Digestive system

Digestion

breakdown of large organic molecules into smaller molecules that can be absorbed

Digestive system

• performs the task of digestion

• food is taken into it, where it is enzymatically broken down into smaller and smaller particles for absorption

• consists of the digestive tract and specific associated organs

Digestive system functions

• ingestion of solids and liquids

• digestion of organic molecules

• absorption of nutrient 

• elimination of waste 

Digestive tract / gastrointestinal tract

one long tube from the mouth to the anus

Digestive tract components

• oral cavity (mouth)

• pharynx

• esophagus

• stomach

• small intestines

• large intestines

• rectum

• anus

Associated organs

organs not directly in the digestive tract but have ducts that lead into the tract

• salivary glands

• liver

• gallbladder

• pancreas

Tunics

layers of tract wall 

Layers of digestive tract wall

• mucosa

• submucosa

• muscularis

• serosa / adventitia

Mucosa

• innermost

• secretes mucus

Submucosa

• above mucosa

• contains blood vessels, nerves, small glands

Muscularis

• above submucosa

• longitudinal, circular, and oblique muscles

Serosa / adventitia

• outermost layer

• peritoneum is present - serosa

• no peritoneum - adventitia

Peritoneum

layer of smooth epithelial tissue

Mesenteries

connective tissue organs in abdominal cavity

Lesser omentum

mesentery connecting lesser curvature of stomach to liver and diaphragm

Greater omentum 

mesentery connecting greater curvature of stomach to transverse colon and posterior colon and posterior body wall

Oral cavity

• first part of digestive system

• contains stratified squamous epithelia

Salivary glands

• produce saliva which contain enzymes to breakdown carbohydrates into glucose

• cleanse mouth

• dissolve and moisten food

Amylase

salivary enzyme that breaks down carbohydrates

Lysozyme

salivary enzymes that are active against bacteria

Tongue

house taste buds and mucus

Teeth

• 32 teeth in normal adult

• incisors, canine, premolars, molars, wisdom

• 20 primary teeth (baby teeth)

• each tooth has crown, cusp, neck, root

Pulp cavity

center of tooth

Enamel

hard covering protects against abrasions

Cavities 

breakdown of enamel by acids from bacteria 

Palate

roof of oral cavity

Hard palate

anterior part of palate

Soft palate

posterior part of palate

Salivary glands

• includes submandibular, sublingual, parotid

• produce saliva contains enzymes to breakdown food

Mumps

inflammation of parotid gland

Pharynx

• throat

• connects the mouth to the esophagus

• has 3 parts:

  • nasopharynx

  • oropharynx

  • laryngopharynx

Esophagus 

• tube that connects the pharynx to the stomach 

• transports food to the stomach 

• joins stomach at cardiac opening 

Heartburn

• occurs when gastric juices regurgitate into esophagus

• caused by caffeine, smoking, or eating or drinking in excess

Swallowing

• voluntary phase

• pharyngeal phase 

• esophageal phase

• peristalsis

Voluntary phase

bolus (mass of food) formed in the mouth and pushed into oropharynx

Pharyngeal phase

swallowing reflex initiated when bolus stimulates receptors in ororpharynx

Esophageal phase

moves food from pharynx to stomach 

Peristalsis

wave-like contractions moves food through digestive tract

• a wave of smooth muscle relaxation moves ahead of the bolus, allowing the digestive tract to expand

• a wave of contraction of the smooth muscles behind the bolus propels it through the digestive tract 

Stomach 

• located in abdomen 

• storage tank for food

• can hold up to 2 liters of food

• produces mucus, hydrochloric acid, protein digesting enzymes

• contains thick mucus layer that lubricates and protects epithelial cells on stomach wall from acidic pH (3)

3 muscular layers of stomach

• outer longitudinal, middle circular, and inner oblique to produce churning action

Rugae

large folds that allow stomach to stretch

Chyme

paste-like substance that forms when food begins to be broken down

Pyloric opening

opening between stomach and small intestine

Pyloric sphincter

thick, ring of smooth muscle around pyloric opening

Hunger pangs

stomach is stimulated to contract by low blood glucose levels usually 12-24 hours after a meal 

Parasympathetic stimulation, gastrin, histamine

increase stomach secretions

regulation of stomach secretions

• cephalic phase

• gastric phase

• intestinal phase

Cephalic phase

• 1st phase

• stomach secretions are initiated by sight, smell, taste, or food thought

Gastric phase 

• 2nd phase 

• partially digested proteins and distention of stomach promote secretion 

Intestinal phase

• 3rd phase

• acidic chyme stimulates neuronal reflexes and secretion of hormones that inhibit gastric secretions by negative feedback loops

Cephalic phase of stomach control 

• the taste, smell, or though of food or tactile sensations of food in the mouth stimulate the medulla oblongata 

• vagus nerves carry parasympathetic action potentials to the stomach where enteric plexus neurons are activated

• postganglionic neurons stimulate secretion by parietal and chief cells and stimulate gastrin and histamine secretion by endocrine cells

• gastrin is carried through the blood back to the stomach where along with histamine it stimulates secretion

Gastric phase of stomach control 

• distention of the stomach stimulates mechanoreceptors (stretch receptors) and activates a parasympathetic reflex. action potentials generated by the mechanoreceptors are carried by the vagus nerves to the medulla oblongata

• the medulla oblongata increases action potentials in the vagus nerves that stimulate secretion by parietal and chief calls and stimulate gastrin and histamine secretion by endocrine cells

• distention of the stomach also activates local reflexes that increase stomach secretions

• Gastrin is carried through the blood back to the stomach where along with histamine it stimulates secretion

Interstitial phase of stomach

• chyme in the duodenum with pH less than 2 or containing fat digestion products (lipids) inhibits gastric secretions by three mechanisms

• chemoreceptors in the duodenum are stimulated by H+ (low pH) or lipids. action potentials generated by the chemoreceptors are carried by the vagus nerves to the medulla oblongata where they inhibits

• local reflexes activated by H+ or lipids inhibit gastric secretion

• secretin and cholecystokinin produced by the duodenum decrease gastric secretions in the stomach

Movements in stomach

• mixing waves

• peristaltic waves

Mixing waves 

• weak contraction 

• thoroughly mix food to form chyme 

Peristaltic waves

• stronger contraction

• force chyme toward and through pyloric sphincter

Hormonal and neural mechanisms

stimulate stomach secretions

Regular meal

stomach empties every 4 hours after this

High fatty meal

stomach empties every 6 - 8 hours after it

Movement in stomach

Small intestine

• measures 6 metes in length

• major absorptive organ

• chyme tales 3 to 5 hours to pass through

• contains enzymes to further breakdown food

• contains secretions for protection against the acidity

Parts of small intestine 

• duodenum 

• jejunum 

• ileum 

Duodenum

• first part of small intestine

• 25 cm long

• contains absorptive cells, goblet cells, granular cells, endocrine cells

• contains microvilli and many folds

• contains bile and pancreatic ducts

Jejunum

• second part of small intestine

• 2.5 meters long and absorbs nutrients

Ileum

• third part of small intestine

• 3.5 meters long

Mucosa of the small intestine 

a simple columnar epithelium with 4 major cell types 

• absorptive cells

• goblet cells

• granular cells

• endocrine cells

Absorptive cells

• have microvilli

• produce digestive enzymes

• absorb digested food

Goblet cells

produce a protective mucus

Granular cells

may help protect the intestinal epithelium from bacteria

Endocrine cells

produce regulatory hormones

Crypts of Lieberkuhn / intestinal glands

epithelial cells are located within tubular glands of mucosa called,,, at the base of the villi 

Glandular and endocrine cells

located in the bottom of the glands

Duodenal glands

mucous glands contained in the submucosa of the duodenum which open into the base of the intestinal glands

Free surfaces

epithelial cells in the walls of the small intestine have enzymes bound to their,,, 

Peptidases

enzymatically breakdown proteins into amino acids for absorption

Disaccharidases

enzymatically breakdown disaccharides into monosaccharides for absorption

Mixing and propulsion of chyme 

primary mechanical events that occur in the small intestine

Peristaltic contractions

proceed along the length of the intestine for variable distances and cause the chyme to move along the small intestine

Segmental contractions 

propagated for only short distances and mix intestinal contents 

Ileocecal sphincter (juncture of ileum) and large intestine

remains mildly contracted most of the time

Peristaltic contraction reaching the ileocecal sphincter from the small intestine

cause the sphincter to relax and allow chyme to move from the small intestine into the cecum

Ileocecal valve

prevents movement from the large intestine back into the ileum

Segmental contraction in small intestine

Liver

• weighs about 3 lbs

• located in the right upper quadrant of the abdomen under the diaphragm

• consists of right, left, caudate, and quadrate lobes

Porta

• gate where blood vessels, ducts, nerves enter and exit

• receives arterial blood from the hepatic artery

Lobules 

divisions of liver with portal triads at corners

Portal triad

contain hepatic artery, hepatic portal vein, hepatic duct

Hepatic cords

• between center margins of each lobule

• separated by hepatic sinusoids

Hepatic sinusoids

contain phagocytic cells that remove foreign particles from blood

Central vein

• center of each lobule

• where mixed blood flows towards

• forms haptic veins

Hepatic duct

transport bile out of liver

Common hepatic duct

formed from left and right hepatic duct

Cystic duct

• joins common hepatic duct

• from gallbladder 

Common bile duct

formed from common hepatic duct and cystic duct

Bile and pancreatic secretions

Functions of liver

• digestive and excretory functions

• stores and processes nutrients

• detoxifies harmful chemicals

• synthesizes new molecules

• secretes 700 milliliters of bile each day

Bile

dilutes and neutralizes stomach acid and breaks down fats

Control of bile secretion and release 

Pancreas

• located to stomach in inferior part of left upper quadrant

• head near midline of body

• tail extends to the left and touches spleen

• endocrine tissues have pancreatic islets that produce insulin and glucagon

• exocrine tissues produce digestive enzymes that travel through ducts

Endocrine tissues

have pancreatic islets that produce insulin and glucagon