Chapter 24: The Digestive System

Organisms

Need to acquire nutrients from environment

Metabolism Anabolism

Uses raw materials to synthesize essential compounds (ex: cell membrane lipids)

Catabolism Decomposes organic molecules to provide energy

(ATP) that cells need to function Usually requires oxygen

Components of the Digestive System

Figure 24–1

Digestive Tract

Gastrointestinal (GI) tract or alimentary canal

Is a muscular tube Extends from oral cavity to anus

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 tract

3. 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

6 Functions of the Digestive System

6 Functions of the Digestive System

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

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

Peristalsis

Figure 24–4

Peristalsis

Consists of waves of muscular contractions Circular and longitudinal

Moves a bolus along the length of the digestive tract

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

The Regulation of Digestive Activities

Figure 24–5

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

At least 18 hormones that affect most aspects of digestive function

Are produced by enteroendocrine cells in digestive tract

Reach target organs after distribution in bloodstream

The Oral Cavity

Figure 24–6

4 Functions of the Oral Cavity

1. Sensory analysis: of material before swallowing

2. Mechanical processing: through actions of teeth, tongue, and palatal

surfaces

3. Lubrication: mixing with mucus and salivary gland secretions

4. Limited (chemical) digestion: of carbohydrates and lipids

The Epithelial Lining

Of cheeks, lips, and inferior surface of tongue is nonkeratinized, and delicate

Mucosa under tongue and inside cheeks is thin and vascular enough to rapidly absorb lipid-soluble drugs

Lingual Papillae

Fine projections on superior surface (dorsum) of tongue

Covered in thick epithelium Assists in moving materials

Lingual Glands

Small glands extend into underlying lamina propria

Secretions flush tongue’s epithelium Contain water, mucins, and enzyme

lingual lipase

Salivary Glands

3 pairs secrete products into oral cavity Each pair has distinctive cellular

organization and produces saliva with different properties

Produce 1.0–1.5 liters of saliva each day

Saliva

99.4% water 0.6% includes:

electrolytes (Na+, Cl—, and HCO3—)

buffers glycoproteins (mucins) antibodies Enzymes (salivary amylase) waste products

Control of Salivary Secretions

By autonomic nervous system: parasympathetic and sympathetic innervation

_______________ stimulation accelerates secretion by all salivary glands

The Swallowing Process

Figure 24–11

Swallowing

Also called deglutition (2,400 X a day) Can be initiated voluntarily Proceeds automatically Is divided in 3 phases:

buccal phase pharyngeal phase esophageal phase

Primary Peristaltic Waves

Movements coordinated by afferent and efferent fibers in glossopharyngeal and vagus nerves

Controlled by swallowing center of medulla oblongata

The Stomach Lining

Figure 24–13

4 Functions of the Stomach

1. Storage of ingested food

2. Mechanical breakdown of ingested food

3. (Preliminary) Disruption of chemical bonds in food material:

by acids and enzymes

4. Production of intrinsic factor: glycoprotein required for absorption of vitamin B12 in

small intestine

Gastric Pits

Are shallow depressions that open onto the gastric surface

Mucous cells: at base, or neck, of each gastric pit actively divide, replacing superficial cells

Gastric Glands

In fundus and body of stomach: extend deep into underlying lamina propria

Each gastric pit leads to several gastric glands

2 main types of cells found in gastric glands: parietal cells chief cells

Parietal and Chief Cells

Parietal cells Secrete hydrochloric acid (HCl)

Chief cells Are most abundant near base of gastric gland

(secrete pepsinogen) Pepsinogen is converted by HCl in the gastric

lumen to pepsin

Enteroendocrine Cells ofPyloric Glands

Are scattered among mucus-secreting cells of pylorus:

G cells Produce gastrin (hormone that stimulates

both the Chief and Parietal cells)

D cells Release somatostatin (hormone that

inhibits release of gastrin)

The Phases of Gastric

Secretion

Figure 24–15

The Cephalic Phase

Begins when you see, smell, taste, or think of food: directed by CNS prepares stomach to receive food

The Gastric Phase

Begins with arrival of food in stomach: builds on stimulation from cephalic phase lasts 3-4 hours

The Intestinal Phase

Begins when chyme first enters small intestine:

After several hours of mixing contractions: when waves of contraction sweep down

length of stomach

Digestion in the Stomach

Stomach performs preliminary digestion of proteins by pepsin: some digestion of carbohydrates (by salivary

amylase) lipids (by lingual lipase) Chyme

become more fluid pH approaches 2.0

pepsin activity increases (proteins)

Absorption in the Stomach

Although some digestion occurs in the stomach, nutrients are not absorbed there Only small lipid-soluble particles can cross

stomach lining Alcohol Drugs (aspirin)

The Small Intestine

Plays key role in digestion and absorption of nutrients

90% of nutrient absorption (lipids, proteins, carbohydrates) occurs in the small intestine

The Intestinal Wall

Figure 24–17

The Duodenum

The segment of small intestine closest to stomach

25 cm (10 in.) long “Mixing bowl” that receives:

chyme from stomach digestive secretions from pancreas and liver

The Jejunum

Is the middle segment of small intestine 2.5 meters (8.2 ft) long Is the location of most:

chemical digestion nutrient absorption

The Ileum

The final segment of small intestine 3.5 meters (11.48 ft) long

Brush Border Enzymes

Integral membrane proteins On surfaces of intestinal microvilli Break down materials in contact with

brush border Ex: Enterokinase

A brush border enzyme Activates pancreatic proenzyme Trypsinogen

Enteroendocrine Cells

In intestinal glands Produce intestinal hormones:

Gastrin Cholecystokinin (CCK) Secretin

Functions of the Duodenum

Has few plicae and Small villi receives chyme from stomach neutralizes acids before they can damage

the absorptive surfaces of the small intestine

Intestinal Secretions

Watery intestinal juice 1.8 liters per day enter intestinal lumen Moistens chyme Assists in buffering acids Keeps digestive enzymes and products of

digestion in solution

Intestinal Movements

Chyme arrives in duodenum Weak peristaltic contractions move it

slowly toward jejunum Segmentation will periodically “mix”

everything up

The Gastric Reflexes

The Gastroenteric Reflex Stimulates motility and secretion along entire

small intestines

The Gastroileal Reflex Triggers relaxation of ileocecal valve: Allows materials to pass from small intestine

into large intestines

Functions of the Pancreas

1. Endocrine cells: of pancreatic islets secrete insulin and glucagon into

bloodstream

2. Exocrine cells: acinar cells Produce pancreatic juice (alkaline mixture

of digestive enzymes, water, ions)

Pancreatic Secretions

1000 ml (1 L) pancreatic juice per day Controlled by hormones from duodenum Contain pancreatic enzymes

Pancreatic Enzymes

Pancreatic alpha-amylase: a carbohydrase breaks down starches similar to salivary amylase

Pancreatic lipase: breaks down complex lipids releases products (e.g., fatty acids) that are

easily absorbed

Pancreatic Enzymes

Nucleases: break down nucleic acids

Proteolytic enzymes: break certain proteins apart proteases break large protein complexes peptidases break small peptides into amino

acids

Trypsin

An active protease Enterokinase in duodenum:

converts trypsinogen to trypsin

Hepatic Blood Supply

1/3 of blood supply: arterial blood from __________________

2/3 venous blood from _________________, originating at: esophagus stomach small intestine most of large intestine

Liver Histology

Figure 24–20

Liver Lobules

The basic functional units of the liver Each lobe is divided:

by connective tissue into about 100,000 liver lobules about 1 mm diameter each

Hepatocytes

liver cells Adjust circulating levels of nutrients:

through selective absorption and secretion form a series of irregular plates arranged

like wheel spokes Many Kupffer Cells (immune system

macrophages) are located in sinusoidal lining

A Portal Area

Contains 3 structures: branch of hepatic portal vein branch of hepatic artery proper small branch of bile duct

Hepatocyte Function

As blood flows through sinusoids: hepatocytes absorb solutes from plasma and secrete materials such as plasma

proteins

Pressures in Hepatic Portal System

Are usually low (average 10 mm Hg or less) Can increase markedly:

if blood flow is restricted by blood clot or damage

Portal hypertension: an abnormal rise in portal pressure can be a symptom of liver cirrhosis can cause esophageal varices

Liver functions

1. Metabolic regulation

2. Hematological regulation

3. Bile production

1. Metabolic Regulation

• Removal and storage of carbohydrates, lipids, amino acids

• Mobilizing or synthesizing energy reserves• Vitamin and mineral storage• detoxification

2. Hematological Regulation

• Phagocytosis and antigen presentation• Synthesis of plasma proteins• Removal of circulating hormones• Removal of antibodies• Removal of RBC

3. Bile production

• Synthesis and secretion of bile• Dietary lipids are not water soluble Mechanical processing in stomach

creates drops containing lipids Pancreatic lipase is not lipid soluble:

interacts only at surface of lipid droplet

Bile needed for emulsification Helps pancreatic lipase do it’s job

The Gallbladder and Bile Ducts

Figure 24–21

The Gallbladder

Is a pear-shaped, muscular sac Stores and concentrates bile prior to

excretion into small intestine Releases bile into duodenum:

only under stimulation of hormone cholecystokinin (CCK)

Gallstones

Are crystals of insoluble minerals and salts Form if bile is too concentrated Small stones may be flushed through bile

duct and excreted

Activities of Major

Digestive Tract Hormones

Figure 24–22

Hormones of Enteroendocrine Cells

Coordinate digestive functions Secretin cholecystokinin (CCK) gastric inhibitory peptide (GIP) vasoactive intestinal peptide (VIP) gastrin enterocrinin

Secretin

Is released when chyme arrives in duodenum

Increases secretion of bile, buffers and enzymes by pancreas and liver

Cholecystokinin (CCK)

Is secreted by the duodenum: when chyme contains lipids and partially

digested proteins Relaxes hepatopancreatic sphincter

and gallbladder: ejects bile and pancreatic juice into

duodenum

Gastric Inhibitory Peptide (GIP)

Is secreted when fats and carbohydrates enter small intestine

Causes pancreas to release insulin Inhibits Gastrin

Vasoactive Intestinal Peptide (VIP)

Dilates capillaries of the villi

Gastrin

Is secreted by G cells in duodenum: when exposed to incompletely digested

proteins

Promotes increased stomach motility Stimulates acids and enzyme production

in the stomach

Enterocrinin

Is released when chyme enters small intestine

Stimulates mucin production by submucosal glands of duodenum

Intestinal Absorption

It takes about 5 hours for materials to pass: from duodenum to end of ileum

Movements of the mucosa increases absorptive effectiveness: stir and mix intestinal contents constantly change environment around

epithelial cells

Right colic(hepatic) flexure

TRANSVERSECOLON

Middle colicartery and vein

Right colicartery and vein

ASCENDINGCOLON

Fatty appendices

Ileocecal valve

Cecum

Appendix

Ileum

Ileocecalvalve

Cecum(cut open)

Appendix

Cecum and appendix

Anal canalAnal

columnsInternal anal

sphincterExternal anal

sphincterAnus

RectumRectum

Rectum, sectioned

SIGMOID COLON

Sigmoid flexure

Taenia coli

Sigmoid arteriesand veins

Haustra

Left colic artery

Inferiormesentericartery

Left colic vein

Intestinal arteriesand veins

Rectalartery

Greateromentum (cut)

DESCENDINGCOLON

Left colic(splenic)flexure

Splenic vein

Superior mesenteric artery

Inferior mesenteric vein

AortaHepatic portal vein

Superiormesenteric vein

Inferior vena cava

Functions of the Large Intestine

1. Reabsorption of water

2. Compaction of intestinal contents into feces

3. Absorption of important vitamins released by bacteria

4. Storage of fecal material prior to defecation

The Large Intestine

Also called large bowel Is about 1.5 meters (4.9 ft) long and 7.5

cm (3 in) wide Three regions:

Cecum Colon Rectum

The Rectum

Forms last 15 cm of digestive tract Is an expandable organ for temporary

storage of feces Movement of fecal material into rectum

triggers urge to defecate

Anal Sphincters

Internal anal sphincter: circular muscle layer of muscularis externa has smooth muscle cells, not under voluntary

control External anal sphincter:

encircles distal portion of anal canal a ring of skeletal muscle fibers, is under

voluntary control

Characteristics of the Colon

Lack villi Presence of distinctive intestinal glands

Are deeper than glands of small intestine Are dominated by goblet cells Provides lubrication for fecal material

Does not produce enzymes

Physiology of the Large Intestine

Less than 10% of nutrient absorption occurs in large intestine

Prepares fecal material for ejection from the body

Absorption in the Large Intestine

Reabsorption of water Reabsorption of bile salts:

in the cecum transported in blood to liver

Absorption of vitamins released by bacteria

Vitamins

Are organic molecules Important as cofactors or coenzymes in

metabolism Normal bacteria in colon make 3 vitamins

that supplement diet Vitamin K, Biotin, Pantothenic acid

Organic Wastes

Bacteria convert bilirubin to urobilinogens and stercobilinogens

Bacteria break down peptides in feces and generate ammonia hydrogen sulfide

Action on indigestible carbohydrates produce flatus, or intestinal gas

Movements of the Large Intestine (1 of 3)

Gastroileal and gastroenteric reflexes: move materials into cecum while you eat

Peristaltic waves move material from cecum to transverse colon very slow allowing hours for water absorption

Movements of the Large Intestine (2 of 3)

Segmentation movements (haustral churning) mix contents of adjacent haustra

Movement from transverse colon through rest of large intestine results from powerful peristaltic contractions (mass movements)

Movements of the Large Intestine (3 of 3)

Stimulus is distension of stomach and duodenum; relayed over intestinal nerve plexuses

Distension of the rectal wall triggers defecation reflex: positive feedback loops triggered by stretch

receptors in rectum

Essential Nutrients

A typical meal contains: Carbohydrates (complex carbohydrates) Proteins (polypeptides) Lipids (triglycerides) water Electrolytes (Na+, K+) Vitamins (A,B,C, D,E,K)

Digestion and Absorption

Digestive system handles each nutrient differently: large organic molecules:

must be digested before absorption can occur

water, electrolytes, and vitamins: can be absorbed without processing may require special transport

Digestive Enzymes

secreted by salivary glands, tongue, stomach, pancreas, (liver)

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

acids in a process called hydrolysis

Salivary Amylase and Pancratic Alpha-Amylase

From parotid and submandibular salivary glands and pancreas

Breaks down starches (complex carbohydrates)

Produces: disaccharides (2 simple sugars) trisaccharides (3 simple sugars)

Carbohydrates

Fragment disaccharides and trisaccharides into monosaccharides (simple sugars): lactase splits lactose into glucose and

galactose

Absorption of Monosaccharides

Intestinal epithelium absorbs monosaccharides

Lipid Digestion

Involves: lingual lipase from glands of tongue pancreatic lipase from pancreas Bile salts improve chemical digestion by

emulsifying lipid drops into tiny droplets

Triglycerides

Are the most important and abundant dietary lipids

Consist of 3 fatty acids attached to 1 molecule glycerol

Lipid Absorption

Triglycerides and other absorbed molecules are coated with proteins: creating chylomicrons

Lacteals pick up chylomicrons and they later enter left subclavian vein

Protein Digestion (1 of 2)

Is complex and time-consuming: mechanical processing in oral cavity

(mastication) and chemical processing in stomach acid (HCl) allows proteolytic enzymes to attack proteins

Protein Digestion (2 of 2)

pepsin: proteolytic enzyme works at pH 1.5–2.0 breaks peptide bonds within polypeptide chain

when chyme enters duodenum: enterokinase from small intestine triggers

conversion of trypsinogen to trypsin pH is adjusted to 7–8 Other pancreatic proteases work through SI

Digestive Secretion

and Absorption

Figure 24–27

Water Absorption

Cells cannot actively absorb or secrete water

All movement of water across lining of digestive tract: involves passive water flow down osmotic

gradients

Vitamins

Are organic compounds required in very small quantities

Are divided in 2 major groups: fat-soluble vitamins water-soluble vitamins

Vitamin B12

Cannot be absorbed by intestinal mucosa in normal amounts: unless bound to intrinsic factor (glycoprotein

secreted by parietal cells of stomach)

5 Effects of Aging on the Digestive System

1. Division of epithelial stem cells declines: digestive epithelium becomes more

susceptible to damage by abrasion, acids, or enzymes

2. Smooth muscle tone and general motility decreases:

peristaltic contractions become weaker

5 Effects of Aging on the Digestive System

3. Cumulative damage from toxins (alcohol, other chemicals) to liver and pancreas

4. Rates of colon cancer and stomach cancer rise

5. Decline in olfactory and gustatory sensitivities:

lead to dietary changes that affect entire body