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PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College

C H A P T E R

Copyright © 2010 Pearson Education, Inc.

23 The Digestive System: Part A


Gastrointestinal Tract Activities• There are six essential activities: • Ingestion – taking food into the digestive tract • Propulsion – swallowing and peristalsis

• Peristalsis – waves of contraction and relaxation of muscles in the organ walls

• Mechanical digestion – chewing, mixing, and churning food

• Chemical digestion – catabolic breakdown of food• Absorption – movement of nutrients from the GI tract to

the blood or lymph• Defecation – elimination of indigestible solid wastes


Peritoneum and Peritoneal Cavity

• Peritoneum – serous membrane of the abdominal cavity

• Visceral – covers external surface of most digestive organs

• Parietal – lines the body wall

• Peritoneal cavity

• Lubricates digestive organs

• Allows them to slide across one another



• Mesentery – double layer of peritoneum that provides:• Vascular and nerve supplies to the viscera• Hold digestive organs in place and store fat



Figure 23.5b

• Retroperitoneal organs – organs outside the peritoneum

• Peritoneal organs (intraperitoneal) – organs surrounded by peritoneum


Histology of the Alimentary Canal

• From esophagus to the anal canal the walls of the GI tract have the same four tunics

• From the lumen outward they are the • mucosa • submucosa • muscularis externa • serosa

• Each tunic has a predominant tissue type and a specific digestive function


Mucosa: innermost layer• A layer of epithelium that is in direct contact with the content of the

GI tract –

• In the mouth, pharynx and anal canal – stratified squamous (protection);

• In the rest of GI tract simple columnar (secretion and absorption)

• A layer of connective tissue- lamina propria

• Areolar CT containing blood and lymphatic vessels

• Support epithelium

• Contains most cells of MALT (mucosa-associated lymphatic tissue)

• Muscularis mucosa – thin layer of smooth muscles that is responsible for the folding of the GI tract (increased surface area)


Mucosa: Other Sub layers

• Submucosa – dense connective tissue containing elastic fibers, blood and lymphatic vessels, lymph nodes, and nerves

• Muscularis externa – responsible for segmentation and peristalsis (GI motility)

• Serosa – the protective visceral peritoneum• Replaced by the fibrous adventitia in the

esophagus


Enteric Nervous System• The GI tract has its own nerve supply – the enteric neurons

• The system is made of interconnected ganglia found in the walls of the GI tract – short reflexes

• Composed of two major intrinsic nerve plexuses:

• Submucosal nerve plexus – regulates glands and smooth muscle in the mucosa

• Myenteric nerve plexus – Major nerve supply that controls GI tract mobility

• Linked to the CNS via afferent visceral fibers (long autonomic reflex arc)

• The motor fibers of the ANS synapse with the enteric neurons


http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/basics/peristalsis.html


Mouth• The mouth is lined with stratified squamous epithelium

• The gums, hard palate, and dorsum of the tongue are slightly keratinized

• Hard palate – made of the palatine bones and palatine processes of the maxillae

• Assists the tongue in chewing

• Soft palate – mobile fold formed mostly of skeletal muscle

• Closes off the nasopharynx during swallowing

• Uvula projects downward from its free edge


Salivary Glands

• Parotid – lies anterior to the ear between the masseter muscle and skin

• Parotid duct opens next to second upper molar• Submandibular – lies along the medial aspect of the

mandibular body

• Its ducts open at the base of the lingual frenulum• Sublingual – lies anterior to the submandibular gland

under the tongue

• It opens via 10-12 ducts into the floor of the mouth


Saliva: Source and Composition

• Secreted from serous and mucous cells of salivary glands

• 97-99.5% water, hypo-osmotic, slightly acidic solution containing

• Electrolytes – Na+, K+, Cl–, PO42–, HCO3

–

• Digestive enzyme –

• salivary amylase – begins starch digestion in the oral cavity

• lingual lipase – activated by stomachacid to digest fat after food is swollowed

• Proteins – mucin, lysozyme, defensins, and IgA

• Metabolic wastes – urea and uric acid


Salivary glands function

• Lubrication and binding

• the mucus in saliva binds masticated food into bolus that slides easily through the esophagus without inflicting damage to the mucosa.

• Saliva also coats the oral cavity and esophagus, and food basically never directly touches the epithelial cells of those tissues.

• Solubilizes dry food• in order to be tasted, the molecules in food must

be solubilized.


Salivary glands function

• Oral hygiene: • The oral cavity is almost constantly flushed with

saliva, which floats away food debris and keeps the mouth relatively clean.

• Saliva contains lysozyme, an enzyme that lyses many bacteria and prevents overgrowth of oral microbial populations.

• Initiates starch digestion• the serous cells secrete an alpha-amylase which

can begin to digest dietary starch into maltose.


Teeth

• Primary – 20 deciduous teeth that erupt at intervals between 6 and 24 months

• Permanent – enlarge and develop causing the root of deciduous teeth to be reabsorbed and fall out between the ages of 6 and 12 years


Tooth Structure

• Crown: the exposed part above the gingiva (gum)

• Covered by enamel

• Enamel is not a tissue but cell secretions

• It is produced before the tooth erupts – if damaged can not regenarate

• the hardest substance in the body (calcium salts and hydroxyapatite crystals)

• Root: portion embedded in the jawbone


Tooth Structure• Cementum: calcified connective tissue that contains cell –

cementocytes in lacunae

• Covers root and attaches it to the periodontal ligament

• Dentin: bonelike material under enamel

• Maintained by odontoblasts that line the pulp cavity

• Periodontal ligament

• Forms fibrous joint called a gomphosis

• Pulp cavity: cavity surrounded by dentin

• Pulp: connective tissue, blood vessels, and nerves

• Root canal: extends from pulp cavity to the apical foramen of the root


• Carries solids and liquids from the pharynx to the stomach

• Passes through esophageal hiatus in diaphragm

• Travels through the mediastinum and pierces the diaphragm

• Joins the stomach at the cardiac orifice

The esophagus


Histology of the esophagus

• Esophageal mucosa contains stratified squamous epithelium

• Changes to simple columnar at the stomach

• Esophageal glands in submucosa secrete mucus to aid in bolus movement

• Muscularis: skeletal superiorly; smooth inferiorly

• Adventitia instead of serosa


Figure 23.14a

Stomach• Chemical breakdown of proteins begins and food is

converted to chyme


Microscopic Anatomy of the Stomach

• Epithelial lining is composed of:

• Simple columnar ET with goblet cells that produce a coat of alkaline mucus

• Gastric pits contain gastric glands that secrete gastric juice, mucus, and gastrin

• Muscularis – has an additional oblique layer that:

• Allows the stomach to break and mix, food physically

• Breaks down food into smaller fragments


Digestion in the Stomach

• The stomach:

• Holds ingested food

• Breaks this food both physically and chemically

• Delivers chyme to the small intestine

• Enzymatically digests proteins with pepsin

• Secretes intrinsic factor required for absorption of vitamin B12


• Preliminary digestion of proteins by pepsin (not completed). Pepsin breaks down complex proteins into smaller peptides

• Permits digestion of carbohydrates - until pH fall below 4.5 the salivary amylase continue its function.

• Very little absorption of nutrients:

• The epithelial cells are covered with mucus

• Epithelial cells lack transport mechanisms

• Gastric lining is relatively impermeable to water

• Most carbohydrates, lipids and proteins are too big (not completely broken down)

• Some drugs, however, are absorbed (ethyl alcohol, aspirin)

Digestion and absorption in the stomach


Small Intestine: Gross Anatomy

• Major organ of digestion and absorption

• 2–4 m long; from pyloric sphincter to ileocecal valve

• Subdivisions

1. Duodenum (retroperitoneal)

2. Jejunum (attached posteriorly by mesentery)

3. Ileum (attached posteriorly by mesentery)


Small Intestine: Microscopic Anatomy

• Structural modifications of the small intestine wall increase surface area

• Plicae circulares: deep circular folds of the mucosa and submucosa

• Villi – fingerlike extensions of the mucosa

• Microvilli – tiny projections of absorptive mucosal cells’ plasma membranes (brush border)


Structure of Small Intestine1

45

2

3


Duodenum • Main function is to receive chyme and neutralize it

• Duodenal glands in the mucosa secrete part of the intestinal juice in response to local reflexes, enterocrinin hormone, vagus nerve stimulation.

• Local reflexes and hormonal mechanisms start working only after chyme enters the duodenum.

• The vagus nerve activates the duodenal glands during the cephalic phase of gastric secretion

• Brunner’s glands in the submucosa

• produce mucus, buffers, hormone urogastrone that inhibit gastric acid production and stimulates ET division

Small Intestine - regional specializations


Small Intestine - regional specializations

Ileum

• Distal region lack plicae

• aggregated lymphoid nodules (Peyer’s patches) that protect the small intestine from bacteria populations that are normally found in the large intestine


Intestinal Juice

• Secreted in response to distension or irritation of the mucosa

• Slightly alkaline and isotonic with blood plasma

• Largely water, enzyme-poor, but contains mucus

• Facilitates transport and absorption of nutrients


Liver• The largest gland in the body• The falciform ligament:

• Separates the right and left lobes anteriorly• Suspends the liver from the diaphragm and anterior abdominal

wall• The lesser omentum anchors the liver to the stomach• The gallbladder rests in a recess on the inferior surface of the right

lobe


The hepatic portal system begins in the capillaries of the digestive organs and ends in the portal vein. Consequently, portal blood contains substances absorbed by the stomach and intestines. Portal blood is passed through the hepatic lobules where nutrients and toxins are absorbed, excreted or converted.


Liver: Microscopic Anatomy

• Liver lobules

• Hexagonal structural and functional units

• Filter and process nutrient-rich blood

• Composed of plates of hepatocytes (liver cells)

• Longitudinal central vein

Copyright © 2010 Pearson Education, Inc.Figure 23.24d


• Portal triads consist of a • bile duct

• Hepatic artery – supplies oxygen-rich blood to the liver

• Hepatic portal vein – carries venous blood with nutrients from digestive viscera

• Liver sinusoids – enlarged, leaky capillaries located between hepatic plates

• Kupffer cells – hepatic macrophages found in liver sinusoids



• Hepatocytes’ functions include:

• Production of bile

• Processing bloodborne nutrients

• Storage of fat-soluble vitamins

• Detoxification• Secreted bile flows between hepatocytes toward the bile

ducts in the portal triads


Liver physiology

• Removing and excreting body wastes, hormones, drugs and other foreign substances Enzymes in the liver alter some toxins so they can be more easily excreted in urine.

• Synthesizing plasma proteins, including those necessary for blood clotting Most of the 12 clotting factors are produced by the liver. Other plasma proteins produced by the liver include albumin, fibrinogen and certain globulins which transport substances such as cholesterol and iron.

• Producing bile to aid in digestion Bile salts aid in fat digestion and absorption. Bile is continuously secreted by the liver and stored in the gallbladder until a meal, when bile enters the beginning of the small intestine.


Liver physiology• Excretion of bilirubin Bilirubin is one of the few waste products

excreted in bile. Macrophages in the liver remove worn out red blood cells from the blood. Bilirubin results from the breakdown of the hemoglobin is excreted into bile by hepatocytes.

• Storing certain vitamins, minerals, and sugars The liver stores enough glucose in the form of glycogen to provide about a day's worth of energy.

• The liver also stores fats, iron, copper, and many vitamins including vitamins A, D, K, and B12.

• Processing nutrients absorbed from digestive tract

• The liver converts glucose into glycogen, its storage form.

• The fatty acids produced by the digestion of lipids are used to synthesize cholesterol and other substances.


Liver: bile secretion

• Bile leaves the liver via:

• Bile ducts, which fuse into the common hepatic duct

• The common hepatic duct, which fuses with the cystic duct

• These two ducts form the bile duct


Composition of Bile

• A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, neutral fats, phospholipids, and electrolytes

• Bile salts emulsify fat to facilitate fat and cholesterol absorption

• The main bile pigment is bilirubin, a waste product of heme


The Gallbladder

• Thin-walled, green muscular sac on the ventral surface of the liver

• Stores and concentrates bile by absorbing its water and ions

• Releases bile via the cystic duct, which flows into the bile duct


Gallbladder and Associated Ducts

Figure 23.20


Pancreas

• Location• Lies deep to the greater curvature of the stomach• The head is encircled by the duodenum and the tail

is close to the spleen• Exocrine function

• Secretes pancreatic juice which breaks down all food

• The pancreas also has an endocrine function – release of insulin and glucagon


Composition and Function of Pancreatic Juice• Water solution of enzymes and electrolytes (primarily

HCO3–)

• Neutralizes acid chyme

• Provides optimal environment for pancreatic enzymes

• Enzymes

• Amylase, lipases, nucleases are secreted in active form but require ions or bile for optimal activity

• Proteases secreted in inactive form

• Protease activation in duodenum

• The pancreas starts to synthesize enzymes before food even arrive to the stomach


Anatomy of the large intestine• The large intestine extends from the ileum to the anus

• The opening from the ileum to the large intestine is guarded by the ileocecal sphincter

• 4 major regions are:

• Cecum - expended pouch close to the ileum entrance to the large intestine; Collects and stores materials from the ileum.

• The appendix is attached to the cecum. The mucosa and the submucosa are dominate by lymphoid nodules and it primary function is as an organ of the lymphatic system

• Colon – divided into ascending, transverse, descending and sigmoid portions

• Rectum – last part of GI tract

• Anal canal – last portion of rectum; anus is the opening of the anal canal


Large Intestine

• Has unique features:

• Teniae coli – three bands of longitudinal smooth muscle in its muscularis

• Haustra – pocketlike sacs caused by the tone of the teniae coli


Large intestine histology

• The ET of the mucosa contain mainly absorptive and goblet cells.

• both cell types are found in long, straight intestinal glands – crypts of Lieberkuhn – that extend the full thickness of the mucosa.

• Colon mucosa is simple columnar epithelium except in the anal canal

• Anal canal mucosa is stratified squamous epithelium


mucosa

crypts of Lieberkuhn

Goblet cell


Functions of the Large Intestine

• Digestion of enteric bacteria

• Reabsorption Vitamins, water, and electrolytes

• Propulsion of fecal material toward the anus

• Though essential for comfort, the colon is not essential for life


Bacterial Flora

• The bacterial flora of the large intestine consist of:• Bacteria surviving the small intestine that enter the

cecum• Those entering via the anus

• These bacteria: • Colonize the colon• Ferment indigestible carbohydrates• Synthesize B complex vitamins and vitamin K


Valves and Sphincters of the Rectum and Anus

• The anus has two sphincters:

• Internal anal sphincter composed of smooth muscle

• External anal sphincter composed of skeletal muscle

• These sphincters are closed except during defecation


Defecation

• Distension of rectal walls caused by feces:

• Stimulates contraction of the rectal walls

• Relaxes the internal anal sphincter• Voluntary signals stimulate relaxation of the external anal

sphincter and defecation occurs


• Luminal digestion: enzymes secreted into the lumen of GI tract from salivary glands, stomach and pancreas

• Membrane or contact digestion : hydrolytic enzymes synthesized by enterocytes and inserted into the brush border membranes.

• cytoplasmic digestion: digestive enzymes in the cytoplasm

Chemical digestion and absorption


Digestive Secretion and Absorption of Water

Figure 24.27


• organic substances needed in small quantities for normal metabolic function, growth and maintenance of the body.

• Fat-soluble vitamins: • Vitamins A, D, E and K

• Water-soluble vitamins:

• Vitamins B1, B2, B6, B12, niacin, biotin and folic acid

Vitamins


Vitamin A (retinol)

Present in many animal tissues, especially fish and liver. Carotinoids in green plants serve can be converted to vitamin A following ingestion.

production of vision pigments, resistance to infectious agents and maintenance of health in many epithelial cells.

Vitamin DSynthesized in the skin when exposed to sunlight.

Major effect is to facilitate absorption of calcium from the intestine.

Vitamin E

Vegetable oils, leafy green vegetables and whole grains.

function as antioxidants, particularly to prevent oxidation of unsaturated fatty acids and maintain the integrity of cell membranes.

Vitamin K

Majority is synthesized by bacteria in the large intestine. Dietary sources include the photosynthetic (green) parts of plants.

Necessary for formation of several blood-clotting factors in the liver, and deficiency leads to bleeding disorders.

Fat-Soluble Vitamins


Vitamin C(Ascorbic acid)

Present in fruits and vegetables.

A major function is synthesis of hydroxyproline, an important component of collagen and, thereby, all connective tissues. Essential for growth of cartilage, bone and teeth, and for wound healing.

Vitamin B1

Present in meats, leafy green vegetables, grains and legumes.

Functions as a coenzyme for several enzymes.

Vitamin B2

Present milk, meats and grains.

Precursor to the coenzymes FAD and FMN, which serve as hydrogen carriers in a number of important oxidation-reduction (respiration) reactions within mitochondria.

Niacin

Present in meats, leafy green vegatables, potatoes and peanuts.

Precursor to the coenzymes NAD and NADPH which serve as hydrogen carriers in glycolysis, Kreb's cycle and oxidative phosphorylation.

Water-Soluble Vitamins


Vitamin B6

Present in meat, fish and poultry. Also present in a number of vegetables, including potatoes and tomatoes.

The precursor to pyridoxal phosphate, a coenzyme for several important reactions involving protein metabolism, including the transamination reactions necessary for synthesis of amino acids.

Biotin

Found in egg yolk, legumes, nuts and liver. Also synthesized by intestinal bacteria.

coenzyme for several enzymes that catalyze carboxylation, decarboxylation and deamination reactions. One example is pyruvate carboxylase, an essential enzyme of Kreb's cycle.

Vitamin B12

the sole source in nature is microbial synthesis.

coenzyme involved in numerous metabolic pathways. Deficiency usually results from failure to absorb the molecule due to inadequate quantities of intrinsic factor

Folic Acid

Present in dark-green vegetables, beef, eggs, whole grains. Also synthesized by intestinal bacteria.

Serves as a coenzyme in the synthesis of several amino acids, as well as purines and thymine, and therefore DNA.

Water-Soluble Vitamins


• Begins in the mouth by salivary amylase.

• Salivary amylase continue to work in the stomach until pH reaches 4.5 (destroys the enzyme)

• Most of starch is not broken down and arrive like that to the small intestine

• In the small intestine pancreatic amylase breaks down the starch (polysaccharide from plant source) as well as the glycogen (polysaccharide of animal source) into disaccharides and trisaccharides

• Brush border enzymes digest into monosaccharides

• Absorption of monosaccharides occurs across the intestinal epithelia

Carbohydrate digestion and absorption



• Low pH and pepsin in the stomach destroy tertiary structure and break down the protein into peptides

• Pancreatic enzymes: trypsin, chymotrypsin, and elastase continue to break down proteins to peptides and amino acids

• Liberated amino acids are absorbed

• Luminal protein digestion produces single amino acids and small peptides (dipeptides, tripeptides and tetrapeptides)

• Brush border peptidases produce single amino acids and smaller peptides from tetrapeptides and larger peptides.

• Intracellular cytoplasmic peptidases break down dipeptides and tripeptides into single amino acids.

Protein digestion and absorption


• Most of lipids in diet are triglycerides (glycerol attached to 3 fatty acids)

• 3 Enzymes for lipid digestion:

• lingual lipase - from salivary secretion

• gastric lipase - secreted by chief cells

• Pancreatic lipase

• Most lipid digestion occurs in the small intestine

Lipid digestion and absorption


Bile and emulsification

• Large lipid droplets insoluble in water - Require emulsification

• Arrival of lipids in the duodenum serves as a stimulus for secretion of bile.

• Bile salts improve chemical digestion by emulsifying lipid drops. Lipid-bile salt complexes are called micelles

• Digested by lipase (water soluble)

• Pancreatic lipase hydrolyzes triglycerides to free fatty acids and monglycerides.

Copyright © 2010 Pearson Education, Inc. http://www.med.unibs.it/~marchesi/mixed_micell.gif

• Emulsification: In the small intestine lipids are emulsified by bile acids (i.e. formation of small droplets of lipids coated with bile acids). Bile salts are polar and water soluble, and function as detergents. Emulsion droplets allow access of the water-soluble lipolytic enzymes by increasing surface area.


Chemical Digestion and Absorption of Lipids

• Absorption of monoglycerides and fatty acids

• Cluster with bile salts to form micelles

• Released by micelles to diffuse into epithelial cells

• Combine with proteins to form chylomicrons

• Enter lacteals and are transported to systemic circulation

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