Chapter 16The Digestive System

Functions of the Digestive SystemFunctions of the Digestive System

1. take in food

2. break down food

3. absorb digested molecules

4. provide nutrients

5. eliminate wastes

Two Divisions of the Digestive SystemTwo Divisions of the Digestive System

1. alimentary canal (digestive tract)

• digests and absorbs

• oral cavity to anus

2. accessory digestive organs

Organs of the Digestive SystemOrgans of the Digestive System

Figure 14.1

Digestive TractDigestive Tract

muscular tube approximately 9 M long

open at both ends

Wall has 4 layers with some variation due to specific function.

Layers of Digestive TractLayers of Digestive Tract

innermost: mucosa

epithelial cells which line cavity

attached to connective tissue

surrounded by thin muscular layer

protects tissues of canal

carries out secretion and absorption

Layers of Digestive TractLayers of Digestive Tract

next: submucosa

thick layer of loose connective tissue housing blood vessels, lymph vessels, and nerves

nourishes surrounding layers of tract

Layers of Digestive TractLayers of Digestive Tract

next: muscular layer

inner: circular smooth muscle

outer: longitudinal smooth muscle

propels food through digestive tract

contains lots of nerves to control movement and secretion

Layers of Digestive TractLayers of Digestive Tract

outermost: serosa

visceral peritoneum

protects underlying tissue

secretes serous fluid to keep tract from sticking to other tissues of abdominal cavity

Layers of Alimentary Canal OrgansLayers of Alimentary Canal Organs

Figure 14.3

Mouth (Oral Cavity) AnatomyMouth (Oral Cavity) Anatomy teeth – 32 total in adults

incisors, premolars, molars, wisdom teeth

tongue – thick, muscular organ functions in digestion,

taste and speech attaches to posterior

part of mouth and by frenulum

covered by mucous membrane

houses taste buds within papillae salty, sweet, sour,

bitter

Mouth (Oral Cavity) AnatomyMouth (Oral Cavity) Anatomy

tonsils lingual and palatine

palate – roof of oral cavity hard = bone soft = muscle and

connective tissue separates oral and

nasal cavities uvula – fleshy

projection of the soft palate

Salivary GlandsSalivary Glands

produce saliva – mixture of mucous and serous fluids keeps mouth moist contains enzymes that aid in chemical digestion

3 pairs in oral cavity parotid – located anterior to ears

secrete serous and mucous fluids submandibular – base of jaw

ducts open below tongue secrete more serous than mucous fluids

sublingual – smallest, below tongue duct opens into floor of oral cavity secretes thick, stringy mucous fluid

SalivaSaliva produce 1 liter a day serous fluid: parotid and submandibular glands

contains amylase, enzyme to break down starches lysozyme: reduces bacterial growth in mouth mucous: contains mucin for lubrication regulated by parasympathetic stimulation (involuntary

nervous system) increase in secretions with sight, smell, thought of

food

Processes of the MouthProcesses of the Mouth

mechanical digestion mastication (chewing) of food mixing of food with saliva by tongue

chemical digestion salivary amylase breaks down starches

allow for the sense of taste NO food absorption occurs in mouth.

SwallowingSwallowing

tongue, soft palate, pharynx and esophagus phase 1: voluntary (mouth)

Bolus (food mass) is forced into pharynx by tongue. phase 2: involuntary (pharynx)

Pharynx connects mouth and esophagus. Receptors in pharynx sense food which triggers

swallowing reflexes. Epiglottis covers larynx.

phase 3: involuntary (esophagus) Esophagus connects pharynx to stomach. Peristalsis moves food from esophagus to stomach. Lower esophageal sphincter helps prevent regurgitation

of food.

Stomach AnatomyStomach Anatomy

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located on the left side of the abdominal cavity

Food enters at the cardiac opening.

Stomach FunctionsStomach Functions

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acts as a storage tank for food

site of food breakdown

start of protein digestion

very little absorption

delivers chyme (processed food) to the small intestine

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Regions of the stomach

Cardiac region – near the heart

Fundus – most superior part

Body

Pylorus – funnel-shaped terminal end

Food empties into the small intestine at the pyloric sphincter.

Stomach AnatomyStomach Anatomy

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Figure 14.4a

Muscles of the stomach

• different than rest of digestive tract• 3 layers

1. outer longitudinal layer

2. middle circular layer

3. inner oblique layer

• produce churning action – mechanical digestion

Specialized Mucosa of the StomachSpecialized Mucosa of the Stomach

gastric glands – secrete gastric juice

3 types of cells

1. mucus cells – produce a sticky alkaline mucus which protects stomach lining

2. chief cells – secrete protein-digesting enzymes (pepsinogen which becomes pepsin)

3. parietal cells – secrete hydrochloric acid

rugae – internal folds of the mucosa allows stomach to stretch when full

Other Stomach SecretionsOther Stomach Secretions

• gastrin – hormone that increases stomach secretions• intrinsic factor – aids in absorption of vitamin B12• cholecystokinin – hormone that stimulates gastric

secretions

Production of gastric juices is regulated by hormones and the nervous system.

Humans produce about 2 L per day.

How Stomach Digests FoodHow Stomach Digests Food

1. As food enters digestive tract, gastrin is produced which causes production of gastric juices.

2. Food mixes with gastric juices and forms chyme which passes to pyloric region.

3. As food empties, secretions from stomach wall are inhibited by nervous system.

Stomach MovementStomach Movement

2 types resulting from stomach wall muscle contractions1. mixing – from weak contractions

mix ingested food with stomach secretions to form chyme

2. peristalsis – from stronger contractionsforce chyme toward and through pyloric sphincter

Structure of the Stomach MucosaStructure of the Stomach Mucosa

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Figure 14.4b, c

Small IntestineSmall Intestine

absorbs products of digestion

receives secretions from pancreas and liver

transports remaining residue to large intestine

6 meters long

ileocecal junction – joins ileum and large intestine

Ileocecal sphincter and ileocecal valve keep materials moving in right direction.

Subdivisions of the Small IntestineSubdivisions of the Small Intestine

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Duodenum

attached to the stomach

curves around the head of the pancreas

25 cm long

Common bile duct from liver and pancreatic duct join each other and empty into duodenum.

Jejunum

2.5 meters; attaches anteriorly to the duodenum

Ileum

3.5 meters, extends from jejunum to large intestine

Subdivisions of the Small IntestineSubdivisions of the Small Intestine

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Duodenum, jejunum and ileum have same structure except:

there is a gradual decrease in diameter

decrease in thickness of the walls

decrease in # of circular folds

decrease in # of villi

Chemical Digestion in the Small IntestineChemical Digestion in the Small Intestine

3 modifications to increase surface area up to 600 times

1. circular folds – run perpendicular to long axis of tract

2. villi – tiny finger-like projections formed by mucosa

3. microvilli – extensions of villi found in absorptive cells

Mucous Lining of Small IntestineMucous Lining of Small Intestinecomposed of simple columnar epithelium1. absorptive cells

have microvilliproduce digestive enzymesabsorb digested food

2. goblet cellsproduce protective mucus

3. granular cells protect intestinal epithelium from bacteria

4. endocrine cellsproduce regulatory hormones

Secretions of the Small IntestineSecretions of the Small Intestine

• lubricate and protect intestinal wall from acidic chyme and digestive enzymes

• keep chyme in liquid form

• produced by mucosa, pancreas, liver

• peptidases – break proteins into amino acids

• disaccharidases – break down complex sugars into monosaccharides

Absorption in the Small IntestineAbsorption in the Small Intestine

major function of small intestinemost in duodenum and jejunum

monosaccharides – absorbed through active transport or facilitated diffusion by villi

amino acids – absorbed through active transport by villi and carried away in blood

fatty acids – dissolve into cell membranes of villi and carried away by lymphatic vessels

water – absorbed through osmosis into villi electrolytes – absorbed by active transport by villi

Movement in the Small IntestineMovement in the Small Intestine

• mixing and propulsion – mechanical breakdown

• peristaltic contraction – causes chyme to move along tract; occur along entire digestive tract

• segmental contraction – short movements to mix contents

Large IntestineLarge Intestine

larger in diameter, but shorter than the small intestine 1.5 meters extends from ileocecal valve to anus major functions:

absorption of water, electrolytes, vitamins K and B elimination of indigestible food (feces) production of mucus for lubrication No digestion of food occurs here. Bacteria produce vitamin K and B.

Divisions of the Large IntestineDivisions of the Large Intestine

cecum – saclike first part of the large intestine appendix

accumulation of lymphatic tissue hangs from the cecum

colon ascending transverse descending S-shaped sigmoid (in pelvis)

rectum

anus external body opening contains 2 sphincters

Large IntestineLarge Intestine

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Figure 14.8

Structures of the Large IntestineStructures of the Large Intestine

3 bands of longitudinal muscle arranged in teniae coli create haustra (pocket like sacs in wall)

large numbers of goblet cells

Absorption in the Large IntestineAbsorption in the Large Intestine

no digestive enzymes absorption of water and electrolytes and vitamins K and B

Propulsion in the Large IntestinePropulsion in the Large Intestine mass movements

slow, powerful contractions move contents toward anus occur two to three times per day cover more distance than peristaltic contractions

Presence of feces in the rectum causes a defecation reflex. Internal anal sphincter is relaxed. Defecation occurs with relaxation of the voluntary

(external) anal sphincter.

PancreasPancreas

located behind the stomach produces digestive enzymes that break down all categories

of food trypsin – protein digestion pancreatic amylase – starch digestion lipidase – fat digestion secreted into the duodenum

Alkaline fluid introduced with enzymes neutralizes acidic chyme.

LiverLiver

located on the right side of the body under the diaphragm

consists of four lobes capable of regeneration connected to the gall bladder via the common hepatic

duct metabolizes carbohydrates, lipids, proteins stores glycogen, vitamins A,D and B12, iron and blood filters blood, removes toxins and damaged blood cells secretes bile

BileBile

yellowish, green liquid includes water, bile salts, bile pigments, cholesterol

and electrolytes Bile salts

emulsify fats. aid in absorption of fatty acids, cholesterol and some

vitamins

Gall BladderGall Bladder

sac found beneath liver stores bile from the liver releases bile into the duodenum in the presence of fatty

food release controlled by sphincter muscles Crystallized bile causes gallstones.

Processes of the Digestive SystemProcesses of the Digestive System

Figure 14.11