Digestive system

Digestive system• Note:

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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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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


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


© 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


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


© 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


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


© 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


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


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


ER

Golgiapparatus

Lacteal



Lumen ofintestine


ER

Golgiapparatus

Lacteal



1


Figure 23.36

Lumen ofintestine


ER

Golgiapparatus

Lacteal




1

2


Figure 23.36

Lumen ofintestine


ER

Golgiapparatus

Lacteal





1

2

3


Figure 23.36

Lumen ofintestine


ER

Golgiapparatus

ChylomicronLacteal






1

2

3

4


Figure 23.36

Figure 23.36

Lumen ofintestine


ER

Golgiapparatus

ChylomicronLacteal






Lymph in the lacteal transports chylomicrons away from intestine.

1

2

3

4

5


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

Digestive system

Education

digestive tract propulsion

digestive process

digestive system25

digestive system note

tongue figure

food alimentary canal

tongue roughness

sublingual parotid