The Digestive System

Organs of digestion

Gastrointestinal tract (alimentary canal)– mouth– pharynx– esophagus– stomach– small intestine– large intestine

Organs of digestion Accessory

structures of digestive system

– teeth– tongue– salivary glands– liver– gallbladder– pancreas

Digestive processes Ingestion

– Taking of food and liquid into the mouth (eating) Secretion

– Cells secrete ~7 litres of fluids per day Mixing and Propulsion

– Mixing and movement of material along GI tract is termed motility

Digestion– Mechanical– Chemical

Absorption Defecation

– Indigestible material eliminated as feces during defacation

Layers of GI tract Mucosa

– Epithelium cells firmly sealed by tight

junctions secretes mucus, digestive

enzymes and hormones Absorption

– Lamina propria Contains mucosa-

associated lymphatic tissue– Muscularis mucosae

causes folds which increase surface area

Layers of GI tract

Submucosa– Blood and lymphatic

vessels– Glands– Submucosal plexus

Regulates movement of mucosa and vasoconstriction of blood vessels

Layers of GI tract Muscularis

– Mouth, pharynx, upper esophagus and external anal sphincter contain skeletal muscle

– Rest of GI tract contains smooth muscle

inner circular and outer longitudinal layers

– Myenteric plexus between layers

• Controls motility

Layers of GI tract

serosa (visceral

peritoneum)– Forms portion of

peritoneum

Peritoneum Peritoneum

Largest serous membrane in body

– Parietal layer

• Lines wall of abdomino-pelvic cavity

– Visceral layer

• Covers some organs in cavity

– Peritoneal cavity

• Contains serous fluid

Folds bind organs to each other and to walls of abdominal cavity

Mouth - Salivary glands

Mucous membranes of mouth and tongue secrete small amount of saliva

Most saliva secreted by major salivary glands which lie outside the mouth

– Parotid– Submandibular– Sublingual

Mouth - composition and functions of saliva Saliva is ~99.5% water

– contains: IgA Lysozyme Salivary amylase Various solutes, organic substances, etc

Saliva functions to:– keep mucous membranes of mouth and pharynx moist– cleanse mouth and teeth– dissolve food– begin chemical digestion of carbohydrates (salivary amylase)

Mouth - control of salivation

Salivation under nervous

control– salivary nuclei in brain stem

Receive input from cortex,

taste buds, olfactory apparatus– Parasympathetic output

increases salivation

– Sympathetic output reduces

salivation (dry mouth when

stressed)Dr Ivan Pavlov, Winner 1904 Nobel Prize for

MedicineFrom: http://www.uic.edu/depts/mcne/founders/page0072.html

Mouth – digestion

Mechanical and chemical digestion occur in the

mouth

– Mechanical digestion results from mastication

– Chemical digestion begins Salivary amylase

– Initiates breakdown of starch

Lingual lipase– Hydrolyses triglycerides into fatty acids and glycerol

– Secreted in inactive form by glands in tongue

– Becomes activated in acid environment of stomach

Pharynx - deglutition

Swallowing occurs in 3 stages:– Voluntary stage

Bolus passed into oropharynx by tongue

– Pharyngeal stage Bolus stimulates stretch receptors

in oropharynx– Send impulses to deglutition

centre in brain stem Involuntary passage of bolus into

esophagus– Esophageal stage

Involuntary passage of bolus through esophagus into stomach

Esophagus

Collapsible muscular tube behind trachea– Secretes mucus and transports food into stomach

Passes through mediastinum

Pierces diaphragm through esophageal hiatus

Upper and lower sphincters

– lower sphincter is physiological sphincter

– Diaphragm helps keep lower sphincter closed when not swallowing

Food moves via peristalsis

– Controlled by neurons in medulla oblongata

Stomach

Stomach

– acts as a mixing chamber and

holding reservoir

– Starch digestion continues

– protein and triglyceride digestion

begins Muscularis has 3 layers

– Additional oblique layer

Stomach Columns of secretory cells form gastric

glands which line gastric pits– Mucous cells

Secrete mucus

– Parietal cells secrete: hydrochloric acid Intrinsic factor

– Required for absorption of vitamin B12

– Chief cells secrete: Pepsinogen Gastric lipase

– G cells Secrete gastrin

Stomach

Mechanical digestion– After food enters stomach mixing waves occur every 15-25 sec

Aid in mechanical digestion to form chyme Forces chyme into duodenum (start of small intestine)

Chemical digestion– HCL

denatures proteins

– Pepsinogen converted to pepsin in presence of HCL and other pepsin molecules Hydrolyses peptide bonds

– Gastric lipase hydrolyses triglycerides

Regulation of gastric secretion and motility

Cephalic phase– Sight, smell, taste or thought of food recognised by cerebral cortex or feeding

centre in hypothalamus Nerve impulses sent to medulla oblongata

– Sends impulses to submucosal plexus (in submucosa)• Increases secretion from gastric glands and stomach motility (via gastrin

secretion) Gastric phase

– when food reaches stomach stimulates Stretch receptors Chemoreceptors (monitor pH)

– Stimulate secretion of Gastrin (by G cells)• Maintains gastric secretion and motility

Intestinal phase– Stimulation of intestinal receptors as food enters small intestine stimulates

secretion of Secretin – reduces gastric secretion CCK-inhibits gastric emptying

– Slows exit of chyme from stomach into duodenum

Pancreas

Pancreas connected to

duodenum

– Secretes pancreatic juice into

duodenum

99% of cells arranged in

clusters called acini

– Exocrine portion of gland

Secrete pancreatic juice

Pancreas – pancreatic juice Pancreatic juice contains:

– Water– Salts– Sodium bicarbonate– Several enzymes which digest:

carbohydrates– Pancreatic amylase

Proteins– Trypsin - secreted in inactive form and activated by brush-border enzyme called

enterokinase– Chymotrypsin – secreted in inactive form and activated by trypsin– Cartboxypeptidase – secreted in inactive form and activated by trypsin– Elastase – secreted in inactive form and activated by trypsin

Triglycerides– Pancreatic lipase

Nucleic acids– Ribonuclease– Deoxyribonuclease

Liver Heaviest gland in body Two lobes

– Large right lobe– Smaller left lobe

Lobes made up of functional units called lobules

– Six-sided structure with hepatocytes arranged around central vein

– Blood from hepatic artery and hepatic portal circulation passes through sinusoids and drains into central veins

– Bile enters bile canaliculi and moves to gallbladder

Blood supply

Hepatic artery – delivers oxygenated

blood

Hepatic portal circulation– delivers nutrient rich

blood from intestines

Liver Functions of liver:

– CHO metabolism glycogenolysis gluconeogenesis glycogenesis

– Lipid metabolism Synthesise lipoproteins and cholesterol Store triglycerides -oxidation

– Protein metabolism Deaminate amino acids Synthesise plasma proteins

– Bile production Detergent-like acidic buffer

– Emulsifies lipids– Process drugs and hormones– Excrete bilirubin

Derived from heme of worn out RBC (secreted into bile)– Store vitamins and minerals– Phagocytosis of RBC, WBC and bacteria (Kupffer’s cells)– Activation of vitamin D

Gall bladder

Bile production signalled by parasympathetic activity

secretin - released when acidity in

duodenum (secretin also inhibits

gastric secretion)

Bile stored and concentrated in

gallbladder

– released when fatty acids and amino

acids enter duodenum

signalled by CCK (also inhibits

gastric emptying)

Small intestine

Extends from pyloric sphincter to ileocecal valve

3 parts:– duodenum– jejunum– ileum

Small intestine

Most digestion and absorption of nutrients occurs in SI

Length gives large surface area (3m in living person)

– Surface area increased by: circular folds (plicae

circulares) - mix chyme villi - capillaries and

lacteals microvilli

Small intestine

Mechanical digestion

– Segmentation mixes chyme

– Peristalsis (migrating motility complex) occurs

once absorption complete Slowly migrates along SI over 90-120 min period

– Chyme remains in SI for 3-5 hours

Small intestine Chemical digestion

– CHO Pancreatic amylase splits starch into smaller fragments

– Brush border enzyme ( -dextrinase) then breaks down to glucose Disaccharides (sucrose, lactose and maltose) broken down by brush border enzymes

– Proteins Trypsin, chymotrypsin, carboxypeptidase and elastase break protein down into peptides

– Each breaks different peptide bonds Brush border enzymes aminopeptidase and dipeptidase break peptides into amino acids

– Lipids Bile salts emulsify triglycerides into small droplets Pancreatic lipase hydrolyses triglycerides

– Nucleic acids Ribonuclease and deoxyribonuclease break nucleic acids into nucleotides Brush border enzymes (nucleosidases and phosphatases) break nucleotides into

pentoses, phosphates and nitrogenous bases

Absorption Monosaccharides

– Secondary active transport with sodium

– facilitated diffusion (fructose)

Amino acids, dipeptides, tripeptides– amino acids primary or secondary

active transport

– di- and tripeptides secondary active transport

All move into capillaries in villus

Absorption Lipids

– Absorbed via simple diffusion Short-chain fatty acids move into

capillaries in villus Others move into lacteals

– bile combines with long-chain fatty acids and monoglycerides to form micelles

micelles contact epithelial cell membrane

lipids diffuse through membrane resynthesised to triglycerides inside

epithelial cells coated with proteins to form

chylomicrons chylomicrons too large to move into

capillaries and move into lacteals

Absorption

Large molecules (eg

complete proteins) not

absorbed

– How then can foods

containing functional

proteins exert their effects?

eg bovine colostrum Perc

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Bovine colostrumWhey proteinControl

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Adapted from: Buckley JD, GD Brinkworth, E Southcott, RN Butler. Bovine colostrum and whey protein supplementation during running training increase intenstinal

permeability. Asia Pacific Journal of Clinical Nutrition 13:s81, 2004.

Large intestine

Approx 1.5 m long Extends from ileocecal sphincter

to anus Tonic contraction of three

longitudinal muscles (teniae coli) form pouches (haustra)

4 divisions:– cecum– colon– rectum– anal canal

internal sphincter - smooth muscle

external sphincter - skeletal muscle

Large intestine

No villi or circular folds in mucosa

Epithelium contains mostly absorptive cells (water absorption) and goblet cells (secrete mucus)

– Located mostly in intestinal glands

Large intestine Mechanical digestion

– Movements of large intestine begin when substances pass iliocecal sphincter

Haustral churning– distention of haustra as chyme enters LI initiates haustral churning

Peristalsis occurs at slower rate than in SI Mass peristalsis

– Strong peristaltic wave that begins at mid-transverse colon drives contents into rectum

• Occurs during or immediately after meal when food enters stomach

Chemical digestion– Final stage of digestion occurs in LI through activity of bacteria

Produces gases and other by-products– Eg vitamins