Digestive System

Digestive System

Digestive System Objectives 2. To understand the mechanism of defecation 3. To understand the functional anatomy of GIT 4. To understand the actual mechanical aspects of food ingestion and digestion 5. To understand how foods are chemically digested by the different GIT segments 6. To know the basic principles of GI absorption

Digestive System

Digestive System Processes of Digestion: 2. Movt of food through the alimentary tract 3. Secretion of digestive juices and digestion of food 4. Absorption of H2O, various electrolytes and digestive products 5. Circulation of blood thru GI organs to carry away the absorbed substances 6. Control of all these fxns by local, nervous, and hormonal systems

Digestive System GI Wall

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Motor fxns of the gut performed by the different layers of smooth ms Individual smooth ms fibers arranged in bundles of parallel fibers

Digestive System Ms fibers electrically connected with one another thru large numbers of gap junctions (more rapid along length of bundle) • Each ms layer fxns as a syncytium • N resting membrane potential: -50 - -60 mV (ave: -56 mV) • Has 2 types of electrical waves •

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Slow vs spikes

Digestive System Slow waves Rhythmic Not action potentials (AP) Intensity:5-15 mV Freq: 3-12/min Cause: interstitial cells of Cajal (electrical pacemakers for smooth ms cells); Na ions only so (-) ms cx Not cause ms cx except in stomach (precx for spikes)

Spikes (peaks of slow waves) *Tonic (continuous repetitive spikes) True AP Has longer duration of AP (large number of Ca ions) Occur with resting membrane potential more positive than -40 mV

Digestive System Enteric NS 2 plexuses: Myenteric/Auerbach’s (outer) 1° fxn:controls GI movts Extends entire length of GIT Excitatory + inhibitory (to pyloric and ileocecal valve sphincters)

Submucosal/Meissner’s (inner) Controls local intestinal secretion and local blood flow and absorption amd local cx of submucosal ms Inner wall of each minute segment of intestine

Digestive System Principal effects of myenteric plexus: 2. Inc tonic cx/ “tone” of gut wall 3. Inc intensity of rhythmical cxs 4. Slightly inc rate of rhythm of cx 5. Inc velocity of conduction of excitatory waves along the gut wall

Digestive System Neurotransmitters: Acetylcholine (excitatory) vs Nor/epinephrine (inhibitory) Autonomic NS: SNS PNS Cranial (vagus n which is T5 – L2 more afferent; more for Innervate all of GIT upper digestive tract Norepinephrine (1 °), (including pancreas) epinephrine Sacral (2nd-4th segments; for Inhibits intestinal tract defecation reflexes) smooth ms (except mucosal Inc activity of ENS ms) More extensive inn to oral cavity and anus

Digestive System SNS

PNS

Intense vasoconstriction to Inc local blood flow due to dec blood flow (direct) inc glandular sec (indirect) Needed for emergency for inc blood flow to other areas during heavy ex and circulatory and hemorrhagic shocks

Digestive System GI Reflexes: Within gut wall ENS

From gut to prevertebral sympathetic ganglia and back to GIT For GI secretion, Gastrocolic peristalsis, Enterogastric mixing cx, local Colonoileal inhibition

From gut to SC/brainstem and back to GIT

By vagus n Pain (inhibitory) defecation

Digestive System 

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Gastrocolic – from stomach to cause evacuation of colon Enterogastric – from colon and small intestine to inhibit stomach moility and secretion Colonoileal – from colon to inhibit emptying of ileal contents into colon Defecation – from colon and rectum to SC and back for powerful colonic, rectal, and abdominal cxs for defecation

Digestive System GI Blood Flow (Splanchnic) Portal vein ↓liver sinusoids Hepatic vein ↓ Vena cava Arterial Circulation: sup and inf mesenteric a (small and large intestines) Celiac artery (stomach)

Digestive System Hormonal Control of GI Motility Hormone

Location

Stimulus

Action

Gastrin

“G” cells of antrum of stomach

Ingestion of meal (stomach distention, products of protein)

Stimulation of gastric acid secretion and growth of gastric mucosa; promotes stomach emptying

Cholecystokinin

“I” cells in mucosa of duodenum and jejunum

Digestive products of fat, fatty acids, and monoglycerides in intestinal contents

contracts gallbladder, expelling bile to small intestine (Strong; fat emusification); inhibits stomach cx for slow emptying of food from stomach (moderate)

Secretin

“S” cells in mucosa of duodenum

Acidic gastric juice emptying into duodenum from pylorus of stomach

Slows motility of GIT (mild);promotes pancreatic sec of bicarbonate to neutralize acid in small intestine; opposes stomach sec

Digestive System Hormone

Location

Stimulus

Action

Gastric inhibitory peptide

Mucosa of upper small intestine

Fatty acids and amino acids, carbohydrate (less)

Dec stomach motor activity (mild), slowing emptying of gastric contents into duodenum when upper small intestines overloaded); stimulates sec of insulin by pancreas

Motilin

Upper duodenum

during fasting; inhibited p ingestion

Inc GI motility

Digestive System Functional Types of Movs in GIT: 2. Propulsive  By peristalsis stimulated by gut distention (2-3 cm behind); heavily depends to active myenteric plexus (myenteric/peristaltic reflex)+ anal dir (Law of the Gut) 2. Mixing  By peristalsis and local intermittent constrictive cxs

Digestive System Mechanics of Ingestion 2. Mastication (Chewing)- teeth *chewing reflex *rate of digestion depends on total surface area exposed to digestive sec 2. Swallowing (Deglutition) 3 stages: voluntary, pharyngeal (reflex), esophageal

Digestive System

Digestive System Motor Functions of Stomach 2. Storage *vagovagal reflex Accommodating capacity: 0.8-1.5L 2. Mixing *chyme; by mixing (slow) waves, retropulsion (peristalsis and upstream squeezing action) 3. Slow emptying at an appropriate rate – pyloric pump (peristalsis + mixing); regulated more by inhibition in duodenum

Digestive System Motor Functions of Small Intestine 2. Mixing Cxs (Segmentation) By slow waves; also depends on myenteric n plexus

Digestive System 2. Propulsive Cxs - By peristalsis (weaker and very slow; faster in proximal than distal intestine); - Net movt: 1 cm/min (35 hrs passage from pylorus to ileocecal valve)

Enhance Motility Gastrin CCK Insulin Motilin Serotonin

Inhibits Motility in Small Int Secretin Glucagon

The function of peristalsis in small intestine is not only to cause progression of chyme toward ileocecal valve but also to spread out the chyme along the intestinal mucosa

Digestive System Motor Functions of Large Intestine (Colon) – very slow movts 2. Absorption of water and electrolytes from chyme to form solid feces (upper ½)

Digestive System 2. Storage of feces until expelled (lower ½) haustrations – mixing and minor propulsion Propulsive – slow but presistent haustrations (8-15 hrs) vs mass movts (facilitated by *gastrocolic and duodenocolic reflexes)

Digestive System Defecation (both voluntary and subconscious control by relaxation of external and internal anal sphincters)

Digestive System Secretory glands: 2. Digestive enzymes – all are proteins 3. Mucous glands – lubrication and protection of GIT * salivary glands and pancreas lie outside the walls of alimentary tract, with glandular cells emptying secretions into the GIT

Digestive System Autonomic Regulation SNS Slight to moderate inc in sec Vasoconstriction (so reduces sec more than inc)

PNS Inc rates of alimentary glandular sec (upper tract and of large intestine)

Digestive System Secretion of Saliva (800-1500 mL/day; pH: 6.0-7.0, suitable for ptyalin; has K and bicarbonate) Salivary Glands

Secretion

Parotid

serous

Submandibular

Serous and mucus

Sublingual

Serous and mucus

Buccal

mucus

Serous – has ptyalin for starch digestion Mucus – has mucin for lubrication and surface protection

Digestive System ↓ Salivation

Rough

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Sour taste Smooth objects in mouth Favorite foods PNS

Digestive System Esophageal Secretion – mucous (lubrication and protection, esp to peptic ulcer) Gastric Secretion 3. mucus-secreting cells 4. Tubular glands 1.

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Oxyntic/gastric (acid-forming) – HCl, pepsinogen (+HCl = pepsin for protein digestion in acid solution of pH of 5), intrinsic factor (for vit B12 absorption in ileum), mucus (prox 80% stomach) Pyloric – mucus (alkaline), gastrin (distal 20%)

Digestive System Phases of Gastric Secretion Even before food enters stomach; 20% of total gastric sec; inc c greater appetite

70% of total gastric sec; stimulated by gastrin

Digestive System Pancreatic Secretion  secreted most abundantly in the presence of chyme in the upper portion of small intestine  contains bicarbonate ions for neutralizing acidity of chyme Digestion

Inactive Secretions

Activated Enzymes (p secreted into intestinal tract)

Protein

Trypsinogen Chymotrypsinogen

trypsin (most), chymotrypsin, etc

Carbo

pancreatic amylase

Fat

Pancreatic lipase, cholesterolesterase, phospholipase

Digestive System *papilla of Vater – where pancreatic sec and bile empty into the duodenum

Digestive System Secretions of Small Intestine Brunner’s glands – secrete alkaline mucus *duodenum – frequent site of peptic ulcer (~50%) * Small intestine has villi for absorption of nutrients and their digestive products ino the blood Enzyme

Action

Peptidases

Peptides into amino acids

Sucrase, maltase, isomaltase, lactase

Dissac into monosaccharides

Intestinal lipase

Neutral fats into glycerol and fatty acids

Digestive System Secretions of Large Intestine  Mucosa has no villi  Epithelial cells mainly of mucous cells secreting alkaline mucus (ph: 8.0) – inc sec along with inc peristaltic motility of colon by PNS

Digestive System Digestion in GIT

Digestion

Sources

Enzyme

Carbohydrates

Sucrose, lactose, starches

Mouth and stomach:ptyalin (parotid g) Small int:pancreatic amylase (lactase, sucrase, maltase, alphadextrinase) *before chyme goes to duodenum, virtually all carbo are digested

Proteins

Meats

Stomach: Pepsin, esp for collagen (most active: 2.0-3.0 pH; inactive:> 5.0) Upper small int duodenum and jejunum (most): proteolytic enzymes from pancreas: trypsin, chymotrypsin, elastase; enterocytes

lining the villi of small int – for final digestion into amino acids into the blood

Fats Animal Neutral/Triglyce (most) rides

Stomach: lingual lipase (secreted by lingual glands in mouth and swallowed with saliva) Small intestine: all occurs; also has enteric lipase from enterocytes *fat emusification in duodenum by bile acids *pancreatic lipase – most impt

Digestive System Absorption in GIT  Total quantity of fluid that must be absorbed each day by the intestines = ingested fluid (1.5 L)+ secreted in GI secretions (7 L) – 8-9 L (1.5L not absorbed in the intestine)  Depends on absorptive capacity of villus (- in stomach, which can absorb lipid soluble substances like alcohol and aspirin)

Digestive System Less in distal small int; inc Absorptive area another 10x

Or folds Kerckring – well developed in duodenum and jejunum; inc absorptive area 3x

Digestive System On each epithelial cell on each villus; inc absorptive capacity for another 20x

Combination of 3 inc total absorptive area of mucosa ~1000x

Digestive System Absorption in Small Intestine Substance

Mechanism

Water

Diffusion

Ions

Ca, Fe – active from small int

Nutrients Carbo

All

Proteins

Fats

monosaccharides- active thru facilitated diffusion (+ Na in glucose; - in fructose) Same (Na co-transport/facilitated diffusion) Direct diffusion to portal blood then absorbed thru lymphatics (hort and medium)

Digestive System Diarrhea – secretion of NaCl and water into intestinal lumen greater than reabsorption (loss of 5-10L/day)

Digestive System Absorption in Large Intestine *Feces – only has <100 mL of fluid (1-5mEq); composition: ¾ H20; ¼ solid  Most in prox ½ (absorbing colon) vs distal ½ (storage colon)  Can absorb max of 5-8 L of fluid and electrolytes/day

Digestive System

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