1. Ch. 24 – The Digestive System
The overall idea is to obtain nutrients from the environment
There are 2 general types of digestive organs involved:
1. Parts of the digestive tract (= a long muscular tube from the mouth to the anus)
This is a.k.a. the gastrointestinal (GI) tract or alimentary canal
It’s made up of the oral cavity (mouth), pharynx (throat), esophagus, stomach, small intestine, and large intestine
Food passes through these
2. Accessory organs = the teeth, tongue, salivary glands, liver, gallbladder, and pancreas
Food does not pass through these
Fig. 24-1, p. 864

2. The general functions of the digestive system
1. Ingestion = eating
2. Mechanical processing (e.g. by the teeth and stomach) = crushing, shearing, and mixing
This increases the surface area on food particles for attack by enzymes
Smaller pieces are easier to move
3. Digestion = chemical breakdown (usually by enzymes)
Proteins → amino acids
Triglycerides → glycerol + fatty acids
Polysaccharides → monosaccharides
4. Secretion of water, acids, enzymes, buffers, and ions
5. Absorption of nutrients across the digestive tract epithelium, and eventually into the blood
6. Excretion = the removal of waste (via defecation)
Fig. 24-1, p. 864

3. An overview of specific digestive organ functions
Fig. 24-1, p. 864

4. The peritoneum and mesenteries
We’ve previously learned about the parietal peritoneum, visceral peritoneum (serosa), and peritoneal cavity (in our discussion of serous membranes in Ch. 4 of BIOL& 251)
Mesenteries = double sheets of peritoneal membrane that connect the parietal peritoneum to the visceral peritoneum
Function: stabilize and provide a route for N.A.V.a.L to the abdominopelvic organs
Fig. 24-2bd, p. 866

5. The general histology of the GI tract
Most parts of the tube have most (if not all) of these features
Additional details are found in the textbook and on p. 16 of the Lab Manual
Lumen (interior)
Exterior
Fig. 24-3, p. 867

6. An example of the general histology of the GI tract: the duodenum of the small intestine
Lumen (interior)
Serosa not shown

7. The movement of digestive materials
Fig. 24-4, p. 868
The movement of digestive materials
The muscular layers of the GI tract have gap junctions between cells and pacesetter cells that spontaneously depolarize
They can also be stimulated autonomically by the CNS and ENS (see the next slide)
Peristalsis = waves of rhythmic contractions that propel the “food mass” (= bolus) forward through the tract
Segmentation (not shown here) = irregular contractions in the intestines that mix/churn/fragment the bolus

8. The regulation of digestive activities
These short reflexes are part of the enteric nervous system (ENS), the lesser known third division of the ANS
E.g. prostaglandins and histamine
E.g. gastrin, secretin, etc. We’ll discuss some of the main ones later in the chapter.
Fig. 24-5, p. 869

9. The oral (buccal) cavity
The oral mucosa has stratified squamous epithelium (some parts of it are keratinized)
Fig. 24-6, p. 870

10. The tongue
Functions:
Mechanical processing and manipulation of food for chewing and swallowing
Sensation (taste, touch, and temperature)
Secretion of mucins (= mucus glycoproteins)
Secretion of lingual lipase, which initiates the digestion of fats
Fig. 17-2, p. 554

11. Salivary glands There are 3 pairs:
Parotid salivary glands – their secretion is all serous (serous cells stain darker)
Sublingual salivary glands – their secretion is almost all mucous (mucous cells stain lighter)
Submandibular salivary glands – their secretion is a mix of serous and mucous (so there are both darker and lighter staining areas—shown below)
Fig. 24-7, p. 872

12. Saliva
Functions: moisten and lubricate food, rinse/flush the mouth, dissolve chemicals for taste bud stimulation, initiate the chemical digestion of complex carbos (by salivary amylase)
Composition:
99.4% water; the rest is solutes, which include:
Ions, salivary amylase, buffers (so pH ~ 7.0), waste products, IgA antibodies, lysozyme, mucus (which helps with the lubrication of food)
Secretion:
1-1.5 liters per day!
Parasympathetic stimulation → salivation
Sympathetic stimulation → dry mouth
Secretion is increased by:
1. Food in the mouth, the taste of food, and chewing (even without food in the mouth)
2. The smell, sight, or sound of food
3. Stomach or small intestine irritation (in order to dilute, rinse, or buffer the unpleasant stimulus)

13. Teeth Periodontal ligament = Tooth composition:
Dense CT that, along with cementum, anchors the tooth in its alveolus (bony socket)
Tooth composition:
1. Dentin = the bulk of a tooth
It’s similar to bone (but it’s harder and acellular)
2. The pulp cavity within the crown
Contains pulp (= N.A.V.a.L.), which exits via the root canal and apical foramen
3. Enamel (the hardest biologically- made substance)
= crystalline calcium phosphate
Covers and protects the dentin of the crown
Teeth
Function: mechanical processing
Mastication = chewing
Fig. 24-8a, p. 873

14. Fig. 24-9, p. 875
Types of teeth
Deciduous (1°) teeth = 20; typically appear between 6-24 months of age
Per quadrant: 2 incisors, 1 cuspid (canine), and 2 molars
Permanent (2°) teeth = 32; most appear between 6-12 years of age
Per quadrant: 2 incisors, 1 cuspid (canine), 2 bicuspids (premolars), and 3 molars
Fig. 24-8b, p. 873

15. = a common passageway for solid food, liquids, and air
Has underlying skeletal muscle for swallowing
Has 3 regions:
1. Nasopharynx – is lined with PCCE; contains the pharyngeal tonsil (adenoid)
2. Oropharynx – is lined with stratified squamous epithelium (nonkeratinized); contains the palatine and lingual tonsils
3. Laryngopharynx – has the same epithelium as the oropharynx
The pharynx (throat)
Fig. 24-6a, p. 870

16. The esophagus
= a hollow muscular tube that passes through the diaphragm at the esophageal hiatus, and connects to the stomach
Function: transport solid food and liquids from the pharynx to the stomach
Fig , p. 876

17. Histology of the esophagus
1. The mucosa includes a stratified squamous epithelium (nonkeratinized)
2. The submucosa contains esophageal (mucous) glands for lubrication of the bolus
The mucosa and submucosa are folded, allowing for expansion during swallowing
3. The muscularis externa gradually transitions (moving from superior to inferior) from skeletal muscle to smooth muscle
Muscle tone keeps the upper and lower ends closed (except when a bolus passes through), acting as “sphincters”
These are not true anatomical sphincters (= circular muscular valves) like the pyloric sphincter and ileocecal valve
4. Adventitia (not serosa) = fibrous CT for attachment
Fig , p. 876

18. Swallowing (deglutition)
The entire process takes ~ 9 seconds for a typical bolus
It consists of 3 phases:
1. The buccal phase – the tongue moves the bolus from the oral cavity to the oropharynx (voluntary)
The soft palate and uvula move upward (to seal off the nasopharynx)
2. The pharyngeal phase – involuntary movement of the bolus into the esophagus
The bolus stimulates receptors in the posterior oropharynx, which triggers the swallowing reflex:
The larynx moves up, the epiglottis closes, and there is inhibition of the breathing centers (breathing ceases for less than a second)
3. The esophageal phase – involuntary peristalsis pushes the bolus toward the stomach
Fig , p. 878

19. Stomach terminology
Fundus = the dome-shaped region superior to the esophageal opening
Body = the main central region
Pylorus = the region that connects with the duodenum via the pyloric sphincter
Rugae = folds of mucosa that allow the stomach to stretch
Chyme = a viscous, acidic, soupy mix of partially digested food
Fig b, p. 879

20. Functions of the stomach
Temporary storage of ingested food
Mechanical processing
Chemical digestion via acid and enzymes
Production of intrinsic factor = a glycoprotein needed for the intestinal absorption of vitamin B12
Fig a, p. 879

21. Histology of the stomach lining
1. Mucosa – is folded into gastric pits that open into deeper gastric glands
2. Submucosa
3. Muscularis externa = 3 layers of smooth muscle
An inner oblique layer is present in addition to the circular and longitudinal layers
4. Serosa
Fig a, p. 880

22. The stomach mucosa in the fundus and body
Under the ‘scope:
Mucous cells – light-staining; most located near the lumen
Parietal cells = “fried eggs”; central round nucleus, more numerous in the midregion of the gastric glands
Chief cells = not “fried eggs”; grainy; in clusters with nuclei toward the outer edge of the cluster; more numerous in the deeper region of the gastric glands
The stomach mucosa in the fundus and body
Gastric glands are made up of the following cells, which together secrete gastric juice (~ 1.5 L per day)…
Mucous cells secrete an alkaline mucus (to protect the stomach’s epithelium from the HCl)
Chief cells secrete the inactive proenzyme pepsinogen…
Which is converted by HCl to the active enzyme pepsin, which begins protein digestion
Parietal cells secrete HCl and intrinsic factor
Fig b, p. 893

23. HCl secretion by parietal cells
Keeps the stomach contents at a pH of about
Functions:
Kill most microbes
Denature the proteins (including enzymes) in food
Help break down plant materials and the CT in meat
Help convert inactive pepsinogen to active pepsin
Fig , p. 881

24. The stomach mucosa in the pylorus
Pyloric glands
Consist mostly of mucous cells that secrete an alkaline mucus, which helps neutralize the HCl before chyme enters the duodenum
G cells – secrete gastrin (a hormone), which:
↑ Stomach motility
↑ The secretion of gastric juice
Mucous
Mucous
Fig b, p. 880

25. Regulation of gastric activity
Gastric acid and enzyme secretion can be controlled/regulated by:
1. The CNS
2. Short reflexes via the stomach wall
These are part of the enteric nervous system (ENS)
3. Hormones secreted by the GI tract
There are 3 specific phases of gastric secretion:
1. The cephalic phase
2. The gastric phase
3. The intestinal phase
On the next two slides, for each phase, note the differences with respect to the stimulus (what causes the phase to begin), some of the key responses, and the overall function

26. The 3 phases of gastric secretion (continued on the next slide…)
Fig , p. 884

27. The 3 phases of gastric secretion (continued from the previous slide)
Fig , p. 885
The 3 phases of gastric secretion (continued from the previous slide)
Additional notes:
CCK, GIP, and secretin also target the pancreas; CCK also targets the gallbladder; secretin also targets the liver (more details later)
There are also two central gastric reflexes (described above) that stimulate activity of the small intestine

28. Digestion and absorption in the stomach
A. Limited digestion of proteins to peptides and small polypeptides by pepsin begins in the stomach; why is it limited?
Pepsin works best when the pH of all of the stomach contents reaches ~ 2, which takes a while
Chyme spends a relatively short duration in the stomach
Pepsin only attacks certain peptide bonds (not all of them)
B. The digestion of carbohydrates and lipids continues…
By salivary amylase and lingual lipase, which continue to work for about 1-2 hours after eating (until the pH of the stomach contents < 4.5)
Absorption
No nutrient absorption occurs in the stomach because:
Mucus lines the stomach lumen and blocks the epithelium
No membrane transport proteins for nutrients are present
The epithelium is not very permeable to H2O
Digestion has not yet been completed when chyme leaves the stomach
There is some absorption of alcohol, aspirin, and other lipid-soluble drugs

29. The small intestine
Is the site of most of the chemical digestion and nutrient absorption that occurs in the GI tract
Has 3 segments:
1. Duodenum – has duodenal (Brunner’s or submucosal) glands in the submucosa that secrete an alkaline mucus to help neutralize incoming chyme
2. Jejunum
3. Ileum – has many Peyer’s patches (MALT) that protect the small intestine from the large intestine’s resident bacteria
It connects to the cecum of the large intestine at the ileocecal valve (not shown here)
Fig , p. 886

30. Small intestine histology
The wall of the small intestine is modified to increase surface area for secretion and absorption; there are 3 levels of folding:
1. Plicae circulares (circular folds) = large (macroscopic), transverse, permanent folds
These folds contain submucosa (which is useful for lab slides)
They increase the luminal surface area by 3X
2. Villi = fingerlike projections of mucosa (so they contain lamina propria – not submucosa)
They contain lacteals (= lymphatic capillaries), which…
Absorb fatty acids, and transport them as chylomicrons (lipid protein) to the blood via the lymphatic system
They increase the luminal surface area by X
Fig ab, p. 887

31. Small intestine histology
3. Microvilli = microscopic projections of the epithelial cell membrane that contain cytoplasm (not lamina propria or submucosa)
They form the brush border and contain brush border (digestive) enzymes
They increase the surface area by 20X
So the total increase in surface area (for digestion and absorption) compared to the wall of a simple, unfolded tube is:
3 x 10 x 20 = 600X! (~ 2200 ft2 total)
Fig cd, p. 887

32. Intestinal glands
Are a.k.a. intestinal crypts or crypts of Lieberkuhn, and contain…
Mucous cells
Stem cells that replace the epithelial cells that are shed into the lumen
Some of the shed cells release brush border enzymes into the lumen
Enteroendocrine cells that secrete gastrin, GIP, secretin, and CCK
1.8-2 L per day of intestinal juice is secreted, which consists of…
Water due to osmosis from the mucosa to the relatively concentrated chyme in the lumen, plus the mucus and shed epithelial cells from the intestinal glands
Fig b, p. 887

33. Fig a, p. 889
The pancreas
Is a mixed gland; i.e., it has both exocrine and endocrine functions
Exocrine ducts:
The pancreatic duct (duct of Wirsung) joins the common bile duct and empties into the duodenum at the duodenal papilla
In 3-10% of the population, the pancreatic duct branches to form the accessory pancreatic duct (duct of Santorini), which also opens into the duodenum

34. Pancreatic histology
1. The exocrine portion = acini and ducts (= ~ 99% of the pancreas)
The acini secrete digestive enzymes
Duct cells secrete water and buffers (which help neutralize chyme)
Enzymes + H20 + buffers = pancreatic juice
~ 1 L per day is secreted
2. The endocrine portion = pancreatic islets (islets of Langerhans)
These secrete (mostly) the hormones glucagon and insulin
Fig b, p. 889

35. More on pancreatic juice
Its pH =
Its secretion is controlled by the vagus nerve and duodenal hormones, especially…
Secretin, which is released when chyme enters the duodenum
It targets pancreatic duct cells to secrete a watery buffer solution
CCK, which also is released when chyme (especially chyme that contains lipids and proteins) enters the duodenum
It targets pancreatic acini to produce and secrete pancreatic enzymes
Its enzymes include (see Table 24-3 for much more detail)…
Pancreatic alpha-amylase
Pancreatic lipase
Nucleases
Proteolytic enzymes (= proteases and peptidases)
These are secreted as inactive proenzymes (to protect the cells of the pancreas itself)
E.g. trypsinogen, chymotrypsinogen, procarboxypeptidase, and proelastase are converted into active trypsin, chymotrypsin, carboxypeptidase, and elastase, respectively

36. The liver = the largest visceral organ in the body
Has 4 lobes (the right and left are the largest ones)
The falciform ligament separates the right and left lobes
The round ligament = a thickening in the falciform ligament that is the remnant of the fetal umbilical vein
Hepatocytes = liver cells
Fig , p. 891

37. Liver histology
The liver is divided by CT into roughly six-sided lobules
Portal areas (portal or hepatic triads) at the corners of each lobule contain branches of the:
1. Hepatic portal vein
2. Hepatic artery proper
3. Bile ducts
Hepatocytes surround sinusoids (= large leaky capillaries)
Arterial and portal blood empties into the sinusoids
The sinusoids empty into the central veins
Kupffer cells = fixed macrophages…
Which phagocytize worn out RBCs, bacteria, debris, etc.
Hepatocytes secrete bile into the bile duct system (part of which is shown here in green), which carries bile out of and away from the liver
Fig ab, p. 892

38. More liver histology
Under the ‘scope!
Fig c, p. 892

39. Some functions of the liver
It has over 200 functions! Here are just a few…
1. Metabolic regulation
A. Carbohydrate metabolism (e.g. glycogenolysis, gluconeogenesis, and conversion into lipids)
B. Lipid metabolism (regulation of blood [lipid])
C. Amino acid metabolism (e.g. protein synthesis and conversion into lipids or glucose)
D. Waste removal (e.g. conversion of ammonia → less harmful urea)
E. Vitamin storage (B12 and fat-soluble vitamins A, D, E, and K)
F. Mineral storage (iron)
G. Drug inactivation (by the smooth ER of hepatocytes)
2. Hematological regulation
A. Phagocytosis and antigen presentation (by Kupffer cells)
B. Synthesis of plasma proteins
C. Removal of circulating hormones
D. Removal of circulating antibodies
E. Removal (or storage) of toxins
3. The synthesis and secretion of bile (see the next slide for the functions of bile)

40. Bile About 1 liter is produced per day by hepatocytes Contains:
Mostly water, minor amounts of bile salts (derived from cholesterol), cholesterol, pigments (bilirubin), and ionic buffers
Functions:
A. The emulsification of fats
Bile salts have hydrophilic and -phobic ends, so they surround large fat globules and break them into small lipid droplets in a watery environment
This increases the surface area for digestion by lipases, and aids absorption
B. The excretion of bilirubin
Remember, bilirubin is a pigment derived from the heme of hemoglobin
Bilirubin (liver) → (small intestine to large intestine) → urobilinogens and stercobilinogens → urobilins and stercobilins (excreted in feces)
Note: > 90% of the bile salts in bile are reabsorbed by the ileum and cecum for recycling back to the liver

41. The gallbladder Functions: 1. Bile storage
2. Bile modification – via the reabsorption of water, concentrating the bile
3. Bile release – the presence of lipids in chyme in the duodenum causes the release of CCK, which causes the hepato pancreatic sphincter (sphincter of Oddi) to relax and the gallbladder to contract
Fig , p. 893

42. The main actions of the major digestive tract hormones
Fig , p. 897

43. The large intestine = cecum + colon + rectum Functions:
1. Reabsorption of water, forming feces
2. Absorption of vitamins produced by bacteria (e.g. vitamin K)
3. Storage of feces prior to defecation
Fig. 24-2b, p. 866

44. The large intestine
Fig a, p. 899

45. Fig , p. 899
The cecum and rectum
Cecum = a pouch that is the 1st part of the large intestine
The appendix is attached
It connects with the ileum of the small intestine via the ileocecal valve (which is a sphincter)
Rectum = the last, straight part of the large intestine
Function: temporary storage of feces before defecation
Anal canal = the last part of the rectum
Anus = the exit of the anal canal, which contains
Stratified squamous epithelium
The internal anal sphincter
It’s made of smooth muscle (involuntary)
The external anal sphincter
It’s made of skeletal muscle (voluntary)

46. Histology of the large intestine
No villi are present, and no digestive enzymes are produced
The mucous (goblet) cells of the intestinal glands (crypts) produce mucus for lubrication
Lymphoid nodules are present in the mucosa and submucosa
The outer, longitudinal layer of the muscularis externa has been reduced to 3 distinct bands called taeniae coli
Their muscle tone produces haustra (= pouches)
Fig , p. 901

47. Physiology of the large intestine
1. The large intestine is the site of absorption of:
Water (you lose only ~ 2% of the total water in the GI tract to the feces!)
Ions (electrolytes)
Vitamins produced by normal resident bacteria (e.g. K, biotin, and B5)
Bile salts (by the cecum)
Some urobilinogens (to be excreted in the urine)
2. Movements of the large intestine include:
A. Peristalsis (which is typically slow)
B. Segmentation movements (= haustral churning)
C. Mass movements = powerful peristalsis a few times a day
The stimulus = the stretch of the stomach or duodenum
These move feces from the transverse colon onward into the rectum
D. Defecation (see the next slide)

48. The defecation reflex
Fig p. 902

49. CH2O digestion and absorption
FYI
CH2O digestion and absorption
Glucose is cotransported with Na+ into intestinal epithelial cells
Na+ is pumped out of the cell (by the Na+/K+ exchange pump) into the lumen
Na+ and glucose bind to the carrier at the brush border
Both enter the cell by moving down Na+’s concentration gradient
The Na+ is pumped back out
Fig , p. 904

50. Lipid digestion and absorption
FYI
Lipid digestion and absorption
Bile salts in the small intestine emulsify fats
Triglycerides → monoglycerides + fatty acids via pancreatic lipase
Bile salts then coat monoglycerides and fatty acids to form tiny micelles, which aid absorption
Fig , p. 904

51. Protein digestion and absorption
FYI
Protein digestion and absorption
Is complex and time-consuming
Each protease targets specific types of peptide bonds and breaks polypeptides into smaller peptides
Peptidases break small peptides into individual amino acids
Fig , p. 904

52. The secretion and absorption of water
FYI
The secretion and absorption of water
Total water ingested + secretions = 9200 mL
Water lost in feces = 150 mL
Most absorption of water occurs in the small intestine
Water is absorbed osmotically following the absorption of solutes (nutrients and ions)
See Table 24-2 for more FYI info on the absorption of vitamins and ions
Fig , p. 907