The Digestive System

1
The Digestive System
2
Components of the Digestive System

• The digestive system consists of the
  gastrointestinal (GI) tract and the accessory
  digestive organs
• The GI tract is a continuous tube that includes
  the mouth, pharynx, esophagus, stomach, small
  intestine, and large intestine
• The accessory organs include the teeth, tongue,
  salivary glands, liver, gallbladder, and pancreas

3
Functions of the Digestive System

• Ingestioneating involves taking foods and
  liquids into the mouth
• Secretionrelease of water, acids, enzymes, etc.
  into the digestive tract
• Mixing and propulsionchurning and movement of
  food through the GI tract

4
Functions of the Digestive System (continued)

• Digestionmechanical and chemical breakdown of
  food
• Absorptionpassage of digested products from the
  GI tract into the blood and lymph
• Defecationthe elimination of feces (wastes,
  indigestible substances, bacteria, sloughed-off
  cells) from the GI tract

5
Layers of the GI Tract

• The wall of the GI tract from the lower esophagus
  to the anal canal has the same basic 4-layered
  arrangement of tissues
• Mucosa
• Submucosa
• Muscularis
• Serosa

6
Mucosa

• The inner lining of the GI tract
• A mucous membrane
• Composed of
• a layer of cells called the epithelium (which is
  in contact with the contents of the GI tract)
• a layer of connective tissue called the lamina
  propria
• a thin layer of smooth muscle called the
  muscularis mucosae

7
Mucosa (continued)

• The epithelium in the mouth, pharynx, esophagus,
  and anal canal serves primarily as a protective
  lining of the tract
• The stomach and intestines are lined with
  epithelium specialized for secretion and
  absorption
• Every 5-7 days cells of the epithelium are
  sloughed off and replaced by new cells

8
Mucosa (continued)

• The lamina propria is connective tissue
  containing many blood and lymphatic vessels
• This layer supports the epithelium and binds it
  to the muscularis mucosae
• It also contains lymphatic tissue (nodules) that
  protects against disease

9
Mucosa (continued)

• The muscularis mucosae has numerous folds to
  increase the surface area of the mucous membrane
  for digestion and absorption

10
Submucosa

• Connective tissue layer that binds the mucosa to
  the muscularis
• Contains many blood and lymphatic vessels
• Also contains the submucosal plexus, a network of
  neurons involved in control of secretions within
  the digestive tract
• May contain glands and lymphatic tissue

11
Muscularis

• The muscularis of the mouth, pharynx, and middle
  to upper parts of the esophagus contains skeletal
  muscle to allow for voluntary swallowing
• The external anal sphincter is also composed of
  skeletal muscle to allow for voluntary control of
  defecation

12
Muscularis (continued)

• Throughout the remainder of the GI tract, though,
  the muscularis consists of smooth muscle
  generally found in two sheets
• Circular muscleinner
• Longitudinal muscleouter
• The myenteric plexus is a network of neurons
  found between the layers of smooth muscle in the
  muscularis this is involved in motility
  (movement) within the digestive tract

13
Serosa

• The portions of the GI tract that are suspended
  in the abdominopelvic cavity have a superficial
  layer called the serosa
• This layer is composed of connective and
  epithelial tissue

14
Peritoneum

• The peritoneum consists of a layer of epithelium
  and an underlying layer of connective tissue
• It is divided into two parts, which are separated
  by a peritoneal cavity containing peritoneal
  fluid
• Parietal peritoneumlines the wall of the
  abdominopelvic cavity
• Visceral peritoneumcovers some of the organs in
  this cavity and is their serosa

15
Greater Omentum

• Part of the peritoneum
• A fatty apron that drapes over the intestines
• Varies in the amount of adipose tissue it
  contains (ex. beer belly in some individuals)
• Contains many lymph nodes to help fight infection
  in the digestive tract

16
Lesser Omentum

• Part of the peritoneum
• Suspends the stomach and first part of the small
  intestine from the liver
• Also contains some lymph nodes to help fight
  infection in the GI tract

17
Mesentery and Mesocolon

• Part of the peritoneum
• Binds the intestines to the posterior abdominal
  wall
• Contain blood and lymphatic vessels and lymph
  nodes

18
Falciform Ligament

• Part of the peritoneum
• Attaches the liver to the anterior abdominal wall
  and diaphragm

19
Mouth

• The mouth is also called the oral cavity or
  buccal cavity
• It is formed by the cheeks, tongue, hard palate,
  and soft palate
• The hard palate is the anterior portion of the
  roof of the mouth it is formed by the maxillae
  and palatine bones, and it separates the nasal
  cavity and the oral cavity
• The soft palate forms the posterior portion of
  the roof of the mouth and is primarily muscle
• The uvula and the soft palate close off the
  nasopharynx during swallowing
• The fauces is the opening between the oral cavity
  and the oropharynx

20
Major Salivary Glands

• The salivary glands release a secretion called
  saliva into the oral cavity
• Saliva helps keep the mucous membrane lining of
  the mouth and pharynx moist and helps to cleanse
  the mouth and teeth
• When food enters the mouth, secretion of saliva
  increases, and it lubricates, dissolves, and
  begins the chemical breakdown of food

21
Major Salivary Glands (continued)

• There are 3 pair of major salivary glands, which
  secrete saliva into the mouth through ducts
• Parotid glandslocated in the cheek near the ears
• Submandibular glandslocated in the floor of the
  mouth near the mandible
• Sublingual glandslocated beneath the tongue and
  superior to the submandibular glands

22
Composition of Saliva

• Saliva is slightly acidic (pH 6.35-6.85) and is
  99.5 water and 0.5 solutes (ions, mucus, IgA,
  the enzymes lysozyme and salivary amylase, and
  the wastes urea and uric acid)
• Lysozyme kills some bacteria within the mouth
• Salivary amylase acts on starch

23
Salivation

• Secretion of saliva, or salivation, is controlled
  by the autonomic nervous system (parasympathetic
  stimulates secretion)
• Approximately 1-1.5 liters of saliva are secreted
  per day
• Salivation is generally stimulated by the
  chemicals in foods, but the smell, sight, or
  thought of food may also cause secretion of
  saliva
• Mumps is a specific viral infection of the
  salivary glands, usually the parotid gland
  vaccination has greatly decreased the incidence
  of this disease

24
Tongue

• The tongue is an accessory digestive organ
  composed of skeletal muscle covered with mucous
  membrane
• It forms the floor of the oral cavity, and is
  attached to the hyoid bone, the styloid process
  of the temporal bone, and the mandible
• The tongue helps maneuver food for chewing and
  shape it into a rounded mass called a bolus
• It also is involved in speech and swallowing

25
Tongue (continued)

• The lingual frenulum is a fold of mucous membrane
  underneath the tongue which attaches it to the
  floor of the mouth and limits its movement
  posteriorly
• The upper and lateral surfaces of the tongue are
  covered with papillae, many of which contain
  taste buds, the receptors for gustation (taste)

26
Teeth

• The teeth are accessory digestive organs anchored
  (by ligaments) in the sockets of the alveolar
  processes of the mandible and maxillae
• The gingivae, or gums, cover the alveolar
  processes
• The teeth are composed of dentin internally,
  covered by enamel both of these substances are
  harder than bone because of their higher content
  of calcium mineral salts
• Humans have 2 sets of teeth during their
  lifetime
• Deciduous teeth (baby teeth)usually 20
• Permanent teethusually 32

27
Digestion in the Mouth

• Digestion begins in the mouth
• Mechanical digestion results from mastication, or
  chewing, and a bolus is formed
• The water in saliva dissolves the food somewhat,
  so enzymes can react with it during chemical
  digestion
• Salivary amylase begins the breakdown of starch
  (a polysaccharide) into smaller molecules

28
Pharynx

• The pharynx, or throat, is a muscular tube that
  extends from behind the nasal cavity to the
  esophagus and larynx
• Swallowed food passes from the mouth into the
  oropharynx, then the laryngopharynx, and into the
  esophagus

29
Esophagus

• The esophagus is a muscular tube that extends
  from the inferior laryngopharynx to the stomach
• It pierces the diaphragm through an opening
  called the esophageal hiatus
• The upper esophageal sphincter is composed of
  skeletal muscle and regulates the movement of
  food from the pharynx into the esophagus
• The lower esophageal sphincter is composed of
  smooth muscle and regulates the movement of food
  from the esophagus into the stomach
• The esophagus secretes mucus and transports food
  into the stomach, but it does not produce any
  digestive enzymes or carry out any absorption

30
Deglutition

• Deglutition is the act of swallowing
• The medulla oblongata contains the deglutition
  center
• Swallowing occurs in 3 stages
• Voluntary stagethe bolus passes from the mouth
  into the oropharynx voluntary
• Pharyngeal stagethe bolus passes through the
  pharynx to the esophagus involuntary
• Esophageal stagethe bolus moves to the stomach
  by peristalsis, successive muscular contractions
  in the walls of the esophagus involuntary

31
Stomach

• The stomach is a J-shaped enlargement of the
  digestive tract
• It is located in the left side of the abdomen
  beneath the diaphragm
• The stomach connects the esophagus to the small
  intestine
• It mixes the food and holds it temporarily until
  the small intestine is ready to receive it

32
Stomach (continued)

• Digestion of starch continues in the stomach, and
  digestion of proteins and triglycerides begins
• The semisolid bolus is converted to a liquid,
  called chyme, and various substances are absorbed

33
Stomach (continued)

• The stomach has four main regions
• Cardiasurrounds the opening into the stomach
  from the esophagus
• Fundusthe rounded superior portion
• Bodythe large central portion
• Pylorusthe portion that connects to the duodenum
  of the small intestine (the first part)

34
Stomach (continued)

• Rugae are folds of the mucosa layer of the
  stomach
• The pyloric sphincter is a ring of smooth muscle
  between the pylorus of the stomach and the
  duodenum of the small intestine
• It normally remains almost closed, but allows
  small amounts of partially digested food into the
  duodenum, a little at a time, during a process
  called gastric emptying

35
Stomach (continued)

• The wall of the stomach is composed of the same 4
  layers as most of the rest of the GI tract, with
  a few differences
• The muscularis in the body of the stomach has 3
  layers of smooth muscle (instead of 2, as in the
  intestines and the rest of the stomach)an inner
  oblique layer, in addition to a middle circular
  layer and an outer longitudinal layer

36
Stomach (continued)

• The epithelium of the stomach extends deep down
  into the mucosa, forming channels called gastric
  pits
• Some of the epithelial cells secrete mucus and
  are called surface mucous cells this mucus helps
  to protect the stomach from the acidic pH of
  stomach acid
• At the bottom of the gastric pits are gastric
  glands, which are composed of secretory cells
• The gastric glands secrete substances into the
  gastric pits, and these substances eventually
  reach the lumen of the stomach

37
Stomach (continued)

• The gastric glands contain 4 types of cells
• Mucous neck cellssecrete mucus, along with the
  surface mucous cells
• Chief cellssecrete pepsinogen and gastric lipase
• Parietal cellsproduce intrinsic factor (needed
  for the absorption of vitamin B12) and
  hydrochloric acid
• G cells, which secrete the hormone gastrin into
  the bloodstream

38
Stomach (continued)

• Stomach acid, or gastric juice, is composed of
  the secretions of the mucous, parietal, and chief
  cells
• Approximately 2-3 quarts of gastric juice are
  secreted per day, and it has a pH of about 2
  (strongly acidic)

39
Digestion in the Stomach

• The passage of food from the mouth to the stomach
  generally takes only seconds
• Once food enters the stomach, starch digestion by
  salivary amylase continues for awhile
• Soon, the food is mixed with gastric juice and
  liquified to form chyme salivary amylase is
  inactivated at this time
• Hydrochloric acid in the gastric juice kills many
  microbes in the food and causes the conversion of
  the inactive enzyme pepsinogen into the active
  version pepsin

40
Digestion in the Stomach (continued)

• Pepsin digests proteins by breaking bonds between
  amino acids
• It is secreted by the chief cells in its inactive
  form to prevent it from digesting proteins in the
  cells
• Gastric lipase, also secreted by chief cells,
  digests triglycerides in fat

41
Digestion in the Stomach (continued)

• Not much absorption occurs in the stomach except
  for water, alcohol, certain drugs (ex. aspirin),
  and a few other substances
• After 2-4 hours, the stomach has emptied its
  contents into the duodenum

42
Pancreas

• The pancreas is a retroperitoneal accessory
  digestive organ that is about 5-6 inches long and
  lies posterior to the stomach
• Pancreatic juices pass to the duodenum through
  two ducts
• Pancreatic duct (duct of Wirsung)
• Accessory duct (duct of Santorini)

43
Pancreas (continued)

• The pancreatic duct generally joins the common
  bile duct from the liver and gallbladder and
  enters the duodenum as a common duct called the
  hepatopancreatic ampulla (ampulla of Vater)
• The accessory duct empties into the duodenum
  about an inch superior to the hepatopancreatic
  ampulla

44
Pancreas (continued)

• The pancreas is made up of two main types of
  cells
• Acini99 of the cells are this type secrete
  pancreatic juice
• Pancreatic islets (islets of Langerhans)1 of
  the cells are this type endocrine cells that
  secrete the hormones glucagon, insulin,
  somatostatin, and pancreatic polypeptide

45
Pancreatic Juice

• Each day the pancreas produces about 1.2-1.5
  quarts of pancreatic juice
• This substance is a clear, colorless liquid
  composed of water, some salts, sodium
  bicarbonate, and several enzymes
• The sodium bicarbonate gives pancreatic juice a
  pH of about 7.1-8.2 (slightly alkaline), which
  buffers gastric juice and stops the action of
  pepsin

46
Pancreatic Juice (continued)

• The enzymes in pancreatic juice include
• Pancreatic amylasedigests starch
• Trypsindigests protein
• Chymotrypsindigests protein
• Carboxypeptidasedigests protein
• Elastasedigests protein
• Pancreatic lipasedigests triglycerides (fats)
• Ribonucleasedigests RNA
• Deoxyribonucleasedigests DNA

47
Pancreatic Juice (continued)

• To prevent them from digesting proteins in the
  pancreas, the protein-digesting enzymes are
  produced in an inactive form (as in the stomach)
• Trypsinogeninactive form of trypsin
• Chymotrypsinogeninactive form of chymotrypsin
• Procarboxypeptidaseinactive form of
  carboxypeptidase
• Proelastaseinactive form of elastase

48
Liver and Gallbladder

• The liver and gallbladder are two accessory
  digestive organs involved with the secretion of
  bile, which emulsifies lipids before they are
  digested
• The liver is divided into two lobes, a right and
  a left, by the falciform ligament
• This ligament also suspends the liver in the
  abdominal cavity
• The lobes of the liver are made up of lobules

49
Liver and Gallbladder (continued)

• Hepatocytes are liver cells within the lobules
  that secrete bile
• The bile leaves the lobules through small ducts
  that eventually merge to form the right and left
  hepatic ducts, which exit the right and left
  lobes
• These two ducts unite and exit the liver as the
  common hepatic duct

50
Liver and Gallbladder (continued)

• The common hepatic duct joins the cystic duct
  from the gallbladder to form the common bile duct
• The main function of the gallbladder is to store
  and concentrate the bile produced by the liver
  until it is needed in the small intestine
• The wall of the gallbladder contains smooth
  muscle tissue which contracts and causes the
  ejection of stored bile into the cystic duct

51
Bile

• Hepatocytes secrete about 1 quart of bile per
  day
• Bile is a yellow, brownish, or olive-green liquid
• It has a pH of 7.6-8.6 and consists mostly of
  water, bile salts, cholesterol, a phospholipid
  called lecithin, bile pigments, and several ions

52
Bile (continued)

• The main bile pigment is bilirubin
• As old red blood cells are phagocytized, iron,
  globin (protein), and bilirubin (derived from
  heme) are formed
• The iron and globin are recycled
• The bilirubin is secreted into bile and
  eventually broken down in the small intestine
• One of the products of this breakdown is
  stercobilin, which gives feces its normal brown
  color

53
Bile (continued)

• Bile salts help to emulsify, or breakdown, large
  lipid molecules so that enzymes such as
  pancreatic lipase can digest them
• Jaundice is a yellowish coloration of the whites
  of the eyes and the skin due to a buildup of
  bilirubin
• It can occur when the liver does not function
  properly to eliminate bilirubin

54
Blood Supply of the Liver

• The liver receives blood from two sources, the
  hepatic artery and the hepatic portal vein
• The hepatic artery carries oxygenated blood the
  hepatic portal vein carries deoxygenated blood
  containing newly absorbed nutrients, drugs, and
  microbes and toxins from the GI tract

55
Blood Supply of the Liver (continued)

• Branches of the hepatic artery and the hepatic
  portal vein carry blood into liver sinusoids,
  capillaries that carry oxygen, nutrients, and
  toxic substances to hepatocytes
• Hepatic veins carry deoxygenated blood away from
  the liver

56
Functions of the Liver

• Carbohydrate metabolismhelps to maintain a
  normal blood glucose level
• Lipid metabolismhepatocytes can store
  triglycerides, break down fatty acids, and
  synthesize cholesterol
• Protein metabolismhepatocytes break down amino
  acids, producing ammonia, which is toxic ammonia
  is converted into urea, which is less toxic
  hepatocytes also synthesize most plasma proteins
  (immunoglobulins, albumin, prothrombin, and
  fibrinogen)

57
Functions of the Liver (continued)

• Processing of drugs and hormonesdetoxifies
  alcohol and drugs
• Excretion of bilirubin
• Synthesis of bile saltsfor lipid emulsification
• Storageof glycogen and many vitamins and
  minerals
• Phagocytosisof old blood cells and bacteria
• Activation of vitamin Dalong with the skin and
  kidneys

58
Small Intestine

• Most digestion and absorption of nutrients occur
  in the small intestine
• It is about 1 inch in diameter and 10 feet long
  in a living person it is about 21 feet long
  after death, due to the loss of smooth muscle tone

59
Small Intestine (continued)

• The small intestine is divided into 3 regions
• Duodenumthe first and shortest region about 10
  inches long
• Jejunumthe second region about 3 feet long
• Ileumthe final and longest region about 6 feet
  long the ileocecal sphincter is at the junction
  of the ileum and the large intestine

60
Small Intestine (continued)

• The wall of the small intestine is composed of
  the same 4 layers that make up most of the rest
  of the digestive tract
• The epithelium of the mucosa contains absorptive
  cells, which absorb nutrients, and goblet cells,
  which secrete mucus

61
Small Intestine (continued)

• Intestinal glands are deep crevices in the mucosa
  that secrete intestinal juice they contain
  Paneth cells, which secrete lysozyme and are
  capable of phagocytosis of microbes three types
  of cells that secrete digestive hormones
  (secretincauses the buffering of the acids in
  chyme, cholecystokinin CCKstimulates secretion
  of pancreatic juice and causes the contraction of
  the gallbladder) are also in these glands
• Lymphatic nodules are found in the mucosa of the
  small intestine, particularly in the ileum

62
Small Intestine (continued)

• To increase the surface area for absorption, the
  small intestinal wall has these features
• Circular folds (plicae circulares) are folds of
  the wall of the small intestine they cause the
  chyme to move in a spiral manner as it passes
  through
• Villi are fingerlike projection which contain
  blood and lymphatic vessels (lacteals) into which
  absorbed nutrients pass
• Microvilli are plasma membrane extensions of the
  absorptive cells

63
Digestion in the Small Intestine

• There are two types of movements in the small
  intestine
• Segmentationsmooth muscles in the wall contract
  and relax to slosh the chyme back and forth
• Peristalsissuccessive muscle contractions along
  the wall that result in movement of the food
  through the tract

64
Digestion in the Small Intestine (continued)

• About 1-2 quarts of intestinal juice, a clear
  yellow fluid, are secreted each day by the small
  intestine
• This liquid has a pH of 7.6 (slightly alkaline)
  and is composed of water, mucus, and digestive
  enzymes

65
Digestion in the Small Intestine (continued)

• Some of the digestive enzymes in intestinal juice
  include
• Maltasedigests the sugar (disaccharide) maltose
• Lactasedigests the sugar (disaccharide) lactose
• Sucrasedigests the sugar (disaccharide) sucrose
• Peptidasedigests proteins

66
Absorption in the Small Intestine

• Passage of digested nutrients from the GI tract
  into the blood or lymph is called absorption
• The absorptive cells of the epithelium of the
  small intestine absorb nutrients

67
Absorption in the Small Intestine (continued)

• Carbohydrates are absorbed as monosaccharides,
  which pass into the capillaries of the villi
• Proteins are absorbed as amino acids, which also
  pass into the capillaries of the villi
• The villi capillaries lead to the hepatic portal
  vein, which carries nutrients to the liver before
  they reach general circulation

68
Absorption in the Small Intestine (continued)

• Lipids are absorbed as monoglycerides and fatty
  acids
• Some of these molecules are rather large and
  insoluble
• Micelles are tiny spheres of bile salts that help
  transport lipids within the chyme in the small
  intestine
• Lipids pass into the lacteals of the villi and
  then into lymphatic vessels instead of directly
  into the bloodstream
• Chylomicrons are large protein spheres that help
  transport lipids within the lymphatic vessels
• Almost all of the water that enters the small
  intestine is absorbed

69
Large Intestine

• The large intestine is about 5 feet long and 2.5
  inches in diameter
• The cecum is a small pouch at the beginning of
  the large intestine the appendix is attached to
  the cecum
• The colon is the long, tubular part of the large
  intestine

70
Large Intestine (continued)

• The regions of the colon include
• Ascending colonlocated on the right side of the
  abdomen
• Transverse colonruns horizontally across the
  abdomen
• Descending colonlocated on the left side of the
  abdomen
• Sigmoid colonS-shaped located at between the
  descending colon and the rectum

71
Large Intestine (continued)

• The rectum is the last portion of the GI tract
• The last inch of the rectum is called the anal
  canal it opens to the exterior at the anus
• The internal anal sphincter is involuntary,
  composed of smooth muscle, and located nearest
  the anal canal the external anal sphincter is
  voluntary, composed of skeletal muscle, and is
  located outside the internal anal sphincter

72
Large Intestine (continued)

• The wall of the large intestine is somewhat
  similar to that of the small intestine
• It has the same 4 layers, epithelial absorptive
  and goblet cells, intestinal glands, and
  lymphatic nodules
• There are no circular folds or villi

73
Large Intestine (continued)

• Teniae coli are longitudinal bands that run most
  of the length of the large intestine
• These bands gather the walls into a series of
  pouches called haustra, which give the colon a
  puckered appearance
• Movement of chyme through the large intestine
  occurs by peristalsis and haustral churning

74
Large Intestine (continued)

• No digestive enzymes are secreted by the large
  intestine
• Mucus is secreted, and bacteria break down
  remaining nutrients
• The action of bacteria allow for the absorption
  of vitamin K and some B vitamins
• Some water, ions, and vitamins are absorbed
• Chyme is changed into feces, which is excreted
  from the body