digestive system
TRANSCRIPT
Digestive system• Note:
All the powerpoint presentations are meant for better understanding of text. These in no way replace the text books .All chapters in the text books must be thoroughly studied.
Digestive system
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Digestive System: Overview• The alimentary canal or gastrointestinal
(GI) tract digests and absorbs food• Alimentary canal – mouth, esophagus,
stomach, small intestine, and large intestine
• Accessory digestive organs – teeth, tongue, gallbladder, salivary glands, liver, and pancreas
Digestive Process
• There are six essential activities: –Ingestion–propulsion –mechanical digestion –Chemical digestion–Absorption–defecation
Gastrointestinal Tract Activities• Ingestion – taking food into the digestive tract • Propulsion – swallowing and peristalsis
– Peristalsis – waves of contraction and relaxation of muscles in the organ walls
• Mechanical digestion – chewing, mixing, and churning food
• Chemical digestion – catabolic breakdown of food• Absorption – movement of nutrients from the GI
tract to the blood or lymph• Defecation – elimination of indigestible solid
wastes
Histology of the Alimentary Canal
• From esophagus to the anal canal the walls of the GI tract have the same four tunics–From the lumen to outward they are
the: – Mucosa– Submucosa– Muscularis externa– Serosa
Histology of the Alimentary Canal
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Regulation of Digestive Tract• Motility and secretion of the digestive tract are controlled by
neural, hormonal, and paracrine mechanisms
• Neural control– Short (myenteric) reflexes – stretch or chemical stimulation acts through
myenteric plexus• Stimulates parastaltic contractions of swallowing
– Long (vagovagal) reflexes - parasympathetic stimulation of digestive motility and secretion
• Hormones– Chemical messengers secreted into bloodstream, and stimulate distant
parts of the digestive tract
• Paracrine secretions– Chemical messengers that diffuse through the tissue fluids to stimulate
nearby target cells
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Enteric Nervous System• Enteric nervous system – a nervous network in the esophagus,
stomach, and intestines that regulated digestive tract motility, secretion, and blood flow– thought to have over 100 million neurons– more than the spinal cord– functions completely independently of the central nervous system
• CNS exerts a significant influence on its action
• composed of two networks of neurons – Submucosal (Meissner) plexus – in submucosa
• controls glandular secretion of mucosa• controls movements of muscularis mucosae
– Myenteric (Auerbach) plexus – parasympathetic ganglia and nerve fibers between the two layers of the muscularis interna
• controls peristalsis and other contractions of muscularis externa
• enteric nervous system contains sensory neurons that monitor tension in gut wall and conditions in lumen
Mouth
• Oral or buccal cavity:–Is bounded by lips, cheeks, palate, and
tongue –Has the following organs:–Teeth–Salivary glands–tongue
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Mouth or Oral Cavity
Figure 25.4
Upper lip
V estibule
T ongue
Lingual frenulum
Lower lip
SubmandibularSublingual
Palatine tonsil
Palatine raphe
Soft palate
Uvula
Palatoglossalarch
Palatopharyngealarch
Inferior labialfrenulum
Hard palateand palatinerugae
Superiorlabialfrenulum
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Salivary ductorifices:
Tongue• Occupies the floor of the mouth and fills the oral cavity when
mouth is closed• Functions include:
– Gripping and repositioning food during chewing– Mixing food with saliva and forming the bolus– Initiation of swallowing, and speech
Lingual Frenulum – median fold that attaches the body to the floor of the mouth
• Sulcus terminalis – groove that separates the tongue into two areas:– Anterior 2/3 residing in the oral cavity– Posterior third residing in the oropharynx
Tongue• Superior surface bears four types of papillae
– Filiform – give the tongue roughness and provide friction . Do not contain taste buds
– Fungiform – scattered widely over the tongue and give it a reddish hue. Contains lots of taste buds.
– Circumvallate – V-shaped row in back of tongue– Foliate –pleatlike shaped
#Study:- Ankyloglossia
Tongue
Figure 23.8
Salivary Glands
• Three pairs of glands – parotid, submandibular, and sublingual– Parotid – lies anterior to the ear between the
masseter muscle and skin. Parotid duct opens into the vestibule next to second upper molar
• Submandibular – lies along the medial aspect of the mandibular body. Its ducts open at the base of the lingual frenulum
• Sublingual – lies anterior to the submandibular gland under the tongue
• # Study : Mumps
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Salivary Glands
Figure 25.9
Parotid duct
Mandible
Parotidgland
Sublingualducts
Massetermuscle
Submandibularduct
Submandibulargland
Sublingualgland
Opening ofsubmandibularduct
Lingualfrenulum
Tongue
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Salivary Glands
Figure 23.9a
Saliva: Source and Composition• Secreted from serous and mucous cells of
salivary glands• 97-99.5% water, hypo-osmotic, slightly
acidic solution containing– Electrolytes – Na+, K+, Cl–, PO4
2–, HCO3–
– Digestive enzyme – salivary amylase– Proteins – mucin, lysozyme, defensins, and IgA– Metabolic wastes – urea and uric acid
Teeth• Primary and permanent dentitions have formed
by age 21• Primary – 20 deciduous teeth that erupt at
intervals between 6 and 24 months• Permanent – enlarge and develop causing the
root of deciduous teeth to be resorbed and fall out between the ages of 6 and 12 years– All but the third molars have erupted by the end of
adolescence– Usually 32 permanent teeth
Classification of Teeth
• Teeth are classified according to their shape and function– Incisors – chisel-shaped teeth for cutting or
nipping–Canines – fanglike teeth that tear or pierce–Premolars (bicuspids) and molars – have
broad crowns with rounded tips; best suited for grinding or crushing
• During chewing, upper and lower molars lock together generating crushing force
Deciduous Teeth
Permanent Teeth
Figure 23.10.2
2I 1C 2PM 3M
X 2 (32 teeth)2I 1C 2PM 3M
Dental Formula: Permanent Teeth
• A shorthand way of indicating the number and relative position of teeth– Written as ratio of upper to lower teeth for the
mouth– Primary: 2I (incisors), 1C (canine), 2M (molars)– Permanent: 2I, 1C, 2PM (premolars), 3M
Tooth Structure
• Two main regions – crown and the root• Crown – exposed part of the tooth above the
gingiva• Enamel – acellular, brittle material composed
of calcium salts and hydroxyapatite crystals; the hardest substance in the body– Encapsules the crown of the tooth
• Root – portion of the tooth embedded in the jawbone
Tooth Structure
Esophagus• – a straight muscular tube 25-30 cm long
– begins at level between C6 and the cricoid cartilage– extends from pharynx to cardiac orifice of stomach passing
through esophageal hiatus in diaphragm– lower esophageal sphincter – food pauses at this point
because of this constriction• prevents stomach contents from regurgitating into the
esophagus• protects esophageal mucosa from erosive effect of the
stomach acid• heartburn – burning sensation produced by acid reflux
into the esophagus
• #Study: Deglutition
Esophagus
Stomach• Chemical breakdown of proteins begins and food is
converted to chyme.it has the following parts:-
• Cardiac region – surrounds the cardiac orifice
• Fundus – dome-shaped region beneath the diaphragm
• Body – midportion of the stomach• Pyloric region – made up of the antrum
and canal which terminates at the pylorus• The pylorus is continuous with the
duodenum through the pyloric sphincter
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Gross Anatomy of Stomach
Figure 25.12blongitudinal wrinkles called rugae can be seen in empty stomach wall.
Esophagus
Cardiac region
Body
Greater curvature
Fundic region
Gastric rugae
Cardiac orificeLesser curvature
Pyloric region:
Antrum
Pylorus
Duodenum
(b)
Pyloricsphincter
Pyloriccanal
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© The McGraw-Hill Companies, Inc./Rebecca Gray, photographer/Don Kincaid, dissections
Microscopic Anatomy of the Stomach
Glands of the Stomach Fundus and Body• Gastric glands of the fundus and body have a
variety of secretory cells
–Mucous neck cells – secrete acid mucus–Parietal cells – secrete HCl and intrinsic
factor–Chief cells – produce pepsinogen and
prorenin–Enteroendocrine cells – secrete gastrin,
histamine, endorphins, serotonin, cholecystokinin (CCK), and somatostatin into the lamina propria
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Functions of Hydrochloric Acid• activates pepsin and lingual lipase
• breaks up connective tissues and plant cell walls– helps liquefy food to form chyme
• converts ingested ferric ions (Fe3+) to ferrous ions (Fe2+)– Fe2+ absorbed and used for hemoglobin synthesis
• contributes to nonspecific disease resistance by destroying most ingested pathogens
• #Study: Ulcer
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Pepsin• zymogens – digestive enzymes secreted as inactive
proteins– converted to active enzymes by removing some of their
amino acids
• pepsinogen – zymogen secreted by the chief cells– hydrochloric acid removes some of its amino acids and
forms pepsin that digests proteins– autocatalytic effect – as some pepsin is formed, it converts
more pepsinogen into more pepsin
• pepsin digests dietary proteins into shorter peptide chains– protein digestion is completed in the small intestine
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Intrinsic Factor• intrinsic factor – a glycoprotein secreted by parietal
cells
• essential to absorption of vitamin B12 by the small intestine– binds vitamin B12 and intestinal cells absorb this complex by
receptor-mediated endocytosis
• vitamin B12 is needed to synthesize hemoglobin– prevents pernicious anemia
• secretion of intrinsic factor is the only indispensable function of the stomach– digestion can continue if stomach is removed (gastrectomy),
but B12 supplements will be needed
Small Intestine• Runs from pyloric sphincter to the ileocecal valve
• Has three subdivisions: • Duodenum, Jejunum, And Ileum• Structural modifications of the small intestine wall
increase surface area
–Plicae circulares: deep circular folds of the mucosa and submucosa
–Villi – fingerlike extensions of the mucosa
–Microvilli – tiny projections of absorptive mucosal cells’ plasma membranes
Small Intestine
Duodenum and Related Organs
Liver
• The largest gland in the body• Superficially has four lobes – right, left,
caudate, and quadrate• The gallbladder rests in a recess on the
inferior surface of the right lobe
Liver:functions
• Hepatocytes’ functions include:– Production of bile– Processing bloodborne nutrients– Storage of fat-soluble vitamins– Detoxification– Deamination– Gluconeogenesis, glucogenesis etc.
Liver: Associated Structures
• Bile produced by the liver leaves it via:
–Bile ducts, which is formed by fusion of cystic duct of gall bladder and hepatic duct of liver
–This bile duct then fuses with the pancreatic duct to form hepatopancreatic duct/ampulla(sphincter of Oddi)
Figure 23.24c
Liver: Microscopic Anatomy
• Hexagonal-shaped liver lobules are the structural and functional units of the liver– Composed of hepatocyte (liver cell) plates
radiating outward from a central vein
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Microscopic Anatomy of Liver
Figure 25.20a
Stroma
Bile ductule
(a)
Hepatocytes
Stroma
Hepatic triad:
Central vein
Branch ofhepaticportal vein
Branch ofproper hepaticartery
Bilecanaliculi
Hepaticsinusoid
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Liver: Microscopic Anatomy
• Liver sinusoids – enlarged, leaky capillaries located between hepatic plates
• Kupffer cells – hepatic macrophages found in liver sinusoids
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Histology of Liver - Hepatic Triad
Figure 25.20b
Stroma
Bile ductule
0.5 mm(b)
Central vein
Hepaticlobule
Branch ofhepaticportal vein
Lymphaticvessel
Branchof properhepatic artery
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© The McGraw-Hill Companies, Inc./Dennis Strete, photographer
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Gross Anatomy of the Gallbladder, Pancreas, and Bile Passages
Hepatic ducts
Common hepatic duct
Cystic duct
Bile duct
Gallbladder: Neck Body Head
Pancreatic duct
Jejunum
Duodenum
Circular folds
Pancreas: Tail Body Head
Minor duodenalpapilla
Hepatopancreaticsphincter
Major duodenalpapilla
Hepatopancreaticampulla
Accessorypancreatic duct
Duodenojejunalflexure
Figure 25.21
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Composition of Bile• A yellow-green, alkaline solution containing bile
salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes
• Bile salts are cholesterol derivatives that:–Emulsify fat–Facilitate fat and cholesterol
absorption–Help solubilize cholesterol
• The chief bile pigment is bilirubin, a waste product of heme
The Gallbladder
• Thin-walled, green muscular sac on the ventral surface of the liver
• Stores and concentrates bile by absorbing its water and ions
• Releases bile via the cystic duct, which flows into the bile duct
• #Study: Gallstones and Lithotripsy
Pancreas
• Location– Lies deep to the greater curvature of the stomach– The head is encircled by the duodenum and the tail abuts
the spleen.
– Pancreas is a heterocrine gland that has both endocrine and exocrine functions:-
• Exocrine function– Secretes pancreatic juice which breaks down all
categories of foodstuff. This pancreatic juice is secreted in acini.
– Acini (clusters of secretory cells) contain zymogen granules with digestive enzymes
• Endocrine function – release of insulin and glucagon
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Pancreatic Acinar Cells
Figure 25.22 a-b
Acinar cells
(a)
Stroma
Ducts
(b)
Exocrineacinar cells
Vein
Zymogengranules
50 µm
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© The McGraw-Hill Companies, Inc./Dennis Strete, photographer
Large Intestine
• Is subdivided into the cecum, appendix, colon, rectum, and anal canal
• The saclike cecum:– Lies below the ileocecal valve in the right iliac
fossa– Contains a wormlike vermiform appendix
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Anatomy of Large Intestine
Haustrum
Ileum
Cecum
Appendix
Rectum
Anal canal
(a) Gross anatomy
Mesocolon
Right colicflexure
Transversecolon
Superiormesentericartery
Ascendingcolon
Ileocecalvalve
Greateromentum(retracted)
Left colicflexure
Descendingcolon
Omentalappendages
Sigmoidcolon
External analsphincter
Taenia coli
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Figure 25.31a
Colon• Has distinct regions:
• ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, and sigmoid colon
• The sigmoid colon joins the rectum • The anal canal, the last segment of the
large intestine, opens to the exterior at the anus
Large intestine
• Has three unique features:– Teniae coli – three bands of longitudinal smooth
muscle in its muscularis– Haustra – pocketlike sacs caused by the tone of
the teniae coli– Epiploic appendages – fat-filled pouches of
visceral peritoneum
Functions of the Large Intestine• Other than digestion of enteric bacteria, no
further digestion takes place• Vitamins, water, and electrolytes are reclaimed• Its major function is propulsion of fecal
material toward the anus• Though essential for comfort, the colon is not
essential for life• #Study :- Diarrhea and Constipation.
Physiology of digestion.
1.Digestion of carbohydrates.2.Digestion of proteins.
3.Digestion of fats.
Digestion of carbohydrates
• Digestion of carbohydrates starts in the oral cavity.
• Salivary amylase present in the saliva causes the breakdown of complex carbohydrates into smaller parts.
• Polysaccharides salivary amylase oligosaccharides,
disaccharides,
monosaccharides.
Carbohydrate digestion in stomach
• Stomach does not have any enzymes for digestion of carbohydrates.
Carbohydrate digestion in small intestine
• Small intestine receives two juices:- intestinal juice and the pancreatic juice
• Pancreatic juice have one enzymes for carbohydrate digestion:- pancreatic amylase
• Pancreatic amylase will act upon complex carbohydrates and break them into small carbohydrates Eg. Starch can be broken down into maltose, isomaltose, etc.
Carbohydrate digestion in small intestine
• Intestine contains many enzymes for carbohydrate digestion:- intestinal amylase, maltase, isomaltase, lactase, sucrase.
• Thus all these enzymes will ultimately convert complex carbohydrates into monosaccharide like glucose, fructose and galactose
Digestion of proteins
• The final product of protein digestion are the amino acids.
• There are no protein digesting enzymes in the mouth and thus Protein digestion starts in the stomach . Enzyme in stomach is pepsinogen– Chief cells of stomach – produce pepsinogen
• Pepsinogen is activated to pepsin by:– HCl in the stomach– Pepsin itself via a positive feedback mechanism
–Pepsin will then act on large protein and will break them into small peptides and amino acids
Digestion of proteins in small intestine
• Enzymes acting in the small intestine–Pancreatic enzymes – trypsinogen,
chymotrypsinogen, and procarboxypeptidase
Pancreatic enzymes are in inactive form and they need to be activated to become functional
- Intestinal enzymes – aminopeptidases, carboxypeptidases, and dipeptidases
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Activation of Pancreatic Enzymes in the Small Intestine
Figure 25.23
Enterokinase
Trypsinogen
ChymotrypsinogenProcarboxypeptidase
ChymotrypsinCarboxypeptidase
Trypsin
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Digestion of fats• Fat (triglycerides) digestion starts in the stomach and
is completed in the intestine. Saliva do not have any fat digesting enzyme.
• Enzymes for fat digestion are :-• Lingual lipase• Gastric lipase• Pancreatic lipase• Intestinal lipase• Bile plays a very important role of emulsifying the
fats.• All these enzymes will break fats into fatty acids
and glycerol.
Action of bile on fat
Absorption• Absorption refers to transport of digested
products of food from the intestinal lumen into the blood circulation.
• Carbohydrate absorption: glucose and galactose are carried by active transport from the intestinal lumen into the epithelial cells of intestine. Then they move out of these cells into the blood capillaries by simple diffusion
• Protein absorption: it is very similar to absorption of carbohydrates. amino acids are carried from lumen to epithelial cells by active transport and then they leave these cells by diffusion.
Figure 23.34
Fatty Acid Absorption• Fatty acids and monoglycerides combine with
bile salt and cholesterol to form micelles• Micelles act as ‘ferry’ and help fatty acids to
enter the intestinal cells by diffusion• Once inside the intestinal cells the free fatty
acids and glycerol are resynthesized into trigylceride. Triglycerides combine with protein to form chylomicron. Resulting chylomicrons are extruded and they enter lacteals and are transported to the circulation via lymph
Figure 23.36
Lumen ofintestine
Absorptiveepithelial cellcytoplasm
ER
Golgiapparatus
ChylomicronLacteal
Fatty acids andmonoglyceridesassociated withmicelles inlumen of intestine
Fatty acids andmonoglyceridesresulting from fatdigestion leavemicelles and enterepithelial cell by diffusion.
Fatty acids are used to synthesize triglycerides in smooth endo-plasmic reticulum.
Fatty globules arecombined with proteins to form chylomicrons(within Golgi apparatus).
Vesicles containingchylomicrons migrate to the basal membrane,are extruded from the epithelial cell,and enter a lacteal (lymphatic capillary).
Lymph in the lacteal transports chylomicrons away from intestine.
1
2
3
4
5
Fatty Acid Absorption
Figure 23.36
Lumen ofintestine
Absorptiveepithelial cellcytoplasm
ER
Golgiapparatus
Lacteal
Fatty acids andmonoglyceridesassociated withmicelles inlumen of intestine
Fatty Acid Absorption
Lumen ofintestine
Absorptiveepithelial cellcytoplasm
ER
Golgiapparatus
Lacteal
Fatty acids andmonoglyceridesassociated withmicelles inlumen of intestine
Fatty acids andmonoglyceridesresulting from fatdigestion leavemicelles and enterepithelial cell by diffusion.
1
Fatty Acid Absorption
Figure 23.36
Lumen ofintestine
Absorptiveepithelial cellcytoplasm
ER
Golgiapparatus
Lacteal
Fatty acids andmonoglyceridesassociated withmicelles inlumen of intestine
Fatty acids andmonoglyceridesresulting from fatdigestion leavemicelles and enterepithelial cell by diffusion.
Fatty acids are used to synthesize triglycerides in smooth endo-plasmic reticulum.
1
2
Fatty Acid Absorption
Figure 23.36
Lumen ofintestine
Absorptiveepithelial cellcytoplasm
ER
Golgiapparatus
Lacteal
Fatty acids andmonoglyceridesassociated withmicelles inlumen of intestine
Fatty acids andmonoglyceridesresulting from fatdigestion leavemicelles and enterepithelial cell by diffusion.
Fatty acids are used to synthesize triglycerides in smooth endo-plasmic reticulum.
Fatty globules arecombined with proteins to form chylomicrons(within Golgi apparatus).
1
2
3
Fatty Acid Absorption
Figure 23.36
Lumen ofintestine
Absorptiveepithelial cellcytoplasm
ER
Golgiapparatus
ChylomicronLacteal
Fatty acids andmonoglyceridesassociated withmicelles inlumen of intestine
Fatty acids andmonoglyceridesresulting from fatdigestion leavemicelles and enterepithelial cell by diffusion.
Fatty acids are used to synthesize triglycerides in smooth endo-plasmic reticulum.
Fatty globules arecombined with proteins to form chylomicrons(within Golgi apparatus).
Vesicles containingchylomicrons migrate to the basal membrane,are extruded from the epithelial cell,and enter a lacteal (lymphatic capillary).
1
2
3
4
Fatty Acid Absorption
Figure 23.36
Figure 23.36
Lumen ofintestine
Absorptiveepithelial cellcytoplasm
ER
Golgiapparatus
ChylomicronLacteal
Fatty acids andmonoglyceridesassociated withmicelles inlumen of intestine
Fatty acids andmonoglyceridesresulting from fatdigestion leavemicelles and enterepithelial cell by diffusion.
Fatty acids are used to synthesize triglycerides in smooth endo-plasmic reticulum.
Fatty globules arecombined with proteins to form chylomicrons(within Golgi apparatus).
Vesicles containingchylomicrons migrate to the basal membrane,are extruded from the epithelial cell,and enter a lacteal (lymphatic capillary).
Lymph in the lacteal transports chylomicrons away from intestine.
1
2
3
4
5
Fatty Acid Absorption
Hormonal regulation of digestion
• The release of digestive secretions is under control of various hormones:-
• Gastrin: stimulate secretion of gastric juice
• Enterogastrone:stops secretion of gastric juice
• Cholecystokinin:release of bile from gall bladder and release of enzymes of pancreatic juice.
Hormonal regulation of digestion• Secretin: speeds up release of bile and
cause release of sodium bicarbonate in pancreatic juice
• Villikinin: accelerates the movement of villi
• Duocrinin: release of mucus from brunner’s glands
• Enterocrinin :release of enzymes from crypts of leiberkuhn