The Digestive System

Anatomy and Physiology II

BIO 232

Overview of the Digestive System

Digestive Processes

• Ingestion - taking food into the system

• Propulsion - moves food through the system

• Swallowing

• Peristalsis - wavelike contraction

• Mechanical digestion

• Chewing

• Churning food in the stomach

• Segmentation (rhythmic local constrictions of the small intestine)

• Chemical digestion

• Absorption

• Defecation

Structure of the Alimentary Canal

• Mucosa - innermost layer

• Major functions - secretion of mucus, digestive enzymes & hormones, absorption & protection against infection

• Layers

• Epithelium (Simple columnar epithelial cells)

ØGoblet cells

• Lamina Propria - connective tissue

• Muscularis - thin smooth muscle layer

Structure of the Alimentary Canal

• Submucosa - just external to mucosa - moderately dense connective tissue

• Blood vessels, lymphatic vessels, lymph nodes and nerve fibers

• Muscularis externa - deep to the submucosa

• Responsible for peristalsis and segmentation

• Inner circular smooth muscle & outer longitudinal smooth muscle

• Sphincter - thicken areas acts as valves

• Sersosa - outermost protective layer

• Connective tissue & outer layer of squamous epithelium

3d View of Alimentary Wall

Mouth

• Also called oral or buccal cavity

• Lips and cheeks - core of skeletal muscle covered externally with skin - lined with mucosa

• The palate - roof of the mouth

• Hard palate - palatine bones and palatine processes of maxillae

• Soft palate - mobile fold mostly of skeletal muscle

• Uvula - finger-like projection

• Rises reflexively during swallowing to close off nasopharynx

• Tongue - floor of the mouth - interlacing bundles of skeletal muscle (intrinsic muscle)

• Extrinsic muscles - from skull to tongue

Mouth

• Salivary glands

• Produce salivary amylase, bicarbonate (buffer), antibodies, lysozyme (antibacterial), defensins (immune function) and mucins (proteins that help soften & lubricate the food)

• Parotid

• Swell with mumps

• Sublingual

• Submandibular

Teeth

• Crown

• Enamel - Calcium compounds

• Dentin - Thick layer bony-like material

• Inner pulp - nerves and blood vessels

• Root - Also has dentin & pulp

Pharynx

Esophagus

• 10 inches long - collapsed when not in use

• Food moves my peristalsis

• Cardiac orifice is surrounded by cardiac or gastroesophageal sphincter

Swallowing (Deglutition)

Swallowing (Deglutition)

Swallowing (Deglutition)

Stomach

Microscopic Anatomy of the Stomach

• Simple columnar epithelium lining

• Renewed every 3 to 6 days

• Goblet cells - bicarbonate rich mucous

• Dotted with millions of deep gastric pits

• Lead to gastric glands - secrete gastric juice

Microscopic Anatomy of the Stomach

• Mucous neck cells - acidic mucous (?)

• Parietal cells

• Middle region of the gland

• HCl & intrinsic factor (glycoprotein - absorption of vitamin B₁₂

• Chief cells

• Produce pepsinogen (inactive form of pepsin)

• Enteroendocrine cells

• Secretes hormones (gastrin, histamine, endorphins, serotonin, cholecystokinin & somatostatin

Microscopic Anatomy of the Stomach

Digestive Processes in the Stomach

• Pepsin - protein digestion

• Rennin - in children - acts on milk

• Aspirin & alcohol - absorbed by the stomach

• Intrinsic factor - essential for life - required for the absorption of vitamin B₁₂ by the intestine - needed to produce mature RBCs

Regulation of Gastric Function

• Gastric motility & secretion - automatic

• Contractility via pacesetter cells in greater curvature of the stomach

• Secretion of HCl & pepsinogen

• Stim of these in the absence of neural & hormonal influences

Regulation of Gastric Function

• Cephalic Phase

• Sight, smell & taste - stimulate Vagus nuclei in the brain

• Vagus stimulates acid secretion

• Direct stimulation of parietal cells - major effect

• Stimulation of gastrin secretion - stimulates acid secretion - lesser effect

Regulation of Gastric Function

• Gastric Phase

• Food arrives in the stomach - gastric secretion via stomach distension

• Stimulates Vagus - this stimulates Vagus induced secretion of acid

Regulation of Gastric Function

• Gastric Phase

• AAs & peptides in stomach - stimulate acid secretion

• Direct stimulation of parietal cells - lesser effect

• Stimulation of gastrin secretion - stimulates acid secretion - major effect

Regulation of Gastric Function

• Gastric Phase

• Gastrin secretion inhibited when pH of gastric juice falls below 2.5

• Stops at pH 1

Regulation of Gastric Function

• Intestinal Phase

• Neural Inhibition of gastric activity when chyme enters duodenum

• Via distension of duodenum & increased osmolality (incr osmotic pressure) - activates neural reflex - inhibits gastric motility & secretion

Regulation of Gastric Function

• Intestinal Phase

• Inhibitory hormone released from duodenum in response to fat in chyme

• Enterogastrone

• Maybe somatostatin, & CCK

• Gastric inhibitory peptide - may be involved

• But may just stimulate insulin secretion from pancreas in response to glucose in the intestine

Small Intestine

• Small diameter - 3 meters long

• Large intestine - 1.5 meters long

• First 20-30 cm - duodenum

• Next 2/5 - jejunum

• Last 3/5 - ileum

• Empties through ileocecal valve in large intestine

Small Intestine

• Products of digestion are absorbed across the epithelial lining

• Carbohydrates, lipids, AAs, calcium & iron

• Primarily absorbed in duodenum & jejunum

Small Intestine

• Bile salts, Vitamin B12, water & electrolytes

• Primarily absorbed in ileum

• The mucosal surface is thrown into large folds - plicae circulares

• This surface is further infolded sharply - villi

• Apical surface of epithelial cells further folded into microvilli

Villi & Microvilli

• Each finger like fold of the mucosa - villus

• Covered with columnar epithelium & mucous secreting goblet cells

• Connective core of the villi

• Lymphocytes, blood capillaries & lacteals (lymphatic vessels)

• Capillaries absorb - monosaccharides & AAs

• Lacteals absorb - fats

Villi & Microvilli

• Microvilli

• Tiny fingerlike folds of the apical surface of epithelial cells

• Looks like a brush - called a brush boarder

Structure of the Villi & Microvilli

Liver

• Hepatocytes - liver cells

• Hepatic plates - one or two hepatocytes thick

• Plates separates from each other by sinusoids (large capillary spaces)

• Lined with Kupffer cells - fixed phagocytes

Hepatic Portal System

• Products of digestion - absorbed by blood capillaries & delivered to liver

• Capillaries of digestive tract drain into the hepatic portal vein - then to liver

• Portal system - capillaries to vein to capillaries to vein

Blood Flow and Bile

• Hepatic plates arranged in unit - Liver lobules

• Middle of each lobule is a central vein

• Periphery of lobule - is hepatic portal vein & hepatic artery

• Open into sinusoids - mixed

• Bile - hepatocytes - secreted into bile canaliculi

• Located within each hepatic plate

• Drain to bile ducts then to hepatic ducts - away from liver

Liver Lobule

Bile Production

• Produces - 250 to 1500 ml of bile/day

• Bile

• Bile salts, bile pigments (bilirubin), phospholipids (mainly lecithin), cholesterol & inorganic ions

Bile Pigment

• Bilirubin - via spleen, bone marrow - mostly liver

• Derived from heme group - minus iron

• Free bilirubin - not very water soluble

• Most carried in the blood attached to albumins

Bile Pigment

• Hepatocyes - pickup bilirubin-albumin combination - active transport

• Inside combined with glucuronic acid - conjugated bilirubin

• Water soluble & secreted into bile

Bile Pigment

• In the intestine - bacteria - to urobilinogen

• 30-50% reabsorbed by hepatic portal system & secreted in bile

• Enterohepatic circulation

• Urobilinogen - soluble in plasma - filtered by kidney

• Gives urine - yellow color

Gall Bladder

• Saclike organ attached to inferior surface of liver

• Stores & concentrates bile - 35 -100ml

• Bile continuously produced & drain through hepatic ducts to common duct to duodenum

• Small intestine empty - Sphincter Oddi - end of common duct - closes

• Forces bile up into cystic duct to be stored in gallbladder

Pancreas

• Pancreatic Juice

• Main Enzymes

• Amylase - digests starch

• Trypsin - digests protein

• Lipase - digests triglycerides

• Most enzymes produced in inactive molecules - zymogens

• Enzymes activated in small intestine

Regulation of Pancreatic Juice Secretion

• Secretion of pancreatic juice controlled by secretin & CCK

• Secretin - in response to a fall in duodenal pH below 4.5

• Stimulates production of bicarbonate by the pancreas

• CCK - in response to fat content in the chyme

• Stimulates the production of pancreatic enzymes

• Trypsin, lipase & amylase

Secretion of Bile

• Secreted continuously - greatly augmented fallowing a meal

• Secretin - major stimulator of bile secretion by the liver

• CCK enhance this effect

• Gallbladder - releases bile via neural reflexes from the duodenum

• Also CCK stimulation

Digestion & Absorption of Carbohydrates

• Digestion begins - salivary amylase (ptyalin)

• Cleaves bonds between adjacent gluc molecules

• Not very effective - don’t chew food long enough

• Action of amylase neutralized in stomach

Digestion & Absorption of Carbohydrates

• Digestion of Starch

• Mainly in duodenum - pancreatic amylase

• Brush boarder enzymes

• Monosaccharides move across epith cells - secondary active transport

• Then glucose secreted by epithelial cells to blood capillaries

Digestion & Absorption of Proteins

• Begins in stomach - pepsin

• Some AAs liberated - mostly short polypeptides

• Most occurs in duodenum & jejunum

• Via pancreatic enzymes

• Endopeptidases

• Trypsin, chymotrypsin & elastase

• Exopeptidases

• Carboxypeptidases & aminopeptidases

Digestion & Absorption of Proteins

• Absorption of AAs, dipeptides & tripeptides

• AAs - cotransport with Na+

• Di & tripeptides - other carriers - further digested inside

• Free AAs into blood capillaries

Digestion & Absorption of Lipids

• Emulsification

• Bile salt micelles secreted into duodenum

• These act to breakdown fat droplets into tiny emulsification droplets of triglycerides

• This creates greater surface area for pancreatic lipase

• Also colipase

• These remove 2 or 3 fatty acids

• Liberates FFAs & monoglycerides

Digestion & Absorption of Lipids

• Phospholipase A - pancreas

• Digests phospholipids

• e.g. Lecithin to FFAs, lysolecithin

Digestion & Absorption of Lipids

• Absorption of Lipids

• FFAs, monoglycerides & lysolecithin - more polar

• Quickly associate with micelles of bile salts, lecithin & cholesterol

• Mixed micelles

Digestion & Absorption of Lipids

• Absorption of Lipids

• Move to brushboarder & absorbed

• FFAs, monoglycerides & lysolecithin leave micelles - enter cell

• Reassembled inside to triglycerides & phospholipids

Digestion & Absorption of Lipids

• Absorption of Lipids

• Now triglycerides & phospholipids & cholesterol combined with protein

• Chylomicrons

• Secreted into lymphatic lacteals - eventually to venous drainage

Transport of Lipids in the Blood

• Once chylomicrons are in the blood

• Triglyceride content removed by lipoprotein lipase

• Hydrolyzes triglycerides to FFAs + glycerol to be used by the tissue cells

• Remains of chylomicrons, containing cholesterol - back to liver

Transport of Lipids in the Blood

• Cholesterol & triglycerides produced by liver

• Combine with apoprotein & secreted into blood as very low density lipoprotein - VLDLP

• Serves to deliver triglycerides to different organs

Transport of Lipids in the Blood

• Once triglycerides removed the VLDLP becomes LDL

• Transports cholesterol about the body

• Contributes to atherosclerosis

• Excess cholesterol returned to liver & attached to HDL

• High ratio of HDL cholesterol to total cholesterol - believed to offer protection against atheorsclerosis

Large Intestine (Colon)

• 1.5 meters in length

• Concentrates & stores undigested material

• Absorbs water

• Absorbs salts

• Absorbs vitamins

• Contains anaerobic bacteria

Gross Anatomy of the Large Intestine