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– thin layer of loose connective tissue– contains BV and lymphatic tissue

• Muscularis mucosae---thin layer of smooth muscle– causes folds to form in mucosal layer – increases local movements - increasing absorption

with exposure to “new” nutrients

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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 divisions– innervation by sensory and motor neurons

• vasoconstriction• local movement by

muscularis mucosa smooth muscle

• supply the secretory cells of the mucosal epithelium• connected to the myenteric plexus (in the muscularis layer) by a series of interneurons

24-6

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 GI tract motility

Serosa• An example of a serous membrane

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

• 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

• Mesentery – small intestines

• Mesocolon – large intestine

• Lesser omentum

• Greater omentum

• Peritonitis = inflammation– trauma

– rupture of GI tract

– appendicitis

– perforated ulcer

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Greater Omentum, Mesentery & Mesocolon

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Lesser Omentum

24-10

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

• Lips and cheeks-----contains buccinator muscle that keeps food between upper & lower teeth

• Vestibule---area between cheeks and teeth• Oral cavity proper---the roof = hard, soft palate and uvula

– floor = the tongue

24-12

Tongue

• 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 –

– duct = Stensen’s duct– blood supply – external carotid artery– parasympathetic nerve supply from IX– sympathetic innervation by superior

cervical ganglion• Submandibular gland –

– duct = Wharton’s duct– opens lateral to the lingual frenulum– blood supply from facial and lingual

arteries– parasympathetic supply – facial nerve

• Sublingual gland –– 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

• minor salivary glands– 600 to 1000 glands– small aggregates of secretory tissue

present in the submucosa– predominantly mucus glands

• 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)• Enzyme (lysozyme) ---helps destroy bacteria• Protects mouth from infection with its rinsing action---1 to 1 and 1/2qts/day• multiple electrolytes: sodium, postassium, chloride, calcium, magnesium• amylase, mucins, peroxidase, lyzozyme, proline-rich proteins

– composition can vary from gland to gland– parotid gland: watery fluid rich in amylase, proline-rich proteins and glycoproteins– submandibular gland also contains higher levels of mucins

• secretory Igs – IgM and IgM and IgA• glucose, amino acids, urea, uric acid and lipids• EGF, insulin, cAMP-binding proteins, albumin• 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

24-15

Saliva• functions:

– 1. buffering – bicarbonate, phosphate ions

– 2. Pellicle formation– 3. maintenance of tooth integrity

– 4. antimicrobial action

– 5. tissue repair

– 6. digestion

– 7. taste

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• Cells in acini (clusters)

• Serous cells secrete a watery fluid

• Mucous cells (pale staining) secrete a slimy, mucus secretion

24-17

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– sight, smell, sounds, memory of food, tongue stimulation---rock in

mouth

– 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

• Stop salivation – dry mouth when you are afraid

– sympathetic nerves

– also inhibits secretion during dehydration

24-18

Digestion in the Mouth• Mechanical digestion (mastication or chewing)

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

• Chemical digestion– salivary amylase

• begins starch digestion at pH of 6.5 or 7.0 found in mouth• when bolus & enzyme hit the pH 2.5 gastric juices hydrolysis

ceases

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

24-19

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

24-20

Teeth:-grinding, tearing and shearing of food

-two main divisions: crown and root-crown: above gumline

-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

24-21

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

24-22

Esophagus• Collapsed muscular tube

• In front of vertebrae

• Posterior to trachea

• Posterior to the heart

• Pierces the diaphragm at hiatus– hiatal hernia or diaphragmatic

hernia

• 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 muscle

24-23

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. Involuntary phase----pharyngeal stage

– breathing stops & airways are closed– soft palate & uvula are lifted to close off nasopharynx– 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

24-24

Anatomy of Stomach

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

• Parts of stomach– cardia– fundus---air in x-ray– body– pylorus---starts to narrow as

approaches pyloric sphincter

• Empties as small squirts of chyme leave the stomach through the pyloric valve

24-25

Histology of the Stomach - Mucosa

• simple columnar epithelium with embedded surface mucus cells

• lamina propria layer under the epithelium (areolar connective tissue) + muscularis mucosae (smooth muscle)

• epithelial cells extend down into the LP where they form columns of secretory cells = gastric glands that line narrow channels called gastric pits

• for the secretion of gastric juice – mix of water, HCl, enzymes and hormones

• food can remain in the fundus of the stomach for up to one hour before being mixed with gastric juice

• during this time – salivary amylase and lipase continue their digestion

• after being mixed with gastric juice – these enzymes become inactivated

24-26

Gastric Mucosa• Hydrochloric acid (parietal cells) converts pepsinogen (from

chief cells) to the enzyme pepsin = protein digestion• stimulated by Ach release by the parasympathetic neurons,

gastrin secreted by G cells and histamine released by mast cells in the LP

• HCl from the parietal cells is secreted as H+ and Cl-ions• proton pumps powered by H+/K+ ATPases actively pump H+

into the lumen of the stomach• causes movement of K+ into the parietal cell• opening of K+ and Cl- channels causes the diffusion of these

ions back into the lumen• 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

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

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

• enzymes: pepsin and gastric lipase– pepsin is secreted as the inactive form pepsinogen– converted by contact with HCl– lipase – splits short-chain triglycerides (milk) into two

fatty acids and a monoglyceride (glycerol + one FA

• absorption of water, alcohol, drugs• mucus cells can absorb some short--chain fatty acids

24-27

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

• most effective at pH 5 to 6 (infant stomach)

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

digested with 1-3mm thick layer of mucous

24-28

Absorption of Nutrients by the Stomach

• Water especially if it is cold• Electrolytes• Some drugs (especially aspirin) & alcohol• Fat content in the stomach slows the passage of alcohol to the

intestine where absorption is more rapid • Gastric mucosal cells contain alcohol dehydrogenase that

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

• Females have less total body fluid that same size male so end up with higher blood alcohol levels with same intake of alcohol

24-29

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 receptors• chemoreceptors monitor pH of the stomach chyme• with the increase in signalling from both these receptors – negative

feedback loop is created• impulses travel to the submucosal plexus– active the parasympathetic

and enteric neurons – cuases waves of peristalisis to mix the food and move food into the SI and stimulates the flow of gastric juice

• as the pH of the stomach chyme decreases (becomes more acidic) – this inhibits this path

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

– hormonal regulation: digestive hormones

24-30

Digestive Hormones

• Gastrin– Stomach (increases gastric juice), gastric & ileocecal

sphincters (relaxes)

• Gastric inhibitory peptide--GIP– stomach & pancreas (inhibits gastrin effects)

• Secretin– Pancreas (increases pancreatic juice), liver (stimulates

bile production) & stomach (decreases gastric juice)

• Cholecystokinin--CCK– Pancreas (increases pancreatic enzyme synthesis),

gallbladder (increases production and secretion of bile) & stomach (slows emptying)

24-31

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

24-32

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

24-33

Small Intestine

• Structures that increase surface area– plica circularis

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

• not found in lower ileum

• can not stretch out like rugae in stomach

– villi• 1 Millimeter tall• Core is lamina propria

of mucosal layer• Contains vascular

capillaries and lacteals (lymphatic capillaries)

– microvilli• 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

• Significant cell division within intestinal glands produces new cells that move up

• Once out of the way---rupturing and releasing their digestive enzymes & proteins

24-34

Intestinal Glands

Small Intestine - Mucosal layer:Epithelial layer• Absorptive cells – digest and absorb nutrients in the

chyme• Intestinal glands (Crypts of Lieberkuhn)

– production of intestinal juice-1 to 2 liters per day-secreted by the absorptive cells of the intestinal glands-enzymes are made and inserted into the microvilli of the absorptive cells – enzymatic digection by these enzymes occurs on the surface of the intestine rather than the lumen of the SI-enzymes for the digestion of carbohydrates, proteins (peptidases) and nucleic acids

-as the cells are turned over – sloughed off and die – releaseing more enzymes that digest food in the chyme

• Goblet cells - Unicellular glands that are part of simple columnar epithelium

• Enteroendocrine cells– within the intestinal glands secretin– cholecystokinin– gastric inhibitory peptide

• Paneth cells– secretes lysozyme– kills bacteria

• Submucosal layer has duodenal glands secretes alkaline mucus

24-35

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

24-36

Mechanical Digestion in the Small Intestine

• 1. Weak peristalsis in comparison to the stomach---chyme remains for 3 to 5 hours

– occurs after most the meal has been absorbed

– also called the migrating motility complex (MMC)

– 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– do NOT push the food through the tract– move chyme back and forth– done in specific segments that are

defined by the smooth muscle layers– most rapid in the duodenum and slows

at it reaches the ileum

24-37

-synthesis of the brush border enzymes by the intestinal glands

-maltase, sucrase, lactase, -dextrinase, enterokinase aminopeptidase, dipeptidase, phosphatases and nucleosidases -these enzymes are not secreted into the SI lumen but are expressed on the surface of the absorptive cell-duodenum is also the site for secretion of the pancreatic juice:

-trypsin, chymotrypsin, elastase, carboxypeptidase-1-2 qt./day------ at pH 7.6-Enzymes are made in the pancreas as inactive forms eg. Chymotrypsinogen, proelastase, procarboxypeptidase-Trypsin synthesized as trypsinogen - converted to trypsin by the enzyme enterokinase -Activated trypsin then converts others into active forms

Small Intestine-Chemical Digestion

24-38

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 (120g/hr)

– fructose passes through the apical membrane of the absorptive cells via facilitated transport

– glucose and galactose are transported into cells via secondary active transport coupled to the active transport of Na+ (symporter)

– glucose and galactose compete for the saccharide site on this transporter– monosacharrides leave through the basal surface and enter the blood via facilitated

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

jejunum– amino acids absorbed come from the food (50%) and from the digestive enzymes and the

degrading epithelial cells– 95-98% of the proteins present in the SI are digested and absorbed– different transporters carry different amino acids– some enter absorptive cells through Na+/amino acid symporters– others through H+.amino acid symporters– enter into the blood via diffusion

24-39

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

breakdown of a fat into 2 fatty acids + a monoglyceride

– the fatty acids are either short-chain or long chain

– once inside the cell – reassembled into triglycerides

– these fats aggregate with phospholipids and cholesterol to form globules (chylomicrons) which enter the lacteal from the absorptive cell via exocytosis

– enter the blood at the subclavian veins– removed from the blood as they pass

through the liver– liver expresses lipoprotein lipase (LPL) –

breaksdown the TGs and the cholesterol in the chylomicron into fatty acids

– hepatocytes and adipose cells recombine these components to reform the triglyceride

– adipose cells are also capable of breaking down TGs (lipolysis) by expressing the LPL enzyme

SI: Absorption of digested nutrients

24-40

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

24-41

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

24-42

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

24-43

Histology of Large Intestine

• Mucosa– smooth tube -----no villi or plica folds– mostly contains absorptive cells with microvilli (water) in its epithelial component + goblet cells– both absorptive and goblet cells are located in long tubular intestinal glands = crypts of Lieberkuhn)– lamina propria and muscularis mucosae also– intestinal glands fill the the mucosa– simple columnar cells absorb water & goblet cells secrete mucus

• Submucosal & mucosa contain lymphatic nodules

24-44

Mechanical Digestion in Large Intestine

• Smooth muscle = 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 –intensifies the peristaltic waves in the ilium and the older chyme then enters the caecum

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

24-45

Chemical Digestion in Large Intestine

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

• chyme is prepared by the action of bacteria• 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

• Bacteria produce vitamin K and B in colon

24-46

Absorption & Feces Formation in the Large Intestine

• food has now been in the GI tract for 3 to 10 hours• solid or semisolid due to water reaborption = feces• feces – water, salts, sloughed-off epithelial cells, bateria, 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• also absorbs some electrolytes---Na+ and Cl- and vitamins• 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 cholesterol to make more bile salts – takes this cholesterol from the

blood• 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

24-47

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

24-48

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

24-49

Histology of the Pancreas

• Acini- dark clusters – 99% of gland

– produce pancreatic juice

• Islets of Langerhans– 1% of gland

– pale staining cells

– produce hormones

24-50

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

24-51

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

24-52

Histology of the Liver

• Hepatocytes arranged in lobules

• bile synthesized and drains into bile canals = canaliculi

• blood flowing into liver from intestine via hepatic portal vein

• divides into sinusoids – “cleaning” by cells

• Kupffer cells phagocytize microbes & foreign matter

24-53

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

24-54

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 brush border of the

absorptive cells– the long-chain fatty acids and monoglycerides then diffuse out into

the absorptive cells cytoplasm – leaving the micelles behind (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

24-55

Liver Functions--Carbohydrate Metabolism

• Turn proteins into glucose

• Turn triglycerides into glucose

• 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

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

24-56

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)

24-57

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

• Intestines– synthesis of brush border

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

24-58

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 (nucleosidease and phosphatase)– pentose, phosphate & nitrogenous bases

• Absorbed by active transport