Chapter 14

The Digestive System – OverviewIs a long, hollow pathway called

the alimentary canal.Begins at mouth and ends with the

anus

Consists of primary and periphery organs

◦ Food passes through primary organs◦ Periphery organs only play a role, food

does not enter these organs.

The Digestive TractThe Digestive Tract

Fig. 14.1

Periphery Organs

4 Stages of DigestionIngestion

◦ taking in of food;

Digestion ◦ breaking down of food into smaller pieces◦ Mechanical digestion

Physical change only Breaks food into smaller pieces to increase surface

area

◦ Chemical digestion Enzymatic breakdown to small organic molecules

Enzymes – protein that chemically breaks down food Produced by the body's glands and organs.

Results in simple molecules that easily pass into the blood.

4 Stages of Digestion cont’dAbsorption

◦transporting of molecules into the blood;

Egestion ◦removing of solid, undigested

materials from the body.

Digestive Tract – MouthDigestive Tract – MouthOverall functions of mouth

◦Begins process of digestion◦Mechanical digestion

Chewing food Mixing with saliva Bolus formation

◦Chemical digestion Begin digestion of starches to

disaccharides

Adult mouthAdult mouth

Fig. 14.2

Digestive Tract – MouthDigestive Tract – Mouth cont’dcont’dTongue

◦Taste buds- chemical sensation of taste◦Rough surface- aids in mechanical

digestion◦Composed of skeletal muscle

Roof of mouth◦Hard palate- composed of bone

ridges for mechanical digestion◦Soft palate- composed of muscle

Uvula Closes off nares during swallowing

Digestive Tract – MouthDigestive Tract – Mouth cont’dcont’d

Tonsils◦Lymphoid tissue

Help protect against infection

◦3 Types Palatine- on both sides of pharynx Pharyngeal (Adenoids)- in nasopharynx Lingual - at base of tongue

◦Tonsillitis- inflammation of tonsils

Digestive Tract – MouthDigestive Tract – Mouth cont’dcont’d

Salivary glands◦3 pairs ◦Collective secretions are called

saliva Lubricates food so it can pass through

canal Dissolves food particles Allows us to taste food!

◦Contains amylase enzyme for starch digestion

Digestive Tract – MouthDigestive Tract – Mouth cont’dcont’d

Teeth◦ Mechanical

digestion◦ 20 Deciduous teeth

Baby teeth, first 2 years

◦ 32 adult teethTooth structure

◦ Crown Outer enamel

covering Dentin Pulp

◦ Root Dentin pulp Fig. 14.2

Digestive Tract – MouthDigestive Tract – Mouth cont’dcont’dTeeth cont’d

4 Types, each with a purpose◦ Incisors

Chisel shaped for cutting an slicing

◦ Canines Forms a single point used to hold and tear

food.

◦ Premolars & Molars Broad flat teeth Crushing and grinding

Digestive Tract – MouthDigestive Tract – Mouth cont’dcont’d

Teeth cont’d

◦Dental caries Cavities in teeth Prevented by flouride Brushing and flossing can help prevent

development

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Digestive Tract - PharynxDigestive Tract - PharynxConnects nasal and oral cavities Part of both respiratory and

digestive systemsMuscular structure, actively pushes

food downSwallowing reflex

◦Uvula closes off nares◦Trachea moves upward under epiglottis◦Epiglottis closes airway off◦Bolus of food moves down into

esophagus

Digestive Tract – Digestive Tract – EsophagusEsophagusPasses from pharynx to stomachPeristalsis

◦Rhythmic wave of contraction throughout tract

◦Propels bolus down esophagus to stomach

◦Sole purpose is conduction of food

Digestive Tract – Digestive Tract – Esophagus Esophagus cont’dcont’dSphinctors

◦Muscles that encircle tubes◦Act like valves

Contraction-closes tube Relaxation- opens tube

◦Esophageal sphinctor At entrance to stomach Not well developed Relaxes with peristaltic wave Bolus pushed through into stomach

Digestive Tract – Digestive Tract – Esophagus Esophagus cont’dcont’dHeartburn

◦Gastroesophageal reflux◦Acidic contents enter esophagus◦Causes irritation

Vomiting◦Abdominal muscles contract◦Diaphragm contracts◦Positive pressure pushes stomach

contents upward through esophagus

Walls of Digestive TractWalls of Digestive TractMucosa

◦Epithelium supported by connective tissue

◦Lines lumen◦Glandular epithelial cells produce

enzymes◦Goblet cells produce mucous

Submucosa◦Loose connective tissue◦Contains blood vessels◦Lymph nodes- Peyer’s patches

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Walls of Digestive Tract Walls of Digestive Tract cont’dcont’d

Muscularis- 2 layers of smooth muscle◦Longitudinal –outer, runs along

length of gut◦Circular-inner, encircles tube

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Walls of Digestive Tract Walls of Digestive Tract cont’dcont’dSerosa

◦Squamous epithelium◦Supported by connective tissue◦Secretes serous fluid◦Lacking in esophagus

14-20

Wall of digestive tractWall of digestive tract

Fig. 14.4

The StomachThe StomachThick-walled J-shaped organLies on left side of abdomenOverall function of the stomach

◦Mechanical digestion Mixing of food with gastric juice Forms semi-liquid called chyme

◦Chemical digestion Initiation of protein digestion

◦Storage of food

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The Stomach The Stomach cont’dcont’d

Rugae: folds in wall◦Aid in mechanical digestion-friction◦Allows expansion

3 muscle layers◦Longitudinal◦Oblique◦Circular

The stomach The stomach cont’d.cont’d.

Gastric glands◦Produce gastric juice◦Chief cells

Pepsinogen Inactive form of proteolytic enzyme

◦Parietal cells HCl

Activates pepsinogen to pepsin Decreases bacterial growth

Mucous cells◦Produce thick protective mucus layer

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14-25

The Stomach The Stomach cont’d.cont’d.

Fig. 14.5

The Stomach The Stomach cont’d.cont’d.

Ulcers◦Open sore in stomach wall◦Helicobacter pyloris ◦Infection decreases mucus

production

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The Small IntestineThe Small IntestineDuodenum

◦First 25 cm◦Principal site of digestion of nutrients◦Receives bile from the liver

Emulsification of fats

◦Receives pancreatic juice from pancreas Many enzymes for digestion of nutrients Bicarbonate to neutralize pH

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The Small Intestine The Small Intestine cont’d.cont’d.

Jejunum and ileum◦Principal site of absorption of

nutrients◦Lining has villi

Increases surface area Outer layer of columnar epithelium

Cells have microvilli- more surface area

Villi contain lymph lacteals Absorb fatty acids and glycerol

Villi contain blood capillaries Absorb sugars and amino acids

Anatomy of the small Anatomy of the small intestineintestine

Fig. 14.6

Regulation of Digestive Regulation of Digestive SecretionsSecretionsNeurological control

◦Pressure and distension◦Presence of particular types of food

Hormonal control◦Gastrin

Produced by stomach Stimulates gastric secretion

◦Gastric inhibitory peptide (GIP) Produced by duodenum Inhibits gastric secretion

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Regulation of Digestive Secretions Regulation of Digestive Secretions cont’dcont’d

Secretin and cholecystokinin (CCK)◦Produced by duodenum◦Acid stimulates secretin release◦Digested protein and fat stimulate

CCK◦Effects of both hormones

Increased pancreatic secretion Increased bile secretion

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Regulation of Digestive Regulation of Digestive Secretions Secretions cont’dcont’dHormonal

control of digestive gland secretions

Fig. 14.7

14-32

The Large IntestineThe Large IntestineCecum

◦Blind end of the large intestine, at juncture of s.i.

◦Appendix Projection of cecum May play a role in fighting infections Rupture may cause peritonitis

Colon◦Ascending, transverse, and descending

portions◦Absorption of water, salts◦Terminates at the rectum

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The Large Intestine The Large Intestine cont’d.cont’d.

Fig. 14.8

14-34

The Large Intestine The Large Intestine cont’d.cont’d.

Defecation reflex◦Feces forced into rectum by

peristalsis◦Stretching of walls initiates reflex◦Rectal muscles contract◦Anal sphinctors relax◦Defecation occurs

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The Large Intestine The Large Intestine cont’d.cont’d.

Fecal composition◦75% water◦Indigestible materials◦Intestinal flora (bacteria)

99% facultative anaerobes Produce vitamin K

◦Color from breakdown of bilirubin and oxidized iron

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Defecation reflex Defecation reflex cont’dcont’dFig. 14.9

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Accessory organsAccessory organsThe pancreas

◦Endocrine function Produces Insulin and glucagon

Regulate blood glucose

◦Exocrine function Produces

sodium bicarbonate: neutralizes chyme pancreatic amylase: chemical starch digestion Trypsin: chemical protein digestion Lipase: chemical fat digestion

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Accessory organs Accessory organs cont’d.cont’d.The liver

◦Largest gland in the body◦Lobules

Structural and functional units◦Triads between lobules

bile duct branch of hepatic artery branch of hepatic portal vein

◦Functions Removes and stores iron and vit. A, D, E, K,

and B12 Makes plasma proteins Regulates cholesterol

Accessory organs Accessory organs cont’dcont’d

The Liver cont’d …

◦Functions cont’d

Absorbs or chemically modifies toxic substances Alcohol and drugs metabolized into less harmful

compounds. Other toxins, pesticides, and carcinogens are also

detoxified. Less harmful compounds filtered from the blood

in the kidneys.

If the liver is chronically exposed to toxins, the cells become damaged and die. (cirrhosis)

Accessory organs Accessory organs cont’dcont’d

The Liver cont’d …

◦Functions cont’d

Bile production Stored in gall bladder Composition

Bilirubin – from breakdown of hemoglobin, greenish color

Bile salts - derived from cholesterol, emulsifies fats in duodenum

Accessory organs Accessory organs cont’dcont’d

The Liver◦The gall bladder, a separate organ on

lower surface of liver Stores bile Secreted through common bile duct to

duodenum

Bile salts contain cholesterol can precipitate out of solution and form crystals Crystals can become gall stones,

If large enough, they can block common bile duct and cause obstructive jaundice

Accessory organs Accessory organs cont’dcont’d

The liver cont’d…Functions cont’d

◦Glucose regulation Our body needs constant, stable levels of

glucose Brain cells store little glucose, and cannot use other

sources 0.1% blood glucose

Intestines cannot regulate the rate of nutrient absorption Different foods digest at different rates

Simple carbohydrates: digest quickly, causes high blood glucose levels after we eat

Complex carbohydrates, proteins: digest more slowly, glucose absorbed into blood gradually.

Accessory organs cont’d Accessory organs cont’d The liver cont’d…Functions cont’d… Glucose

Regulation cont’d High glucose levels in blood triggers the

pancreas to release insulin Insulin in blood signals liver to convert excess glucose

into glycogen.

Low glucose levels trigger the pancreas to release glucagon Glucagon signals liver to convert glycogen back into

useable glucose

When glycogen is depleted, the liver converts lipids (fats) and amino acids (proteins) to glucose

Requires deamination, break down, of amino acids then liver combines ammonia with carbon dioxide to form urea

Accessory organs cont’d Accessory organs cont’d The liver cont’d …Functions cont’d … Glucose

Regulation cont’d

Diabetes Low insulin levels, so no trigger to liver to

convert glucose to glycogen. Insulin injections help to maintain balanced

sugar levels.

Low Carb Diets Insufficient ‘fast-burn’ foods consumed, body

never has excess glucose to store as glycogen Puts body into starvation state, begins to digest

its own fats and proteins.

Accessory Organs Accessory Organs cont’dcont’d

The liver cont’d…Functions cont’d… Glucose Regulation cont’d

◦Both result in insufficient glycogen supplies Causes body to use fats as fuel, this

produces small molecules called ketones

Ketone build-up results in ketoacidosis Side effects include: weakness, nausea, headache, confusion, dehydration and acetone-like smelling breath. 

Hepatic portal systemHepatic portal system

Fig. 14.11