DIGESTIVE SYSTEM

17.1 General Characteristics of the Digestive System

Digestion

Digestion is the mechanical and chemical breakdown of food into forms that can be absorbed by cells.

Mechanical Digestion

  • Physically breaks food into smaller pieces

  • Does not change chemical composition

  • Examples:

    • Chewing (mastication)

    • Churning in stomach

    • Segmentation in intestines

Chemical Digestion

  • Uses enzymes to break food molecules into simpler substances

  • Changes chemical composition

  • Produces nutrients that can be absorbed

Functions of the Digestive System

  1. Ingestion (taking in food)

  2. Propulsion (moving food)

  3. Mechanical digestion

  4. Chemical digestion

  5. Absorption

  6. Defecation (elimination of waste)

Components of the Digestive System

Alimentary Canal (GI Tract)

Food passageway extending from mouth to anus.

The sympathetic and parasympathetic nervous systems control the alimentary canal

The functions of motor movements in the alimentary canal are to mix contents w/fluids and move the contents along the tract

Organs:

  • Mouth

  • Pharynx

  • Esophagus

  • Stomach

  • Small intestine

  • Large intestine

  • Anal canal

Characteristics:

  • Muscular tube

  • About 8 meters long

Accessory Organs

Food does not pass through these organs.

Include:

  • Salivary glands

  • Liver

  • Gallbladder

  • Pancreas

Functions:

  • Produce secretions that aid digestion

Structure of the Alimentary Canal Wall

The wall contains 4 layers.

1. Mucosa

Innermost layer

Composition:

  • Epithelium

  • Connective tissue

  • Smooth muscle

Functions:

  • Protection

  • Absorption

  • Secretion

Features:

  • Folded to increase surface area

2. Submucosa

Composition:

  • Loose connective tissue

  • Blood vessels

  • Lymphatic vessels

  • Nerves

Functions:

  • Nourishes tissues

  • Transports absorbed nutrients

3. Muscularis (Externa)

Composition:

  • Circular smooth muscle

  • Longitudinal smooth muscle (outermost layer)

Functions:

  • Mixing food

  • Propelling food

4. Serosa

Outermost layer

Composition:

  • Epithelium

  • Connective tissue

Functions:

  • Protection

  • Lubrication

  • Reduces friction

Also called:

  • Visceral peritoneum (abdominal organs)

Movements of the Digestive Tract

Mixing Movements

  • Local contractions

  • Mix food with digestive juices

  • Do not move food forward

Examples:

  • Segmentation

  • Stomach churning

Propelling Movements

Peristalsis

Wave-like contractions that move food forward.

Process:

  1. Circular muscle contracts behind food

  2. Food pushed forward

  3. Wave continues along tract

Innervation of the Digestive Tract

Submucosal Plexus

Controls:

  • Secretions

Myenteric Plexus

Controls:

  • Motility (movement)

Autonomic Nervous System Effects

Parasympathetic

"Rest and Digest"

Increases:

  • Secretion

  • Motility

Sympathetic

"Fight or Flight"

Decreases:

  • Secretion

  • Motility

Enteroendocrine Cells

Found in:

  • Stomach

  • Small intestine

Function:

  • Release digestive hormones

17.2 Mouth

Functions

  • Food intake

  • Mastication (chewing)

  • Speech

  • Taste sensation

Consists of:

  • Oral cavity (function: mastication, speech, and sensory perception)

  • Oral Vestibule (The narrow space between the teeth, cheeks, and lips)

Surrounded by:

  • Lips

  • Cheeks

  • Tongue

  • Palate

Cheeks

Functions:

  • Form side walls of mouth

  • Hold food between teeth

  • Aid chewing

Contain:

  • Facial muscles

  • Stratified squamous epithelium

Lips

Functions:

  • Surround mouth opening

  • Detect food temperature and texture

Tongue

Functions:

  • Moves food

  • Mixes food with saliva

  • Helps swallowing

  • Contains taste receptors

  • positioning food between the teeth for chewing

Structures:

Lingual Frenulum

Connects tongue to floor of mouth

Papillae

  • Projections on tongue

  • Contain taste buds

Lingual Tonsils

rounded masses of lymphatic tissue

Palate

Forms roof of mouth.

Hard Palate

Made of:

  • Maxillae

  • Palatine bones

Soft Palate

Muscular posterior portion

Ends in:

  • Uvula

Tonsils

Palatine Tonsils

Sides of the back of the mouth

Pharyngeal Tonsils (Adenoids)

Posterior wall of pharynx

Lingual Tonsils

Base of tongue

Teeth

Functions

  • Mechanical digestion

  • Cut, tear, and grind food

  • Mix food with saliva

Dentition - the pattern of generalized teeth in a human

Primary (Deciduous) Teeth

  • 20 teeth

Permanent Teeth

  • 32 teeth

Types of Teeth

Incisors

Function:

  • Biting

Canines (Cuspids)

Function:

  • Grasping

  • Tearing

Premolars (Bicuspids)

Function:

  • Grinding

Molars

Function:

  • Grinding

Tooth Structure

Crown

Visible portion

Root

Embedded in socket

Neck

Junction of crown and root

Tooth Tissues

Enamel

  • Covers crown

  • Hardest substance in body

Dentin

  • Makes up most of tooth

Pulp Cavity

Contains:

  • Blood vessels

  • Nerves

Cementum

Covers root

Periodontal Ligament

Anchors tooth

Dental Caries (Cavities)

Cause:

  1. Sugars remain on teeth

  2. Bacteria metabolize sugars

  3. Acid forms

  4. Enamel destroyed

Prevention:

  • Brushing

  • Flossing

  • Dental exams

  • Fluoride

  • Sealants

17.3 Salivary Glands

Functions of Saliva

  • Moistens food

  • Binds food particles

  • Dissolves food for taste

  • Begins carbohydrate digestion

  • Protects teeth

Contains:

  • Water

  • Mucus

  • Amylase

  • Bicarbonate ions

Major Salivary Glands

Parotid Glands

  • Largest

  • Near ears

  • Produce watery saliva

  • Rich in amylase

  • located in the cheek, anterior to the ear

Submandibular Glands

  • Floor of mouth

  • Produce serous and mucous secretions

Sublingual Glands

  • Beneath tongue

  • Mostly mucus

Salivary Secretory Cells

Serous Cells

Produce:

  • Watery secretion

  • Salivary amylase

Amylase:

  • Breaks starch and glycogen into disaccharides

Mucous Cells

Produce mucus

Functions:

  • Lubrication

  • Binds food

17.4 Pharynx and Esophagus

Pharynx

Located:

  • Behind the mouth/posterior to the oral cavity

Functions:

  • Passageway for food

  • Passageway for air

Regions

Nasopharynx

  • It communicates with the nasal cavity and provides a passageway for air during breathing. It is connected to the middle ears, through the auditory tubes.

  • posterior to the nasal cavity

Oropharynx

Posterior to the soft palate, located in the back of the oral cavity. It is a passageway for food moving downward from the mouth and for air moving to and from the nasal cavity.

Laryngopharynx

  • The most inferior part of the pharynx, it is a passageway of food to the esophagus.

  • posterior to the larynx and immediately inferior to the oropharynx

Swallowing (Deglutition)

Stage 1 – Voluntary

  • Food chewed

  • Mixed with saliva

  • Forms bolus

Stage 2 – Reflex

Events:

  • Soft palate rises

  • Uvula rises (to block the entrance of food or drink into the nasal cavity)

  • Larynx elevates

  • Epiglottis closes airway

  • Esophagus opens

  • Food enters esophagus

Stage 3

Peristalsis moves food to stomach.

  • moved to the stomach by the pharynx & esophagus

Esophagus

Functions:

  • Moves food to stomach

Length:

  • About 25 cm

Passes through:

  • Esophageal hiatus of diaphragm

Muscle Composition

Upper 1/3:

  • Skeletal muscle

Middle 1/3:

  • Skeletal + smooth

Lower 1/3:

  • Smooth muscle

Lower Esophageal (Cardiac) Sphincter

Functions:

  • Allows food into stomach

  • Prevents reflux

17.5 Stomach

Functions

  • Receives food

  • Stores food

  • Mixes food

  • Begins protein digestion

  • Produces chyme

  • Limited absorption

  • absorbs small amounts of nutrients

Structure

Cardia

Near esophagus

Fundus

Upper dome-shaped region

Body

Largest portion

Pylorus

Connects to small intestine

Contains:

  • Pyloric sphincter

Rugae

Folds allowing expansion

Gastric Juice

Chief Cells

Produce:

Pepsinogen

Inactive enzyme

Converted to:

Pepsin

Digests proteins

Parietal Cells

Produce:

Hydrochloric Acid (HCl)

Functions:

  • Activates pepsin

  • Kills microbes

Intrinsic Factor

Needed for Vitamin B12 absorption

Mucous Cells

Produce:

  • Protective mucus

Gastric Lipase

Function:

  • Digests fats

Regulation of Gastric Secretion

Neural Control

Parasympathetic:

  • Stimulates secretion

Sympathetic:

  • Inhibits secretion

Hormonal Control

Gastrin

Increases gastric juice

Somatostatin

Decreases acid secretion

CCK

Slows stomach emptying

Phases of Gastric Secretion

Cephalic Phase

Triggered by:

  • Sight

  • Smell

  • Taste

  • Thought of food

Gastric Phase

Food enters stomach.

Stimulates:

  • Gastrin release

  • Gastric juice production

Intestinal Phase

Food enters small intestine.

Primarily:

  • Inhibits stomach activity

Gastric Absorption

Absorbs:

  • Water

  • Salts

  • Alcohol

  • Lipid-soluble drugs

Chyme

Semifluid mixture of:

  • Food

  • Gastric juice

Enterogastric Reflex

When duodenum fills:

  • Stomach emptying slows

Vomiting

Controlled by:

  • Vomiting center in medulla

Causes:

  • Toxins

  • Motion sickness

  • Overstretching stomach

Heartburn

Cause:

  • Acid reflux into esophagus

Prevention:

  • Eat slowly

  • Smaller meals

  • Stay upright

  • Avoid caffeine, nicotine, alcohol

17.6 Pancreas

Endocrine Function

Produces:

  • Insulin

  • Glucagon

Exocrine Function

Produces:

  • Pancreatic juice

Pancreatic Juice Components

Pancreatic Amylase

Carbohydrates

Pancreatic Lipase

Fats

Trypsin

Proteins

Chymotrypsin

Proteins

Carboxypeptidase

Proteins

Nucleases

Nucleic acids

Bicarbonate Ions

Neutralize acid

Regulation of Pancreatic Secretion

Secretin

Stimulates bicarbonate secretion

CCK

Stimulates enzyme secretion

Parasympathetic Nerves

Increase secretion

17.7 Liver and Gallbladder

Liver

Largest internal organ.

Location:

  • Right upper quadrant

  • Beneath diaphragm

Liver Lobes

  1. Right lobe

  2. Left lobe

  3. Quadrate lobe

  4. Caudate lobe

Liver Functions

Carbohydrate Metabolism

  • Glycogenesis

  • Glycogenolysis

  • Gluconeogenesis

Lipid Metabolism

  • Fat oxidation

  • Cholesterol synthesis

Protein Metabolism

  • Deamination

  • Urea formation

  • Plasma protein synthesis

Storage

Stores:

  • Glycogen

  • Iron

  • Vitamins A, D, B12

Detoxification

Removes:

  • Drugs

  • Alcohol

  • Toxins

Blood Filtration

Removes:

  • Old RBCs

  • Foreign substances

Blood Reservoir

Bile Production

Bile

Components:

  • Water

  • Bile salts

  • Bile pigments

  • Cholesterol

  • Electrolytes

Functions of Bile Salts

  • Emulsify fats

  • Increase surface area

  • Aid lipase

  • Help absorb vitamins A, D, E, K

  • Form micelles

Jaundice

Yellowing of:

  • Skin

  • Eyes

  • Mucous membranes

Causes:

  • Bile duct blockage

  • Hepatitis

  • Cirrhosis

  • Excess RBC destruction

Hepatitis

Inflammation of liver.

Hepatitis A

Contaminated food/feces

Hepatitis B

Body fluids

Hepatitis C

Blood transmission

Gallbladder

Functions:

  • Stores bile

  • Concentrates bile

  • Releases bile into duodenum

Gallstones

Cause:

  • Cholesterol crystallization

Risk factors:

  • Excess cholesterol

  • Concentrated bile

  • Inflammation

CCK and Bile Release

Fat enters duodenum →
CCK released →
Gallbladder contracts →
Bile released

17.8 Small Intestine

Functions

  • Receives chyme

  • Completes digestion

  • Absorbs nutrients

  • Moves residue to large intestine

Regions

Duodenum

Shortest section

Jejunum

Most active absorption

Ileum

Contains Peyer's patches

Structures Increasing Surface Area

Plicae Circulares

Circular folds

Villi

Fingerlike projections

Microvilli

Brush border

Villi Structure

Contain:

  • Blood capillaries

  • Lacteal

  • Nerves

Small Intestinal Secretions

Mucus

Goblet cells and Brunner's glands

Watery Fluid

Intestinal glands

Brush Border Enzymes

Peptidases

Sucrase

Maltase

Lactase

Lipase

Enterokinase

Absorption

Carbohydrates

Starch → Disaccharides → Monosaccharides

Absorbed into:

  • Blood

Proteins

Proteins → Peptides → Amino acids

Absorbed into:

  • Blood

Fats

Fats →
Fatty acids + glycerol

Micelles form

Triglycerides reassembled

Chylomicrons formed

Absorbed into:

  • Lacteals

Small Intestine Movements

Segmentation

Mixing

Peristalsis

Propulsion

Parasympathetic: via vagus nerve

  • Stimulates

Sympathetic:

  • Inhibits

17.9 Large Intestine

Functions

  • Absorb water

  • Absorb electrolytes

  • Form feces

  • Store feces

  • Defecation

Parts

Cecum

Contains appendix

Colon

  • Ascending

  • Transverse

  • Descending

  • Sigmoid

Rectum

Anal Canal

Anal Sphincters

Internal

Smooth muscle

External

Skeletal muscle

Large Intestinal Wall

Contains:

  • No villi

  • No plicae circulares

Taeniae Coli

Three longitudinal muscle bands

Haustra

Pouches formed by taeniae coli

Functions

Water Absorption

About 90% of entering water

Intestinal Flora

Produce:

  • Vitamin K

  • Vitamin B12

  • Thiamine

Mucus Secretion

Feces Formation

Movements

Mixing Movements

Mass Movements

Strong peristaltic waves
2–3 times daily

Defecation Reflex

  • Rectum fills

  • Internal sphincter relaxes

  • External sphincter voluntarily relaxes

Feces Composition

  • 75% water

  • Electrolytes

  • Mucus

  • Bacteria

  • Bile pigments

Odor from:

  • Phenol

  • Indole

  • Skatole

  • Ammonia

  • Hydrogen sulfide

Large Intestine Disorders

Diverticulosis

Outpouchings in intestinal wall

Ulcerative Colitis

Affects mucosa and submucosa

Symptoms:

  • Bloody diarrhea

  • Cramps

Crohn's Disease

Affects all layers

Symptoms:

  • Pain

  • Diarrhea

Colorectal Cancer

4th most common cancer in U.S.

Screening:

  • Colonoscopy

  • Fecal occult blood test

17.10 Life-Span Changes

As aging occurs:

Teeth

  • Enamel thins

  • Teeth loosen

  • Tooth loss increases

Digestive Tract

  • Peristalsis slows

  • Heartburn increases

  • Gastric secretion decreases

  • Constipation increases

Absorption

  • Nutrient absorption decreases

  • Fat-soluble vitamin absorption decreases

  • Lactose intolerance becomes more common

Accessory Organs

Age gradually but often continue functioning adequately.