Digestive System

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Digestive System

• Introduction
• Organs
• General Structure
• Structures and functions
• Digestion
• Absorption

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Digestive system

• Nutrients required by body cells have to be in
  simple and soluble form.
• Ingested food is in complex form which has to be
  broken into simpler form.
• The conversion of complex form of food into
  simple soluble form brought by digestive system.
• Different organs that makes up digestive system
  are involved in digestion.

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Main Organs (in order)

• Mouth and Oral Cavity
• Pharynx
• Esophagus
• Stomach
• Small intestine
• Large intestine

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Accessory Organs

• Teeth and tongue
• Salivary glands
• Liver
• Gallbladder
• Pancreas

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General Structure

• Walls of the digestive tract consist of four
  layers throughout its length.
• From lumen to outward
• Mucosa/mucous membrane epithelial layer coated
  with mucous
• Submucosa loose connective tissue contain
  blood vessels, lymphoid, nerves and glands
• Muscularis/musculosasmooth muscle that produces
  wave-like contractions (peristalsis) to move
  food, consists of two layers, outer-longitudinal,
  inner-circular
• Serosa/adventitia connective tissue outermost
  covering.

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Baisc structure of Alimentary canal
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Structures and functions

• Oral cavity
• Hollow chamber with a roof, floor and walls.
  Food enters through the mouth.
• Hard palate bony structure in the anterior or
  front portion of the mouth.
• Soft Palate located above the posterior or rear
  portion of the mouth muscle tissue.
• Uvula hangs down from the soft palate helps
  prevent food or liquid from entering the nasal
  cavities above the mouth.
• Tongue skeletal muscle covered with mucous
  membrane. Involved in mixing, swallowing and
  tasting.

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Structures and functions

• Teeth 4 major types
• Incisors cutting function during mastication.
• Canines (cuspids) pierce or tear food.
• Premolars (bicuspids) large flat surfaces with
  2 or 3 grinding or crushing cusps on their
  surface. Grind food into a bolus so it can be
  swallowed.
• Molars (tricuspids) same as premolars.
• Deciduous teeth baby teeth, 20 in number
  adults have 32 teeth

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Structures and functions

• Teeth contd
• Parts of a tooth
• Crown portion that is exposed and visible in
  the mouth covered with enamel.
• Neck narrow portion surrounded by the pink
  gingiva or gum tissue.
• Root fits into the socket (joint) of the upper
  or lower jaw.

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Structures and functions
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Structures and functions

• Salivary Glands(3 pairs)
• Parotids largest, lie just below and in front
  of each ear at the angle of the jaw (mumps).
• Submandibular open into the mouth on either
  side of the lingual frenulum.
• Sublingual open into the floor of the mouth.
• Saliva contains mucus and a digestive enzyme
  called salivary amylase which begins the process
  of chemical digestion of carbohydrates.

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Location of salivary glands
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Structures and functions

• Esophagus
• Muscular, mucus lined tube that connects the
  pharynx with the stomach (about 25 cm or 10
  inches long).
• Peristalsis moves food from the mouth to the
  stomach.

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Structures and functions

• Stomach
• Lies in the upper part of the abdominal cavity
  just under the diaphragm.
  
  
• Serves as a pouch that food enters after it has
  been chewed, swallowed and passed through the
  esophagus.
• At the junction of the esophagus and the stomach
  is the cardiac sphincter (muscle) that keeps food
  from reentering the esophagus
• Divided into three regions
• Fundus enlarged portion to the left of and
  below the cardiac sphincter.
• Body central part of the stomach
• Pylorus lower, narrow section which joins the
  first part of the small intestine.

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Structure of stomach
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Structures and functions

• The lining of the stomach has numerous folds when
  empty called rugae they allow for expansion when
  we eat.
• Has numerous gastric glands that secrete gastric
  juice into the stomach.
• Most common cells found in the glands are
• Parietal cells - produce HCl acid that denatures
  proteins.
• Chief cells - produce pepsin that begins protein
  digestion.
• Mucus cells - produce mucus the protects from HCl
  action.

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Gastric pits and gland
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Structures and functions

• The stomach wall has 3 layers of muscle that
  allow mixing of the food with gastric juice,
  breaking it down into a semisolid mixture called
  chyme.
• Pyloric sphincter (muscle) holds food in the
  stomach or allows for emptying of chyme into the
  small intestine. Controlled by the small
  intestine.

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LS of stomach
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Structures and functions

• Small intestine
• Roughly 7 meters in length.
• Smaller in diameter compared to the large
  intestine.
• Divided into three sections
• Duodenum
• Jejunum
• Ileum

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Structures and functions

• Small intestine
• Mucous lining, contains thousands of microscopic
  glands which secrete intestinal digestive juices.
• Intestinal lining has multiple circular folds,
  plicae, that increase the surface area available
  for absorption of nutrients.
• The surface is covered with tiny finger-like
  projections called villi.

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LS of small intestine
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Structures and functions

• Liver
• Fills the entire upper right section of the
  abdominal cavity and extends part way into the
  left side.
• The largest gland in the body secretes bile into
  hepatic ducts.
• Functions
• Produces bile
• Stores vitamins and minerals
• Plays a part in maintaining blood sugar
  (glycogen) and blood pressure (blood proteins).

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Structures and functions

• Liver contd
• Bile breaks up (emulsifies) large fat globules
  into smaller particles to increase the surface
  area for digestion.
• Bile gives the feces its color (no bile - stools
  become clay colored). Bile absorbed into the
  blood gives the skin a yellowish color jaundice
  or icterus.

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Liver anterior view
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Structures and functions

• Gall Bladder
• Stores bile
• Cystic duct arising from it joins with hepatic
  duct.
• When fat enters the duodenum, the mucosal cells
  release a hormone called cholecystokinin (CCK).
• CCK stimulates the contraction of the gallbladder
  and forces bile into the duodenum via the common
  bile duct.

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Gall bladder
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Structures and functions

• Pancreas
• Lies behind the stomach in the C-shaped curve of
  the duodenum.
• Endocrine portion produces insulin and glucagon.
• Exocrine portion (ascini) secretes pancreatic
  juice into ducts that connect to the duodenum at
  the same area as the bile duct.
• Pancreatic juice contains enzymes such amylase,
  trypsin, chymotrypsin and lipase and also
  bicarbonate that neutralizes the HCl in the
  chyme.

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Pancreas
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Structures and functions

• Large Intestine
• About 5 feet in length, forms the lower portion
  of the digestive tract.
• Undigested and unabsorbed food material enter the
  large intestine after passing through a
  sphincter-like structure called the ileocecal
  valve.

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L/S of Large intestine
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Structures and functions

• Large Intestine
• One primary function of the large intestine is to
  remove water from the feces.
• A few vitamins and minerals are also absorbed.
• Bacteria living in the large intestine break down
  some of the remaining material.
• Bacteria also produce some of the B-complex
  vitamins and vitamin K.

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Structures and functions

• Large Intestine
• Inner surface has no folds or villi not well
  suited for absorption.
• Large Intestine
• Cecum ileocecal valve opens into this pouchlike
  area.
• Ascending colon right side of body
• Transverse colon extends across the front of
  the abdomen
• Descending colon left side of the body
• Sigmoid colon leads to the rectum

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Large intestine
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Digestion

• Complex process that occurs in the alimentary
  canal, consists of mechanical and chemical
  changes that prepare food for absorption.
• Mechanical digestion
• Breaks food into tiny particles, mixes them with
  digestive juices, moves them along the alimentary
  canal and finally eliminates the digestive wastes
  from the body. Includes mastication,
  deglutition, peristalsis and defecation.

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Digestion

• Chemical digestion
• Breaks down large, non-absorbable food molecules
  into smaller, absorbable molecules that can
  easily pass through intestine into blood and
  lymph.
• Process includes numerous chemical reactions
  catalyzed by specific enzymes in saliva, gastric,
  pancreatic, and intestinal juices.

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Digestion

• Carbohydrates
• Digestion begins in mouth (salivary amylase)
• Salivary and pancreatic amylase converts
  starches into disaccharides (double sugars).
• Intestinal enzymes (sucrase, maltase, lactase)
  break down disaccharides into monosaccharides.
• Polysaccharide salivary amylase maltose small
  polysaccharide
• Undigested polysaccharides pancreatic amylase
  maltose diaaccharides maltse,sucrase, lactase
  monosaccharides

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Digestion

• Proteins
• Starts in the stomach and ends in the small
  intestine.
• Pepsin in stomach,trypsin and chymotrypsin in
  pancreatic juice digest protein.
• Aminopeptidase present in intestinal secretion
  finally digest protein into amino acids.
• Protein pepsin short polypeptides
• Short polypeptides trypepsin, chymotrypsin small
  polypeptides peptides carboxypeptidase,
  peptidases,dipeptidases amino acids

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Digestion

• Lipids (fat)
• Bile produced by liver is poured into the
  duodenum.
• It brings about emulsification.
• Pancreatic lipase splits the lipid molecules into
  fatty acids and glycerol.
• Fat bile salts emulsified fat droplets pancreatic
  lipase fatty acids glycerol

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Absorption

• Stomach absorbs only a few substances
• Main absorption occurs in small intestine
• Absorption occurs by a combination of simple
  diffusion, facilitated diffusion and active
  transport
• Amino acids and glucose are directly transported
  to liver by hepatic portal vein
• Fatty acids and glycerol are absorbed by
  intestinal cells
• Resembled as triglycerides in ER
• Packed into protein covered fat
  droplets-chylomicrons
• Chylomicrons enter the lacteals and enter into
  blood circulation

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Hormonal control

• Gastrin(stomach) causes gastric glands in stomach
  to
• secrete pepsinogen.
• Stimulated by distention of stomach by food,
  partially
• digested proteins and caffine
• Secretin(duodenum) causes the pancreas and liver
  to secrete sodium bicarbonate and bile
  respectively.
• Stimulated by the presence of acidic chyme in the
• Duodenum.

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• CCK/cholecytokinin(duodenum)
• Stimulates pancreas to release digestive enzymes
  and gall bladder to empty bile
• Stimulated by the presence of fatty acids and
  partially digested protein in duodenum.
• Gastric inhibitory peptide/GIP(duodenum)
• Decrease stomach churning, thus slowing emptying.
  
• Stimulated by the presence of fatty acids or
  glucose in duodenum