Digestive System (gastrointestinal system)

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Whats in it?

Alimentary Canal: from mouth to anus through ventral body cavity (30ft long)

Accessory organs

teeth

peridontal ligament: ense fibrious connective tissue, anchors teeth to socket walls, shock absorber during chewing.

3 regions: crown, root, neck (of tooth, gingivae connected)

Dentin: calcified connected tissue forming tooth covered by enamel

Enamel: harder than bone material encloses pulp cavity

Pulp: connective tissue containing blood vessels, nerves and lymphatic vessels.

20 baby teeth, 32 adult teeth

tongue

skeletal muscle, voluntary somatic control.

extrinsic muscles: attach to bones and move tongue side to side

intrinsic muscles: within tongue, alter shape size for speech/swallowing

salivary glands

liver

largest gland/second largest organ

made out of repeating functional units called liver lobules

hepatocytes: major functional cells of liver. Chemical factories. Involved in digestive/non-digestive functions. synthesize, transform, and store protein, carbs, fats. Secrete bile.

Bile: excretory product that emulsify fats for watery environment of small intestine digestive juices.

consist of water, bile salt, cholesterol, and bile pigments.

non digestive functions: phagocytosis of old cells, making heparin and other plasma proteins, modifying vitamin D

gallbladder

pancreas

oblong gland posterior to stomach in retroperitoneal space.

connected to duodenum by hepatopancreatic ampulla and accessory duct.

secrete enzymes and sodium bicarbonate (buffers acidic pH of chyme)

99% of pancreatic acini (glandular clusters) participate in exocrine secretion

Pancreatic islets: form endocrine portion of gland. Secrete glucgon, insulin, and somatostatin, and pancreatic polypeptide.

Alkaline pancreatic juice is secreted into duodenum each day. Creates proper environment for:

pancreatic enzyme

enzymes that cleave polypeptide and single amino acids: trypsin, chymotrypsin, carvoxypeptidase, and elastase.

pancreatic lipase

GI tract

Wall of GI from lower esophagus to anal canal have same basic 4 layer of tissue (deep to superficial)

Mucosa: mucous membrane sitting on loose connective tissue called lamina propria.

lamina propria has MALT that protect against disease, underneath is muscularis mucosae, thin lining that throws stomach lining and small intestines to tiny folds (increase surface area).

nonkeratinized stratified squamous epithetium (protects)- in pharynx, essophagus, anus.

Simple columnar epithelium (secrete/absorb): stomach and intestine

among epithelial cells are various glandular cells that secret mucus and fluid into lumen (exocrine).

Submucosa: loose connective tissue binding mucosa to muscularis

Has blood/lymphatic vessels and neurons called submucosal plexus

muscularis: mouth, pharynx, superior and middle esophagus, anal sphincter contain skeletal muscle that allows for swallowing and control of defecation. Rest of tract is smooth muscle with myenteric nerve plexus between them.

serosa/adventitia: outermost layer.

adventitia- if attached to surrounding tissues, fibrous connective tissue arranged around organ it supports.

serosa- in peritonial cavity, slippery mesothelium surface layer. Covers intra abdominal organs as visceral peritoneum.

Peritonium

BODY'S LARGEST SEROUS MEMBRANE. WRAPS AROUND MOST ABDOMINOPELVIC ORGANS.

visceral peritonium forms serosa of alimentary canal and other intra-abdominal organs. Continues around abdominal wall as parietal peritoneum.

Has FIVE LARGE FOLDS:

Greater omentum: largest fold, contain many lymph nodes

falciform: attaches liver to abdominal wall and diaphragm

lesser omentum: suspends stomach and duodenum from inferior edge of liver. Pathway for blood vessels (hepatic portal vein/common hepatic artery) to ever liver and contains common bile duct.

Mesentery and mesocolon: attach bowel to posterior ab wall. Holding intestine loosly in place as muscle contractions mix and move luminal contents in GI tract.

Retroperitoneal space: organs behind peritoneum

kidneys and ureter

Most of pancreas

adrenal glands

aorta and inferior vena cava

Mouth (oral/buccal cavity)

mechanical digestion: mastication mixes saliva with food to make bolus

Saliva: starts chemical digestion.99.5% water,ions, IgA, lysozyme, salivary amylase.

Salivary amylase: breaks down starch.

3 large salivary glands: secrete most saliva.

Parotid, submandibular, and sublingual

3 small glands: labial (lips), cheeks (buccal), palate (palatal), tongue

parasympathic stimulation: increase amt of saliva. Brain stem, efferent nerve impulses transmitted by facial and glossopharyngeal nerve. Touch, smell, taste, psychological factors also factor.

sympathetic stimulation: decrease amt of saliva secreted

Esophagus

only function is propulsion(moving food down to stomach)

muscular tube, begins inferior to laryngopharynx and posterior to trachea.

mucosa is nonkeratinized squamous epithelium

Stomach

between esophagus and duodenum of small intestine

diaphragm pushes it inferiorly with inhalation, and superior with exhalaton

Rugae, large fold in stomach which allows gastric distension depending on how much food is in.

MOST DISPENSIBLE PART OF GI

gastric muscularis has 3rd inner oblique layer to facilitate mixing action of digestion.

Four main regions: cardia, fundus, body, pylorus

Gastric glands open into bottom of narrow channels called gastric pits.

exocrine and endocrine cells populate gastric glands and pits.Exocrine gland cells include:

mucous neck cells: produce mucus

parietal cells: which produce intrinsic factor and HCI

chief cells: secreate protease pepsinogen

ALL THREE FORM GASTRIC JUICE (2-3 l/d)

G cells: located mainly in pyloric antrum, secrete hormone gastrin in bloodstream

Converts pepsinogen into pepsin.

Pepsin: only proteolutic enzyme in stomach

Gastric lipase: splits triglycerides

Intrinsic factor: needed to absorb vitamin B12 in terminal ileum. B12 is needed for rbc production

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Processes

Metabolic processes

GI tract and accessory organs are responsible for these

Catabolism: big molecules broken to smaller ones (mouth, stomach, duodenum). (GI tract digestion.)

Mechanical digestion: all movements that facilitate catabolism (mastication, swallow, mixing, peristalsis: movement of muscles in GI tract that faciliate food movement)

Chemical digestion: water is used to break chemical bonds (hydrolysis)

Fats -> fatty acids and glycerol

Carbs -> polysaccharides -> monosaccharides

Protein -> polypeptides and amino acids

Sucrose -> glucose and fructose

Enzyme mediated hydrolysis: water and emzymes break down substrate into products. Most enzymes are identified by substrate they break down. ex. Dipeptidase breaks down dipeptide.

Anabolism: Smaller molecules used as building blocks for large molecules.(liver)

Digestive activities of gastrointestinal tract

Three phases:

Cephalic phase

smell, sight, thought, initial taste activate neural centers in cerebral cortex, hypothalamus, and brain stem to prepare for digestion.

brain activate facial and glossopharyngeal nerve to secrete salive, and vagus secrete gastric juices.

Gastric phase

When food reaches stomach neural and hormonal (gastrin *) mechanisms promote secretion of gastric juices and motility.

Intestinal phase

when food enter small intestine. Neural and hormonal response promote continued digestion of food.

Carb digestion in mouth

digestion of starches and glycogen

enzyme is amylase made by salivary glands and pancreas. Also brush border enzymes in small intestine.

In mouth: amylase from parotid and submandibular salivary glands start carb digestion

Amylaze converts them to

maltose (disaccharide)

maltotriose (trisaccharide)

alpha-dextrins (starch fragment)

Few starch/glycogen molecules are completely digested into maltose before they enter small intestine.

Stomach acidic ph (2) destroys amylase

Fat digestion in mouth

Happens primarily in small instestine. (can also in mouth/stomach)

Lipases break triglycerides and phospholipids

Lingual/gastric lipase hydrolyze small amt of triglycerides.

fatty acids and monoglycerides are end product

Mastication (chewing)

regulated by CNS, voluntary. Skeletal muscles elevate mandible, food is pushed toward oropharynx

Deglutition (swallowing food)

oropharynx, laryngopharynx swallowed food must transit them to esophagus. Involuntary/volunatry muscles.

3 stages:

Voluntary: tongue forces bolus to back of mouth into oropharynx

pharyngeal: involuntary, bolus passes in oropharynx, receptors send impulses to the deglution center of medulla and pons. Returning impulse cause soft palate to move superiorly and posteriorly to close nasopharynx. Epiglottis is inferior to close glottis

esophageal: peristalsis push bolus onward

Stomach perstalsis

stretch receptors activate reflexes that promote peristaltic movements.

mixing waves begin to mix food with stomach secretions.

Waves force chyme (digesting food) toward and through pyloric sphincter.

Most food does not exit stomach, so it moves back and forth in churning digestive motion

parasympathetic nervous system stimulates digestive movement in stomach

EMPTYING CONTENT TO DUODENUM (SHORTEST TO LONGEST): Carbs, protein, fat

Gastric Acids

stomach mucosa makes hydrochloric acid

Gastric acid breaks hydrogen bonds and alters globular shape of proteins to promote pepsin (enzyme), destruction of microbial proteins.

enzymes more easily break down the resulting polypeptide into smaller peptides.

Protein digestion in stomach

happens to stomach and small intestine

pepsin starts process

pancreatic and intestinal brush border enzymes complete digestive process.

Lipid digestion in stomach

Lingual and gastric lipase hydrolyze small amt of triglycerdies.

End products are fatty acide and monoglycerides

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Diseases

Gastroesophageal Reflux disease (GERD)

heart burn

when the lower esophageal spincter is incompetent in regulating movement of bolus into stomach.

Gastric erosion

disturbed balance between hydrochloric acid production, pepsin secretion, and mucosal defenses can lead to stomach erosion