The Digestive System Alimentary Canal - biology-with …20... · The Digestive System “Alimentary...

The Digestive System “Alimentary Canal”

o  Alimentary: n  Concerning food, nourishment, and the organs of

digestion. n  From the Latin alimentum meaning nourishment.

Digestion Check In Anatomy Nutrient Mechanical

Digestion Chemical Digestion Enyzmes Acidic or

Basic

The Digestive System I) Introduction o  Introduction o  your digestive system is responsible for four major

processes: n  ingestion

o  taking in of food n  digestion

o  breaking down of food, can be: n  physical (mastication) n  chemical

The Digestive System I) Introduction

n  ingestion o  taking in of food

n  digestion o  breaking down of food, can be:

n  physical (mastication) n  chemical

n  absorption o  absorbing nutrients in food and making them

available to your circulatory system for transport n  elimination

o  getting rid of indigestible food.


Ingestion Digestion Absorption Elimination

Undigested material

Chemical digestion (enzymatic hydrolysis)

Nutrient molecules enter body cells

Small molecules

Mechanical digestion

Food

Pieces of food

1 2 3 4


n  absorption o  absorbing nutrients in food and making them available to your

circulatory system for transport n  elimination

o  getting rid of indigestible food.

o  the goal of the digestive system is to ingest food, to digest into small molecules that can cross plasma membranes, to absorb nutrients and to eliminate non-digestible wastes

o  in humans digestion of food is an extracellular process.

The Digestive System II) Physiology

Cecum

Anus

Ascending portion of large intestine

Gall- bladder

Small intestine

Large intestine

Small intestine

Rectum

Pancreas

Liver

Salivary glands

Tongue Oral cavity

Pharynx Esophagus

Sphincter

Stomach

Sphincter

Duodenum of small intestine

Appendix

Anus

Liver

Pancreas

Small intestine

Large intestine

Rectum

Stomach Gall- bladder

A schematic diagram of the human digestive system

Esophagus

Salivary glands

Mouth

The Digestive System II) Physiology A)  Mouth/Oral Cavity (Bouche)

n  food is chewed in the mouth (mastication) and mixed with saliva (chemical digestion)

i)  Teeth n  among mammals, dentition differs according to the

mode of nutrition n  humans are omnivores therefore our dentition is

nonspecific to both vegetables and meat diets.


n  among mammals, dentition differs according to the mode of nutrition n  humans are omnivores therefore our dentition is nonspecific to

both vegetables and meat diets.

n  adult humans have: n  32 teeth n  one-half of each jaw has teeth of four

different types: o  ~ 2 chisel shaped incisor for biting o  ~ 1 pointed canine for tearing o  ~ 2 fairly flat premolars for grinding o  ~ 3 molars, well flattened for crushing


n  adult humans have: n  32 teeth n  one-half of each jaw has teeth of four different types: o  ~ 2 chisel shaped incisor for biting o  ~ 1 pointed canine for tearing o  ~ 2 fairly flat premolars for grinding o  ~ 3 molars, well flattened for crushing


o  ~ 2 chisel shaped incisor for biting o  ~ 1 pointed canine for tearing o  ~ 2 fairly flat premolars for grinding o  ~ 3 molars, well flattened for crushing

n  herbivores like horses have: n  sharp even incisors for clipping plant material n  large flat premolars and molars for grinding

and crushing plant material


n  herbivores like horses have: n  sharp even incisors for clipping plant material n  large flat premolars and molars for grinding and crushing

plant material


n  herbivores like horses have: n  sharp even incisors for clipping plant material n  large flat premolars and molars for grinding and crushing

plant material

n  carnivores have: n  pointed incisors and enlarged canine teeth to

tear off pieces of meat that is small enough to be swallowed.


n  carnivores have: n  pointed incisors and enlarged canine teeth to tear off pieces of

meat that is small enough to be swallowed.


n  ii) Salivary Glands n  there are three pairs of salivary glands

o  parotid o  sublingual o  submandibular

n  all three secrete saliva through the salivary ducts o  lubricates the food o  contains salivary amylase


o  parotid o  sublingual o  submandibular

n  all three secrete saliva through the salivary ducts o  lubricates the food o  contains salivary amylase


o  submandibular n  they secrete saliva through the salivary ducts

o  lubricates the food o  contains salivary amylase

o  this is the first site of chemical digestion n  starch is broken down into maltose

starch + H2O maltose (disaccharide)

salivary

amylase


n  iii) Tongue n  food is manipulated by the tongue

o  tongue has: n  pressure receptors n  chemical receptors called taste buds.

n  the tongue pushes the chewed and saliva mixed food (the bolus) to the back of the pharynx and initiates the swallowing reflex.


n  pressure receptors n  chemical receptors called taste buds.

n  the tongue pushes the chewed and saliva mixed food (the bolus) to the back of the pharynx and initiates the swallowing reflex.

o  B) Esophagus o  the esophagus conducts food o  the digestive and respiratory passages interact at pharynx.

n  a small flap called the epiglottis covers the trachea. o  the bolus passes through the pharynx entering the esophagus o  if food enters the trachea (wind pipe) we choke.

The Digestive System II) Physiology o  B) Esophagus o  the esophagus conducts food o  the digestive and respiratory passages interact at pharynx.

n  a small flap called the epiglottis covers the trachea. o  the bolus passes through the pharynx entering the esophagus o  if food enters the trachea (wind pipe) we choke.

o  when food enters the esophagus a rhythmic contraction of muscles occurs (peristalsis) pushing the bolus down the esophagus to the stomach

Larynx Trachea

Epiglottis up

Pharynx

Tongue

Glottis

Esophagus

Esophageal sphincter contracted

Food

To stomach

To lungs

Larynx Trachea

Epiglottis up

Pharynx

Tongue

Glottis

Esophagus


Food

To stomach

To lungs

Epiglottis down

Esophageal sphincter relaxed

Glottis up and closed

Larynx Trachea

Epiglottis up

Pharynx

Tongue

Glottis

Esophagus


Food

To stomach

To lungs

Epiglottis down

Esophageal sphincter relaxed

Glottis up and closed

Epiglottis up


Sphincter relaxed

Relaxed muscles

Contracted muscles

Relaxed muscles

Stomach

Glottis down and open

http://www.hopkins-gi.org/multimedia/database/intro_250_Swallow.swf


Digestion Chemical Digestion Enyzmes Acidic or

Basic

Mouth

Carbohydrates Yes Yes Salivary Amylase

(polysaccharide to disaccarides) Acidic Proteins Yes No N/A

Fats Yes No N/A

Esophagus

Sphincter

Stomach

Sphincter

Duodenum of small intestine Small

intestine

Stomach

Esophagus

The Digestive System II) Physiology C)  Stomach o  site of:

n  food storage n  initial protein digestion

o  contains three layers of muscle that run in different directions.

n  this allows for muscle contraction to churn the food rather than just squish it.

o  movement of food to and from the stomach is regulated by circular muscles called sphincters.


n  this allows for muscle contraction to churn the food rather than just squish it.

o  movement of food to and from the stomach is regulated by circular muscles called sphincters.

n  sphincters act like draw strings on a bag. o  contraction of the lower esophageal sphincter

(previously known as the cardiac sphincter) , the LES, closes the opening to the stomach. n  the LES prevents food and acid from being

regurgitated up into the esophagus.

Medical Tangent o  Acid Reflux/Heartburn o  Gastroesophageal reflux disease

n  commonly referred to as GERD or acid reflux n  the liquid content of the stomach regurgitates (backs up or

refluxes) into the esophagus. o  can inflame and damage the lining (cause esophagitis)

of the esophagus

Medical Tangent

Medical Tangent n  the liquid content of the stomach regurgitates (backs up or refluxes) into the

esophagus. o  can inflame and damage the lining (cause esophagitis) of the esophagus

n  regurgitated liquid usually contains: o  acid and pepsin that are produced by the stomach. o  may contain bile that has backed-up into the stomach

from the duodenum. o  Acid is believed to be the most injurious component

of the refluxed liquid n  GERD is a chronic condition

o  Once it begins, it usually is life-long.

Medical Tangent o  the body has mechanisms to protect itself from the harmful

effects of reflux n  most reflux occurs during the day when individuals are

upright. o  in the upright position, the refluxed liquid is more

likely to flow back down due to the effect of gravity. o  while individuals are awake, they repeatedly swallow,

whether or not there is reflux. o  the salivary glands in the mouth produce saliva,

which contains bicarbonate. n  the bicarbonate neutralizes the small amount of

acid


n  the LES prevents food and acid from being regurgitated up into the esophagus.

o  the pyloric sphincter regulates the movement of food and stomach acids into the small intestine. Lower esophageal

sphincter (LES)

Pyloric sphincter

Diaphragm

Esophagus

Stomach

Duodenum

Muscle Layers

To Small Intestine

The Digestive System II) Physiology o  the L-shaped stomach has numerous ridges called rugae

n  allow for expansion o  millions of cells line the inner wall of the stomach.

n  these cells secrete gastric fluids or juices that aid digestion.

n  the main components of gastric juice are: o  mucus o  hydrochloric acid o  pepsinogens


n  the main components of gastric juice are: o  mucus o  hydrochloric acid o  pepsinogens

n  parietal cells of the stomach o  secretes hydrochloric acid (HCl(aq))

n  lowers the pH of the stomach to around 2 o  kills many harmful substances that are ingested with

food. o  deactivates salivary amylase

n  converts pepsinogen into its active form pepsin.


o  kills many harmful substances that are indigested with food. o  deactivates salivary amylase o  converts pepsinogen into its active form pepsin. n  peptic/Chief cells of the stomach

o  secretes pepsinogen o  pepsinogen is an inactive enzyme. o  it stays inactive until it reaches the lumen of

the stomach and is activated by HCl(aq) (it is now called pepsin)


Pepsinogen Pepsin

HCl

Long Chain Amino Acids

(Proteins)

Shorter Chain Amino Acids (Peptones)

n  peptic/ Chief cells of the stomach o  secretes pepsinogen

n  pepsinogen is an inactive enzyme. o  it stays inactive until it reaches the lumen of the stomach and

is activated by HCl (it is now called pepsin)


o  mucus cells of the stomach n  secretes mucus

o  mucus provides a protective coating to prevent self digestion of the stomach lining

o  another chemical secreted by the cells of the stomach is renin. n  renin works to slow down the movement of milk through

the digestive system

Interior surface of stomach

Esophagus

Chief cells

Small intestine

Epithelium

Stomach

Sphincter

Parietal cell

Pepsinogen and HCl are secreted.

HCl converts pepsinogen to pepsin.

Pepsin activates more pepsinogen.

Chief cell

Folds of epithelial tissue

Pepsin

Sphincter

Pepsinogen

HCl

H+

Cl–

Parietal cells

Mucus cells

Gastric gland

1

2

2

3

3

1

5 µm


o  another chemical secreted by the cells of the stomach is renin. n  renin works to slow down the movement of milk through the digestive

system

o  after the action of gastric juice (chemical digestion) and stomach contractions (physical digestion) the contents of the stomach have a thick, soupy consistency. n  this is called chyme.

o  the base of the stomach has a sphincter called the pyloric sphincter.

Medical Tangent o  Peptic Ulcer o  when the protective mucus lining of the stomach breaks

down the cell membrane is exposed to HCl and pepsin n  the breakdown of the cell membrane of the stomach is a

peptic ulcer. o  must ulcers are the result of an infection by the bacterium

Helicobacter pylori. o  peptic ulcers can be treated with antibiotics if found early

enough


o  the base of the stomach has a sphincter called the pyloric sphincter.

n  when the sphincter relaxes a small amount of chyme passes into the duodenum (the first part of the small intestine) o  chyme entering the duodenum sets off a reflex that

causes the sphincter to contract and close the opening temporarily. n  this adaptation allows for a slower more thorough

digestion.

Gall- bladder

Small intestine

Large intestine

Small intestine

Pancreas

Liver

Stomach

Sphincter


Liver

Pancreas

Small intestine

Stomach Gall- bladder

The Digestive System II) Physiology D) Small Intestine

n  site of majority of digestion and absorption n  is approximately 7 metres in length n  it is called the small intestine because it is narrower than the large

instestine. n  is made up of three regions

o  duodenum o  jejunum o  ileum

n  the mucus membrane layer of the small intestine has ridges and furrows that give it corrugated appearance.

The Digestive System II) Physiology o  the mucus membrane layer of the small intestine has ridges and furrows that give

it corrugated appearance.

n  on the surface of these ridges and furrows are small fingerlike projections called villi. o  cells on the surface of the villi have minute projections called

microvilli.

n  the villi and microvilli greatly increase the effective surface area of the small intestine.

n  if the small intestine was a smooth tube, it would have to be 500 to 600 meters long to have a comparable surface area.

Fig. 41-15a

Muscle layers

Vein carrying blood to hepatic portal vein

Villi

Intestinal wall

Key

Nutrient absorption

Large circular folds

Fig. 41-15b Microvilli (brush border) at apical (lumenal) surface

Key

Nutrient absorption

Blood capillaries

Epithelial cells

Villi

Lymph vessel

Basal surface

Lacteal Epithelial cells

Lumen


n  the villi and microvilli greatly increase the effective surface area of the small intestine.

n  if the small intestine was a smooth tube, it would have to be 500 to 600 meters long to have a comparable surface area.

o  when chyme enters the duodenum: n  proteins and carbohydrates are partly digested n  fat digestion still needs to be carried out.

o  more digestion is still required before nutrients can be absorbed through the intestinal wall. n  two accessory glands, the liver and the pancreas send

secretions to the duodenum to complete digestion.

The Digestive System II) Physiology o  more digestion is still required before nutrients can be

absorbed through the intestinal wall. n  two accessory glands, the liver and the pancreas send

secretions to the duodenum to complete digestion. i)  liver ii)  pancreas

The Digestive System II) Physiology i)  liver

n  the liver produces bile o  bile is

n  stored in the gall bladder n  is sent to the duodenum via the bile duct n  contains bile salts which are emulsifiying agents n  (Emulsifying agents break up fat into fat

droplets so they can mix with water)


n  contains bile salts which are emulsifiying agents n  (Emulsifying agents break up fat into fat

droplets so they can mix with water)

n  emulsified fats are more easily digested by enzymes (more surface area)

fat fat droplets bile salts


n  emulsified fats are more easily digested by enzymes (more surface area)

n  when the stomach is empty bile is stored and concentrated in the gall bladder.

o  when there is fats in the small intestine the hormone cholecystokinin (CCK) is released. n  CCK is carried in the blood stream to the gall

bladder. o  triggers the release of bile salts

fat fat droplets bile salts


n  CCK is carried in the blood stream to the gall bladder. o  triggers the release of bile salts o  once inside the duodenum the bile salts

emulsify fat into fat droplets o  (this is physical digestion because

chemical bonds are not broken)

Secretin and CCK

Stomach

Gallbladder Liver

+


Bile

Gastrin

Secretin

Pancreas CCK

CCK Key

Stimulation Inhibition

+

–

+

+ + –


o  once inside the duodenum the bile salts emulsify fat into fat droplets

o  (this is physical digestion because chemical bonds are not broken)

o  bile also contains pigments n  the liver breaks down hemoglobin (Hb) from red

blood cells (RBC) and stores the products in the gall bladder for removal o  the characteristics brown colour of feces

results from Hb breakdown


o  the characteristics brown colour of feces results from Hb breakdown

n  other liver functions: o  synthesis o  produces bile salts o  manufactures blood proteins o  breakdown/conversion o  removes toxic nitrogen group from amino acids

forming urea o  converts the toxic components of hemoglobin

(excreted by bile salts)


o  breakdown/conversion o  removes toxic nitrogen group from amino acids forming urea o  converts the toxic components of hemoglobin (excreted by bile salts)

o  converts glucose into glycogen and vice versa to maintain a constant blood sugar level.

o  storage o  stores glycogen o  stores vitamins A, B12 and D o  detoxification o  converts harmful compounds (ie. alohol) to less

harmful products.


o  detoxification o  converts harmful compounds (ie. alohol) to less harmful products.

o  ii) Pancreas o  when acid enters the small intenstine, a chemical

called prosecretin is converted into secretin n  secretin is absorbed into the bloodstream and carried to the

pancreas o  it signals the pancreas to release a solution containing

bicarbonate ions (HClO3-)

o  bicarbonate travels through the pancreatic duct to the small intestine.

o  in the small intestine bicarbonate raises the pH from about 2.5 (acidic) to 9.0 (basic)


o  in the small intestine bicarbonate raises the pH from about 2.5 (acidic) to 9.0 (basic)

o  the basic pH inactivates pepsin. n  the small intestine is protected from the stomach

acids and protein digesting enzymes. o  pancreatic secretions contain enzymes that digest all

three major nutrients: n  ~ protiens n  ~ carbohydrates n  ~ lipids


o  pancreatic secretions contain enzymes that digest all three major nutrients: n  ~ protiens n  ~ carbohydrates n  ~ lipids

o  pancreatic secretions or “juice” contains: n  sodium bicarbonate (for neutralization) n  pancreatic amylase n  trypsinogen/trypsin n  lipase n  erepsin


o  pancreatic secretions or “juice” contains: n  sodium bicarbonate (for neutralization) n  pancreatic amylase n  trypsinogen/tryspin n  lipase n  erepsin

o  pancreatic amylase n  digest any remaining starches to maltose n  (polysaccharides ðdisaccharides)


o  pancreatic amylase n  digest any remaining starches to maltose n  (polysaccharides ðdisaccharides)

o  trypsinogen n  an inactive enzyme n  is activated by another enzyme called enterokinase in the

lumen of the duodenum


o  tyrpsinogen n  an inactive enzyme n  is activated by another enzyme called enterokinase in the lumen of the

duodenum

trypsinogen (inactive) enterokinase

trypsin (active)

proteins & peptones

smaller amino chains (peptides)

erepsins

amino acids


o  tyrpsinogen n  an inactive enzyme n  is activated by another enzyme called enterokinase in the lumen of

the duodenum

o  erepsins n  complete protein digestion by breaking the bonds

between short-chain peptides, releasing amino acids. n  released from the pancreas and the small intestine

o  lipase n  enzymes released by the pancreas n  break down lipids n  two types


o  lipase n  enzymes released by the pancreas n  break down lipids n  two types

o  pancreatic lipase o  ~ most common o  ~ breaks down fats into fatty acids and

glycerol o  phospholipase o  ~ breaks down phospholipids


o  pancreatic lipase o  ~ most common o  ~ breaks down fats into fatty acids and glycerol o  phospholipase o  ~ breaks down phospholipids

o  back to the small intestine n  at this point:

o  fats are fully digested o  carbohydrates are partially digested o  proteins are fully and partially digested.

n  epithelial cells of the villi produce intestinal enzymes (intestinal juice)

o  this completes the digestion of peptides and sugars


o  fats are full digested o  carbohydrates are partially digested o  proteins are fully and partially digested.

n  epithelial cells of the villi produce intestinal enzymes (intestinal juice) o  this completes the digestion of peptides and sugars

peptides amino acids

disaccharides maltase

lactase monosaccharides

peptidase


Digestion Chemical Digestion Enyzmes Acidic

or Basic

Small Intestine

Carbohydrates X

Pancreatic Amylase (glycogen, starch → maltose)

Carbohydrases sucrase (sucrose → glucose + fructose)

maltase (maltose → glucose) lactase (lactose → glucose + galactose)

Basic (pH = 8) Lipids X Lipase

(Lipids → fatty acids + glycerol)

Proteins X

Proteases Trypsin (Peptides → smaller peptides)

Chymotrypsin (Peptides → smaller peptides) Peptidases

(peptides → smaller peptides and amino acids)

The Digestive System II) Physiology o  absorption of nutrients

n  each villus contains a network of blood vessels and a small lymphatic vessel called a lacteal o  after glycerol and fatty acids are absorbed by

epithelial cells, they are recombined into fats within these cells

o  these fats are mixed with cholesterol and coated with protein, forming molecules called chylomicrons, which are transported into lacteals

Lumen of small intestine

Lacteal

Chylomicron

Phospholipids, cholesterol, and proteins

Triglycerides

Monoglycerides

Triglycerides

Fatty acids

Epithelial cell

The Digestive System II) Physiology o  these fats are mixed with cholesterol and coated with protein, forming

molecules called chylomicrons, which are transported into lacteals

o  amino acids and sugars pass through the epithelium of the small intestine and enter the bloodstream

o  capillaries and veins from the lacteals converge in the hepatic portal vein and deliver blood to the liver and then on to the heart

Bloodstream

Veins to heart

Lymphatic system

Small intestine

Esophagus Stomach

Lipids Mouth

Hepatic portal vein

Absorbed food (except lipids)

Absorbed water

Secretions from the gastric glands of the stomach

Secretions from the pancreas and the liver

Liver

Rectum

Anus

Large intestine

The Digestive System II) Physiology o  E) Large Intestine

n  chemical digestion is complete n  the cecum

o  the first area where food from the small intestine arrives.

o  has a small blind pouch attached to it called the appendix.

n  the colon: o  the largest part of the large intestine. o  stores wastes long enough to reabsorb water o  inorganic salts, minerals and vitamins are reabsorbed

with the water. n  home to Escherichia coli (E. coli) bacteria


o  inorganic salts, minerals and vitamins are reabosrbed with the water. n  home to Escherichia coli (E. coli) bacteria

o  uses waste materials to synthesize vitamins B and K. n  as wastes build up receptors in the cell wall send a signal

to the central nervous system promoting a bowel movement. o  bowel movements ensure removal of potentially toxic

wastes from the body. o  people who do not eat enough fibre (cellulose) have

fewer bowel movements.

o  bowel movements ensure removal of potentially toxic wastes from the body.

o  people who do not eat enough fibre (cellulose) have fewer bowel movements.

n  wastes of the digestive tract, the feces, become more solid as they move through the colon

n  feces pass through the rectum and exits via the anus n  feces is stored in the rectum until they can be eliminated n  two sphincters between the rectum and anus control

bowel movements

The Digestive System II) Physiology o  interesting adaption

n  herbivores generally have longer alimentary canals than carnivores, reflecting the longer time needed to digest vegetation

Cecum

Small intestine

Herbivore Carnivore

Colon (large intestine)

Stomach Small intestine

Carbohydrate digestion

Polysaccharides

Smaller polysaccharides, maltose

Polysaccharides

Maltose and other disaccharides

Disaccharides

Protein digestion Nucleic acid digestion Fat digestion

Proteins

Small polypeptides

Monosaccharides

Small peptides

Amino acids

Amino acids

Polypeptides

Smaller polypeptides

DNA, RNA Fat globules

Nucleotides Fat droplets

Nucleosides

Nitrogenous bases, sugars, phosphates

Glycerol, fatty acids, monoglycerides

(starch, glycogen) (sucrose, lactose)

Oral cavity, pharynx, esophagus

Stomach

Lumen of small intestine

Epithelium of small intestine (brush border)

Carbohydrate digestion

Polysaccharides

Smaller polysaccharides, maltose

Polysaccharides

Maltose and other disaccharides

Disaccharides

Protein digestion Nucleic acid digestion Fat digestion

Proteins

Small polypeptides

Pepsin

Pancreatic amylases

Salivary amylase

Disaccharidases

Monosaccharides

Small peptides

Amino acids

Amino acids

Polypeptides

Smaller polypeptides

Pancreatic trypsin and chymotrypsin

Pancreatic carboxypeptidase

Dipeptidases, carboxypeptidase, and aminopeptidase

DNA, RNA

Pancreatic nucleases

Fat globules

Nucleotides Fat droplets

Nucleosides

Nitrogenous bases, sugars, phosphates

Nucleotidases

Nucleosidases and phosphatases

Glycerol, fatty acids, monoglycerides

Bile salts

Pancreatic lipase

(starch, glycogen) (sucrose, lactose)

The Digestive System III) Control of Digestion o  control of digestion is exerted by the nervous and endocrine

(hormonal system) n  seeing, smelling and tasting food will produce gastric

secretions before there is any food in the stomach. n  swallowing stimulates gastric secretions even if the

esophagus is empty. o  hormones play a large role in control

o  hormones play a large role in control

n  secretin o  is released when acid from the stomach moves into

the small intestine. o  travels to the pancreas via the blood stream where it

stimulates the release of bicarbonate to raise the pH. n  gastrin

o  produced when the wall of the stomach are distended by the presence of food and by the presence of partially digested protein.

o  travels in the blood and stimulates the parietal cells of the stomach to release HCl.

n  gastrin o  produced when the wall of the stomach are distended by the presence of

food and by the presence of partially digested protein. o  travels in the blood and stimulates the parietal cells of the stomach to

release HCl.

o  the size of the meal n  a larger meal will trigger more receptors in your stomach

causing more forcefull contractions and faster emptying. n  a fatty meal will cause the small intestine to secrete

enterogastrone which will slow peristaltic movement allowing more time for fat digestion and absorption.

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