Unit 2: Digestion

Digestive System Definition

The organ system that processes food, extracts nutrients from it, and eliminates the residue

Five Stages of Digestion

1. Ingestion

2. Digestion

3. Absorption

4. Compaction

5. Defecation

Stage one: Ingestion

selective intake of food

Stage two: digestion

mechanical and chemical breakdown of food into a form usable by the body

Stage three: absorption

uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood & lymph

Stage four: compaction

absorbing water and consolidating the indigestible residue into feces

Stage five: defecation

elimination of feces

Digestive tract (alimentary canal)

• 30-foot-long muscular tube extending from mouth to anus

• mouth, pharynx, esophagus, stomach, small intestine & large intestine

• Gastrointestinal (GI) tract is the stomach and intestines 

Accessory organs in the digestive system

• teeth

• tongue

• salivary glands

• liver

• gallbladder

• pancreas

Tissue Layers of GI Tract

• Mucosa

• Submucosa

• Muscularis Externa

Mucosa (GI Tract)

• Stratified squamous

• Lamina propria

• Muscularis mucosae

Submucosa (GI Tract)

Esophageal gland

Muscularis externa (GI Tract)

• Inner circular layer 

• Outer longitudinal layer

Lesser Omentum

• Attaches stomach to liver

Greater Omentum

• covers small intestines like an apron

Mesentery

Mesentery of small intestines holds many blood vessels

Mesocolon

Mesocolon anchors colon to posterior body wall

Regulation of Digestive Tract

Motility and secretion of the digestive tract are controlled by

• neural

• hormonal

• paracrine mechanisms

Neural Control

• Short (myenteric) reflexes:

  • stretch or chemical stimulation acts through myenteric plexus

• Long (vagovagal) reflexes:

  • parasympathetic stimulation of digestive motility and secretion

Hormones

• Chemical messengers secreted into bloodstream, and stimulate distant parts of the digestive tract

• gastrin & secretion

Paracrine secretions

• Chemical messengers that diffuse through the tissue fluid to stimulate nearby target cells

Teeth: Alveolus

• Tooth socket in bone

  • gomphosis joint between tooth & bone

Teeth: Periodontal ligament

• Modified periosteum whose collagen fibers penetrate into the bone on one side and into the tooth on the other

  • anchors tooth firmly in alveolus

  • allows slight movement under pressure of chewing

Teeth: Gingiva (gum)

covers the alveolar bone

Tooth structure: Dentin

• Hard yellowish tissue that makes up most of the tooth

Tooth Structure: Enamel

• Covers crown & neck

• Enamel is noncellular secretion formed during development

Tooth structure: Cementum

• Covers root

Tooth structure: Root canal

• Root canal in the roots leading to pulp cavity in the crown

  • nerves & blood vessels 

  • apical foramen — pore at the basal end of each root canal

Tooth structure: Occlusion

• Meeting of the teeth with the mouth closed

Cementum & Dentin

• Both are living tissue & can regenerate

Saliva

• Hypotonic solution of 97.0% to 99.5% water

• pH of 6.8 to 7.0

Saliva Solute: Salivary amylase

• Enzyme that begins starch digestion in the mouth

Saliva Solute: lingual Lipase

• Enzyme that is activated by stomach acid * digests fat after the food is swallowed

Saliva Solute: lysozyme

• Enzyme that kills bacteria

Minor Saliva solutes

• mucus

  • binds & lubricates the mass of food & aids in swallowing

• immunoglobulin A (IgA)

  • an antibody that inhibits bacterial growth

Salivary Glands

• Parotid gland

• Submandibular gland

• Sublingual gland

Salivary ducts

• Parotid duct

• Submandibular duct

• Sublingual duct(s)

Cells of Gastric Glands

• Mucous cells: secrete mucus

• Regenerative (stem) cells

• Parietal cells

  • secrete hydrochloric acid (HCI), intrinsic factor, and a hunger hormone called ghrelin

Cells of Gastric Glands: Chief cells

• Most numerous

• Secrete gastric lipase & pepsinogen

• dominate lower half of gastric glands

• absent in pyloric & cardiac glands

Cells of Gastric Glands: Enteroendocrine cells

• Concentrated in lower end of gland

• Secrete hormones & paracrine messengers that regulate digestion

Hydrochloric Acid

• Gastric juice has a high concentration of hydrochloric acid

  • pH as low as 0.8

• Parietal cells produce HCI & contain carbonic anhydrase (CAH)

Pepsin

• Digests dietary proteins into shorter peptide chains

  • protein digestion is completed in the small intestine

• Pepsinogen & Zymogens

Pepsin: Zymogens

• Digestive enzymes secreted as inactive proteins

  • converted to active enzymes by removing some of their amino acids

Pepsin: Pepsinogen

• Zymogen secreted by the chief cells

  • hydrochloric acid removes some of its amino acids & forms pepsin that digests proteins

  • autocatalytic effect—as some pepsin is formed, it converts more pepsinogen into more pepsin

Intrinsic Factor Definition

• a glycoprotein secreted by parietal cells

Intrinsic Factor (1)

• Essential to absorption of vitamin B12 by the small intestine

  • binds vitamin B12 and intestinal cells absorb this complex by receptor-mediated endocytosis

Intrinsic Factor (2)

• Vitamin B12 is needed to synthesize hemoglobin

  • prevents pernicious anemia

Intrinsic Factor (3)

• Secretion of intrinsic factor is the only indispensable function of the stomach

  • digestion can continue if stomach is removed (gastrectomy), but B12 supplements will be needed

Digestion & Absorption (1)

• Salivary & gastric enzymes partially digest protein & lesser amounts of starch & fat in the stomach

Digestion & Absorption (2)

• Most digestion & nearly all absorption occur after the chyme has passed into the small intestine

Digestion & Absorption (3)

• Stomach does not absorb any significant amount of nutrients

  • aspirin

  • some lipid-soluble drugs

Digestion & Absorption (4)

• Alcohol is absorbed mainly by small intestine

  • intoxicating effects depends partly on how rapidly the stomach is emptied

Protection of the Stomach

• Living stomach is protected in three ways from the harsh acidic & enzymatic environment it creates

• Breakdown of these protective measures can result in inflammation & peptic ulcer

Protection of the stomach (1): Mucous Coat

• thick, highly alkaline mucus resists action of acid & enzymes

Protection of the stomach (2): Tight Junctions

• between epithelial cells prevent gastric juice from seeping between them & digesting the connective tissue of the lamina propria & beyond

Protection of the stomach (3): Epithelial Cell Replacement

• Stomach epithelial cells live only 3 to 6 days

Regulation of Gastric Function: Step One

• Cephalic Phase

  • Vagus nerve stimulates gastric secretion even before food is swallowed

Regulation of Gastric Function: Step Two

• Gastric Phase

  • Food stretches the stomach & activates myenteric & vagovagal reflexes. These reflexes stimulate gastric secretion. Histamine & gastrin also stimulate acid & enzyme 

Regulation of Gastric Function: Step Three

• Intestinal Phase

  • Intestinal gastrin briefly stimulates the stomach, but then secretin, CCK, and the enterogastric reflex inhibit gastric secretion & motility while the duodenum processes the chyme already in it. Sympathetic nerve fibers suppress gastric activity, while vagal (parasympathetic) stimulation of the stomach is now inhibited. 

Functions of Hepatocytes (1)

• After a meal, the hepatocytes absorb from the blood

  • glucose, amino acids, iron, vitamins, & other nutrients for metabolism or storage

Functions of Hepatocytes (2)

• Removes & degrades

  • hormones, toxins, bile pigments, & drugs

Functions of Hepatocytes (3)

• Secretes into the blood

  • albumin, lipoproteins, clotting factors, angiotensinogen, & other products

Functions of Hepatocytes (4)

• Between meals, hepatocytes breaks down stored glycogen & releases glucose into the blood

Bile (1)

• Yellow-green fluid containing minerals, cholesterol, neutral fats, phospholipids, bile pigments, & bile acids

  • bilirubin—principal pigment derived from the decomposition of hemoglobin

  • bacteria in large intestine metabolize bilirubin to urobilinogen

    • responsible for the brown color of feces

Bile (2)

• Bile acids (bile salts)—steroids synthesized from cholesterol 

Bile (3)

• Gallstones may form if bile becomes excessively concentrated

Pancreatic zymogens

• trypsinogen

• chymotrypsinogen—converted to trypsinogen by trypsin 

• procarboxypeptidase— converted to carboxypeptidase by trypsin

Other pancreatic enzymes

• Pancreatic amylase—digests starch

• Pancreatic lipase—digests fat

• Ribonuclease & deoxyribonuclease—digest RNA & DNA respectively

Segmentation in Small Intestine

• Purpose of segmentation is to mix & churn not to move material along as in peristalsis

Carbohydrate Digestion

• Oligosaccharides & maltose contacts brush border enzymes (dextrinase, glucoamylase, maltase, sucrase, & lactase) act upon oligosaccharides, maltose, sucrose, lactose & fructose to glucose

Lactose Intolerance 

• Lactose passes undigested into large intestine

  • increases osmolarity of intestinal contents

  • causes water retention in the colon & diarrhea

  • gas production by bacterial fermentation of the lactose

Lactose Intolerance Population Statistics

• 15% American whites

• 90% of African-Americans

• 70% of Mediterranean descent

• Nearly all of Asian descent

Protein Digestion & Absorption (1)

• Pancreatic enzymes take over protein digestion in small intestine by hydrolyzing polypeptides into shorter oligopeptides

Protein Digestion & Absorption (2)

• Brush border enzymes finish task, producing free amino acids that are absorbed into intestinal epithelial cells

Emulsification

• Fat globule is broken up & coated by lecithin & bile acids

Fat Hydrolysis

• Emulsification droplets are acted upon by pancreatic lipase, which hydrolyzes the first & third fatty acids from triglycerides, usually leaving the middle fatty acid

Micelles

• Micelles in the bile pass to the small intestine & pick up several types of dietary & semi-digested lipide

Lipid Absorption

• Within the intestinal cell, free fatty acids & monoglycerides are transported to the smooth ER

• Resynthesized into triglycerides

• Golgi complex coats these with phospholipids & protein to form chylomicrons

  • taken up by more porous lacteal into the lymph

  • white, fatty intestinal lymph (chyle) flows into larger & larger lymphatic vessels until they reenter the bloodstream

Chylomicrons & the Lymphatics

• Chylomicrons are released into the lymphatic system in the lacteals of the villi. They enter the blood stream when lymphatic fluid enters the subclavian vein via the thoracic duct.

Nucleic Acid

• nucleases (deoxyribonuclease & ribonuclease) hydrolyze DNA & RNA to nucleotides

• nucleosidases & phosphatases of brush border split them into phosphate ions, ribose or deoxyribose sugar, & nitrogenous bases

Vitamins

• are absorbed unchanged

• fat-soluble vitamins— A, D, E, & K absorbed with other lipids

  • if they’re ingested without fat-containing food, they are not absorbed at all, but are passed in the feces & wasted 

• water-soluble vitamins, B complex & C, absorbed by simple diffusion & B12 if bound to intrinsic factor from the stomach

Minerals (electrolytes)

• are absorbed all along small intestine

• Na+ cotransported with sugars & amino acids

• Cl- exchanged for bicarbonate reversing chloride-bicarbonate exchange that occurs in the stomach

• iron & calcium absorbed as needed

  • unable to absorb ferric ions (Fe3+) but stomach acid reduces ferric ions to absorbable ferrous ions

  • transferrin (extracellular protein) transports iron in blood to bone marrow, muscle, & liver

Water Balance

• Digestive system is one of several systems involved in water balance

• Digestive tract receives about 9 L of water/day

  • 0.7 L in food, 1.6 L in drink, 6.7 L in gastrointestinal secretions

  • 8 L is absorbed by small intestine & 0.8 L by large intestine

  • 0.2 L voided in daily fecal output

• Water is absorbed by osmosis following the absorption of salts & organic nutrients