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:1. oxygen2. 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 1. Ingestion:

– occurs when materials enter digestive tract via the mouth2. Propulsion:

- Move food through GI swallowing and peristalsis3. 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:1. Stretching2. Osmolarity (osmotic concentration)

- Measure of solute concentration osmoles of solute particles per unit volume in solution

3. pH4. Substrate concentration5. End product concentration

Control of Digestive Function

1. Neural mechanisms2. Hormonal mechanisms3. Local mechanisms

Modes of control can be extrinsic or intrinsic

1. Neural control- Enteric (or intrinsic) Nervous System (ENS):

- Nerve plexus of the gut (GI)A. Short reflexes: ENS only “gut brain”B. Long reflexes: involve input from ANS (in CNS)

• Higher level control of digestive and glandular activities, control peristaltic waves

2. 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

3. 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

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

Disorders of Peritoneal Cavity

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?

A. provides a layer of insulating fat

B. supports and stabilizes organs in abdominopelvic cavity

C. divides abdominal cavity from pelvic cavity

D. all of the above

Which would be more efficient in propelling intestinal contents from one

place to another—peristalsis or segmentation?

A. peristalsis

B. segmentation

What effect would a drug that blocks the parasympathetic stimulation of the

digestive tract have on peristalsis?

A. increase peristalsis rate

B. decrease peristalsis rate

C. cause irregular peristaltic wave

D. weaken peristaltic contractions

The functional histology of

the digestive system.

Structure of the Digestive Tract

Figure 24–3

• 4 Major layers along entire length of digestive tract:1. mucosa2. submucosa3. muscularis

externa4. serosa

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 digestion3. 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

1. The Mucosa (mucous membrane)

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 areaPlicae (small intestine): - Permanent folds of mucosa and submucosa- Increase surface areaRugae (stomach): - pleats of mucosa and submucosa- Expand to accommodate volume

1. The Mucosa (mucous membrane)

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

• Serosa– 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

• 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

4. The Serosa or Adventitia

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

Figure 24–4

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

Peristaltic Motion

1. Circular muscles contract behind bolus:

– while circular muscles ahead of bolus relax

2. Longitudinal muscles ahead of bolus contract:

– shortening adjacent segments

3. Wave of contraction in circular muscles:

– forces bolus forward

Segmentation

• Cycles of contraction:– 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

1. Oral Cavity/Mouth/Buccal Cavity

• 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

1. Analyze food: taste buds2. Mechanically process food: chew3. Lubricate food: saliva

– mixing with mucus and salivary gland secretions

4. Digest starches: amylase– Limited digestion of carbohydrates and

lipids

A. TongueB. Salivary GlandsC. Teeth

1. Oral Cavity/Mouth/Buccal Cavity

Accessory organs of the Mouth

Accessory organs of the Mouth

A. Tongue- Muscular- Surface covered by papillae

- Provide friction, house taste buds- Functions

1. Speech2. Manipulate food into teeth for mastication3. Compress food in bolus for swallowing4. Analyze food for texture, taste, and temp.5. Produce secretion

1. Mucin – lubrication2. 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

Accessory organs of the Mouth

B. Salivary Glands- Functions of saliva

1. Cleanse mouth, control oral bacteria2. Dissolve food chemicals for taste3. Moisten food for bolus formation4. Begin chemical digestion of

carbohydrates5. Buffer oral pH

Accessory organs of the Mouth

Figure 24–7

Three pairs of salivary glands:1.Parotid salivary glands2.Sublingual salivary glands3.Submandibular salivary glands

1. Oral Cavity/Mouth/Buccal Cavity

• Each pair produces saliva with different properties1. Parotid salivary gland = 25% of saliva

• Inferior to zygomatic arch•Produce serous secretion:

– enzyme salivary amylase (breaks down starches) 2.Sublingual salivary gland = 5% of saliva

- Inferior to tongue- Watery secretion, high in buffers

3.Submandibular salivary glands = 70% of saliva- Posterior floor of mouth- Secrete buffers, glycoproteins (mucins), and salivary amylase

B. The Salivary Glands

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

1. Oral Cavity/Mouth/Buccal Cavity

C. The Teeth• Function

– Mastication, mechanical digestion

• Structure: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 TeethC. 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 enamel2. Root

– Embedded region– Covered in cementum, attached to peridontal ligaments– Hold tooth in alveolus of jaw

C. Teeth

Figure 24–9

• Contain 4 types of teeth:

1. incisors2. cuspids (canines)3. bicuspids (premolars)4. molars

1. Incisors:- Blade-shaped teeth, Located at front of mouth– 8 total used for clipping or cutting– Have a single root

2. Cuspids/canines/eyeteeth- Conical shaped- 4 total used for tearing and piercing- Single root

3. Bicuspids/Premolars- 2 ridges/cusps- 8 total used for crushing, mashing, grinding- One or two roots

C. The Teeth

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

1. Impacted tooth- Fails to erupt- Remains in jaw causing pain- Surgical fix

2. Dental caries- Demineralization of enamel and dentin by bacteria- Exposes nerves pain

3. Peridontal disease- Infection of gingiva (gums)- Immune response erodes bone around teeth

teeth fall out

C. The Teeth

Which type of epithelium lines the oral cavity?

A. simple squamous epithelium

B. stratified cuboidal epithelium

C. stratified squamous epithelium

D. pseudostratified ciliated columnar epithelium

The digestion of which nutrient would be affected by damage to the parotid

salivary glands?

A. carbohydrates

B. proteins

C. lipids

D. nucleic acids

Which type of tooth is most useful for chopping off bits of relatively

rigid foods?

A. molars

B. bicuspids

C. canines

D. 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

• Features1. Stratified squamous epithelium2. Large folds in mucosa and submucosa

to keep lumen closed during rest3. Esophageal glands in submucosa

secrete mucus to reduce friction4. Upper 2/3 of muscularis externa

contains skeletal muscle5. Adventitia anchors esophagus

3. Esophagus

• Pharynx and esophagus function in food propulsion from mouth to stomach

• Deglutitions (swallowing)1. Buccal phase – voluntary2. Pharyngeal phase – involuntary reflex3. Esophageal phase – involuntary reflex

The Deglutition Process

Figure 24–11

1. Buccal phase – voluntary- Soft palate raised to protect nasopharynx- Bolus pushed against hard palate- Bolus pushed into oropharynx

2. Pharyngeal phase – involuntary reflex- Epiglottis blocks entry to trachea- Contractions move bolus past glottis

3. Esophageal phase – involuntary reflex- Peristaltic waves push bolus toward stomach- Gastroesophageal sphincter opens- Bolus enters stomach

The Deglutition Process

What is occurring when the soft palate and larynx elevate and the glottis

closes?

A. swallowing

B. hiccupping

C. speaking

D. coughing

4. StomachFunctions:1. Storage of ingested food (~1 L)2. Mechanical breakdown of ingested food

- churning3. Chemical breakdown of food

- denature and digest proteins- disrupt chemical bonds in food by acids and enzymes

4. Produce intrinsic factor for Vitamin B12 uptake- VitB12 necessary for erythropoiesis

- glycoprotein required for absorption of vitamin B12 in small intestine

4. The Stomach

Figure 24–12a

Stomach holds acidic mixture of enzymes and food called Chyme: typically 1L but up to 4 L

4. The Stomach

Figure 24–12b

4 regions: 1. Cardia, 2. fundus, 3. body, 4. pylorus

4. The Stomach: Four major regions

1. Cardia- Where esophagus connects via gastroesophageal

sphincter- Gastric glands produce mucus to protect esophagus

2. Fundus- Superior region, contact diaphragm

3. Body- Majority of stomach- Holds chyme- Gastric glands secrete enzymes and acids for

digestion4. 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 StomachGastric Glands

• Produce 1-3L gastric juice/day– Secretions vary per region

1.Cardia gastric glands = mucus2.Fundus and Body gastric glands = digestive enzymes

and acids- Two types of gastric cells

•parietal cells & chief cells3.Pyloric gastric glands = mucus and hormones

- 2 important hormone producing cells- G cells & D cells

4. The StomachGastric Glands

2. Fundus and Body gastric glands- Two types of gastric cells

1. Parietal cell secretions1. Intrinsic factor (VitB12 uptake)2. H+ and CL- ions combine to make HCL in stomach

2. Chief cell secretions1. Pepsinogen converted to pepsin by acid in stomach; hydrolyzes proteins2. Rennin infants only, curdles mile protein to aid digestion

4. The StomachGastric Glands

2. Fundus and Body gastric glands- Acid production important to gastric

function1. Kill microbes2. Denature proteins

- digestion, destroy enzymes in food3. Break down plant cell walls and animal

CT4. Activate pepsin

The Secretion of Hydrochloric Acid

Figure 24–14

4. The StomachGastric Glands

3. Pyloric gastric glands = mucus and hormones- 2 important enteroendocrine cells

1. 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

2. D cells = produce somatostatin hormone- Inhibits release of gastrin

- Thus inhibits gastric activity- Secreted in response to sympathetic stimulation

4. The StomachDisorders

1. Gastritis- Inflammation of gastric mucosa- Caused by drugs, stress, infection- Chronic can lead to ulcer

2. Peptic Ulcer- Erosion of stomach lining- Caused by

- Too much acid & Not enough mucus- Most common:

- Helicobacter pylori (bacteria)

4. The StomachRegulation of Gastric Activity

• Secretion and motility controlled by 3 factors

1. Innervations from CNS (ANS)

2. Reflexes of the ENS

3. 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

4. The StomachRegulation of Gastric Activity

A) 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

4. The StomachRegulation of Gastric ActivityB) Gastric Phase

- Initiates stomach digestive activities- Triggered by food entering the stomach

- Stimuli = distension, peptides, low acidity

- Lasts 3-4 hours Three responses1. Neural response2. Hormonal response3. Local response

Gastric Phase – 3 responses1. 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

C) Intestinal Phase- Controls chyme entry into duodenum- Last many hours- Involves excititory and inhibitory control of gastric

activity depending on chyme composition- Two Responses

1. Neural Response- Stretch receptors trigger enterogastric reflex

- turns off ENS and parasympathetic stimulation of G cells

- Stimulates sympathetic stimulation of pyloric sphincter (contraction occurs)

2. Hormonal Response

4. The StomachRegulation of Gastric Activity

Intestinal Phase – Hormonal Response

2. Hormonal response- Different hormones depending on chyme compositionA ) Lipids, carbohydrates, peptides

- Cholecystokinin and Gastric Inhibitory Peptide - Inhibit gastric secretion and motility- Stimulates pancreas + gallbladder secretion

B) Low pH- Secretin inhibits gastric secretion

- Stimulates pancreas and liver secretionsC) 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?

A. pH is higher

B. pH is lower

C. pH fluctuates depending on what was eaten

D. 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?

A. The vagus nerve carries gastric pain sensations.

B. The vagus nerve stimulates gastric motility.

C. The vagus nerve stimulates gastric secretions.

D. 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 IntestinesThe Duodenum

A) 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 IntestinesThe 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 IntestinesHistology

• Same 4 layers, but adapted for absorption1. 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 IntestinesHistology

• 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 IntestinesGlands of small intestine

1. 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 Glands1. 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

1. Myenteric reflexes (ENS)- Peristalsis to move chyme slowly through the small

intestine2. 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 oblongata1.Pyloric sphincter relaxes

- Contents of duodenum and upper jejunum discharged into stomach

2.Salivary secretion enhanced- Buffer stomach acid

3.Soft palate rises to close off nasopharynx4.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

1. 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

2. 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 duodenum1. Secretin

- Released in response to acid chyme- triggers pancreas to secrete bicarbonate and phosphate buffer

2. 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 gut1.Pancreatic alpha-amylase

- Hydrolyzes starch2.Pancreatic lipase

- Hydrolyzes lipids and fatty acids3.Nucleases

- Hydrolyzes nucleic acids (RNA, DNA)4.Proteolytic enzymes MAJORITY

6. The Pancreas• Pancreatic Enzymes

4. Proteolytic enzymes MAJORITY- Many, each digests specific peptide bonds- 2 main classes

1. Proteases = hydrolyze large proteins into peptides2. 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?

A. with plicae circularis

B. with villi

C. with microvilli

D. all of the above

How would a meal that is high in fat affect the level of cholecystokinin in the

blood?

A. It would increase the level of cholecystokinin.

B. It would decrease the level of cholecystokinin.

C. It would cause rapid fluctuations in the level of cholecystokinin.

D. It would have no effect.

How would the pH of intestinal contents be affected if the small intestine did not

secrete the hormone secretin?

A. pH would be higher than normal.

B. pH would be lower than normal.

C. No changes in pH would result.

D. Unpredictable changes would result.

The digestion of which nutrient would be most impaired by damage to the

exocrine pancreas?

A. proteins

B. carbohydrates

C. fats

D. 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

1. Hepatic Artery- Oxygen rich blood

2. Hepatic portal vein- Nutrient rich blood from small intestine

3. 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

1. Remove pathogens, cell debris, damaged erythrocytes

2. Remove and store iron3. Remove and store lipids4. 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

1. Metabolic regulation2. Hematological regulation3. 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 Regulation1. Liver serves as blood reservoir2. Kupffer cells

1. recycle RBCs2. perform antigen presentation to lymphocytes

3. Hepatocytes 1. remove/recycle hormones2. remove antibodies

4. Hepatocytes and Kupffer cells remove, inactivate, or store toxins, drugs, and heavy metals

5. Hepatocytes produce plasma proteins

3. Digestion: Bile• Bile components:

– Water, bilirubin, phospholipids, electrolytes, cholesterol, bile salts (lipids)

• Bile functions:1.Buffer chyme with electrolytes2.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 Liver1. 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

• 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 MechanismA. ANS

•Parasympathetic = increase digestive activity

•Sympathetic = decrease digestive activityB. ENS reflexes

•Coordinates movement of materials from one region to the next

Coordination of Secretion and Absorption in the Small Intestine

2. 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)

Coordination of Secretion and Absorption in the Small Intestine

2. Hormonal MechanismC. Gastric Inhibitory Peptide

•Released when chyme contains lipids and carbohydrates

•Inhibits gastric activityD. Secretin

•Released when chyme is acidic•Stimulates release of bile from liver and

buffers from pancreas•Reduces gastric activity

Coordination of Secretion and Absorption in the Small Intestine

2. Hormonal MechanismE. 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

Coordination of Secretion and Absorption in the Small Intestine

2. Hormonal MechanismF. 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 activity2. Secretion from pancreas and gallbladder3. 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• Less than 10% of nutrient absorption• Prepares fecal material for ejection from the body • Functions:

1. Reabsorb any remaining water and compact feces2. Absorb vitamins and electrolytes3. 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

1. Cecum: – the pouchlike first portion

2. Colon: – the largest portion

3. Rectum: – the last 15 cm of digestive tract

9. The Large IntestineSubdivision

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

9. The Large IntestineSubdivision

B) Colon– Absorbs water, vitamins, electrolytes– Four major regions

1. Ascending colon2. Transverse colon3. Descending colon4. 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

9. The Large IntestineSubdivision

C) Rectum– Stores feces– Has 3 valves to separate feces and flatus– Exits at anal canal– Lined with stratified squamous epitheliumDefecation controlled by two sphincters:1.Internal anal sphincter – Involuntary control

- Smooth muscle from muscularis externa2. External anal sphincter – Voluntary control

- Skeletal muscle under voluntary control

Defecation Reflex

Figure 24–25

9. Large Intestine

Large Intestine Movements1.Haustral contractions2.Mass movements3.Defecation reflex

9. Large Intestine

Large Intestine Movements1.Haustral contractions

- local, slow segmenting contractions- Shift faces for water absorption

2.Mass movements- Powerful peristaltic contractions- Force feces toward rectum- Occur few times/day- Can trigger defecation reflex via stretch

receptors in rectum

9. Large IntestineLarge 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

LipidDigestion 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

ProteinDigestion 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 AcidDigestion 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

WaterDigestion and Absorption

• Digestion Method– No digestion required– 2L from food, 7L from secretions

• Absorption Method– Osmosis (95% in small intestine)– ~ 150ml lost in feces

IonsDigestion 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

VitaminsDigestion 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

1. Epithelium division rates decline- ulcers more likely

2. Smooth muscle tone decrease- Constipation from slow movement- Diverticulitis and hemorrhoids from weak walls

3. Cumulative damage- Teeth wear- Liver toxin, lipid build up

4. Increased cancer rate

What component of food would increase the number of

chylomicrons in the lacteals?

A. electrolytes

B. carbohydrates

C. protein

D. fat

The absorption of which vitamin would be impaired by the removal

of the stomach?

A. vitamin D

B. vitamin C

C. vitamin B12

D. vitamin A

Why is diarrhea potentially life threatening, but constipation is

not?

A. Constipation is easier to treat.

B. Constipation affects only the very young or the elderly.

C. Diarrhea causes edema.

D. 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