicrons: 99% lipid and 1% protein (extremely low
density); enter lymph
• VLDL: 92% lipid, 8% protein
– Form in which lipids leave the liver
– Triglycerides removed from VLDL and stored in
adipose cells. VLDL has been converted to LDL.
• LDL: 75% lipid, 25% protein
– Transports cholesterol to cells
– Cells have LDL receptors
– # of LDL receptors become less once cell’s
lipid/cholesterol needs are met.
• HDL: 55% lipid, 45% protein
– Transports excess cholesterol from cells to liver
24-61
Transport of LDL into Cells
24-62
Amino Acid Transport
24-63
Proteins
• Pepsin breaks proteins into smaller
polypeptide chains
• Proteolytic enzymes produce small peptide
chains
– Dipeptides, tripeptides, amino acids
• After absorption, amino acids are carried
through the hepatic portal vein to the liver.
24-64
Water and Ions
• Water: can move in
either direction across
wall of small intestine
depending on osmotic
gradients
• Ions: sodium,
potassium, calcium,
magnesium, phosphate
are actively
transported
24-65
Effects of Aging
• Decrease in mucus layer, connective tissue,
muscles and secretions
• Increased susceptibility to infections and
toxic agents, increase in incidences of
ulcerations and cancers
24-66