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The Digestive System

Digestive System

• Digestion

�Mechanical & chemical breakdown of food into a form that can be used by cells

�Mechanical – breaks large pieces into smaller pieces

�Chemical – breaks food into simpler chemicals. Ex. Carbohydrates broken into glucose, proteins into amino acids

Digestive Processes

• Ingestion – taking in food

• Digestion – mechanical & chemical

breakdown

• Deglutition – swallowing

• Propulsion – movement of food thru system

• Absorption – nutrients & water move into

surrounding cells

• Defecation – removal of ingestible waste

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Digestive System• 2 main divisions

� Alimentary canal(GI tract) – continuous coiled hollow tube from mouth to anus

� Accessory digestive organs – release secretions into GI tract

Organs of the Alimentary Canal

• Mouth

• Pharynx

• Esophagus

• Stomach

• Small intestine

• Large intestine

• Anal canal

• Salivary glands

• Liver

• Gallbladder

• Pancreas

Accessory Organs

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Anus

ACCESSORY ORGANS

Salivary glandsSecrete saliva, which containsenzymes that initiate breakdownof carbohydrates

LiverProduces bile, whichemulsifies fat

GallbladderStores bile and introduces it into small intestine

PancreasProduces and secretes pancreatic juice, containing digestive enzymes and bicarbonate ions, into small intestine

ALIMENTARY CANAL

MouthMechanical breakdown of food; begins chemical digestion of carbohydrates

PharynxConnects mouth with esophagus

EsophagusPeristalsis pushes food to stomach

StomachSecretes acid and enzymes; mixes foodwith secretions to begin enzymaticdigestion of proteins

Small intestineMixes food with bile and pancreatic juice; final enzymatic breakdown of food molecules; main site of nutrient absorption

Large intestineAbsorbs water and electrolytes to form feces

RectumRegulates elimination of feces

• Muscular tube ~ 8 meters long (~26 ft)

• Peristalsis – wavelike motion of muscular contractions that propel food forward

• 4 layers

� Mucosa membrane

� Inner epithelial layer surrounding lumen

� Villi present – tiny folds or projections that absorb nutrients

� Submucosa layer

� Loose CT containing glands, nerves & blood vessels

� Nourishes surrounding cells

� Transports nutrients to blood stream

� Muscular layers

� Inner layer of circular smooth muscles; movement of food

� Outer layer of longitudinal smooth muscles; movement of food

� Serosa membrane

� Outer visceral peritoneum that secretes serous fluid that lubricate layer

Characteristics of GI tract

Movements of the AI Tract

• Segmentation – movement in the S.I. that

aids in mixing

• Peristalsis – contractions that move food

down the A.I. tract

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Mouth (Oral Cavity)• Lips (labia) – protect anterior opening

• Cheeks – some taste buds; muscles for chewing

• Teeth – mastication; mechanical digestion

• Salivary glands – chemical digestion

• Hard palate – forms anterior roof

• Soft palate – forms posterior roof

• Uvula – fleshy projection of the soft palate; prevents food from entering nasal cavity

• Tongue – attached by the lingual frenulum; contains papillaewhich houses taste buds; mixes food

• Vestibule – space between lips, teeth & gums

• Tonsils – lymphatic tissues

Mouth (Oral Cavity)

Teeth•Hardest structure in body

•Mastication

•Primary teeth (20) – baby

teeth

•Secondary teeth (~ 32) –some lack wisdom teeth

•Incisors – cut

•Cuspid/canine – grasp & tear

•Molars – grind

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• Secretes saliva which moistens food

• Mucosa cells secrete mucus that binds food (bolus)

• Serous cells produce amylase that digest starches

• Solvent for taste buds

• Keeps pH of mouth

about 6.5-7.5

• 3 salivary glands

• Parotid gland- largest

• Submandibular gland

• Sublingual gland

• Lipase - fats

• Sucrase - sugars

Salivary Glands

Pharynx • Connects nasal & oral cavities

• Passage way for food & air

• Nasopharynx –not part of digestive system

• Oropharynx – posterior to oral cavity

• Laryngopharynx –connects to esophagus

• Epiglottis – prevents food entering larynx

Esophagus• Passageway for food only; no digestion

• Runs from pharynx to stomach thru diaphragm

• Esophageal hiatus – point where esophagus passes thru diaphragm

• Esophageal sphincter(cardiac sphincter)

• controls opening into stomach

• prevents regurgitation

• Food propelled forward by peristalsis

• Mucosa cells moisten & lubricate

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Stomach Anatomy• Left side of abdominal cavity

• Avg. capacity ~ 1 liter

• pH ~ 2

• Food enters at the esophageal sphincter

• Regions of the stomach

• Cardiac region – near heart & esophageal sphincter

• Fundus – upper curved area; contains air

• Body region – center

• Pylorus – funnel-shaped terminal end of stomach

Stomach Anatomy• Pyloric sphincter –controls movement of

food to small intestine

• Rugae – internal folds of the mucosa lining of the stomach

• External regions

• Lesser curvature

• Greater curvature

Slide

14.17

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Stomach Functions• Storage tank for food

• Site of mechanical breakdown

• 3 layers of muscles mix food – circular, longitudinal & oblique

• Chemical breakdown of protein begins

• Delivers chyme (processed food; semi-liquid state) to the small intestine

• Only absorption in stomach is alcohol, some H2O & some drugs

Mucosa of the Stomach • Lined w/gastric pits leading to gastric glands

• Gastrin – hormone that regulates secretion of gastric glands

• Mucous neck cells – produce a sticky alkaline mucus that lines stomach walls

• Gastric glands – secrete gastric juice

• Chief cells – produce protein-digesting enzymes (pepsinogens)

• Parietal cells – produce hydrochloric acid (HCl) & intrinsic factor needed for B12 absorption

• Goblet cells – produce mucus

Structure of the Stomach Mucosa

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Small Intestine• ~18-20 ft. (5-6m)

• Receives chyme from stomach

• Receives secretions from liver, gallbladder & pancreas

• Site of nutrient absorption into the blood

• Extends from pyloric sphincter to ileocecal valve

• Mesentery – peritoneal membrane stabilizing intestines; supports blood vessels & nerves to S.I.

• Greater omentum – peritoneal membrane draped from stomach over intestines

Omentum

Subdivisions of Small Intestine• Duodenum

• Attached to stomach at pyloric sphincter

• Shortest part

• Pancreatic & bile ducts empty here

• Jejunum

• Attaches to duodenum

• Thicker & more vascular

• Ileum

• Extends from jejunum to large intestine

• Longest section

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Structures Involved in Absorption of Nutrients

• Villi - fingerlike projections lining mucosa

• ↑ surface area for absorption

• Columnar epithelia

• Blood capillaries present

• Lacteals - specialized lymphatic capillaries

• Plicae circulares – circular folds of mucosa

• Microvilli – smaller projections on villi to ↑absorption

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Small Intestinal Movements

• The small intestine carries on mixing

movements that include:

• Peristalsis – pushing movements that propel chyme

• Segmentation – ring-like contractions that can

move chyme back and forth

Large Intestine• ~ 1.5 m long (~4.5-5 ft)

• Larger diameter than S.I.

• Absorbs water & electrolytes – little or no digestion

• ileocecal sphincter - controls movement of chyme from S.I. to L.I.

• Cecum – beginning of L.I.

• Appendix – hangs down from cecum; lymphatic tissues

• Colon ÷ into 4 areas; segmented pockets called HAUSTRA

• Ascending colon – right side of body; turns left at hepatic flexure

• Transverse colon – horizontal; turns down at splenic flexure

• Descending colon – left side of body; turns toward pelvis to form

• Sigmoid colon – S-shaped curve

• Teniae coli – longitudinal muscles on surface of colon

• Rectum – last section of L.I.

• Anus – opening controlled by internal & external sphincters

Functions of the Large Intestine

• Absorption of water & electrolytes

• Eliminates indigestible food as feces

• No digestion of food

• Goblet cells produce mucus to act as a lubricant

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Accessory Digestive Organs

• Pancreas

• Liver

• Gall bladder

• Salivary glands

Pancreas• Mixed gland – both endocrine & exocrine

• Pancreatic duct merges w/common bile duct & secretions are released into duodenum

• Alkaline fluid introduced with enzymes neutralizes acidic chyme

• 2 types of cells

• Acinar cells – bulk of pancreas; produce pancreatic juices

• Islet cells – produce hormone insulin

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• Pancreatic amylase – splits starch or

glycogen

• Pancreatic lipase – breaks down

triglycerides; fats

• Trypsin & chymotrypsin – digest

proteins

• Nucleases – digest nucleic acids

• Bicarbonate ions – make pancreatic

juice alkaline

Pancreatic Enzymes

Liver• Largest organ in body

• Right upper quadrant of the body, under diaphragm

• 4 lobes suspended from the diaphragm & abdominal wall by the falciform ligament

• Connected to the gall bladder via the hepatic duct

• Produces glycogen from glucose

• Converts glycogen to glucose

• Synthesizes cholesterol

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Right lobe

Coronary ligament

Inferior vena cava

Left lobe

Inferior vena cava

Right lobe

Gallbladder Quadrate lobe

Cystic duct

Left lobe

Hepatic ductHepatic artery

Bile duct

Caudate lobe

Gallbladder

(b)(a)

Roundligament

Hepatic portalvein

Falciform ligament

Liver• Breaks down red blood cells

• Filters & detoxifies blood

• Stores iron, vitamin A, D & glycogen

• Produces bile

• Bile consist of the following

�Bile salts

�Bile pigments (Bilirubin & biliverin)

�Cholesterol

�Phospholipids

Gall Bladder• Sac beneath liver that stores concentrated bile

• Cystic duct carries bile from liver to gall bladder

• Common bile ducts secretes bile into the duodenum in the presence of fatty food

• Hepatopancreatic sphincter controls release

• Emulsification – when bile salts break fat globules into smaller pieces

• Gallstones form when cholesterol solidifies & can cause blockages

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Processes of the Digestive System

• Chemical Digestion

• Enzymes break down food molecules into their building blocks

• Each major food group uses different enzymes

• Carbohydrates are broken to simple sugars

• Proteins are broken to amino acids

• Fats are broken to fatty acids and glycerol