Chapter 26 Physiology of the Digestive System

1
Chapter 26 Physiology of the Digestive System
2
OVERVIEW OF DIGESTIVE FUNCTION

• Primary function of the digestive system to
  bring essential nutrients into the internal
  environment so they are available to every cell
  in the body
• Mechanisms used to accomplish the primary
  function of the digestive system (Figure 26-1,
  Table 26-1)
• Ingestion food is taken in
• Digestion breakdown of complex nutrients into
  simple nutrients
• Motility of the gastrointestinal (GI) wall
  physically breaks down large chunks of food
  material and moves food along the tract
• Secretion of digestive enzymes allows chemical
  digestion
• Nutrients are absorbed into the internal
  environment through the GI mucosa
• Material that is not absorbed is eliminated

3
(No Transcript)
4
Digestion

• All the processes that chemically and
  mechanically break down food into nutrients
• Begins immediately after food is ingested
• 2 types Mechanical and Chemical
• Mechanical movement of the GI tract which
• Changes the physical state of food to facilitate
  chemical digestion
• Churning of the contents in the GI lumen so they
  are well mixed with the GI juices
• Propelling food through the digestive tract

5
Mechanical Digestion

• Mastication reducing the food particle size
  through chewing, and mixes food with saliva
• Deglutition swallowing
• Oral Stage (moth to oropharynx)
• Pharyngeal Stage (oropharynx to esophagus)
• Esophageal Stage (esophagus to stomach)

6
DIGESTION MECHANICAL

• Deglutition process of swallowing complex
  process requiring coordinated, rapid movements
  (Figure 26-2)
• Oral stage (mouth to oropharynx) voluntarily
  controlled formation of a food bolus in the
  middle of the tongue tongue presses bolus
  against the palate and food is then moved into
  the oropharynx
• Pharyngeal stage (oropharynx to esophagus)
  involuntary movement to propel bolus from the
  pharynx to the esophagus, the mouth (tongue),
  nasopharynx (soft palate), and larynx
  (epiglottis) must be blocked a combination of
  contractions and gravity move bolus into
  esophagus
• Esophageal stage (esophagus to stomach)
  involuntary movement contractions and gravity
  move bolus through esophagus and into stomach

7
(No Transcript)
8
DIGESTION MECHANICAL (cont.)

• Peristalsis and segmentation two main types of
  motility produced by the smooth muscle of the GI
  tract can occur together in an alternating
  fashion
• Peristalsis wavelike ripple of the muscle layer
  of a hollow organ progressive motility that
  produces forward movement of matter along the GI
  tract (Figures 26-3 and 26-4)

9
DIGESTION MECHANICAL (cont.)

• Segmentation mixing movement digestive reflexes
  cause a forward-and-backward movement with a
  single segment of the GI tract helps break down
  food particles, mixes food and digestive juices,
  and brings digested food in contact with
  intestinal mucosa to facilitate absorption
  (Figure 26-5)

10
DIGESTION MECHANICAL (cont.)

• Regulation of motility
• Gastric motility
• emptying the stomach takes approximately 2 to 6
  hours
• while in the stomach, food is churned (propulsion
  and retropulsion) and mixed with gastric juices
  to form chyme
• chyme is ejected approximately every 20 seconds
  into the duodenum
• gastric emptying is controlled by hormonal and
  nervous mechanisms (Figure 26-6)
• Hormonal mechanism fats in duodenum stimulate
  the release of gastric inhibitory peptide (GIP),
  which decreases peristalsis of gastric muscle and
  slows passage of chyme into duodenum
• Nervous mechanism receptors in the duodenal
  mucosa are sensitive to presence of acid and
  distention impulses over sensory and motor
  fibers in the vagus nerve cause a reflex
  inhibition of gastric peristalsis enterogastric
  reflex

11
(No Transcript)
12
DIGESTION MECHANICAL (cont.)

• Intestinal motility includes peristalsis and
  segmentation
• Segmentation in duodenum and upper jejunum mixes
  chyme with digestive juices from the pancreas,
  liver, and intestinal mucosa
• Rate of peristalsis picks up as chyme approaches
  end of jejunum, moving it through the rest of the
  small intestine into the large intestine
• Chyme normally takes approximately 5 hours to
  pass through the small intestine
• Peristalsis is regulated in part by intrinsic
  stretch reflexes but also is stimulated by the
  hormone cholecystokinin (CCK)

13
DIGESTION

• Chemical digestion changes in chemical
  composition of food as it travels through the
  digestive tract these changes are the result of
  hydrolysis
• Digestive enzymes
• Extracellular, organic (protein) catalysts
• Principles of enzyme action
• Specific in their action (Figure 26-7)
• Function optimally at a specific pH (changing pH
  changes the shape of the enzyme)
• Most enzymes catalyze a chemical reaction in both
  directions
• Enzymes are continually being destroyed or
  eliminated from the body and must continually be
  synthesized
• Most digestive enzymes are synthesized as
  inactive proenzymes

14
DIGESTION Chemical Digestion (contd)

• Carbohydrate digestion (Figure 26-9)
• Carbohydrates are saccharide compounds
• Polysaccharides are hydrolyzed by amylases to
  form disaccharides
• Carbohydrate digestion begins in the mouth and
  continues in the small intestine with amylase
• Final steps of carbohydrate digestion are
  catalyzed by sucrase, lactase, and maltase, which
  are found in the cell membrane of epithelial
  cells covering the villi that line the intestinal
  lumen
• Protein digestion (Figure 26-10)
• Protein compounds are composed of twisted chains
  of amino acids
• Proteases catalyze hydrolysis of proteins into
  amino acids
• Main proteases pepsin in gastric juice, trypsin
  in pancreatic juice, peptidases in intestinal
  brush border

15
(No Transcript)
16
(No Transcript)
17
DIGESTION Chemical Digestion (contd)

• Fat digestion (Figure 26-12)
• Since fats are insoluble in water, they must be
  emulsified by bile in small intestine before
  being digested (Figure 26-11)
• Bile is made of lecithin and bile salts
• Lecithin and bile salts form micelles around fat
  droplets, making them more water soluble and
  easier to break down
• Pancreatic lipase is the main fat-digesting
  enzyme
• Residues of digestion some compounds of food
  resist digestion and are eliminated as feces

18
(No Transcript)
19
(No Transcript)
20
Secretion

• Saliva is made up of
• Water helps liquify food into chyme
• mucus protects mucosa and lubricates food for
  easy passage
• salivary amylase carbohydrate digesting enzyme
  that works best in a neutral pH
• Sodium bicarbonate helps maintain neutral pH
  for amylase

21
Secretion

• Gastric juice
• Made up of water, mucus and digestive enzymes
  secreted by gastric glands within gastric pits
• Chief cells secrete inactive pepsinogen which is
  activated by HCl to form pepsin
• Parietal cells secrete HCl
• Chloride shift and Proton pump help stomach
  become more acidic and blood more basic
• Produce IF which binds to B12 and facilitates its
  absorption in the SI

22
(No Transcript)
23
(No Transcript)
24
Secretion

• Pancreatic juice
• Made up of water and inactive prozymes (zymogens)
• Trypsinogen is activated by enterokinase to
  become trypsin (protein digesting)
• Trypsin activates chymotrypsin (protein), lipases
  (fats), nucleases (DNA, RNA) and amylase (starch)
• reverse chloride shift secretes H into the
  blood and bicarbonate into GI lumen neutralizes
  the pH of the chyme

25
(No Transcript)
26
(No Transcript)
27
Secretion

• Bile
• Secreted by the liver and stored in the
  gallbladder
• Stimulated by CCK
• Delivered to the duodenum via the cystic duct
  into the common bile duct
• Contains cholesterol, bile pigments and products
  of detoxification for removal through feces
• Intestinal juice
• Basic, mucus solution that buffers and lubricates
  materials in the intestine

28
(No Transcript)
29
CONTROL OF DIGESTIVE GLAND SECRETION

• Digestive glands secrete when food is present in
  the digestive tract or when it is seen, smelled,
  or imagined
• Nervous and hormonal mechanisms control the flow
  of digestive juices
• Salivary secretion
• Only reflex mechanisms control the secretion of
  saliva
• Chemical and mechanical stimuli come from the
  presence of food in the mouth
• Olfactory and visual stimuli come from the smell
  and sight of food

30
CONTROL OF DIGESTIVE GLAND SECRETION (cont.)

• Gastric secretion three phases (Figure 26-18)
• Cephalic phase called psychic phase because
  mental factors activate the mechanism (sight,
  smell, taste and thought of food) in the medulla
  oblongata
• parasympathetic fibers in branches of the vagus
  nerve conduct stimulating efferent impulses to
  the glands to produce gastrin (by G cells in the
  stomach)
• Gastric phase when products of protein digestion
  reach the pyloric portion of the stomach, they
  stimulate release of gastrin
• Gastrin accelerates secretion of gastric juice,
  ensuring enough enzymes are present to digest
  food also secreted with stomach distension
• Intestinal phase various mechanisms seem to
  adjust gastric secretion as chyme passes to and
  through the intestinal tract endocrine reflexes
  involving gastric inhibitory peptide, secretin,
  and CCK inhibit gastric secretions

31
(No Transcript)
32
CONTROL OF DIGESTIVE GLAND SECRETION (cont.)

• Pancreatic secretion stimulated by several
  hormones released by intestinal mucosa
• Secretin, stimulated by acid in the duodenum,
  evokes production of pancreatic fluid low in
  enzyme content but high in bicarbonate to
  neutralize chyme
• Cholecystokinin (CCK) several functions
• Causes increased exocrine secretion from the
  pancreas
• Opposes gastrin, thus inhibiting gastric
  hydrochloric acid secretion
• Stimulates contraction of the gallbladder so that
  bile is ejected into the duodenum
• Bile secretion bile secreted continually by the
  liver secretin and CCK stimulate ejection of
  bile from the gallbladder
• Intestinal secretion little is known about how
  intestinal secretion is regulated suggested that
  the intestinal mucosa is stimulated to release
  hormones that increase the production of
  intestinal juice

33
ABSORPTION

• Process of absorption
• Passage of substances through the intestinal
  mucosa into the blood or lymph (Figure 26-19)
• Most absorption occurs in the small intestine due
  to the large surface area created by the villi
  and microvilli

34
Absorption

• Mechanisms
• Water is absorbed through osmosis
• Movement through secondary active transport
• Na is pumped out of the cell creating a
  concentration gradient allowing Na to move out
  of the GI lumen
• Glucose is too large and hydrophilic to move
  easily through the membrane must be transported
  via Na cotransport
• Amino Acids move similarly to glucose
• After absorption, nutrients move through the
  hepatic portal system to the liver
• Liver stores excess nutrients and the rest leave
  via the hepatic vein into systemic circulation

35
(No Transcript)
36
Lipid Absorption

• Include triglycerides, phospholipids, steroids,
  fat-soluble vitamins
• Bile salts and lecithin surround fatty acid and
  glycerol to form micelles
• Micelles approach the brush border, simple lipid
  molecules (FA) are released to pass through the
  membrane
• Inside the cell, the FA reunite to form large fat
  molecules and finally form chylomicrons
• Absorbed by lacteals and move through lymphatic
  system
• Water soluble chylomicrons allow fat to travel
  through the lymph and enter blood stream.

37
(No Transcript)
38
Absorption

• Fat soluble vitamins
• A,D,E,K depend on bile salts for absorption
• Water soluble vitamins require transport
• Most drugs are lipid soluble and can pass through
  the membrane

39
(No Transcript)
40
ELIMINATION

• Definition the expulsion of feces from the
  digestive tract act of expelling feces is called
  defecation
• Defecation occurs as a result of a reflex brought
  about by stimulation of receptors in the rectal
  mucosa that is produced when the rectum is
  distended (Figure 26-23)
• Constipation contents of the lower part of the
  colon and rectum move at a slower than normal
  rate extra water is absorbed from the feces,
  resulting in a hardened stool
• Diarrhea result of increased motility of the
  small intestine, causing decreased absorption of
  water and electrolytes and a watery stool

41
(No Transcript)
42
(No Transcript)