Digestive Tract

• Gastrointestinal (GI) tract or alimentary canal

• Is a muscular tube

• Extends from oral cavity to anus

Components of the Digestive System

6 Functions of the Digestive System 1. Ingestion:

– occurs when materials enter digestive tract via the mouth

2. Mechanical processing: – crushing and shearing– makes materials easier to propel along

digestive tract3. Digestion:

– is the chemical breakdown of food– into small organic fragments– for absorption by digestive epithelium

4. Secretion:– is the release of water, acids, enzymes, buffers, and salts

– by epithelium of digestive tract

– by glandular organs

5. Absorption:

– movement of organic substrates, electrolytes, vitamins, and water

– across digestive epithelium– into interstitial fluid of digestive

tract

6. Excretion: – removal of waste products from body fluids

Lining of the Digestive Tract• Protects surrounding tissues against:

– corrosive effects of digestive acids and enzymes– mechanical stresses, such as abrasion– bacteria

• Is ingested with food or resides in digestive tract

• Attacked by macrophages, and immune system cells

• In lamina propria (underlying layer of areolar tissue)

Histological Organization of the Digestive Tract

• Major layers of the digestive tract:– mucosa– submucosa– muscularis externa– serosa

The Mucosa

• Is the inner lining of digestive tract

• Is a mucous membrane consisting of:– epithelium, moistened by glandular secretions

• Lamina propria composed of areolar tissue

Lining of Digestive Tract

• Folding increases surface area for absorption:

1. longitudinal folds, disappear as digestive tract fills

2. permanent transverse folds (plicae)

• Consists of a layer of areolar tissue that contains:– blood vessels– sensory nerve

endings– lymphatic vessels– smooth muscle cells– scattered areas of

lymphoid tissue

The Lamina Propria

Muscularis Mucosae• Narrow band of smooth

muscle and elastic fibers in lamina propria

• Smooth muscle cells arranged in 2 concentric layers:

The Submucosa• Is a layer of dense

irregular connective tissue

• Surrounds muscularis mucosae

• Has large blood vessels and lymphatic vessels

• May contain exocrine glands:– secrete buffers and

enzymes into digestive tract

Submucosal Plexus

• Also called plexus of Meissner

• Innervates the mucosa and submucosa

Muscularis Externa Structure

• Is dominated by smooth muscle cells

• Involved in:– mechanical processing– movement of materials along

digestive tract

• Movements coordinated by enteric nervous system (ENS)

The Serosa• Serous membrane

covering muscularis externa:– except in oral

cavity, pharynx, esophagus, and rectum

The Movement of Digestive Materials• By muscular layers of digestive tract:

– consist of visceral smooth muscle tissue

Smooth Muscle

• Along digestive tract:– has rhythmic cycles of activity– controlled by pacesetter cells

• Cells undergo spontaneous depolarization:– triggering wave of contraction through entire

muscular sheet

Pacesetter Cells• Located in muscularis

mucosae and muscularis externa:– surrounding lumen of

digestive tract

Peristalsis

Figure 24–4

• Consists of waves of muscular contractions

• Moves a bolus along the length of the digestive tract

• Bolus is a small, oval mass of digestive contents

Peristaltic Motion

1. Circular muscles contract behind bolus:– while circular muscles ahead of bolus relax

2. Longitudinal muscles ahead of bolus contract:– shortening adjacent segments

3. Wave of contraction in circular muscles:– forces bolus forward

Segmentation

• Cycles of contraction:– Churn and fragment bolus– mix contents with intestinal secretions

• Does not follow a set pattern:– does not push materials in any 1 direction

Control of Digestive Function

• Neural mechanisms

• Hormonal mechanisms

• Local mechanisms

Neural Mechanisms• Control:

– movement of materials along digestive tract

– secretory functions

• Motor neurons:– control smooth

muscle contraction and glandular secretion

– located in myenteric plexus

Digestive Hormones Mechanisms• At least 18 hormones that affect:

– most aspects of digestive function– activities of other systems

• Are peptides • Are produced by enteroendocrine cells in

digestive tract• Reach target organs after distribution in

bloodstream

Local Mechanisms• Prostaglandins, histamine, and other chemicals

• Released into interstitial fluid

• Affect adjacent cells within small segment of digestive tract

• Coordinating response to changing conditions:– e.g., variations in local pH, chemical, or

physical stimuli

• Affect only a portion of tract

Chemical Events in Digestion

Figure 24–26

Processing Nutrients• The digestive system:

– breaks down physical structure of food– disassembles component molecules

• Molecules released into bloodstream are:– absorbed by cells

• Broken down to provide energy for ATP synthesis:– used to synthesize carbohydrates, proteins,

and lipids

Digestive Enzymes• Are secreted by:

– salivary glands– tongue– stomach– pancreas

• Break molecular bonds in large organic molecules:– carbohydrates, proteins, lipids, and nucleic

acids– in a process called hydrolysis

• Are divided into classes by targets:– carbohydrases:

• break bonds between simple sugars

– proteases:• break bonds between amino acids

– lipases:• separate fatty acids from glycerides

Figure 24.1

Figure 24.1 The Components of the Digestive System

• Ingestion

• Mechanical processing

• Digestion

• Secretion

• Absorption

• Excretion

Functions of the digestive system

Figure 24.3 The Structure of the Digestive Tract

Figure 24.3

• Visceral smooth muscle shows rhythmic cycles of activity– Pacemaker cells

• Peristalsis – Waves that move a bolus

• Segmentation – Churn and fragment a bolus

Movement of digestive materials

Figure 24.4 Peristalsis

Figure 24.4

• Movement of materials along the digestive tract is controlled by:– Neural mechanisms

• Parasympathetic and local reflexes

– Hormonal mechanisms • Enhance or inhibit smooth muscle contraction

– Local mechanisms • Coordinate response to changes in pH or chemical stimuli

Control of the digestive system

Figure 24.5 The Regulation of Digestive Activities

Figure 24.5

Figure 24.21a, b

Figure 24.21 The Gallbladder

• Neural and hormonal mechanisms coordinate glands• GI activity stimulated by parasympathetic innervation

– Inhibited by sympathetic innervation

• Enterogastric, gastroenteric and gastroileal reflexes coordinate stomach and intestines

Coordination secretion and absorption

Figure 24.22 The Activities of Major Digestive Tract Hormones

Figure 24.22

GI System--Structure

• Mouth

• Esophagus

• Stomach

• Small Intestine

• Large intestine

• Rectum

• Anus

GI System--Structure

• GI tract is about 30 feet long from mouth to anus

• Layers from inner to outer– Mucosa– Submucosa– Muscle- 2 layers

• Circular (inner)• Longitudinal (Outer)

– Serosa except esophagus (fibrous)

GI System--Structure

• Innervation– Sympathetic-inhibitory– Parasympathetic-excitatory– Example:

• Peristalsis is increased with parasympathetic stimulation

GI System--Structure

• GI tract receives 25-30% of cardiac output• Venous blood empties into portal vein which

peruses liver• Upper portion of GI tract receives blood from

splanchic artery• Small intestine receives blood from hepatic

and mesentery arteries• Large intestine receives blood supply from

superior and inferior mesenteric arteries

GI System--Structure

• Two types of movement– Mixing (Segmentation)– Propulsion (Peristalsis)– Hormones and enzymes for digestion– Mucous provides protection, lubrication, water,

electrolytes

GI System--Structure

• Abdominal organs are covered by peritoneum• Two layers of peritoneum

– Parietal-lines abdominal cavity wall

– Visceral-covers abdominal organs

– Peritoneal cavity- space between parietal and visceral

– Two folds of peritoneum• Mesentery-attaches small intestine to posterior abdominal wall

• Lesser omentum-hangs from stomach like apron, contains fat and lymph nodes

GI System—Intrinsic/Enteric layer

• GI tract has its own nervous system• Enteric or intrinsic• Two intrinsic layers

– Lies between mucosa and the circular muscle layer– Other layer lies between circular and longitudinal layer

• Innervation contributes to coordination of GI motor and secretory activities

• Enteric nervous system is also known as the “gut brain”• Ability to control movement and secretion of GI tract

GI System-Function

• Supply nutrients to body cells– Ingestion (taking in) and propulsion

(movement)– Digestion-breakdown– Absorption—transfer of food products into

circulation– Elimination-Excreting waste products

GI System-Function/Ingestion

• Swallowing (deglutition)-mechanical component of ingestion

• Mouth– Lips– Oral cavity

• Three pairs of salivary glands– Parotid– Submaxillary– Sublingual– Glands produce saliva (water, protein, mucin, salts,

amylase(pytalin))

GI System-Function/Ingestion

• Pharynx• Esophagus

– Upper third – striated muscle– Distal two thirds-smooth muscle– With swallowing, peristaltic wave moves bolus into

esophagus– Lower esophageal sphincter (LES) remains contracted

except when swallowing, vomiting, or belching– LES-important barrier that prevents acid reflux from

stomach

GI System-Digestion/Absorption

• Digestion is mechanical and chemical

• Mouth– Lubricates and softens mass

• Stomach– Stores food– Mixes food with gastric secretions– Empties contents into small intestine

GI System-Digestion/Absorption-Stomach

• Parts of stomach– Fundus

• Chief cells (secrete pesinogen) • Parietal cells (secrete HCL, water, and intrinsic

factor) • Acid Ph protects against ingested organisms• Intrinsic factor promotes cyclocobalamin (B 12)

absorption in the small intestine

– Body– Antrum

GI System-Digestion/Absorption-Stomach

• Pylorus– Small portion of antrum– Lies proximal to pyloric sphincter– Pyloric sphincter guards entrance to and from

entrance to stomach– Rugae-folds in stomach, contains glands

GI System-Digestion/Absorption-Small Intestine

• Two functions– Digestion– Absorption-uptake of nutrients from gut to

bloodstream– Approximately 23 feet long– Extends from pylorus to ileo cecal valve (prevents

reflux from small intestine to large intestine)– Large surface area for absorption

GI System-Digestion/Absorption-Small Intestine

• Villi– Functional units of small intestine

– Contains goblet cells• Secrete mucous

– Epithelial cells• Produce digestive enzymes

– Mirovilli-assist villi to increase surface area

– Crypts of Liberkuhn-contain base columnar cells

– Brunner’s glands-secrete mucous in duodenum

GI System-Physiology of Digestion

• Salivary gland-breakdown of starch

• Stomach-releases of pepsinogen from chief cells

• Acid in stomach converts of pepsinogen in to pepsin-initial breakdown of proteins

• Stomach has minimal impact on breakdown of starches and fat

• Food stays in stomach 3-4 hours

GI System-Physiology of Digestion-Small Intestine

• Carbohydrates broken down into monosaccharides– Fats to glycerol– Proteins to amino acids– Chyme (food mixed with gastric secretions)

GI System-Physiology of Digestion-Small Intestine

• Enzymes secreted from pancreas• Bile from liver • Intestinal secretions• Hormones secretin stimulates pancreas to produce

bicarb (neutralizes acid in chyme)• Duodenum secretes mucous to protect small

intestine from acid• Hormone cholecystokinin stimulates gall bladder

contraction and relaxation of the sphincter of Oddi-bile released-digest fat

GI System-Physiology of Elimination-Large Intestine

• 5-6 feet long

• Four parts– Cecum and Appendix– Colon– Rectum– Anus

• Most important function-absorption of water and electrolytes

GI System-Physiology of Elimination-Large Intestine

• Forms feces until defecation

• Feces– Water– Bacteria– Unabsorbed minerals– Undigested food – Bile pigment– Epithelial cells

GI System-Physiology of Elimination-Large Intestine

• Bacteria-– Responsible for breakdown of proteins not

digested by small intestine– Synthesize Vitamin K and some B vitamins– Production of flatus– Slow movement-haustral churning– Defecation

• Voluntary and involuntary muscles

• Valsalva maneuver

Physiology of Elimination-Liver

• Largest internal organ in body• Right and left lobes• Lobules-functional unit

– Rows of hepatic cells (hepatocytes)

– Kupfer cells-phagocytic activity-removes toxins/bacteria from blood

– Portal circulatory system brings blood to liver from stomach, intestine

– Involved in metabolism, detoxification, destruction of RBC

Physiology of Elimination-Biliary Tract

• Consists of– Gallbladder-concentrate, store bile– Duct system– Bile-produced by hepatic cells – Moves down common bile duct, enters duodenum

at ampulla of Vater– In intestine, bilirubin converts to stercobilirubin

(color of stool) and urobilirubin (color of urine)– Sphincter of Oddi keeps ampulla closed

Physiology of Elimination-Biliary Tract

• Bilirubin metabolism

• Breakdown of Hemoglobin

• Bound to albumin for transport to liver (uncongugated)

• In liver, conjugated with glucuronic acid

• Conjugated bilirubin is soluable

Physiology of Elimination-Pancreas

• Exocrine and endocrine functions

• Exocrine-digestion-pancreatic enzymes

• Endocrine– Islets of Langerhans– Beta cells-insulin– Alpha cells-glucagon– Delta-somatostatin– F cells-pancreatic polypeptides

GI System and Aging

• Periodontal disease-loss of teeth• Taste buds, smell, salivary secretions decrease• Delayed gastric emptying• Motility of GI tract diminishes• Decrease in HCL secretion• Reduction in secretion of acid and intrinsic factor• Liver size decreases

GI System-Factors that Influence Function

• Stress/anxiety

• Dietary intake

• Ingestion of ETOH/caffeine

• Cigarette smoking

• Fatigue

• Organic diseases

• Others?

Chapter 21, part A

Digestion

About this Chapter• Overview of the digestive system & how it is

organized

• How products are moved and the role of digestive secretions

• How and where food is broken down and absorbed

• How digestive wastes are concentrated and eliminated

• How digestion is regulated in the gastrointestinal (GI) tract

Processes of the Digestive System: Overview

Figure 21-1: Processes of the digestive system

• Ingestion

• Digestion: enzymatic

• Motility

• Secretion

• Absorption

• Elimination

• (Self protection)

Digestive Anatomy: Overview• Oral cavity

• Salivary glands

• Esophagus

• Stomach– Fundus– Body– (rugae)– Antrum– pyloris

Digestive Anatomy: Overview

Figure 21-2a: ANATOMY SUMMARY: The Digestive System

More Digestive Anatomy• Small intestine

– Duodenum– Jejunum– Ileum

• Pancreas• Liver• Large intestine

– Colon– Rectum

• Anus

Digestive Anatomy: Histological Overview• Stomach wall

– Mucosa– Gastric glands– Muscularis mucosa– Submucosa– Muscularis Externa– Serosa

• Small intestine wall– Vili & microvili– Pyer's patches

Digestive Anatomy: Histological Overview

Figure 21-2c: ANATOMY SUMMARY: The Digestive System

Digestive Anatomy: Histological Overview

Figure 21-2e: ANATOMY SUMMARY: The Digestive System

Motility: Smooth Muscle Contractions

Figure 21-4: Contractions in the GI tract

• Tonic – support

• Phasic – move products– Parastalsis – moves

– Segmentation –

mixes

Digestive Secretions: (7 L / Day From Tissues into

Lumen)

Figure 21-5: Daily mass balance in the digestive system

• Salivary glands

• Pancreas

• Water

• Enzymes

• Mucus

• Ions: H+, K+, Na+

• HCO3-, Cl-

• Mass Balance (H2O)