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Ch. 24 – The Digestive System

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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

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