Chapter 24: The Digestive System

Chapter 24: The Digestive System
Primary sources for figures and content: Marieb, E. N. Human Anatomy & Physiology. 6th ed. San Francisco: Pearson Benjamin Cummings, 2004. Martini, F. H. Fundamentals of Anatomy & Physiology. 6th ed. San Francisco: Pearson Benjamin Cummings, 2004.

The organs of the digestive system and their major functions.

Digestive System Responsible for providing raw materials to support life: Food molecules catabolized energy and building blocks to supply anabolic reaction Anabolism = Uses raw materials to synthesize essential compounds Anabolic rxn: Cell division, repair, secretions, etc. Catabolism = Decomposes substances to provide energy cells need to function

Catabolic Reactions Require two essential ingredients: oxygen
organic molecules broken down by intracellular enzymes: e.g., carbohydrates, fats, and proteins

Components of the Digestive System
Figure 24–1

Components of the Digestive System
Alimentary canal / Gastrointestinal Tract (G.I.) Accessory Digestive Organs Oral Cavity Teeth Pharynx Tongue Esophagus Salivary Glands Stomach Liver Small Intestine Gall bladder Large Intestine Pancreas Rectum Anus

Digestive Tract Gastrointestinal (GI) tract or alimentary canal
Is a muscular tube Extends from oral cavity to anus

7 Functions of the Digestive System
Ingestion: occurs when materials enter digestive tract via the mouth Propulsion: - Move food through GI swallowing and peristalsis Mechanical processing: Chewing, churning, mixing, compacting makes materials easier to propel along digestive tract 4. Chemical Digestion: enzymatic chemical breakdown of large food molecules into small organic fragments (building blocks) for absorption by digestive epithelium

7 Functions of the Digestive System
5. Secretion: release of water, acids, enzymes, mucus, salts, and cell waste by epithelium of digestive tract and glandular organs 6. Absorption: movement of organic substrates, electrolytes, vitamins, and water from the gut, into interstitial fluid, lymph, and blood 7. Excretion: removal of cell waste products, secretions, indigestible foodstuffs from the body Defecation of feces

Control of Digestive Function
Digestive activity (gland secretion or lumen movement) is controlled by chemical or mechanical stimuli: Stretching Osmolarity (osmotic concentration) - Measure of solute concentration  osmoles of solute particles per unit volume in solution pH Substrate concentration End product concentration

Control of Digestive Function
1. Neural mechanisms 2. Hormonal mechanisms 3. Local mechanisms

Modes of control can be extrinsic or intrinsic
Neural control Enteric (or intrinsic) Nervous System (ENS): Nerve plexus of the gut (GI) Short reflexes: ENS only “gut brain” Long reflexes: involve input from ANS (in CNS) Higher level control of digestive and glandular activities, control peristaltic waves Hormonal control 18 hormones produced by enteroendocrine cells in GI: target may be same organ or distant organ Specific hormones for specific foods

Modes of control can be extrinsic or intrinsic
Local mechanism - Prostaglandins and histamine can trigger localized secretion based on contents of GI tract

The Regulation of Digestive Activities
Figure 24–5

Peritoneal Cavity Most in peritoneal cavity of abdomen
Is located within the abdominopelvic cavity Include two layers of serosa Cavity  lined with serosa, parietal peritoneum Organs  covered with serosa, visceral peritoneum Both serous membrane lining (parietal and visceral) secrete peritoneal fluid Provides essential lubrication Allows sliding without friction or irritation

Disorders of Peritoneal Cavity
1. Ascites = excess peritoneal fluid  swelling of abdomen and distortion of organs Can cause heart burn, indigestion, back pain 2. Peritonitis = inflammation of peritoneum from damage or infection Can cause pain and organ failure

Mesenteries Sheets of peritoneum are called mesenteries
Mesenteries support a bulk of the digestive system Peritoneal organs = supported by mesenteries Double sheets of peritoneum serous membrane Retroperitoneal organs = anchored to body wall Mesenteries also hold blood vessels, lymphatic vessels, nerves, and adipose  protection, insulation, energy reserve

Functions of Mesenteries
Connect parietal peritoneum with visceral peritoneum Provide an access route to and from the digestive tract for passage of blood vessels, nerves, and lymphatic vessels Stabilize positions of attached organs Prevent intestines from becoming entangled

Special Mesentery Folds
The Lesser Omentum Stabilizes position of stomach Provides access route for blood vessels and other structures entering or leaving liver Falciform Ligament Helps stabilize position of liver The Greater Omentum holds intestines hanging like an apron from lateral and inferior borders of stomach Adipose tissue in greater omentum: pads and protects surfaces of abdomen provides insulation to reduce heat loss stores lipid energy reserves

Mesenteries Figure 24–2a, b

Mesenteries Figure 24–2c, d

Blood Supply Digestive organs receive ¼ cardiac output
This can increase following a meal All venous return from GI enters hepatic portal circulation Delivered to liver Liver processes or absorbs nutrients from gut before blood returns to circulation

What is the importance of the mesenteries?
provides a layer of insulating fat supports and stabilizes organs in abdominopelvic cavity divides abdominal cavity from pelvic cavity all of the above

Which would be more efficient in propelling intestinal contents from one place to another—peristalsis or segmentation? peristalsis segmentation

What effect would a drug that blocks the parasympathetic stimulation of the digestive tract have on peristalsis? increase peristalsis rate decrease peristalsis rate cause irregular peristaltic wave weaken peristaltic contractions

The functional histology of the digestive system.

Structure of the Digestive Tract
4 Major layers along entire length of digestive tract: 1. mucosa 2. submucosa 3. muscularis externa 4. serosa Figure 24–3

1. The Mucosa (mucous membrane)
Is the inner lining of digestive tract Functions to: 1. Secrete mucus, digestive enzymes, and hormones 2. Absorb end products of digestion 3. Provide protection from pathogens

A. Epithelium Continuously renewed, surface cells last 2-6 days Stratified squamous  mechanical stresses Oral cavity, pharynx, esophagus, anus Simple columnar  absorption Stomach, intestines with goblet cells (mucus) and enteroendocrine cells (hormones) B. Lamina propria Loose areolar connective tissue with  blood vessels, lymphatic vessels, nerves, mucous glands lymphoid tissue (extending from submucosa) MALT (mucosa associated lymphatic tissue e.g. Peyer’s patches) and Tonsils C. Muscularis muscosae

C. Muscularis muscosae Bands of smooth muscles and elastic fibers One layer circular, one layer longitudinal Functions to change shape of plicae and villi Villi: finger-like projections on the mucosa layer Increase surface area Plicae (small intestine): Permanent folds of mucosa and submucosa Rugae (stomach): pleats of mucosa and submucosa Expand to accommodate volume

Enteroendocrine Cells
Are scattered among columnar cells of digestive epithelium Secrete hormones that: coordinate activities of the digestive tract and accessory glands

2. The Submucosa Layer of dense irregular connective tissue
Surrounds muscularis mucosae Contains: large blood vessels and lymphatic vessels exocrine glands: secrete digestive enzymes and mucus into digestive tract Houses the submucosal Nerve Plexus Autonomic nervous system control of glands and smooth muscle of mucosa

Submucosal Plexus Also called plexus of Meissner
Innervates the mucosa and submucosa Contains: sensory neurons parasympathetic ganglionic neurons sympathetic postganglionic fibers

3. Muscularis Externa Is dominated by smooth muscle cells
Are arranged in: inner circular layer outer longitudinal layer Functions of smooth muscle Mixing and moving lumenal contents Circular layer thickened to create sphincters at junctions to prevent backflow

3. Muscularis Externa Contains the Myenteric Nerve Plexus (enteric nervous system - ENS): Control G.I. mobility via local reflex arcs and ANS stimulation (mostly parasympathetic)

4. The Serosa or Adventitia
membrane covering muscularis externa Visceral peritoneum Areolar connective tissue plus mesothelium Covers all abdominal/peritoneal G.I. tract organs except Does not cover oral cavity, pharynx, esophagus, and rectum

4. The Serosa or Adventitia
Covers muscularis externa of oral cavity, pharynx, esophagus, and rectum dense irregular connective tissue sheath of collagen fibers Function: Anchors organs to surrounding tissues

How the materials moves through the digestive system.

The Movement of Digestive Materials
Movement from visceral smooth muscle tissue Coordinated by short local reflex arcs of ENS Peristalsis = waves of contraction move food bolus along length of gut Segmentation = single point contractions chop up bolus allow enzymes to access inner regions

Peristalsis ENS also innervated by ANS allowing extrinsic control of digestive activity Parasympathetic = incr. muscle activity + secretion Sympathetic = decr. Muscle activity + secretions Most ANS to the gut is Parasympathetic Figure 24–4

Peristaltic Motion Circular muscles contract behind bolus:
while circular muscles ahead of bolus relax Longitudinal muscles ahead of bolus contract: shortening adjacent segments Wave of contraction in circular muscles: forces bolus forward

Segmentation Cycles of contraction: Does not follow a set pattern:
Churn and fragment bolus mix contents with intestinal secretions Does not follow a set pattern: does not push materials in any 1 direction

Anatomy of the GI by Region.

1. Oral Cavity/Mouth/Buccal Cavity
Figure 24–6

Connects environment to pharynx Lined with stratified squamous epithelium Walls = muscular cheeks Floor contains tongue Roof = hard palate (anterior) soft palate (posterior) close off nasopharynx during swallowing

4 Functions of the Oral Cavity
Analyze food: taste buds Mechanically process food: chew Lubricate food: saliva mixing with mucus and salivary gland secretions 4. Digest starches: amylase Limited digestion of carbohydrates and lipids

Oral Cavity/Mouth/Buccal Cavity Accessory organs of the Mouth
A. Tongue B. Salivary Glands C. Teeth

Accessory organs of the Mouth
A. Tongue Muscular Surface covered by papillae Provide friction, house taste buds Functions Speech Manipulate food into teeth for mastication Compress food in bolus for swallowing Analyze food for texture, taste, and temp. Produce secretion Mucin – lubrication Lingual lipase – start lipid digestion

B. Salivary Glands Produce 1-2 L saliva/day Saliva = 99% water plus Enzymes  amylase for starch digestion Electrolyte buffers Mucin  lubrication Antibodies Antimicrobials  lysozyme and defensins

B. Salivary Glands Functions of saliva Cleanse mouth, control oral bacteria Dissolve food chemicals for taste Moisten food for bolus formation Begin chemical digestion of carbohydrates Buffer oral pH

Three pairs of salivary glands: Parotid salivary glands Sublingual salivary glands Submandibular salivary glands Figure 24–7

B. The Salivary Glands Each pair produces saliva with different properties 1. Parotid salivary gland = 25% of saliva Inferior to zygomatic arch Produce serous secretion: enzyme salivary amylase (breaks down starches) Sublingual salivary gland = 5% of saliva Inferior to tongue Watery secretion, high in buffers Submandibular salivary glands = 70% of saliva Posterior floor of mouth Secrete buffers, glycoproteins (mucins), and salivary amylase

B. Salivary Glands Low levels saliva produced continuously
Parasympathetic stimulate Increase secretion: food cue Sympathetic stimulate Decrease secretion: “dry mouth”

C. Teeth Figure 24–8

C. The Teeth Function Structure: Mastication, mechanical digestion
A. Pulp cavity Soft center that receive blood vessels & nerves in CT from the root canal  called pulp B. Dentin Bone like, surrounds pulp cavity, contains odontoblasts which secrete and maintain dentin throughout life C. External surface Crown Root

C. The Teeth C. External surface 1. Crown Exposed region
Covered in enamel composed of hydroxyapatite Calcium salt crystals, like bone but no collagen Cells that produce enamel degenerate after eruption No repair of enamel 2. Root Embedded region Covered in cementum, attached to peridontal ligaments Hold tooth in alveolus of jaw

C. Teeth Contain 4 types of teeth: incisors cuspids (canines)
bicuspids (premolars) molars Figure 24–9

C. The Teeth Incisors: - Blade-shaped teeth, Located at front of mouth
8 total used for clipping or cutting Have a single root Cuspids/canines/eyeteeth Conical shaped 4 total used for tearing and piercing Single root Bicuspids/Premolars 2 ridges/cusps 8 total used for crushing, mashing, grinding One or two roots

C. The Teeth Under age 12  4. Molars 4-5 ridges/cusps
12 total used for crushing and grinding Three or more roots Under age 12  You have 20 deciduous/milk teeth primary dentition Replaced by age 21  You get 32 permanent teeth secondary dentition

C. The Teeth Disorders of teeth Impacted tooth Fails to erupt
Remains in jaw causing pain Surgical fix Dental caries Demineralization of enamel and dentin by bacteria Exposes nerves  pain Peridontal disease Infection of gingiva (gums) Immune response erodes bone around teeth  teeth fall out

Which type of epithelium lines the oral cavity?
simple squamous epithelium stratified cuboidal epithelium stratified squamous epithelium pseudostratified ciliated columnar epithelium

The digestion of which nutrient would be affected by damage to the parotid salivary glands?
carbohydrates proteins lipids nucleic acids

Which type of tooth is most useful for chopping off bits of relatively rigid foods?
molars bicuspids canines incisors

2. The Pharynx A common passageway for solid food, liquids, and air
Oropharynx = back of mouth Laryngopharynx = superior to voice box Connects oral cavity to esophagus Shared space with respiratory system Stratified squamous epithelium Lamina propria has tonsils and mucus glands Skeletal muscles surround for swallowing

3. The Esophagus Figure 24–10

3. The Esophagus A hollow muscular tube, connects pharynx to stomach
About 25 cm long and 2 cm wide Passes through diaphragm to abdomen At rest superior and inferior regions constricts to keep air out and stomach contents in Inferior constriction point = gastroesophageal sphincter “heartburn” = gastroesophaegel reflux disease (GERD) Gastric juice regurgitates into esophagus causing erosion

3. The Esophagus Features Stratified squamous epithelium
Large folds in mucosa and submucosa to keep lumen closed during rest Esophageal glands in submucosa secrete mucus to reduce friction Upper 2/3 of muscularis externa contains skeletal muscle Adventitia anchors esophagus

3. Esophagus Pharynx and esophagus function in food propulsion from mouth to stomach Deglutitions (swallowing) Buccal phase – voluntary Pharyngeal phase – involuntary reflex Esophageal phase – involuntary reflex

The Deglutition Process
Figure 24–11

The Deglutition Process
Buccal phase – voluntary Soft palate raised to protect nasopharynx Bolus pushed against hard palate Bolus pushed into oropharynx Pharyngeal phase – involuntary reflex Epiglottis blocks entry to trachea Contractions move bolus past glottis Esophageal phase – involuntary reflex Peristaltic waves push bolus toward stomach Gastroesophageal sphincter opens Bolus enters stomach

What is occurring when the soft palate and larynx elevate and the glottis closes?
swallowing hiccupping speaking coughing

4. Stomach Functions: Storage of ingested food (~1 L)
Mechanical breakdown of ingested food - churning Chemical breakdown of food denature and digest proteins - disrupt chemical bonds in food by acids and enzymes Produce intrinsic factor for Vitamin B12 uptake VitB12 necessary for erythropoiesis glycoprotein required for absorption of vitamin B12 in small intestine

4. The Stomach Chyme: typically 1L but up to 4 L
Stomach holds acidic mixture of enzymes and food called Chyme: typically 1L but up to 4 L Figure 24–12a

1. Cardia, 2. fundus, 3. body, 4. pylorus
4. The Stomach 4 regions: 1. Cardia, 2. fundus, 3. body, 4. pylorus Figure 24–12b

4. The Stomach: Four major regions
Cardia Where esophagus connects via gastroesophageal sphincter Gastric glands produce mucus to protect esophagus Fundus - Superior region, contact diaphragm Body Majority of stomach Holds chyme Gastric glands secrete enzymes and acids for digestion Pylorus

4. The Stomach: Four major regions
4. Pylorus Inferior region Connects to duodenum via pyloric sphincter Regulates chyme entry into duodenum Gastric glands secrete hormones to stimulate gastric activity

4. The Stomach: Features Figure 24–13

4. The Stomach Features: Muscularis externa has 3 layers (+oblique)
assist mixing chyme When empty mucosa and submucosa folds into rugae Can expand for large volume Mucosa has simple columnar epithelium with goblet cells that secrete alkaline mucus Mucosa perforated by deep gastric pits which connect to gastric glands in lamina propria Stem cells in gastric pits replace stomach epithelium every 3-6 days

4. The Stomach Gastric Glands
Produce 1-3L gastric juice/day Secretions vary per region Cardia gastric glands = mucus Fundus and Body gastric glands = digestive enzymes and acids Two types of gastric cells parietal cells & chief cells Pyloric gastric glands = mucus and hormones 2 important hormone producing cells G cells & D cells

2. Fundus and Body gastric glands Two types of gastric cells 1. Parietal cell secretions Intrinsic factor (VitB12 uptake) H+ and CL- ions combine to make HCL in stomach 2. Chief cell secretions 1. Pepsinogen  converted to pepsin by acid in stomach; hydrolyzes proteins 2. Rennin  infants only, curdles mile protein to aid digestion

Fundus and Body gastric glands Acid production important to gastric function Kill microbes Denature proteins - digestion, destroy enzymes in food Break down plant cell walls and animal CT Activate pepsin

The Secretion of Hydrochloric Acid
Figure 24–14

Pyloric gastric glands = mucus and hormones 2 important enteroendocrine cells G cells = produce gastrin hormone Stimulates secretion by parietal and chief cells Promote contraction of gastric wall Secreted in response to food or parasympathetic stimulation D cells = produce somatostatin hormone Inhibits release of gastrin Thus inhibits gastric activity Secreted in response to sympathetic stimulation

4. The Stomach Disorders Gastritis Inflammation of gastric mucosa
Caused by drugs, stress, infection Chronic can lead to ulcer Peptic Ulcer Erosion of stomach lining Caused by Too much acid & Not enough mucus Most common: Helicobacter pylori (bacteria)

4. The Stomach Regulation of Gastric Activity
Secretion and motility controlled by 3 factors Innervations from CNS (ANS) Reflexes of the ENS Hormones Mechanisms rely on stimuli from three regions - Head, stomach, and small intestines 3 phase of regulation Cephalic, Gastric, and Intestinal phases - May all act simultaneously to alter gastric activity

The Phases of Gastric Secretion 1. Cephalic 2. Gastric 3. Intestinal
Figure 24–15

The Cephalic Phase Prepares the stomach for food Triggered by seeing, smelling, or thinking of food Lasts a few minutes Neural response: Parasympathetic ANS triggers increase in all gastric secretions Mucus, enzymes, and acid Triggers G cells to release Gastrin Causes secretion and motility

Gastric Phase Initiates stomach digestive activities Triggered by food entering the stomach Stimuli = distension, peptides, low acidity Lasts 3-4 hours  Three responses 1. Neural response 2. Hormonal response 3. Local response

Gastric Phase – 3 responses
1. Neural response Stretch receptors activate ENS reflexes and parasympathetic ANS innervation Both stimulate secretions from parietal cells (acid), Chief cells (pepsin) and G cells (Gastrin) 2. Hormonal response Triggered by neural responses, peptides, increased pH G cells release Gastrin which trigger secretions by parietal and chief cells Gastric mobility 3. Local response Triggered by distortion Mast cells release histamine  stimulates parietal cells

Intestinal Phase Controls chyme entry into duodenum Last many hours Involves excititory and inhibitory control of gastric activity depending on chyme composition Two Responses Neural Response Stretch receptors trigger enterogastric reflex turns off ENS and parasympathetic stimulation of G cells Stimulates sympathetic stimulation of pyloric sphincter (contraction occurs) Hormonal Response

Intestinal Phase – Hormonal Response
Different hormones depending on chyme composition A ) Lipids, carbohydrates, peptides Cholecystokinin and Gastric Inhibitory Peptide  Inhibit gastric secretion and motility Stimulates pancreas + gallbladder secretion Low pH Secretin  inhibits gastric secretion Stimulates pancreas and liver secretions Proteins Intestinal Gastrin  Stimulates parietal and chief cells Stimulates gastric mobility

The Phases of Gastric Secretion
Table 24–1

4. The Stomach Much digestion occurs in the stomach but not much absorption Except alcohol and drugs Food does not usually remain in the stomach for more than 4 hours but total time depends on the chemical makeup of food How long it will take to digest in the small intestines Carbohydrate rich: pass quickly Fatty foods can cause chyme to remain in the stomach for 6+ hrs

How would a large meal affect the pH of blood that leaves the stomach?
pH is higher pH is lower pH fluctuates depending on what was eaten pH remains stable regardless of digestive activity

When a person suffers from chronic ulcers in the stomach, the branches of the vagus nerve that serve the stomach are sometimes severed. Why? The vagus nerve carries gastric pain sensations. The vagus nerve stimulates gastric motility. The vagus nerve stimulates gastric secretions. The vagus nerve causes ulcers.

5. Small Intestines Major digestive organ Chemical digestion completed
90% of nutrients absorbed Remaining absorbed in large intestine 20 ft long, 3 major subdivisions based on histology (No clear anatomical divisions) Duodenum Jejunum Ileum

Segments of the Intestine
Figure 24–16

5. Small Intestines The Duodenum
First 10 inches Retroperitoneal (behind the peritoneum) Receives chyme from stomach through pyloric sphincter Receives digestive secretions from pancreas and liver through duodenal ampula controlled by hepatopancreatic sphincter Mixing pot

5. Small Intestines The Jejunum and Ileum
B) Jejenum Peritoneal Is the middle segment of small intestine  2.5 meters (8.2 ft) long Is the location of most: chemical digestion nutrient absorption C) Ileum - Peritoneal - last 12 ft - Mucosa rich in lymphoid tissue - Connects to cecum at ileocecal valve

The Intestinal Wall Figure 24–17

5. Small Intestines Histology
Same 4 layers, but adapted for absorption 1. Plicae Mucosa and submucosa folded into circular plicae Cause chyme to spiral slowly 2. Villi Plicae covered with finger-like projections of mucosa called intestinal villi Base of each has crypt/intestinal gland 3. Microvilli Simple columnar epithelial cells have microvilli on apical surface Membrane called brush border of intestines

5. Small Intestines Histology
Plicae + villi + microvilli = 2200ft2 surface area (compare to 3.6ft2 for flat wall) Lamina propria of each villus contains capillaries Capillaries carry small nutrient molecules to liver via hepatic portal vein Larger molecules that cannot enter capillaries (lipid-protein complexes) are collected by special lymphatic capillary called a lacteal Contractions of muscularis mucosae Move villi to expose surface to new chyme Squeeze lacteal to move lymph

5. Small Intestines Glands of small intestine
Goblet cells Between columnar epithelial cells Secret mucus (mucin) 2. Intestinal glands Variety of cells in the crypts Located in the lamina propria at the base of each villus Produce many products Intestinal juice, lysozyme, hormones, epithelial cells

Intestinal Glands Intestinal juice (1-2L/day)
Watery mucus, aids solubilization and absorption of nutrients 2. Lysozyme From Paneth cells, lyse bacteria 3. Hormones  from Enteroendocrine cells All control GI activity Intestinal gastrin, Cholecystokinin, Secretin, Gastric inhibitory peptide, Vasoactive intestinal peptide, Somatostatin, Enterocrinin 4. Epithelial cells Stem cells in glands

Intestinal Glands New cells are created from epithelial stem cells
Cells migrate up villus Shed at tip Complete turnover 3-6 days Shed cells carry digestive enzymes in plasma membrane that function in lumen Brush border enzyme  complete digestion of carbohydrates and proteins

Specializations in Small Intestines
Duodenum has duodenal glands in submucosa Produce mucus to protect against acidic chyme from stomach Number and size of crypts decreases along the length of the small intestines Number of goblet cells increase along length of small intestines Jejunum has the most plicae and villi Ileum has aggregated lymphoid nodules called Peyer’s Patches for immune defense

Small Intestines Secretions
~ 2 L intestinal gland secretions/day Secretion begins before chyme enters due to parasympathetic stimulation When chyme is present Stretch receptors and enterocrinin stimulate secretions of mucus, hormones and juice No digestive enzymes in liquid secretions Sympathetic stimulation inhibits secretion Stress  ulcers, no mucus

Small Intestinal Movements
Myenteric reflexes (ENS) - Peristalsis to move chyme slowly through the small intestine Parasympathetic reflexes (stretch receptors)  accelerate movement Gastroenteric reflex Stimulates motility and secretion along whole small intestine Gastroileal reflex Relaxes ileocecal valve Materials pass from ileum to cecum (large intestines)

Small Intestinal Movements
Irritation to GI triggers reflexes to empty quickly Emesis/Vomiting reflex Controlled by emetic center of medulla oblongata Pyloric sphincter relaxes - Contents of duodenum and upper jejunum discharged into stomach Salivary secretion enhanced - Buffer stomach acid Soft palate rises to close off nasopharynx Diaphragm and abdominal wall muscles contract - Stomach contents are regurgitated

6. The Pancreas Figure 24–18

6. Pancreas Retroperitoneal Inferior to stomach Exocrine and endocrine
Pancreatic islets (endocrine) 1% - Cells secrete insulin (increase storage of glucose  decreases blood sugar levels) and glucagon (increase blood sugar levels) to control blood sugar Pancreatic acini Acinar cells = simple cuboidal epithelium Produce digestive enzymes and buffers  pancreatic juice

6. The Pancreas Pancreatic juice is released into pancreatic ducts
Pancreatic ducts joins with the common bile duct Common bile duct and Pancreatic duct enters duodenum at duodenal ampula This entry is controlled by the hepatopancreatic sphincter

6. The Pancreas Pancreatic Juice
~1.5L/day in response to parasympathetic and hormonal control Water + electrolytes (buffer) Hormonal control from duodenum Secretin Released in response to acid chyme triggers pancreas to secrete bicarbonate and phosphate buffer Cholecystokinin Released in response to lipids and peptides in chyme OR parasympathetic stimulation Triggers pancreatic enzyme secretion

6. The Pancreas Pancreatic Enzymes
~70% secreted as proenzymes, activated in the gut Pancreatic alpha-amylase - Hydrolyzes starch Pancreatic lipase - Hydrolyzes lipids and fatty acids Nucleases - Hydrolyzes nucleic acids (RNA, DNA) Proteolytic enzymes  MAJORITY

6. The Pancreas Pancreatic Enzymes 4. Proteolytic enzymes  MAJORITY
Many, each digests specific peptide bonds 2 main classes Proteases = hydrolyze large proteins into peptides Peptidases = hydrolyze peptide chains into amino acids All proteolytic enzymes are secreted inactive MUST be activated in the gut, this prevents autolysis Enterokinase (Brush border enzyme) Activates pancreatic trypsinogen  trypsin Trypsin  Activates all other pancreatic proteolytic pro-enzymes via cleavage

Disorders of the Pancreas
Pancreatitis Inflammation of the pancreas Inflammation  blocked ducts  injury of acinar cells Necrotic cells release lysosome enzymes  activate pro-enzymes  autolysis Diabetes mellitus Destruction of Islet cells Cause  pancreatitis or autoimmune attack Result  loss of regulation of blood sugar levels

How is the small intestine adapted for the absorption of nutrients?
with plicae circularis with villi with microvilli all of the above

How would a meal that is high in fat affect the level of cholecystokinin in the blood?
It would increase the level of cholecystokinin. It would decrease the level of cholecystokinin. It would cause rapid fluctuations in the level of cholecystokinin. It would have no effect.

How would the pH of intestinal contents be affected if the small intestine did not secrete the hormone secretin? pH would be higher than normal. pH would be lower than normal. No changes in pH would result. Unpredictable changes would result.

The digestion of which nutrient would be most impaired by damage to the exocrine pancreas?
proteins carbohydrates fats nucleic acids

7. The Liver Figure 24–19

7. The Liver Is the largest visceral organ (1.5 kg)
Right side, inferior to diaphragm Four lobes Anterior: large right and left lobes Separated by falciform ligament = fetal umbilical vein Posterior: tiny caudate lobe and quadrate lobe Lobules = function units

Liver Lobule

7. The Liver Lobule: hexagonal functional units
Separated by interlobular septa Central vein in the middle Six portal triads on hexagonal corners Hepatic Artery - Oxygen rich blood Hepatic portal vein - Nutrient rich blood from small intestine Bile duct Collect bile produced by hepatocytes

7. The Liver Lobule: hexagonal functional units Hepatocytes
Function in nutrient regulation and bile production Arranged in rows around central veins with sinusoids between rows Blood flow in lobule Blood from arteries and veins of triads flows through sinusoids Allows molecule exchange with hepatocytes Blood flows out through the central vein

7. The Liver Kupffer Cells
Sinusoids also contain Kupffer cells (fixed macrophages) Functions Remove pathogens, cell debris, damaged erythrocytes Remove and store iron Remove and store lipids Remove and store heavy metals

7. The Liver Bile Flow Bile secreted by hepatocytes 
Flows through bile canaliculi between cells Canaliculi  Bile ducts of triads  merge into common hepatic duct  Common hepatic duct exits the liver  joins cystic duct from gallbladder creating common bile duct  Common bile duct connects to duodenum at duodenal ampula, controlled by hepatopancreatic sphincter

Bile Flow

3 Functions of the Liver Metabolic regulation Hematological regulation
Digestive: Bile production and secretion

1. Metabolic Regulation Hepatocytes regulate blood nutrient levels
Nutrient rich blood from GI goes to liver  excess can be removed and deficits can be corrected E.g. Carbohydrates metabolism Too much glucose  hepatocytes store glucose an glycogen Too little  hepatocytes break down glycogen or perform gluconeogenesis (synthesis from non-carb) to release glucose Hepatocytes also carry out: Lipid and amino acid metabolism Vitamin and mineral storage Waste removal, Drug and toxin inactivation

Composition of Circulating Blood
All blood leaving absorptive surfaces of digestive tract: enters hepatic portal system flows into the liver Liver cells extract nutrients or toxins from blood: before it reaches systemic circulation through hepatic veins Liver removes and stores excess nutrients: corrects nutrient deficiencies by mobilizing stored reserves or performing synthetic activities

2. Hematological Regulation
Liver serves as blood reservoir Kupffer cells recycle RBCs perform antigen presentation to lymphocytes Hepatocytes remove/recycle hormones remove antibodies Hepatocytes and Kupffer cells remove, inactivate, or store toxins, drugs, and heavy metals Hepatocytes produce plasma proteins

3. Digestion: Bile Bile components:
Water, bilirubin, phospholipids, electrolytes, cholesterol, bile salts (lipids) Bile functions: Buffer chyme with electrolytes Emulsify fats Break large lipid globs into small droplets of phospholipids and bile salts Functions of emulsification Allow enzymes to access lipids Promote solubilization and absorption of lipids in blood and lymph

3. Digestion: Bile Enterohepatic circulation of bile
Bile salts absorbed in lipid droplets Bile salts recycled back to liver Bile salts are not metabolized Secretin from duodenum Promote secretion of bile from the liver

Disorders of the Liver Hepatitis Inflammation of liver
Cause  viral infection Result  restricts blood flow to liver Six known viruses with different severity 2. Cirrhosis Chronic inflammation Cause  severe hepatitis or alcoholism Result  damaged hepatocytes are replaced by fibrous tissue and adipose Results in portal hypertension  veins swell and rupture

8. The Gallbladder Figure 24–21

8. The Gallbladder Gall stones Anterior and inferior to liver
Functions: 1. concentrate 2. and store bile produced by liver Cholecystokinin from duodenum causes: 1. release of bile by stimulating contraction of gallbladder 2. relaxation of hepatopancreatic sphincter Gall stones Crystalizations of over concentrated bile

Disorders of the Gallbladder
1. Gall stones Crystalizations of over concentrated bile 2. Cholecystitis Inflammation caused by large gall stones that block or damage the gallbladder

Coordination of Secretion and Absorption in the Small Intestine
Neural and hormonal mechanisms coordinate activities of digestive glands 1. Neural Mechanism A. ANS Parasympathetic = increase digestive activity Sympathetic = decrease digestive activity B. ENS reflexes Coordinates movement of materials from one region to the next

Hormonal Mechanism - Hormones from intestinal glands of duodenum control small intestine, stomach, and accessory organs to coordinate digestive activities A. Enterocrinin Released when chyme enters duodenum Stimulates mucus production in duodenum B. Intestinal Gastrin Released when chyme contains proteins Stimulates gastric activity (secretion and motility)

Hormonal Mechanism C. Gastric Inhibitory Peptide Released when chyme contains lipids and carbohydrates Inhibits gastric activity D. Secretin Released when chyme is acidic Stimulates release of bile from liver and buffers from pancreas Reduces gastric activity

Hormonal Mechanism E. Cholecystokinin Released when chyme contains lipids and peptides, stimulates: Secretion of enzyme from pancreas Contraction of gallbladder for bile release Relaxes hepatopancreatic sphincter to allow entry of bile and enzymes into duodenum Inhibits gastric activity Reduces hunger sensation 20 min post food consumptions

Hormonal Mechanism F. Vasoactive Intestinal Peptide Released when chyme enters duodenum Inhibits gastric secretion Stimulates intestinal secretion Dilates local capillaries for absorption G. Somatostatin Released in response to sympathetic stimulation Inhibits: 1. Gastric activity 2. Secretion from pancreas and gallbladder 3. Blood flow to intestine thus inhibiting absorption

Hormones of Duodenal Enteroendocrine Cells
Coordinate digestive functions: Enterocrinin Intestinal gastrin Gastric inhibitory peptide (GIP) Secretin Cholecystokinin (CCK) Vasoactive intestinal peptide (VIP) Somatostatin

Activities of Major Digestive Tract Hormones
Figure 24–22

9. The Large Intestine Figure 24–23

9. The Large Intestine ~ 5 feet long Functions:
Less than 10% of nutrient absorption Prepares fecal material for ejection from the body Functions: Reabsorb any remaining water and compact feces Absorb vitamins and electrolytes Storage of feces and defection No digestion, except by microbes Water absorption important to feces consistency Too much water = diarrhea Too little water = constipation

9. Large Intestines Bacteria of Large Intestines
~ 2 lb bacteria in gut Some produce vitamins: Vitamin K  clotting factor synthesis Biotin  glucose metabolism Vitamin B5  steroid hormone and neurotransmitter synthesis Bacterial metabolism produces characteristics of feces Bilirubin  urobilins and stercobilins = brown Fermentation of organics  ammonia, indole H2S = odor Fermentation of carbs  methane, CO2 = flatus You will produce ~ 500ml flatus/day

9. Large Intestines Histology Specialization
Muscularis externa made up of three longitudinal bands of smooth muscle called  taeniae coli Contraction of taeniae coli produces pouches called  haustra Mucosa has deep crypts with intestinal glands that produce mucus No plicae and vili Lamina propria has large lymphoid nodules Epithelium is simple columnar with abundance of goblet cells

9. The Large Intestine Figure 24–23

3 Parts of the Large Intestine
Cecum: the pouchlike first portion Colon: the largest portion Rectum: the last 15 cm of digestive tract

9. The Large Intestine Subdivision
A) Cecum Attached to ileum via ileocecal valve Function  begin compaction of feces Appendix on side Has lymphoid nodules that are part of the MALT Appendicitis Blockage of appendix Results in bacterial growth causing inflammation and swelling Rupture will release bacteria into abdomen  peritonitis  sepsis  death

B) Colon Absorbs water, vitamins, electrolytes Four major regions Ascending colon Transverse colon Descending colon Sigmoid colon Diverticulitis Pockets form in the colon wall Site of recurrent inflammation and infection Due to lack of fiber

The Colon Figure 24–24

C) Rectum Stores feces Has 3 valves to separate feces and flatus Exits at anal canal Lined with stratified squamous epithelium Defecation controlled by two sphincters: Internal anal sphincter – Involuntary control - Smooth muscle from muscularis externa 2. External anal sphincter – Voluntary control - Skeletal muscle under voluntary control

Defecation Reflex Figure 24–25

9. Large Intestine Large Intestine Movements Haustral contractions
Mass movements Defecation reflex

9. Large Intestine Large Intestine Movements Haustral contractions
local, slow segmenting contractions Shift faces for water absorption Mass movements Powerful peristaltic contractions Force feces toward rectum Occur few times/day Can trigger defecation reflex via stretch receptors in rectum

9. Large Intestine Large Intestine Movements 3. Defecation reflex
Stretch receptors parasympathetic stimulation  1. Contraction of sigmoid colon 2. Relaxation of internal anal sphincter Voluntary control of external anal sphincter controls timing of defecation Time in colon controls water absorption - Movement too fast  too much water in feces = diarrhea - Movement too slow  too little water in feces = constipation

Chemical Digestion Large molecules catabolized into monomers
Monomers absorbed by mucosa Enzymatic breakdown = hydrolysis

Digestive Secretion and Absorption
Figure 24–27

Carbohydrate Digestion and Absorption
Digestion Method Amylases (saliva, pancreas) Polysacchardes  di- and trisaccharides Brush Border Enzymes (small intestine) di- and trisaccharides  monosaccharides Absorption Method Facilitated diffusion OR Cotransport of monosaccharides Cellulose not digest: “bulk” fiber, aids intestinal mobility

Carbohydrate Digestion and Absorption
Lactose intolerant Fail to produce lactase = brush border enzyme Undigested lactose creates osmotic gradient that causes feces to remain hydrated (diarrhea) Bacteria ferment lactose producing flatus

Lipid Digestion and Absorption
Digestion Method Bile salts (liver)  emulsification Lipases (tongue, pancreas) Triglycerides  monoglycerides and fatty acids Absorption Method Micelles form  monoglycerides, fatty acids and bile salts Micelles absorbed by intestinal epithelium Proteins added = chylomicron (water soluble) Chylomicrons exocytosed into lumen Chylomicrons absorbed by lacteal

Protein Digestion and Absorption
Digestion Method Mastication (mouth) Churning (stomach) Pepsin + Acid (stomach): protein  polypeptide Protease + Peptidases (pancreas, brush border) Polypeptide  amino acids Absorption Method Facilitated diffusion OR Cotransport of amino acids

Summary: Chemical Events in Digestion
Figure 24–26

Nucleic Acid Digestion and Absorption
Digestion Method Nucleases (pancreas) Nucleic acid  nucleotides Brush Border Enzymes (small intestine) Nucleotides  nitrogenous bases + sugar + phosphate ions Absorption Method Active transport of nitrogenous bases + sugar + phosphate ions

Water Digestion and Absorption
Digestion Method No digestion required 2L from food, 7L from secretions Absorption Method Osmosis (95% in small intestine) ~ 150ml lost in feces

Ions Digestion and Absorption
Ions from food and secretions Must be regulated for homeostasis Control osmosis and pH, needed for enzyme function Digestion Method No digestion required Na+, Ca++, K+, Mg++, Fe++, Cl-, I-, HCO3- Absorption Method Diffusion Cotransport Active Transport

Vitamins Digestion and Absorption
Digestion Method No digestion required Fat soluble: A, D, E, K Water soluble: most B vitamins, C Vitamins B12 Absorption Method Fat soluble: Mixed with fats in micelle  chylomicrons Water soluble B and C: Diffusion Water soluble B12: Bound to intrinsic factor  binds receptors  endocytosed

Absorption of Ions and Vitamins
Table 24–4

Aging on the Digestive System
Epithelium division rates decline - ulcers more likely Smooth muscle tone decrease Constipation from slow movement Diverticulitis and hemorrhoids from weak walls Cumulative damage Teeth  wear Liver  toxin, lipid build up Increased cancer rate

What component of food would increase the number of chylomicrons in the lacteals?
electrolytes carbohydrates protein fat

The absorption of which vitamin would be impaired by the removal of the stomach?
vitamin D vitamin C vitamin B12 vitamin A

Why is diarrhea potentially life threatening, but constipation is not?
Constipation is easier to treat. Constipation affects only the very young or the elderly. Diarrhea causes edema. Diarrhea causes rapid water and electrolyte loss.

SUMMARY Digestive system: digestive tract accessory organs
Digestive system functions: ingestion mechanical processing digestion secretion absorption excretion

SUMMARY Oral cavity Buccal cavity: oral mucosa Tongue:
intrinsic tongue muscles extrinsic tongue muscles Salivary glands Teeth Pharynx Esophagus Stomach: cephalic phase gastric phase intestinal phase

SUMMARY Small intestine: gastroenteric reflex gastroileal reflex
Pancreas Liver: bile Gallbladder Intestinal hormones: secretin cholecystokinin (CCK) gastric inhibitory peptide (GIP) vasoactive intestinal peptide (VIP) gastrin enterocrinin

SUMMARY Large intestine: cecum colon Rectum
Processing and absorption of nutrients Carbohydrate digestion and absorption Lipid digestion and absorption Protein digestion and absorption Water absorption Ion absorption Vitamin absorption

Chapter 24: The Digestive System

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Chapter 24: The Digestive System

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