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)
Cells secrete ~7 litres of fluids per day Mixing and movement of material along GI tract is termed motility Mechanical Chemical
Secretion
–
Mixing and Propulsion
–
Digestion
– –
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 Contains mucosaassociated lymphatic tissue causes folds which increase surface area
–
Lamina propria
–
Muscularis mucosae
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 abdominopelvic 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 Medicine
From: 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 Bolus stimulates stretch receptors in oropharynx
–
–
Pharyngeal stage
Send impulses to deglutition centre in brain stem
Involuntary passage of bolus into esophagus Involuntary passage of bolus through esophagus into stomach
–
Esophageal stage
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 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 Pancreatic lipase Ribonuclease Deoxyribonuclease
Proteins
– – – –
Triglycerides
–
Nucleic acids
– –
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 Synthesise lipoproteins and cholesterol Store triglycerides -oxidation
–
Lipid metabolism
–
Protein metabolism
Deaminate amino acids Synthesise plasma proteins
Detergent-like acidic buffer
–
–
Bile production
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 Trypsin, chymotrypsin, carboxypeptidase and elastase break protein down into peptides
–
–
Proteins
Each breaks different peptide bonds
Brush border enzymes aminopeptidase and dipeptidase break peptides into amino acids Bile salts emulsify triglycerides into small droplets Pancreatic lipase hydrolyses triglycerides Ribonuclease and deoxyribonuclease break nucleic acids into nucleotides Brush border enzymes (nucleosidases and phosphatases) break nucleotides into pentoses, phosphates and nitrogenous bases
–
Lipids
–
Nucleic acids
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
–
Percentage change in lactulose:rhamnose ratio
500 400 300 200 100 0 -100 -200 Bovine colostrum Whey protein Control
*
How then can foods containing functional proteins exert their effects?
eg bovine colostrum
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