Digestive System

Objectives1. Identify the organs of the digestive system, list their major functions, describe the

functional histology of the digestive tract, and outline the mechanisms that regulate digestion.

2. Discuss the anatomy of the oral cavity, and list the functions its major structures and regions.

3. Describe the structure and functions of the pharynx.4. Describe the structure and functions of the esophagus.5. Describe the anatomy of the stomach, including its histologic features, and

discuss its roles in digestion and absorption.6. Describe the anatomical and histological characteristics of the small intestine,

explain the functions and regulation of intestinal secretions, and describe the structure, functions, and regulation of the accessory digestive organs.

7. Describe the gross and histological structure of the large intestine, including its regional specializations and role in nutrient absorption.

8. List the nutrients required by the body, describe the chemical events responsible for the digestion of organic nutrients, and describe the mechanisms involved in the absorption of organic and inorganic nutrients.

9. Summarize the effects of aging on the digestive system.10. Give examples of interactions between the digestive system and the other organ

systems.

GI Tract - Overview

Alimentary Canal

Oral Cavity

Pharynx

Esophagus

Stomach

Small intestine

Duodenum

Jejunum

Ileum

Large intestine

Cecum

Colon

Ascending colon

Transverse colon

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Descending colon

Sigmoid colon

Rectum

Anal canal

Support Organs

Teeth

Tongue

Salivary Glands

Parotid

Submandibular

Sublingual

Liver

Gallbladder

Pancreas

vermiform appendix

Terms

Ingestion: taking food in

Mechanical Processing: e.g., mastication (chewing)

Digestion: breakdown of food

Secretion: release to lumen

Absorption: across digestive epithelium and into interstitial fluid

Excretion: removal of waste products from body fluids

Defecation: waste discharge

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Layers of GI Tract

Mucosa

Simple columnar

Secretive/ absorptive layer

Submucosa

Vascularized/ lacteal

Muscularis

Peristaltic movement

Inner circular and outer longitudinal

layers of smooth muscle

Serosa (adventitia in thorax)

Visceral peritoneum

Movement of Digestive Materials

Peristalsis

Segmentation

Control of Digestive Function

Neural Mechanisms

Myenteric plexus

Local reflexes

Short reflexes

Long Reflexes

Hormonal Mechanisms

Enteroendocrine cells

Local Mechanisms

Physical stimulation, pH,

chemical stimulation

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Serous Membranes

Parietal peritoneum

Dorsal mesentery

Ventral mesentery

Visceral peritoneum

Retroperitoneal

Mesenteries

Falciform ligament

Lesser omentum

Greater omentum

Peritoneal cavity

Oral Cavity

Lips

Vestibule

Teeth

Tongue

Papillae

Filiform

Fungiform

Vallate

Taste buds

Palate

Hard

Soft

Uvula

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Salivary Glands

Parotid, submandibular, sublingual

Ducts

Produce saliva

Cleans teeth

IgA kills oral bacteria

Digest starch (salivary amylase)

Lubricates: mucins

1-1.5 L per day

Mumps

Viral disease

Teeth

Heterodont dentition

Incisors

Canines (cuspids)

Premolars (bicuspids)

Molars

Tooth Structure

Disorders of Mouth and Teeth

Pharynx and Esophagus

Heartburn or acid reflux

GERD

Gastroesophageal Reflux

Disease

Hiatal hernia

Drugs

Mild

Acid-blockers, H2-receptor antagonists

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Tagamet

Zantac

Pepcid

Moderate to severe

Proton-pump inhibitors

Prilosec

Prevacid

Promotility agents

Reglan

Pharynx and Esophagus

Heartburn or acid reflux

GERD

Gastro-esophageal Reflux Disease

Hiatal hernia

Surgical treatments

Stretta Procedure

Bard endoscopic suturing system

Swallowing Process

Oral or Buccal phase

Pharyngeal phase

Esophageal phase

Upper sphincter

Lower sphincter

Clinical concerns

Achalasia

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Stomach

Functions:

Churn food w/gastric juice

Initiate protein digestion

Limited absorption

Move chyme to small intestine

Histology of the Stomach

Parietal Cells

Intrinsic factor

H2CO3 and Cl

Chief Cells

pepsinogen

Enteroendocrine Cells

Regulation of Gastric Activity

Cephalic Phase

Parasympathetic

Minutes

Gastric Phase

Neural response

Hormonal response

Local response

3-4 hours

Intestinal Phase

Neural response

Hormonal response

Disorders of Stomach

Gastroenterology

Gastritis

Anorexia

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Nausea

Emesis

Ulcer

Helicobacter pylori

Triple therapy

Bismuth subsalicylate

Tetracycline

Flagyl or Biaxin

Stomach cancer

Alcohol

Chewing tobacco

Smoked or heavily processed foods

Infants

Pylorospasm

Pyloric stenosis

LOWER DIGESTIVE SYSTEM

Small Intestine

Functions:

Reception of secretions from liver and pancreas

Chemical and mechanical breakdown

Absorption of nutrients

Transportation of undigested material to large intestine

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Small Intestine

Regions:

Duodenum

~25 cm (10 in.)

receives common bile duct

Jejunum

~2.5 m (8ft.)

Bulk of chemical digestion

Ileum

~3.6 m (12 ft.)

Structures of Small Intestine

Histology of Small Intestine

Mucosa

Intestinal villi

Brush border

Lacteal

Submucosal glands

Muscularis mucosae

Submucosa

Submucosal glands

Muscularis externa

Serosa

Submucosal glands

Submucosal lymphoid

Peyer’s Patches

Plicae

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Movement into Small Intestine

Submucosal glands increase in activity to protect duodenal epithelium from

gastric secretions

Local reflexes

Hormonal release

Parasympathetic response

Vagus nerve

Prepares the duodenum before acidic gastric contents are added

Pancreas

Endo- and exocrine functions

Pancreatic islets (Islets of

Langerhans)

Endocrine - glucogon, insulin,

somatostatin

Acini

Exocrine – controlled by hormones

from duodenum

Alkaline mixture of water ions and

digestive enzymes (trypsin, chymotrypsin,

etc.)

Hormones and Enzymes

Hormones controlling pancreatic secretions

Secretin- watery buffer raising pH.

Cholecystokinin- opens the hepatopancreatic sphincter and stimulates the

contraction of the gallbladder

Enzymes releases by pancreas (Cystic Fibrosis)

Pancreatic alpha-amylase

Pancreatic lipase

Nucleases

Proteolytic enzymes

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Enterokinase

Trypsinogen

Chymotrypsenogen

Procarboxypeptidase

Proelastase

Liver

Falciform ligament

Left lobe

Right lobe

Round ligament

Caudate lobe

Quadrate lobe

Hilus

Liver

Hepatic Lobules

Hepatocytes

Hepatic triads

Hepatic portal branch

Hepatic artery

Bile duct branch

Kupffer cells (stellate

reticuloendothelial cells)

Liver

Functions:

Metabolic Regulation

Carbohydrate Metabolism

Lipid Metabolism

Amino Acid Metabolism

Removal of Wastes

Vitamin Storage

Mineral Storage

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Drug inactivation

Hematological Regulation

Phagocytosis and Antigen presentation

Synthesis of Plasma Proteins

Removal of Hormones, Antibodies and Toxins

Synthesis and secretion of Bile

Bile

Emulsifies fat

Lipases unable to break down large droplets

Form chylomicrons (micelles)

Diffuse across cell membranes and enter cytoplasm, then are secreted by

exocytosis and are eventually taken up by lacteals

Lacteals drain into lymphatic system and triglycerides enter blood stream at

subclavian vein via the thoracic duct.

Then reach liver for processing

Gallbladder

Stores and

concentrates bile

Gallstones

solid crystals of insoluble minerals and salts

Release of bile controlled by CCK

Coordination of Secretion

Neural Mechanisms

Preparing the digestive tract for activity

(parasympathetic) or inhibiting gastrointestinal activity (sympathetic)

Coordinating the movement of materials along the length of the digestive tract

(reflexes)

Hormonal Mechanisms

Secretin: increases the secretion of bile and buffers by the liver and pancreas

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Cholecystokinin (CCK): accelerates the production of digestive enzymes in

pancreas and relaxes the hepatopancreatic sphincter and contraction of the gallbladder

Gastric Inhibitory Peptide (GIP): stimulates lipid synthesis in adipose tissue and

increases glucose usage by skeletal muscles

Large Intestine

Regions:

Cecum--appendix--ascending colon

Hepatic flexure--transverse colon

Splenic flexure

Descending colon

Sigmoid colon

Rectum

Function:

Absorb water, electrolytes

Absorb vitamins released by

bacteria

Pass waste to rectum and storage

~1.5 m (5 ft.)

Large Intestine

Absorption in Large Intestine

Water

9000 mL ingested or secreted – 150mL lost in feces

Bile salts

Recycled in liver

Vitamins

Vitamin K – fat soluable – clotting factors

Biotin – water soluable – glucose metabolism

Vitamin B3 – water soluable – steroid hormone and neurotransmitter production

Histology of Large Intestine

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Mucosa

Columnar cells

Goblet cells

Intestinal gland

Muscularis mucosae

Submucosa

Muscularis externa

Serosa

Anal Canal

Rectal valve

Internal sphincter

External sphincter

Pectinate line

Keratinized

Disorders of GI Tract

Peptic ulcers

Diarrhea

Hiatal hernia

Diverticulitis

Stomach cancer

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Metabolism and Energetics

Objectives: 1. Define metabolism, and explain why cells must synthesize new organic

components.2. Describe the basic steps in glycolysis, the TCA cycle, and the electron transport

system, and summarize the energy yields of glycolysis and cellular respiration.3. Describe the pathways involved in lipid metabolism, and summarize the

mechanisms of lipid transport and distribution.4. Summarize the main processes of protein metabolism, and discuss the use of

proteins as an energy source.5. Differentiate between the absorptive and postabsorptive metabolic states, and

summarize the characteristics of each.6. Explain what constitutes a balanced diet and why such a diet is important.7. Define metabolic rate, discuss the factors involved in determining an individual’s

BMR, and discuss the homeostatic mechanisms that maintain a constant body temperature.

Terms to know

Anabolic reactions

Glycogenesis

Gluconeogenesis

Lipogenesis

Protein synthesis

Catabolic reactions

Glycogenolysis

Glycolysis

Lipolysis

Protein degradation

Deamination

Nutrient Use in Cellular Metabolism

Remember

Glycolysis, Kreb’s cycle (TCA-tricarboxylic cycle or citric acid cycle) and the

Electron Transport System: a series of reactions that convert one molecule of glucose to 6

molecules of water, 12 molecules of carbon dioxide and 38 molecules of ATP.

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

Water-soluble vitamins: includes all the B vitamins and vitamin C

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BMR (basal metabolic rate)

Metabolism and Eating Disorders

Anorexia nervosa

Bulimia

Obesity

Protein-calorie malnutrition

Marasmus

Kwashiorkor

The New Food Pyramid

Basic Nutrients

Carbohydrates: sugars

Fats: lipids, fatty acids, glycerol

Proteins: amino acids

Mineral salts: inorganic ions released from disassociation of electrolytes

Vitamins: fat and water soluble

Water

Minerals

Functions

Determine Osmotic Concentration of body fluids

Major roles in important physiological processes

Membrane transport

Action potential generation

Maintain skeleton

Neurotransmitter and hormone production and release

Cofactors in a variety of enzymatic reactions

Glycolysis-ATP production

Gastric acid secretion

Minerals

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Bulk Minerals

Sodium

Potassium

Chloride

Calcium

Phosphorus

Magnesium

Trace Minerals

Iron

Zinc

Copper

Manganese

Cobalt

Selenium

Chromium

Vitamins

Fat-soluble

A: maintains epithelia, required to synthesize visual pigments, immunity, bone

health

D: bone growth, calcium and phosphorus absorption in intestine

E: prevents breakdown of vitamin A and fatty acids

K: essential for liver synthesis of prothrombin and clotting factors

Water-soluble

B1 (thiamine): coenzyme in decarboxylations

B2 (riboflavin): part of Kreb’s cycle

Niacin: part of Kreb’s cycle

B5 (pantothenic acid): parts of Kreb’s cycle

B6 (pyridoxine): coenzyme in amino acid and lipid metabolism

Folacin (folic acid): coenzyme in amino acid and nucleic acid metabolism

B12 (cobalamin): coenzyme in nucleic acid metabolism

Biotin: coenzyme in decarboxylations

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C9ascorbic acid): coenzyme, delivers hydrogen ions, antioxidant

Aging and The Digestive System

The rate of epithelial stem cell division decreases

Smooth muscle tone decreases

The effects of cumulative damage become apparent

Cancer rates increase

Changes in other systems have direct or indirect effects on the digestive system

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