Biology 233
Human Anatomy and Physiology
Chapter 24 Lecture Outline
DIVISIONS OF THE DIGESTIVE SYSTEM
Gastrointestinal (GI) Tract – path that food follows
mouth, pharynx, esophagus, stomach, small intestine,
large intestine, rectum, anus
Accessory Digestive Structures
tongue, teeth, salivary glands
liver, gallbladder, pancreas
FUNCTIONS OF THE DIGESTIVE SYSTEM
1) ingestion – eating and drinking
2) secretion – lubricate, soften and digest foods, regulate GI function,
protect GI tract
3) motility – smooth muscle contractions
mix food with digestive secretions (churning, segmentation)
propel food from esophagus to anus (peristalsis)
4) digestion (catabolism) – chemically breaking down food into molecules
that can be absorbed (digestive enzymes)
5) absorption – passage of digested materials through cells lining intestine
into blood or lymph
6) excretion – secretion of cellular wastes into lumen of GI tract
7) defecation – elimination of feces
excreted wastes, undigested or unabsorbed materials
ABDOMINOPELVIC CAVITY – contains most of GI tract
Peritoneum – serosa of cavity
parietal peritoneum – lines walls
visceral peritoneum – lines organs
peritoneal cavity – space between parietal and visceral peritonea
contains serous fluid – lubricates surfaces
ascites – fluid accumulation in peritoneal cavity
peritonitis – inflammation of peritoneum
Folds of Peritoneum – support organs, contain vessels and nerves to organs
greater omentum – covers intestines
attaches from stomach and duodenum to transverse colon
stores large amounts of fat
lesser omentum – from liver to stomach and duodenum
mesentery – posterior body wall to intestines
forms fan between loops of small intestines
mesocolon – posterior body wall to large intestine
falciform ligament – diaphragm and anterior body wall to liver
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Retroperitoneal Organs – behind parietal peritoneum, against posterior wall
kidneys, adrenals, pancreas and duodenum
GENERAL HISTOLOGY OF THE GI TRACT
4 Main Layers:
mucosa – inner epithelial membrane
submucosa – connective tissue
muscularis – smooth muscle
serosa (visceral peritoneum) or
adventitia (connective tissue connected to surrounding tissues)
1) Mucosa – 3 layers
epithelium (inner layer) – 2 types depending on region
nonkeratinized stratified squamous – protects from abrasion
mouth, pharynx, esophagus, anal canal
simple columnar – secretion, absorption
stomach, small and large intestines
tight junctions between cells prevent leakage
goblet cells – secrete mucus
enteroendocrine cells – secrete regulatory hormones
lamina propria (middle layer)
areolar connective tissue w/ blood and lymph vessels
mucous glands
MALT (mucosa-associated lymphatic tissue)
immune protection – lymphocytes and macrophages
muscularis mucosae (outer layer)
smooth muscle – tension produces folds in epithelium
2) Submucosa
dense irregular connective tissue w/ blood and lymph vessels
various digestive glands
submucosal plexus – nerves that regulate mucosa and submucosa
regulates mucosal movement, vascular diameter, secretions
3) Muscularis (externa)
mainly smooth muscle
inner circular layer
outer longitudinal layer
myenteric plexus – nerves that regulate GI motility
(skeletal muscle – mouth, pharynx, upper esophagus, anal sphincter)
voluntary control of swallowing and defecation
4) Serosa – visceral peritoneum (lines abdominopelvic organs)
serous membrane – secretes serous fluid
simple squamous epithelium
areolar connective tissue
(Adventitia – connective tissue around mouth, pharynx, esophagus, rectum)
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MOUTH (ORAL OR BUCCAL CAVITY) – space containing teeth and tongue;
functions in ingestion of food and fluids
Cheeks – form walls of mouth; end anteriorly at lips
skeletal muscle – positions food in mouth
integument lines externally
keratinized stratified squamous epithelium
oral mucosa lines internally
nonkeratinized stratified squamous w/mucous glands
Vestibule – space between cheeks (or lips) and teeth
Hard and Soft Palates – roof of mouth; lined by oral mucosa
hard – maxillae and palatine bones
soft – skeletal muscle; covers nasopharynx when swallowing
uvula – dangling process
Fauces – opening from oral cavity into pharynx
Lingual Frenulum – fold of mucosa from floor or mouth to tongue
TONGUE
functions in sensation and manipulation of food, and bolus formation
bolus – ball of food and saliva
ANATOMY OF TONGUE
skeletal muscle
extrinsic muscles – originate on bone and connective tissue around
tongue and insert into tongue
anchor tongue
form floor of mouth
move tongue in and out, side to side
intrinsic muscles – originate and insert within tongue
alter size and shape of tongue
aid in manipulating food and producing speech
oral mucosa – somewhat keratinized on superior surface
papillae
filiform – tactile receptors
fungiform, circumvallate – taste buds
SALIVARY GLANDS – secrete products into mouth
saliva – secretions that moisturize and cleanse mouth structures
lubricates food and begins chemical digestion
COMPOSITION OF SALIVA
water (99.4%)
solutes (0.6%)
mucin (glycoprotein) – forms mucus; lubricates mouth and food
ions – Na, Cl, bicarbonate (buffers acidic foods)
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IgA – antibodies; immune protection
lysozyme – bactericide
digestive enzymes – salivary amylase (digests starch)
lingual lipase (digests triglycerides)
Production of Saliva – salivation
mucous acini – clusters of cells producing mucus
lubricate mouth and food
serous acini – clusters of cells producing watery secretions containing
digestive enzymes
MAJOR SALIVARY GLANDS
parotid glands – near ears; serous secretions
parotid duct – opens near upper second molar
submandibular glands – base of tongue, near mandibles
open under tongue; seromucous secretions
sublingual glands – under tongue, superior to submandibular glands
open under tongue; mucous secretions
small salivary glands – found throughout oral mucosa; most secrete mucus
lingual glands (tongue) – secrete lingual lipase
ANS REGULATION OF SALIVATION
parasympathetic – stimulates salivation
average 1-1.5 liters/day – swallowing moistens esophagus
stimulated by taste, feel, smell, sight, or thought of food
sympathetic – inhibits salivation
TEETH
alveolar processes – maxillae and mandible
alveoli – sockets for teeth
periodontal ligaments – dense fibrous CT
anchors tooth in alveolus
shock absorption
gingivae (gums) – oral mucosa lining alveolar processes
ANATOMY OF A TOOTH
Regions of Tooth
crown – visible portion above gumline
neck – narrowed region in gingival sulcus (pocket)
root – portion embedded in alveolus
1-3 roots/ tooth
Layers of Tooth
pulp cavity – cavity in center of tooth
pulp – loose connective tissue w/blood & lymph vessels
and nerves
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root canal – extension of pulp cavity into root of tooth
apical foramen – opening at root tip where vessels
and nerves enter and exit
dentin – main component of tooth
harder than bone – 70% calcium salts
cementum – outer lining of root; similar to bone
enamel – outer lining of crown only
hardest substance in body – 95% calcium salts
protects tooth from abrasion and acids
HUMAN DENTITION – sets of teeth
4 Types of Teeth:
1) incisors – front teeth
chisel-shaped; cut food
1 root
2) cuspids (canines) – one cusp (point on crown)
pointed; grasp and tear food
1 root
3) bicuspids (premolars) – 2 cusps
crush and grind food
1-2 roots
4) molars – back teeth; 4 cusps
crush and grind food
usually 3 roots
Deciduous (primary) Dentition – baby teeth
20 teeth; replaced by adult teeth
erupt beginning at 6 months (all by 2-3 years)
incisors (8) – central and lateral
cuspids (4)
primary molars (8) – first and second
Permanent (secondary) Dentition – adult teeth
32 teeth
erupt beginning at 6 years and into adulthood
incisors (8) – central and lateral
cuspids (4)
bicuspids (8) – first, second
molars (12) – first, second, and third
first – erupts at 6yr
second – 12yr molar
third – wisdom tooth (impacted)
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SUMMARY OF DIGESTIVE FUNCTIONS OF MOUTH
ingestion – incisors, cuspids, cheeks & lips, tongue
mastication (chewing)
teeth grind, tongue and cheeks manipulate, mixed with saliva
bolus formation – soft ball of food w/saliva
tongue pushes bolus to pharynx for swallowing
beginning of chemical digestion
salivary amylase - breaks down polysaccharides
lingual lipase – breaks down lipids
PHARYNX – (nasopharynx), oropharynx, laryngopharynx
conducts food and liquid from mouth to esophagus
Mucosa of Pharynx:
nasopharynx – respiratory epithelium
oropharynx and laryngopharynx – oral mucosa
PHASES OF DEGLUTITION (swallowing)
1) buccal phase – bolus moved from mouth to oropharynx
voluntary
tongue moves up and back pushing bolus through fauces
2) pharyngeal phase – bolus moved from pharynx to esophagus
involuntary
swallowing reflex – integrated in swallowing center (medulla)
tactile receptors in pharynx and uvula detect bolus
motor neurons stimulate skeletal muscles in pharynx
larynx elevated – epiglottis covers glottis
soft palate elevated – covers nasopharynx
respiratory center inhibited – breathing stops
pharyngeal muscles contract – push bolus into esophagus
3) esophageal phase – bolus moves through esophagus into stomach
involuntary
peristalsis – waves of muscular contraction
ESOPHAGUS
10 inch tube from laryngopharynx to stomach
posterior to trachea / posterior mediastinum
esophageal hiatus – opening through diaphragm
hiatal hernia – abdominal organs slip through opening
HISTOLOGY OF ESOPHAGUS
mucosa – nonkeratinized stratified squamous (abrasion)
submucosa – mucous glands (esophageal glands)
muscularis – transitions from skeletal to smooth muscle
(upper 1/3 is skeletal, lower 1/3 is smooth)
adventitia – connective tissue attaches to surrounding tissues of neck
and mediastinum
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FUNCTION OF ESOPHAGUS
secretes mucus and transports bolus to stomach
upper esopohageal sphincter – relaxes during pharyngeal stage of
swallowing
esophageal phase of swallowing
peristalsis – coordinated contraction and relaxation of circular
and longitudinal muscularis
regulated by swallowing center and local ENS reflexes
lower esophageal sphincter – closed until bolus arrives
prevents gastroesophageal reflux
heartburn – acid from stomach damages esophageal mucosa
STOMACH (GASTRO-)
FUNCTIONS OF STOMACH
mechanical
storage of food
churning – mixing and mechanical breakdown of food
chyme – soupy mixture of food and secretions
secretory – secretes gastric juice (combination of secretions)
hydrochloric acid (HCl)
denatures proteins – breaks down food tissues
activates digestive enzymes
pepsinogen  pepsin
kills bacteria
digestive enzymes
pepsinogen (active form – pepsin) – digests proteins
infants – gastric lipase, rennin – digest milk
mucus – protects mucosa of stomach
regulatory secretions
intrinsic factor – needed to absorb vit. B12
gastrin – hormone that stimulates gastric secretion and
motility
absorption – minor function
some drugs, alcohol
ANATOMY OF STOMACH
J-shaped; just inferior to liver
greater curvature – large, inferior/medial curve
lesser curvature – small, superior/lateral curvature
rugae – folds in mucosa of empty stomach
REGIONS OF STOMACH
1) cardia – superior portion attached to esophagus
many mucous glands protect esophagus
2) fundus – rounded superior portion
storage portion – stretches to contain large meals
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3) body – main central portion
churning – muscular contractions mix food and secretions
4) pylorus – connects to duodenum
pyloric sphincter – allows small amounts of chyme to pass
through to duodenum periodically
HISTOLOGY OF STOMACH
mucosa – simple columnar epithelium
surface mucous cells – secrete mucus
gastric pits – pits in gastric surface
mucous neck cells – mucus
gastric glands – produce various secretions
parietal cells – HCl, intrinsic factor
chief cells – pepsinogen, gastric lipase
enteroendocrine cells – gastrin, other hormones
submucosa
muscularis – 3 layers – assist in churning
outer longitudinal
middle circular
inner oblique
serosa
REGULATION OF GASTRIC ACTIVITY
1) cephalic phase (head) – taste, smell, sight, thought of food
cerebral cortex and hypothalamus stimulated
parasympathetic stimulation (CN X)
increased secretions and motility
2) gastric phase – food entering stomach
stretch receptors and chemoreceptors in stomach
submucosal and myenteric plexus reflexes (ENS)
increased secretions and motility
relaxation of pyloric sphincter
enteroendocrine stimulation – increased gastrin secretion
stimulates gastric secretions and motility
3) intestinal phase – chyme entering duodenum
stretch receptors and chemoreceptors in duodenum
enteroendocrine cells in duodenum stimulated
release hormones that inhibit gastric functions
SMALL INTESTINE – main site of digestion and absorption
FUNCTIONS OF SMALL INTESTINE
mechanical
segmentation – mixing contractions of circular muscles
occur when intestine is full
peristalsis – slow waves move contents from stomach to large intestine
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secretory
intestinal juice – fluid medium for nutrients and enzymes to interact
mucus – alkaline to buffer HCl from stomach
enzymes to digest carbohydrates, lipids, proteins, and nucleic acids
intestinal enzymes
brush border enzymes – attached to mucosal membrane
pancreatic enzymes (from pancreas) – secreted into small
intestine
bile (from liver) – secreted into small intestine
emulsifies lipids – breaks up lipid droplets so they can be digested
by enzymes
enteroendocrine secretions – hormones that coordinate activities of
stomach, intestines, liver, and pancreas
secretin – inhibits gastric function
stimulates liver and pancreatic secretions
cholecystokinin (CCK) – inhibits gastric function
stimulates pancreatic and gallbladder secretions
absorption (site of 90% of absorption)
molecules pass through mucosa into blood or lymph
circular folds, villi, and microvilli increase surface area for absorption
Substances Absorbed in Small Intestine
into blood capillaries
monosaccharides (carbohydrates)
amino acids (proteins)
pentoses, phosphates, nitrogenous bases (nucleic acids)
electrolytes (ions)
water-soluble vitamins
water
into lymphatic capillaries
fatty acids, monoglycerides (lipids)
lipid-soluble vitamins
micelles – lipids surrounded by bile salts
dissolve lipids in intestinal lumen and transport them to
mucosa to be absorbed
chylomicrons – lipids surrounded by protein coat in mucosa
secreted into interstitial space  lymph  blood
ANATOMY OF SMALL INTESTINE
Regions of Small Intestine
1) duodenum – proximal portion; attached to stomach
retroperitoneal
2) jejunum – middle portion
3) ileum – distal portion; longest
ileocecal valve – opening into colon; regulated by a
smooth muscle sphincter
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plicae – folds in mucosa & submucosa from duodenum to mid-ileum
increase surface area of mucosa for absorption
HISTOLOGY OF SMALL INTESTINE
mucosa
simple columnar epithelium w/goblet cells
villi – finger-like extensions of mucosa and lamina propria
increase surface area for absorption
microvilli – finger-like extensions of epithelial cell membrane
increase surface area for absorption
brush border (fuzzy appearance) – brush border enzymes
intestinal glands (crypts) – pockets of secretory cells
intestinal juice
enteroendocrine cells – secretin, CCK, other hormones
Peyer’s patches (MALT)
lacteals – large lymphatic capillaries in villi
extensive capillary network
submucosa
duodenal glands (Brunner’s glands)
secrete alkaline mucus – neutralizes HCl from stomach
muscularis – 2 layers
serosa
REGULATION OF SMALL INTESTINE
submucosal and myenteric reflexes (ENS)
touch & stretch receptors and chemoreceptors detect chyme
increased intestinal motility and secretions
enteroendocrine secretions stimulate intestine, liver, pancreas
and gallbladder
coordination with stomach - parasympathetic
stretch reflexes – stomach filling (more stretch) stimulates
intestinal function
cephalic phase parasympathetic stimulation
stimulates intestinal function
LARGE INTESTINE
FUNCTIONS OF LARGE INTESTINE
mechanical
haustral churning – haustra (pouches) fill, churn contents, pass contents
to the next haustra
mass movement – rapid contraction drives contents forward in colon and
into rectum
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secretory – mucus to lubricate feces
bacterial flora – normal, symbiotic bacteria living in GI tract
ferment carbohydrates – release gases
digest peptides – odor
bilirubin – converted to a brown pigment
produce vitamins – K, biotin, B5
absorption
water, vitamins (K, biotin, B5), bile salts
form feces – sloughed mucosal cells, bacteria, salts, unabsorbed or
indigestible materials (fiber)
ANATOMY OF THE LARGE INTESTINE
extends from ileocecal valve to rectum
Regions of Large Intestine
1) cecum – blind pouch at proximal end
appendix – finger-like extension; many lymphoid nodules
2) ascending colon – right abdomen
3) right colic flexure (hepatic flexure)
4) transverse colon – superior
5) left colic flexure (splenic flexure)
6) descending colon – left abdomen
7) sigmoid colon – S-shaped distal end
rectum – expands to store feces
mucosa – stratified squamous epithelium
anus – exit for digestive tract (defecation)
internal anal sphincter – involuntary (smooth muscle)
external anal sphincter – voluntary (skeletal muscle)
HISTOLOGY OF LARGE INTESTINE
mucosa
simple columnar epithelium
reduced surface area (no villi)
intestinal glands – long, straight pits into lamina propria
many goblet cells – mucus lubricates feces
lymphatic nodules (MALT)
submucosa
muscularis
teniae coli – 3 longitudinal bands of smooth muscle
tension forms pouches in colon (haustra)
serosa
fatty appendices – serosal pouches with fat
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REGULATION OF LARGE INTESTINE
activity of small intestine fills proximal colon
local ENS reflexes stimulate haustral churning and slow peristalsis
gastrocolic reflex – stretch of stomach and duodenum triggers mass
movement
defecation reflex – triggered by stretch receptors in wall of rectum
parasympathetic stimulation of teniae coli in descending and
sigmoid colons
parasympathetic inhibition of internal anal sphincter
somatic reflex (skeletal muscle)
contraction of external anal sphincter
conscious component
relaxation of external anal sphincter
contraction of abdominal muscles
PANCREAS
endocrine function – pancreatic islets regulate blood glucose level
exocrine function – secretes pancreatic juice into small intestine
COMPOSITION OF PANCREATIC JUICE
water, ions
sodium bicarbonate – alkaline pH (7.1-8.2)
buffers HCl from stomach
pancreatic enzymes
many – carbohydrases – digest carbohydrates
proteases/peptidases – digest proteins/peptides
lipases – digest lipids
nucleases – digest nucleic acids
proenzymes – inactive until activated by intestinal enzymes
ANATOMY OF PANCREAS
retroperitoneal – behind stomach
head – attached to duodenum
body and tail
pancreatic duct – carries pancreatic juice to duodenum
hepatopancreatic ampulla – pancreatic duct and bile duct from
liver join
duodenal papilla – hepatopancreatic ampulla opens into duodenum
hepatopancreatic sphincter – regulates opening
(accessory pancreatic duct – empties directly into duodenum)
HISTOLOGY OF PANCREAS
pancreatic acini – clusters of cuboidal cells; secrete digestive enzymes
ducts – epithelium secretes water and bicarbonate
pancreatic islets (islets of Langerhans) – secrete hormones
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REGULATION OF PANCREAS
cephalic phase of gastric stimulation
parasympathetic (CN X) – stimulates enzyme secretion
chyme entering duodenum
stimulates enteroendocrine cells
CCK – stimulates enzymatic secretions
secretin – stimulates bicarbonate secretion
LIVER
hepatic portal system carries blood and absorbed materials from GI tract to
liver for processing before they enter general circulation
liver regulates composition of the blood
FUNCTIONS OF LIVER
1) carbohydrate metabolism – maintains blood glucose level
low glucose
glycogenolysis – stored glycogen broken down to glucose
gluconeogenesis – amino acids, lactic acid, other monsaccharides
converted to glucose
high glucose
glycogenesis – glucose stored as glycogen
lipogenesis – glucose converted to triglycerides (fat)
2) lipid metabolism
stores or breaks down triglycerides
breaks down fatty acids for energy
synthesizes cholesterol
3) amino acid metabolism
deamination – removes amino groups (NH2)
then converted to carbs or fats, or broken down for energy
ammonia (NH3) – toxic by-product of deamination
4) synthesizes plasma proteins (except Igs)
albumin, alpha and beta globulins, fibrinogen, prothrombin
5) removal or inactivation of many products in blood
detoxification – breaks down may drugs and toxins (eg. alcohol)
converts toxic ammonia to urea
excretes or stores other drugs and toxins (eg. antibiotics, pesticides)
excretes bilirubin – from old blood cells
recycles antibodies and hormones (eg. thyroxine and steroids)
6) synthesizes and secretes bile
7) storage
glycogen
vitamins (A,B12,D,E,K)
minerals (iron, copper)
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8) phagocytosis and antigen presentation
Kupffer’s cells – fixed macrophages
remove old blood cells, debris, pathogens
9) activation of Vit D (calcitriol)
skin, liver, and kidneys participate
ANATOMY OF LIVER AND GALL BLADDER
largest gland of body
immediately inferior to diaphragm
falciform ligament – attaches liver to diaphragm and anterior
abdominal wall
Lobes of Liver
right and left lobes (falciform ligament separates)
quadrate lobe – inferior, posterior
caudate lobe – superior, posterior
gallbladder – pear-shaped sac in depression on posterior surface
of liver; stores bile from liver
BLOOD SUPPLY TO LIVER
hepatic artery – branch of aorta
hepatic vein – drains from liver to inferior vena cava
hepatic portal vein – drains venous blood from GI tract
carries nutrients absorbed by GI tract to liver
branches into a second capillary bed in liver
liver processes nutrients before blood returns to hepatic vein
HISTOLOGY OF LIVER
lobules – functional units of liver
hexagonal (6 sides)
central vein – drains into hepatic vein
triads (portal areas) at each corner of lobule
branch of hepatic artery – brings O2 and hormones to lobule
branch of hepatic portal vein – brings GI nutrients to lobule
bile duct – drains bile produced in lobule
hepatic sinusoids – large capillary spaces connecting central vein
to blood vessels of triad
Kupffer cells – fixed macrophages in sinusoids
hepatocytes – liver cells lining sinusoids
process incoming blood
produce bile
bile canaliculi – ducts which collect bile from hepatocytes
drain into bile ducts
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right and left hepatic ducts – formed by bile ducts converging
drain bile from right and left liver lobes
common hepatic duct – drains bile into common bile duct OR cystic duct
common bile duct – indirectly empties bile into duodenum
cystic duct – drains into gall bladder to store bile when hepatopancreatic
sphincter is closed
COMPOSITION OF BILE
1 quart/day; olive green/yellowish/brownish
bicarbonate ions – alkaline pH (7.6-8.6)
buffers acid from stomach
bile salts – lipids derived from cholesterol
amphipathic – have charged and uncharged regions
emulsification – break large, insoluble lipid globules into tiny
droplets coated by a layer of bile salts (micelles)
increases surface area for digestion
makes lipids soluble and aids in their absorption
(bile salts are removed from lipids as they pass into mucosal cells)
bile pigments – from bilirubin; give bile its color
REGULATION OF BILE SECRETION
following a meal
parasympathetic impulses stimulate bile secretion
chyme in duodenum stimulates enteroendocrine cells
CCK – stimulates contraction of gall bladder
relaxes hepatopancreatic sphincter
secretin – stimulates bicarbonate secretion from liver
bile salts are reabsorbed by large intestine
bile salts returning to liver in hepatic portal vein stimulate liver
more bile is secreted
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