24-1

The Digestive System

• Mouth---bite, chew, swallow

• Pharynx and esophagus----transport

• Stomach----mechanical disruption; absorption of water & alcohol

• Small intestine--chemical & mechanical digestion & absorption

• Large intestine----absorb electrolytes & vitamins (B and K)

• Rectum and anus---defecation

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Types of Digestion

• Mechanical – mouth, stomach,SI, LI

• Chemical – mouth, stomach, SI

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Layers of the GI Tract

1. Mucosal layer

2. Submucosal layer

3. Muscularis layer

4. Serosa layer

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Mucosa• Epithelium

– stratified squamous (in mouth, esophagus & anus) = tough

– simple columnar in the rest

• secretes enzymes and absorbs nutrients

• specialized cells (goblet) secrete mucous onto cell surfaces

• enteroendocrine cells---secrete hormones controlling organ function

• Lamina propria

– loose connective tissue

– contains BVs and lymphatic tissue

• Muscularis mucosae

– thin layer of smooth muscle

– causes folds to form in mucosal layer

– responsible for local food movements

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Submucosa

• Loose connective tissue– containing BV, glands and lymphatic tissue

• Meissner’s plexus– part of the enteric nervous system - “brain of the gut”– parasympathetic division only - sensory and motor neurons

• vasoconstriction of blood vessels to gut• controls contraction of muscularis mucosa• controls the secretory cells of the mucosal epithelium• connected to the myenteric plexus (in the muscularis layer)

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Muscularis • Skeletal muscle = voluntary control

– in mouth, pharynx , upper esophagus and anus

– control over swallowing and defecation

• Smooth muscle = involuntary control– inner circular fibers & outer longitudinal fibers

– mixes, crushes & propels food along by peristalsis

• Auerbach’s plexus (myenteric plexus)– both parasympathetic & sympathetic innervation of circular and

longitudinal smooth muscle layers

– part of the Enteric nervous system

– controls overall GI tract motility

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Serosa

• An example of a serous membrane

• Covers all organs and walls of cavities not open to the outside of the body

• Covering the digestive organs in the peritoneal cavity = visceral peritoneum

• Secretes a serous fluid

• Consists of connective tissue covered with simple squamous epithelium

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Peritoneum

• Peritoneum– visceral layer covers

organs

– parietal layer lines the walls of body cavity

• Peritoneal cavity– potential space containing

a bit of serous fluid

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The Mesenteries of the GI tract• Mesentery – small intestines parietal

peritoneum

• Mesocolon – large intestine parietal peritoneum

• Lesser omentum – liver stomach

• Greater omentum

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The path of food:oral cavity/teeth/salivary glands

oropharynx/epiglottis

esophagus

stomach

small intestine: duodenum

small intestine: ileum

small intestine: jejunum

large intestine: ascending colon

large intestine: transverse colon

large intestine: descending colon

sigmoid colon rectum anus

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Mouth

• Oral cavity proper---the roof = hard, soft palate and uvula– floor – geniohyoid, mylohyoid – contains the tongue– lips and cheeks-----contains buccinator muscle that keeps food between upper &

lower teeth– Vestibule---area between cheeks and teeth

• Lined with an oral mucosa (stratified squamous epithelium & lamina propria)– Lining of the cheeks = buccal mucosa– Lining of the maxilla and mandible = alveolar mucosa (gingiva)

• Landmarks: lingual frenulum, labial frenulum, uvula– Shortened lingual frenulum can impede movement of the tongue within the mouth

24-12

Tongue• physiological functions– manipulation of food for chewing

and swallowing– production of digestive enzyme– speech

• made of extrinsic and intrinsic muscles that control movement and the shape of the tongue

• extrinsic – control the movement of the tongue in and out of the mouth, manipulates food, hold the tongue in position and forms the floor of the mouth

• intrinsic – originate from and insert into the connective tissue of the tongue, alter the shape and size of the tongue for speech and swallowing

• lamina propria layer of the mucosa contains the lingual glands for the secretion of mucus and lingual lipase

24-13

Salivary Glands• Parotid Gland – development at 4 to 6

weeks– duct = Stensen’s duct– blood supply – external carotid

artery– parasympathetic nerve supply from

IX– sympathetic innervation by superior

cervical ganglion• Submandibular gland – development

at 6 weeks– duct = Wharton’s duct– opens lateral to the lingual

frenulum– blood supply from facial and

lingual arteries– parasympathetic supply – facial

nerve• Sublingual gland – development at 8

to 12 weeks– small ducts = ducts of Rivinus– larger duct = Bartholin’s duct– empty with the SM duct at the

sublingual caruncle– blood supply – sublingual and

submental arteries– facial nerve innervation

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Salivary Glands• minor salivary glands

– 600 to 1000 glands– small aggregates of secretory

tissue present in the submucosa– not found in the gingiva and

anterior hard palate– predominantly mucus glands– except the lingual glands (von

Ebner’s glands) found on the tongue – open into troughs surrounding the circumvallate papilla

• labial, lingual, palatal, buccal, glossopalatine and retromolar glands

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Saliva• 600-1000ml/day• Wet food for easier swallowing

• Dissolves food for tasting

• Bicarbonate ions buffer acidic foods

– bulemia---vomiting hurts the enamel on your teeth

• Chemical digestion of starch begins with enzyme (salivary amylase)

• Protects mouth from infection with its rinsing action---1 to 1 and 1/2qts/day

• oral fluid is mixed or whole saliva – includes secretions from all three major glands + minor glands plus desquamated epithelial cells, microorganisms, food, debris, inflammatory cells and serum components

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Saliva• Components:• 1. water• 2. enzymes: amylase, lipase, lyzozyme• 3. mucins, mucus• 4. multiple electrolytes: sodium, postassium,

chloride, calcium, magnesium• 5. glucose, amino acids, urea, uric acid and lipids• 6. secretory Igs – IgG and IgM and IgA• 7. growth factors & hormones: EGF, insulin,

24-17

Saliva• functions:

– 1. buffering – bicarbonate, phosphate ions• protection against demineralization caused by bacterial

acids resulting from the breakdown of sugars– 2. Pellicle formation

• many salivary proteins bind to the surface of the teeth and oral mucosa – forms a thin film = salivary pellicle

• several of these proteins bind calcium to protect the tooth– 3. maintenance of tooth integrity

• saliva is saturated with calcium and phosphate ions • at the tooth surface the high concentration of calcium and

phosphate results in maturation of the enamel – increases surface hardness

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– 4. antimicrobial action• barrier function provided by mucins• saliva also contains a spectrum of proteins with

antimicrobial activity – histatins, lysozyme, lactoferrin and peroxidase

• also the presence of antibodies – IgA is the major salivary Ig (results in agglutination of microbes and prevents their adherence to oral tissues)

– 5. tissue repair• variety of growth factors are present in saliva• many promote tissue growth and differentiation

– 6. digestion• amylase and lipase

– 7. taste• solubilizes food substances – allows binding to taste

receptors located in taste buds

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Salivation• controlled by the ANS

• parasympathetic system provides a constant supply of saliva to keep the mucus membranes moist and to lubricate the food

• Increased salivation – parasympathetic system– stimulation of taste receptors are conveyed to the cerebral cortex to the

salivatory nuclei in brainstem

– returning impulses via the parasymp system travel via CN 7 & 9

• Decreased salivation – sympathetic system

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Digestion in the Mouth• Mechanical digestion = mastication or chewing

• breaks food into pieces• mixes food with saliva so it forms a bolus

• Chemical digestion– salivary amylase

• breakdown of complex carbohydrates into smaller pieces• activity dependent upon pH

– lingual lipase• secreted by glands in tongue• begins breakdown of triglycerides into fatty acids and

glycerol

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Teeth:-grinding, tearing and shearing of food

-two main divisions: crown and root-crown: above gum-line

-innermost layer - pulp (nerves/blood vessels) -outer covering of specialized calcified connective tissue - dentin

-covered with a layer of enamel-root: entry of nerves and blood vessels

-secures the tooth into the jaw (cementum)-covered by a periodontal membrane - unites with gums

24-22

Primary and Secondary Dentition

-primary: 20 teeth starting at 6 months-secondary/adult: between 6 and 12 years = 32 teeth 8 incisors - biting 4 canines (cuspids) - tearing 8 premolars (bicuspids) - grinding 12 molars (tricupids) - grinding

** third pair of molars (wisdom teeth) may not erupt -impacted

24-23

Pharynx

• Funnel-shaped tube extending from internal nares to the esophagus (posteriorly) and larynx (anteriorly)

• Skeletal muscle lined by mucous membrane• Deglutition or swallowing is facilitated by saliva

and mucus– starts when bolus is pushed into the oropharynx– sensory nerves send signals to deglutition center in

brainstem (medulla oblongata)

Esophagus• Collapsed muscular tube

• In front of vertebrae

• Posterior to trachea

• Posterior to the heart

• Pierces the diaphragm at hiatus

• Mucosa = stratified squamous

• Submucosa = large mucous glands

• Muscularis = upper 1/3 is skeletal, middle is mixed, lower 1/3 is smooth– upper & lower esophageal

sphincters are prominent circular arrangements of smooth muscle

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Physiology of the Esophagus - Swallowing

• 1. Voluntary phase---tongue pushes food to back of oral cavity

• stimulates receptors in the oropharynx• message travels to the deglutition center in

the MO and lower pons• 2. Pharyngeal stage (involuntary)

– breathing stops & air passages are closed– vocal cords close– epiglottis becomes bent over airway (glottis)

as larynx is lifted– controlled by autonomic nervous system

• 3. Esophageal phase (involuntary) - Peristalsis pushes food down

– circular fibers behind bolus contract

– longitudinal fibers in front of bolus also contract to shorten the distance of travel and widens the espophagus

• lower esophageal sphincter relaxes as food approaches the stomach

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Anatomy of Stomach

• Size when empty– large sausage– stretches due to rugae

• Parts of stomach– cardic portion– fundus– body– pyloric portion – narrows at the

pyloric sphincter

• bolus mixes with gastric juice in the stomach to form chyme

• stomach empties as small squirts of chyme leave the stomach through the pyloric sphincter

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Muscularis• Three layers of

smooth muscle--outer longitudinal, circular & inner oblique

• Permits greater churning & mixing of food with gastric juice

Serosa• Simple squamous

epithelium over a bit of connective tissue

• Also known as visceral peritoneum

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Histology of the Stomach - Mucosa

• simple columnar epithelium with embedded surface mucus cells

• epithelial cells form columns of secretory cells = gastric glands that line narrow channels called gastric pits

• glands - for the secretion of gastric juice

• mix of water, HCl, enzymes and hormones

24-29

Gastric Mucosa• Hydrochloric acid (parietal cells) converts

Pepsinogen (from chief cells) to the enzyme pepsin = protein digestion

• HCl from the parietal cells is secreted as H+ and Cl-ions

• the H+ comes from the absorption of water into the parietal cell – combination of water and CO2 by carbonic

anhydrase creates carbonic acid which dissociates into HCO3- and H+ ions

• proton pumps actively pump H+ into the lumen of the stomach

• the bicarbonate is pumped into the blood in exchange for Cl-ions

• Cl- channels then allow the diffusion of these Cl- into the lumen

• Intrinsic factor (parietal cells)– absorption of vitamin B12 for RBC

production

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• Parietal cells: HCl and Intrinsic Factor• Chief cells: enzymes: pepsin and gastric lipase

– pepsin is secreted as the inactive form pepsinogen

– converted by contact with HCl– gastric lipase – splits short-chain triglycerides

(e.g. in milk) into two fatty acids and a monoglyceride (glycerol + one FA)

Gastric Mucosa Secretions

24-31

Stomach--Chemical Digestion

• Protein digestion begins– HCl denatures (unfolds) protein molecules– HCl transforms pepsinogen into pepsin that breaks

peptides bonds between certain amino acids

• Fat digestion continues– gastric lipase splits the triglycerides in milk fat

• HCl kills microbes in food• Mucous cells protect stomach walls from being

digested

24-32

Absorption of Nutrients by the Stomach

• Water especially if it is cold• Electrolytes• Some drugs (especially aspirin) & alcohol• Gastric mucosal cells contain alcohol dehydrogenase that

converts some alcohol to acetaldehyde– more of this enzyme found in males than females

– 5 different forms of ADH – some more efficient then others

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Gastric phase of digestion• starts once food reaches the stomach• neural and hormonal regulation to promote gastric

secretion and motility– neural regulation:

• distension of the stomach stimulates stretch mechanoreceptors – stimulates production of gastric juices

• impulses travel via the parasympathetic neurons – stimulates the flow of gastric juice, causes waves of peristalisis to mix the food with the juice and move food into the SI

• chemoreceptors monitor pH of the stomach chyme as juice is being made

• as the pH of the stomach chyme decreases (becomes more acidic) –creates a negative feedback loop

• as the food leaves the stomach and the stretching of the wall lessens – this inhibits this path

– hormonal regulation: digestive hormones• gastrin, secretin, CCK, GIP

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Anatomy of the Small Intestine

• 20 feet long----1 inch in diameter

• Large surface area for majority of absorption

• 3 parts– duodenum---10 inches– jejunum---8 feet– ileum---12 feet

• ends at ileocecal valve

Small Intestine

• SI has specific structures that increase surface area– plica circularis

• permanent ½ inch tall folds that contain part of submucosal layer

• not found in lower ileum

• cannot stretch out like rugae in stomach

– villi• Contains vascular

capillaries and lacteals (lymphatic capillaries)

– microvilli• produces a cell surface

feature known as brush border

Functions of Microvilli

• Absorption and digestion

• Digestive enzymes found at cell surface on microvilli - digestion occurs at cell surfaces

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Intestinal Glands

Small Intestine - Mucosal layer:

Epithelial layer• Absorptive cells –absorb nutrients in the

chyme• cells at the bottom of the mucosa form

Intestinal Glands• Goblet cells - Unicellular glands that are

part of simple columnar epithelium• Enteroendocrine cells

– found within the Intestinal glands– secretin– cholecystokinin– gastric inhibitory peptide

• Paneth cells– secretes lysozyme– kills bacteria

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• Absorptive cells: – production of brush-border enzymes (“intestinal juice”)-1 to 2 liters per day-enzymes are made and inserted into the microvilli of the absorptive cells -BB enzymes: enzymes for the digestion of carbohydrates, proteins (peptidases) and nucleic acids

-

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SI mucosal layer cont....

• lamina propria of the SI contains areolar connective tissue

• plus an abundance of mucosa-associated lymphatic tissue – MALT– solitary lymphatic nodules in the distal part of

the ileum– groups of nodes in the ileum – Peyer’s patches

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Mechanical Digestion in the Small Intestine• 1. Weak peristalsis in comparison to the

stomach---chyme remains for 3 to 5 hours– starts at the lower portion of the stomach and

pushes the chyme forward– reaches the end of the ileum after 90 – 120

minutes– then another wave starts in the stomach

• 2. Segmentation---local mixing of chyme with “digestive juices” in the SI– does NOT push the food through the tract– move chyme back and forth over the lining of

the SI– done in specific segments of the SI– most rapid in the duodenum and slows at it

reaches the ileum

24-40

-protein and carbohydrate digestion via synthesis of the brush border enzymes by the intestinal glands

-enterokinase-maltase, sucrase, lactase, -dextrinase -aminopeptidase, dipeptidase-phosphatases and nucleosidases -enzymes are expressed on the surface of

absorptive cells – external digestion-resulting monosaccharides, amino acids and

nucleotides are internalized by the absorptive cell-SO carbohydrate and protein digestion stops

in the interior of the absorptive cell

Small Intestine-Chemical Digestion

24-41

– BUT - duodenum is also the site for secretion of the pancreatic juice:

– pancreatic amylase, pancreatic lipase + 4 proteases: trypsin, chymotrypsin, elastase, carboxypeptidase

• 1-2 qt./day------ at pH 7.6• proteases are made in the pancreas as inactive forms

– eg. Trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase

• Trypsinogen converted to trypsin by the brush border enzyme enterokinase

• Activated trypsin then converts other three proteases into their active forms

Small Intestine-Chemical Digestion

24-42

SI: Absorption of digested nutrients

• occurs via diffusion, facilitated transport, osmosis and active transport

• water: 90% absorption occurs in the SI – 10% in the stomach and LI

• carbohydrates – absorbed as monosaccharides by either facilitated or secondary active transport– fructose passes through the apical

membrane of the absorptive cells via facilitated transport

– glucose and galactose are transported via secondary active transport created by the active transport of Na+ (eg. Na/glucose symporter)

– glucose and galactose compete for the saccharide site on this transporter

– all three monosacarrides leave through the basal surface of the absorptive cell and enter the blood via facilitated transport

24-43

SI: Absorption of digested nutrients

• proteins – absorbed as amino acids by active transport in the duodenum and jejunum– amino acids absorbed come from

the food and from the digestive enzymes and the degrading absorptive cells

– different transporters carry different amino acids

– some enter through Na+/amino acid symporters others through H+ amino acid symporters

– enter into the blood via diffusion

- 95-98% of the proteins present in chyme in the SI are digested and absorbed

24-44

• fats – absorbed via simple diffusion– 95% absorbed in the small intestine– bile induced emulsification results in breakdown of

a fat into 2 fatty acids + a monoglyceride (one fatty acid + glycerol)

– the fatty acids are either short-chain or long chain– short chain fatty acids and cholesterol move into

absorptive cell via diffusion– long chain fatty acids and monoglycerides require

combination with bile in the form of a micelle– micelle “ferries” the fatty acids and MGs into the

absorptive cells • bile returns to the SI lumen to “pick up” more FAs and

MGs– once inside the cell – FAs and MGs are reassembled

into triglycerides – these fats aggregate with phospholipids,

apolipoproteins and cholesterol in the absorptive cell to form chylomicrons which enter the lacteal

– large pores of the lacteal allow for the passage of chylomicrons

SI: Absorption of digested nutrients

Fat dropletscoated withbile salts

Fat globule

Bile salts

Micelles madeup of fatty acids,monoglycerides,and bile salts

Epitheliumof smallintestineEpitheliumof lacteal

Lacteal

24-45

– chylomicrons enter the blood at the subclavian veins

– removed from the blood as they pass through the liver

– liver expresses lipoprotein lipase (LPL) – breaks down the chylomicron into fatty acids, monoglycerides and cholesterol

– cholesterol is used to make bile

– cholesterol, TGs and apoplipoproteins are assembled into HDL or VLDL

• VLDL breaks down into LDL (by LPL enzyme)

– adipose cells are also capable of taking up chylomicrons and breaking them down via the LPL enzyme

Fat Processing

24-46

Absorption in the SI

• Blood that enters the SI villi is rich in oxygen, poor in nutrients

• Blood that exits the villi is poor in oxygen and rich in nutrients

• BUT: venous blood may also be contaminated with toxins

• SO: blood from SI flows into the liver via the hepatic portal vein – to be filtered prior to emptying via the hepatic vein into the inferior vena cava

24-47

Absorption of Water

• 9 liters of fluid dumped into GI tract each day

• Small intestine reabsorbs 8 liters

• Large intestine reabsorbs 90% of that last liter

• Absorption is by osmosis through cell walls into vascular capillaries inside villi

24-48

Anatomy of Large Intestine

• 5 feet long by 2½ inches in diameter• Ascending & descending colon are retroperitoneal• Cecum & appendix

• Rectum = last 8 inches of GI tract anterior to the sacrum & coccyx• Anal canal = last 1 inch of GI tract

– internal sphincter----smooth muscle & involuntary – external sphincter----skeletal muscle & voluntary control

24-49

Histology of Large Intestine

• Muscular layer– internal circular layer is normal– outer longitudinal muscle

• taeniae coli = shorter bands• tonic contractions of these bands

puckers the LI into pouches = haustra (pouches) formed (also called diverticulum)

• epiploic appendages

• Serosa = visceral peritoneum• Appendix

– contains large amounts of lymphatic tissue

24-50

Histology of Large Intestine

• Mucosa– smooth tube -----no villi or plica folds– mostly contains absorptive cells with microvilli in its epithelial component + goblet

cells– both absorptive and goblet cells are located in long tubular structures called Intestinal

glands– absorptive cells are for the absorption of water and salt ONLY!– goblet cells secrete mucus

• Submucosa & mucosa contain lymphatic nodules – contribute to immunity

24-51

Mechanical Digestion in Large Intestine

• Smooth muscle of muscularis = mechanical digestion• Peristaltic waves (3 to 12 contractions/minute)

– haustral churning----relaxed pouches are filled by muscular contractions in the haustra below it

– gastroilial reflex = when stomach is full, gastrin hormone relaxes ileocecal sphincter so small intestine will empty and make room for new chyme

• Also intensifies the peristaltic waves in the ileum and the older chyme then enters the caecum

– gastrocolic reflex = when stomach fills, a strong peristaltic wave moves contents of transverse colon into rectum

24-52

Chemical Digestion in Large Intestine

• No enzymes are secreted only mucous – by the goblet cells in the intestinal glands

• BUT - chyme is aacted upon by the action of bacteria – bacterial based chemical digestion NOT human-based

• Bacteria ferment: – undigested carbohydrates - carbon dioxide & methane

gas– undigested proteins - simpler substances (indoles,

skatoles, hydrogen sulfide)----odor– turn bilirubin into simpler substances that produce

color of feces

• Bacteria also produce vitamin K and B in colon

24-53

Absorption & Feces Formation in the Large Intestine

• food has now been in the GI tract for 3 to 10 hours• solid or semisolid form in the LI due to water reaborption =

feces• feces – water, salts, sloughed-off epithelial cells, bacteria,

products of bacterial decomposition, unabsorbed and undigested materials

• 90% of all water absorption takes place in the SI – 10% in the LI

• but the LI is very important in maintaining water balance– Excess water in feces = diarrhea– Insufficient water in feces = constipation

• also absorbs some electrolytes---Na+ and Cl- • can also absorb some vitamins

24-54

Fiber

• feces also contains materials undigestible by human enzymes = Fiber

• Three types of fiber:• dietary fiber - indigestible plant carbohydrates (cellulose, lignin and

pectin)• soluble fiber – dissolves in water (beans, barley, broccoli, prunes,

apples and citrus)– forms a gel that slows the passage of materials through the colon– also helps to lower blood cholesterol – binds to bile salts to prevent their

reabsorption– liver must make more bile – so it gets the cholesterol component of bile from

LDL in the blood – lowers LDL levels– also acted upon by bacteria – carbon dioxide and methane gas

• insoluble fiber – woody or structural parts of the plant (skins of fruits and vegetables, coatings around bran and corn)– passes though the colon relatively unchanged

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Defecation

• Gastrocolic reflex moves feces into rectum

• Stretch receptors signal sacral spinal cord

• Parasympathetic nerves contract muscles of rectum & relax internal anal sphincter

• External sphincter is voluntarily controlled

Digestive Hormones• production of digestive hormones

– gastrin – by the G cells of the stomach lining• stimulates production of gastric juice and encourages emptying of the

stomach• also relaxes the sphincter between the ileum and cecum (ileocecal)

– gastric inhibitory peptide – antagonist to gastrin– CCK – by the enteroendocrine cells of the SI (presence of fatty acids)

• CCK stimulates the release of pancreatic juice and bile (synthesis and increased gallbladder contraction)

• also slows the emptying of the stomach• decreases gastric juice production

– secretin – by the enteroendocrine cells of the SI• secretin stimulates the release of bicarbonate from the pancreas – neutralizes

chime• decreases gastric juice production• decreases gastrin production and release• increases pepsinogen• role in water regulation – activates the release of ADH from posterior

pituitary

Digestive Feedback systems• emptying of the stomach:

– production of gastrin – stimulates emptying– production of GIP/enterogastrone and CCK – inhibits emptying

• pancreatic juice production:– secretin and CCK – stimulation of production

• bile production:– CCK – stimulation of secretion

Pancreas

Stomach

Entero-gastrone

Gall-bladder

Liver

DuodenumSecretin

CCK

CCK

Stimulation

Inhibition

Gastrin

Key

24-58

Anatomy of the Pancreas

• 5" long by 1" thick• Head close to curve in

C-shaped duodenum• pancreatic duct joins

common bile duct from liver

• Opens 4" below pyloric sphincter

24-59

Histology of the Pancreas

• Acini- dark clusters – 99% of gland

– produce pancreatic juice

• Islets of Langerhans– 1% of gland

– pale staining cells

– produce hormones

– alpha cells, beta cells,

delta cells, F cells

24-60

• Endocrine cells secrete near a capillary

• 1 to 2 million pancreatic islets

• Contains 4 types of endocrine cells

• Alpha cells (20%) produce glucagon• Beta cells (70%) produce insulin• Delta cells (5%) produce somatostatin• F cells produce pancreatic polypeptide

24-61

Composition and Functions of Pancreatic Juice

• 1 + 1/2 Quarts/day at pH of 7.1 to 8.2• Contains water, enzymes & sodium bicarbonate• Digestive enzymes

– pancreatic amylase, pancreatic lipase, proteases– trypsinogen---activated by enterokinase (a brush border enzyme)– chymotrypsinogen----activated by trypsin– procarboxypeptidase---activated by trypsin– proelastase---activated by trypsin– trypsin inhibitor---combines with any trypsin produced inside

pancreas

– ribonuclease----to digest nucleic acids– deoxyribonuclease

24-62

Anatomy of the Liver and Gallbladder

• Liver– weighs 3 lbs.

– below diaphragm

– right lobe larger

– gallbladder on right lobe

– size causes right kidney to be lower than left

• Gallbladder– fundus, body & neck

24-63

Histology of the Liver

• Hepatocytes arranged in lobules comprised of hepatocytes

• Lobules are surrounded by branches off the hepatic portal vein

• Running between the hepatocytes of the lobules are blood filled spaces = sinusoids

• Also running between hepatocytes of a lobule are bile canals = bile canaliculi – join to form the bile ducts of the liver

• Kupffer cells phagocytize microbes & foreign matter

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Gallbladder• Simple columnar epithelium

• No submucosa

• Three layers of smooth muscle

• Serosa or visceral peritoneum

Bile Production

• One quart of bile/day is secreted by the liver– yellow-green in color & pH 7.6 to 8.6

• Components– water & cholesterol

– bile salts = Na & K salts of bile acids

– bile pigments (bilirubin) from hemoglobin molecule

Flow of Bile

• Bile capillaries

• Hepatic ducts connect to form common hepatic duct

• Cystic duct from gallbladder & common hepatic duct join to form common bile duct

• Common bile duct & pancreatic duct empty into duodenum

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Bile functions• emulsification – breakdown of fats into fatty acids (long or short

chain) + monoglycerides• make the long-chain fatty acids and monoglycerides (which

are large) more soluble in the watery environment of the chyme• bile salts surround the long-chain fatty acids and form tiny

spheres called micelles– amphipathetic nature of bile salts – hydrophobic portion interacts with the

fatty acids• micelles are absorbed into the absorptive cell

– the long-chain fatty acids and monoglycerides then separate from the bile and move into the absorptive cells cytoplasm – leaving the micelles behind (move back into the chyme)

– micelles act as a lipid “ferry”– also solubilize other large hydrophobic molecules like the fat-soluble

vitamins (A, D, E, K) and cholesterol

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Liver Functions--Carbohydrate Metabolism

• Turn amino acids into glucose (gluconeogenesis)– Alanine and glutamine

• Turn triglycerides into glucose– Turns glycerol into glucose (gluconeogenesis)– Turns odd number FAs into glucose (gluconeogenesis)

• Turn excess glucose into glycogen & store

in the liver

• Turn glycogen back into glucose as needed

Liver Functions --Lipid Metabolism

• Synthesize cholesterol - Synthesize lipoproteins----HDL and LDL (used to transport fatty acids in bloodstream)

• Stores some fat

• Breaks down some fatty acids for energy production (ATP)

Liver Functions--Protein Metabolism

• Deamination = removes NH2 (amine group) from amino acids

• Converts resulting toxic ammonia (NH3) into urea for excretion by the kidney

• Synthesizes plasma proteins utilized in the clotting mechanism and immune system

• Convert one amino acid into another

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Other Liver Functions

• Detoxifies the blood by removing or altering drugs & hormones (thyroid & estrogen)

• Releases bile salts help digestion by emulsification• Stores fat soluble vitamins-----A, B12, D, E, K• Stores iron and copper• Phagocytizes worn out blood cells & bacteria• Activates vitamin D (the skin can also do this with 1 hr

of sunlight a week)

24-68

Chemical Digestion

Digestion of Carbohydrates

• Mouth---salivary amylase– inactivated in the stomach by

the low pH• Esophagus & stomach---

nothing happens• Duodenum

– secretion of pancreatic juice (amylase)

– synthesis of the brush border enzymes (maltase, sucrase & lactase) act on disaccharides

• produces monosaccharides--fructose, glucose & galactose

• lactose intolerance (no enzyme; bacteria ferment sugar)--gas & diarrhea

Digestion of Proteins

• Stomach– HCl denatures or unfolds

proteins

– secretion of pepsinogen and activation by HCl - pepsin turns proteins into peptides

• Pancreas– secretion of pancreatic juice

which contains trypsin, chymotrypsin etc…

• Intestines– synthesis of brush border

enzymes-----aminopeptidase or dipeptidase------split off amino acid at amino end of molecule

24-69

Digestion of Lipids

• Mouth----lingual lipase

• Small intestine– emulsification by bile

– pancreatic lipase within the pancreatic juice---splits into 2 fatty acids + a monoglyceride

– no lipid-digesting enzymes made by the brush border

Digestion of Nucleic Acids

• Pancreatic juice contains 2 nucleases– ribonuclease which digests RNA

– deoxyribonuclease which digests DNA

• Nucleotides produced are further digested by brush border enzymes (nucleosideases and phosphatases)– break the nucleotide into pentose,

phosphate & nitrogenous bases

• Absorbed by active transport