The digestive system

1
The digestive system

• Three fundamental processes
• Secretion Delivery of enzymes, mucus, ions and
  the like into the lumen, and hormones into blood.
  
• Absorption Transport of water, ions and
  nutrients from the lumen, across the epithelium
  and into blood.
• Motility Contractions of smooth muscle in the
  wall of the tube that crush, mix and propel its
  contents.

2

• The following slides are a quick overview.
• Howstuffworks "The Digestive System

3
Alimentary canal

• 2 main functions
• Digesting and absorbing nutrients
• Protecting from invasion

Trachea - windpipe
4
Begins in the mouth
5
The Mouth and Pharynx

• In the mouth, teeth, jaws and the tongue begin
  the mechanical breakdown of food into smaller
  particles. Most vertebrates, except birds (who
  have lost their teeth to a hardened bill), have
  teeth for tearing, grinding and chewing food. The
  tongue manipulates food during chewing and
  swallowing mammals have tastebuds clustered on
  their tongues.

6

• Salivary glands secrete salivary amylase, an
  enzyme that begins the breakdown of starch into
  glucose.
• Mucus moistens food and lubricates the
  esophagus.
• Bicarbonate ions in saliva neutralize the acids
  in foods.
• Animation player go to animation of swallowing.
  

7

• Food travels through the esophagus to the
  stomach.
• Moved by paristaltic waves.
• http//images.encarta.msn.com/xrefmedia/aencmed/ta
  rgets/illus/ilt/0007760a.gif
• Animation Quizzes go to digestion mouth to
  stomach. Pay attention to glands involved.

8
(No Transcript)
9
Stomach
Short term storage reservoir (1L for up to 4h)
Digestion chemical (HCl and enzymes) - proteins
mechanical - liquefication of food
Slowly releases food into intestine
http//35.9.122.184/images/41-AnimalNutrition/41-1
6-Duodenum-L.gif
10
Stomach Epithelium
Mucous goblet cells
Prevents self-digestion
Enzymes (pepsinogen) chief cells
Acid (HCl) parietal cells
pH 1-2 Kills bacteria Loosens fibrous
foods Activates pepsinogen Denatures salivary
amylase
Hormone (gastrin) G cells
Controls gastric motility and acid secretion
Stomach epithelial cells are some of the fastest
growing cells in the body, typically replacing
themselves about every 3 days
11
Cardiac sphincter allows passage from esophagus
to stomach
12
The stomach
13

• Stomach Histology
• Digestive System WebQuest ( go to virtual body)

14

• The wall of the stomach is lined with millions of
  gastric glands, which together secrete 400800 ml
  of gastric juice at each meal. Three kinds of
  cells are found in the gastric glands
• parietal cells
• "chief" cells
• mucus-secreting cells
• Parietal cells
• Parietal cells secrete hydrochloric acid and
  intrinsic factor

15

• "Chief" Cells
• synthesize and secrete pepsinogen, the precursor
  to the proteolytic enzyme pepsin.
• Pepsin cleaves peptide bonds. Its action breaks
  long polypeptide chains into shorter lengths.
• Secretion by the gastric glands is stimulated by
  the hormone gastrin. Gastrin is released by
  endocrine cells in the stomach in response to the
  arrival of food

16
Parietal and chief cells
17

• Epithelial cells secrete mucus that forms a
  protective barrier between the cells and the
  stomach acids
• . Pepsin is inactivated when it comes into
  contact with the mucus.
• Bicarbonate ions reduce acidity near the cells
  lining the stomach.
• Tight junctions link the epithelial
  stomach-lining cells together, further reducing
  or preventing stomach acids from passing.

18
Mucous secreting cells
19

• Carbohydrate digestion, begun by salivary amylase
  in the mouth, continues in the bolus as it passes
  to the stomach.
• The bolus is broken down into acid chyme in the
  lower third of the stomach, allowing the
  stomach's acidity to inhibit further carbohydrate
  breakdown. Protein digestion by pepsin begins.
• Alcohol and aspirin are absorbed through the
  stomach lining into the blood.

20

• McGraw-Hill Online Learning Center TestltBLURTgt go
  to stomach digestion

21

• Walls of the stomach contract vigorously and mix
  food with juices secreted when food enters.
•  gastric juice contains hydrochloric acid and
  another digestive substance, pepsin.
•  gastric juices are produced independently of
  protective mucous secretions.

22

• Hydrochloric acid (HCl) lowers pH of the gastric
  contents to about 2. This acid kills most
  bacteria and other microorganisms. Low pH also
  stops activity of salivary amylase and promotes
  activity of pepsin. Pepsin is a hydrolytic
  enzyme that acts on proteins to produce peptides.
  
• pepsin protein
  H2O ? peptides

23

• A thick layer of mucus protects wall of the
  stomach and first part of duodenum from HCl and
  pepsin
•   Ulcers develop when lining is exposed to
  digestive action recent research indicates this
  is usually due to infection by Helicobacter
  pylori bacteria.
• Stomach contents, a thick, soupy mixture, are
  called chyme.       

24

• At base of the stomach is a narrow opening
  controlled by a sphincter (a circular muscle
  valve).
• When the sphincter relaxes, chyme enters
  duodenum a neural reflex causes the sphincter to
  contract closing off the opening.
• Duodenum is first part of the small intestine.
           

25
Absorption in the stomach

• Very little absorption
• As the contents of the stomach become thoroughly
  liquefied, they pass into the duodenum, the first
  segment (about 10 inches long) of the small
  intestine.
• Two ducts enter the duodenum
• one draining the gall bladder and hence the liver
  
• the other draining the exocrine portion of the
  pancreas.

26
Exocrine gland

• Exocrine glands are glands whose secretions pass
  into a system of ducts that lead ultimately to
  the exterior of the body. So the inner surface of
  the glands and the ducts that drain them are
  topologically continuous with the exterior of the
  body (the skin). Endocrine glands, in contrast,
  place their secretions into the internal
  environment - the blood.

27

• Examples of exocrine glands are
• salivary glands (shown next slide) that secrete
  saliva into the mouth
• bile-producing glands of the liver
• prostate gland
• the portion of the pancreas that secretes
  pancreatic fluid into the duodenum. (The pancreas
  is also an endocrine gland - its islets of
  Langerhans secrete several hormones into the
  blood.)
• gastric glands
• sweat glands

28
Salivary glands of mouth
29
(No Transcript)
30
From the stomach to the small intestines

• Animation Quizzes ( go to stomach to small
  intestines )

31
Small Intestine
Around 6m in an adult Food takes 1-6 h to pass
through 2 main tasks digestion, absorption
3 parts Duodenum Jejenum Ileum
32
Small Intestine cont.
Jejenum digestion/ absorption. 2.5m long
Ileum absorption. 4m long
Walls only one cell thick Villi, microvilli
increase surface area for absorption Rich blood
supply capillaries absorb water and soluble
nutrients (glucose, amino acids, vitamins,
minerals) and the blood carries the nutrients to
the liver, which stores nutrients and releases
them as required
Lacteal contains lymph. Fatty acids and
glycerol are absorbed by the epithelial cells
where they reform into fats. They become coated
in protein (chylomicrons) and pass into the lymph
in the lacteals. It takes around 18h for lymph to
rejoin the blood, the protein coat dissolves and
fats are absorbed into cells
33
Small Intestines

• Human small intestine is a coiled muscular tube
  about three meters long.

34
The small intestines

• Digestion within the small intestine produces a
  mixture of disaccharides, peptides, fatty acids,
  and monoglycerides.
• The final digestion and absorption of these
  substances occurs in the villi, which line the
  inner surface of the small intestine.
• Hole's Human Anatomy Physiology  Animation
  Activities go to small intestine digestion

35

• The villi increase the surface area of the small
  intestine to many times what it would be if it
  were simply a tube with smooth walls. In
  addition, the apical (exposed) surface of the
  epithelial cells of each villus is covered with
  microvilli (also known as a "brush border").
  Thanks largely to these, the total surface area
  of the intestine is almost 200 square meters,
  about the size of the singles area of a tennis
  court and some 100 times the surface area of the
  exterior of the body.

36

• The electron micrograph (courtesy of Dr. Sam L.
  Clark) shows the microvilli of a mouse intestinal
  cell.

37
Large Intestine
1.5m long, 6cm diameter
Food stays 10h to a few days
Colon Reabsorbs water so waste is converted to
semi-solid faeces egested Diarrhoea,
constipation (fibre helps stimulate peristalsis)
38

• While the contents of the small intestine are
  normally sterile, the colon contains an enormous
  (1014) population of microorganisms. (Our bodies
  consist of only 1013 cells!) Most of the
  bacteria (of which one common species is E. coli)
  are harmless And some are actually helpful, for
  example, by synthesizing vitamin K. Bacteria
  flourish to such an extent that as much as 50 of
  the dry weight of the feces may consist of
  bacterial cells.

39
Bacteria
1-2kg of bacteria in your gut 4000 species
Bad - bacteria that can cause illness e.g. H
pylori (ulcers), Salmonella, E. coli, Listeria
(food poisoning)
Good symbiotic bacteria. These live in close
harmony with the body without causing harm, and
have additional health benefits. Probiotics are
live micro-organisms that, when consumed in
adequate amounts, confer a health benefit to the
host. e.g. bifidobacteria, lactobacillus

• Aid digestion
• Break down toxins
• Produce vitamins B12 and K
• Stimulate the immune system
• Help prevent growth of cancers
• Convert prodrugs to drugs

40
Accessory glands

• Salivary glands ( already covered)
• Pancreas
• Liver
• gallbladder

41
Duodenum digestion 25cm long
Pancreas pancreatic juice NaHCO3, enzymes
(insulin, glucagon) pH of duodenum 7-8
Amylase, lipase, trypsinogen, chymotrypsinogen
Liver bile made in liver, stored in gall
bladder Water, salts, bile salts Neutralise
HCl Digestion and absorption of fats and fat
soluble vitamins (emulsification) Waste products
eliminated by secretion into bile and elimination
in feces (e.g. bilirubin, biliverdin)

42
More on pancreas

• The pancreas consists of clusters if endocrine
  cells (the islets of Langerhans) and exocrine
  cells whose secretions drain into the duodenum.
• Pancreatic fluid contains
• sodium bicarbonate (NaHCO3). This neutralizes the
  acidity of the fluid arriving from the stomach
  raising its pH to about 8.
• pancreatic amylase. This enzyme hydrolyzes starch
  into a mixture of maltose and glucose.
• pancreatic lipase. The enzyme hydrolyzes ingested
  fats into a mixture of fatty acids and
  monoglycerides. Its action is enhanced by the
  detergent effect of bile.

43
(No Transcript)
44
Pancreas both endocrine and exocrine

• Exocrine TissueThe exocrine tissue consist of
  Acinar cells and pancreatic ducts. The pancreas'
  exocrine function produces a variety of digestive
  enzymes (trypsin, chymotrypsin, lipase, and
  amylase, among others). These enzymes are passed
  into the duodenum through a channel called the
  pancreatic duct. In the duodenum, the enzymes
  begin the process of breaking down a variety of
  food components, including, proteins, fats, and
  starches.

45

• Endocrine TissueThe endocrine tissue consists of
  cells clusters known as islets of Langerhans.
  These endocrine (endo within) cells of the
  pancreas produce and secrete hormones into the
  bloodstream.
• The main pancreatic hormones, insulin and
  glucagon, work together to maintain the proper
  level of sugar in the blood. Failure diabetes

46
Liver
Weighs about 1.5kg Holds about 13 of total
blood Liver cell hepatocyte Unique ability to
regenerate average life 150 days
Right lobe
Left lobe
Blood rich in food from ileum
http//www.britishlivertrust.org.uk/content/liver/
about.asp
47
(No Transcript)
48

• liver --  Encyclopædia Britannica go to
  animation of liver
• Hemoglobin taken apart in liver Animation
  Hemoglobin Breakdown

49
Bile salts

• Bile is a complex fluid containing water,
  electrolytes and a battery of organic molecules
  including bile acids, cholesterol, phospholipids
  and bilirubin that flows through the biliary
  tract into the small intestine. There are two
  fundamentally important functions of bile in all
  species

50

• Bile contains bile acids, which are critical for
  digestion and absorption of fats and fat-soluble
  vitamins in the small intestine.
• Many waste products are eliminated from the body
  by secretion into bile and elimination in feces.

51

• The secretion of bile can be considered to occur
  in two stages
• Initially, hepatocytes secrete bile into
  canaliculi, from which it flows into bile ducts.
  This hepatic bile contains large quantities of
  bile acids, cholesterol and other organic
  molecules.
• As bile flows through the bile ducts it is
  modified by addition of a watery,
  bicarbonate-rich secretion from ductal epithelial
  cells.

52
Control of eating

• Animations

53
Review what you know

• Look at the following slides and review what the
  system is about.

54
Big Picture 07a, p. 119
55
Fig. 7.4c, p. 122
parotid gland
submandibular gland
sublingual gland
56
Fig. 7.5a, p. 123
Voluntary Phase
Involuntary Phase
hard palate
bolus
epiglottis
Larynx rises trachea closes muscle
contractions squeeze food into esophagus.
Contracted muscles close off esophagus.
trachea open
57
Fig. 7.5b,c, p. 123
muscles relaxed
muscles relaxed
Circular muscles contract, squeezing bolus
toward the stomach.
bolus
Lower esophageal sphincter opens and food enters
stomach.
stomach
58
Fig. 7.7a, p. 124
serosa
esophagus
longitudinal muscle
circular muscle
pyloric sphincter
oblique muscle
submucosa
mucosa
duodenum
59
Fig. 7.10c,d, p. 125
epithelium
villi
blood capillaries
lymph vessel
connective tissue
vesicles
artery
vein
lymph vessel
Villi on one of the folds, longitudinal section
One villus
60
Fig. 7.8e(3), p. 125
Microvilli at free surface of absorptive cells
ctyoplasm
mucus secretion (goblet cell)
phagocytosis lysozyme secretion
hormone secretion
absorption
61
Fig. 7.9ab, p. 127
INTESTINAL LUMEN
carbohydrates
monosaccharides
proteins
amino acids
EPITHELIAL CELL
INTERNAL ENVIRONMENT
62
Fig. 7.9c-f, p. 127
bile salts
bile salts

micelles
fat globules (triglycerides)
fatty acids, monoglycerides
emulsification droplets
triglycerides proteins
EPITHELIAL CELL
chylomicrons
INTERNAL ENVIRONMENT
63
Fig. 7.10, p. 128
(inferior vena cava)
hepatic vein
(liver capillary beds)
liver
stomach
gallbladder
(spleen)
hepatic portal vein
pancreas
ascending colon
descending colon
small intestine
appendix
rectum
64
Fig. 7.11, p. 129
ascending portion of large intestine
last portion of small intestine
cecum
appendix
65
Fig. 7.12,p. 130
sight, smell, taste of food
emotional states
CENTRAL NERVOUS SYSTEM
smooth muscle or gland
Gut wall
sensory receptors
nerve network
Stimulus
Response
Gut lumen
change in food volume, composition in lumen
gut wall moves or substances secreted into lumen
66
Fig. 7.13, p. 131
67
Table 7.5 Calories Expended in Some Common
Activities
Hours needed to lose 1lb. fat
Kcal/hour per pound of body weight
Activity
120 lbs.
155 lbs.
185 lbs.
Basketball Cycling (9mph) Hiking Jogging Mowing
lawn (push mower) Racquetball Running (9-minute
mile) Snow skiing (cross-country) Swimming (slow
crawl) Tennis Walking (moderate pace)
3.78 2.70 2.52 4.15 3.06 3.90 5.28 4.43 3.48 3.00
2.16
7.7 10.8 11.6 7.0 9.5 7.5 5.5 6.6 8.4 9.7 13.5
6.0 8.4 8.9 5.4 7.4 5.8 4.3 5.1 6.5 7.5 10.4
5.0 7.0 7.5 4.5 6.2 4.8 3.6 4.3 5.4 6.3 8.7
Table 7.5, p. 139
68
Table 7.6 Summary of the Digestive System
MOUTH (oral cavity) PHARYNX ESOPHAGUS STOMACH
SMALL INTESTINE COLON (large
intestine) RECTUM ANUS
Star of digestive system, where food is chewed,
moistened, polysaccharide digestion
begins Entrance to tubular parts of digestive
and respiratory systems Muscular tube, moistened
by saliva, that moves food from pharynx to
stomach Sac where food mixes with gastric fluid
and protein digestion begins stretches to store
food taken in faster than can be processed
gastric fluid destroys many microbes The first
part 9duodenum) receives secretions from the
liver, gallbladder, and pancreas Most nutrients
are digested, absorbed in second part
jejunum) Some nutrients absorbed in last part
(ileum), which delivers unabsorbed material to
colon Concentrates and stores undigested matter
(by absorbing mineral ions and water) Distension
triggers expulsion of feces Terminal opening of
digestive system
Table 7.6, p. 140
69
Table 7.6 Summary of the Digestive System
Accessory Organs SALIVARY GLANDS PANCREAS L
IVER GALLBLADDER
Glands (three main pairs, many minor ones) that
secrete saliva, a fluid with polysaccharide-digest
ing enzymes, buffers, and mucus (which moistens
and lubricates ingested food) Secretes enzymes
that digest all major food molecules and buffers
against against HCI from stomach Secretes bile
(used in fat emulsification) role in
carbohydrate, fat, and protein,
metabolism Stores and concentrates bile from the
liver
Table 7.6, p. 140
70
Table 7.2 Ways the Liver Contributes to
Homeostasis

• Plays a role in carbohydrate metabolism
• Partially controls synthesis of proteins in
  blood assembles and disassembles certain other
  proteins
• Forms urea from nitrogen-containing wastes
• Assembles and stores some fats forms bile to aid
  in fat digestion
• Inactivates many chemicals (hormones, some drugs)
• Detoxifies many poisons
• Breaks down worn-out red blood cells
• Aids immune response (removes some foreign
  particles)
• Absorbs, stores factors needed for red blood cell
  formation

Table 7.2, p. 128