Digestive system

Alimentary canal/Gastrointestinal tract organs

continuous tube

nourish the body

Accessory structures

facilitate digestion

• teeth

• mouth

• salivary glands

• tongue

• liver

• gallbladder

• pancreas

Digestive processes

• ingestion

• Propulsion

• Mechanical digestion

• Chemical digestion

• Absorption

• Defacation

Propulsion

moving forward

• swallowing(oropharynx)

• Peristalsis( esophagus, stomach, small intestine, large intestine)

Mechanical digestion

• churning- stomach mixing (mouth, salivary glands)

• chewing- mouth

• segmentation (small intestine)

Absorption

• by the colon

• nutrients and water to the blood vessels and lymph vessels (small intestine)

• water to the blood vessels (large intestine)

Peristalsis

Alternating contraction sequence of both inner circular and outer longitudinal muscle layers

Segmentation/Mixing

Localized contraction of the circular muscle layer facilitates the blending of materials by back and forth movements of material

Alimentary canal (GI tract) histology- innermost → outermost

mucosa ( epithelium, lamina propia, muscularis mucosae)→ submucosa → muscularis(circular/longitudinal muscle) → serosa(areolar connective tissue)

Mucosa layer

• epithelium (simple/stratified)

• lamina propia (Areolar CT, MALT)

• Muscularis mucosae (narrow band of smooth muscle and elastic fibers)

Submucosa

• Areolar CT & Dense irregular CT

• Blood and lymphatic vessels

• Submucosal plexus (innervates the mucosa and submucosa)

Muscularis

• Circular muscle (inner layer)

• Longitudinal muscle (outer layer)

• Myenteric plexus (innervates muscle layers)

Serosa/Adventitia

• Areolar CT with collagen and elastic fibers

• Serosa (intraperitoneal); Adventitia (retroperitoneal)

Enteric Nervous system (ENS) - intrinsic set of nerves

• brain of the gut

• Submucosal plexus: control secretion

• Myenteric plexus: GI tract motility

Autonomic Nervous system (ANS)- extrinsic set of nerves

• Parasympathetic→ increases secretion and motility by stimulating ENS

• Sympathetic→ decreases secretions and motility by inhibiting ENS

Peritoneum

Largest serous membrane in body

• visceral peritoneum

• parietal peritoneum

• peritoneal cavity

Retroperitoneal organs

• SAD PUCKER

• Supra-adrenal glands

• Aorta and inferior vena cava

• Duodenum

• Pancreas

• Urinary bladder and ureters

• Colon( ascending and descending)

• Kidneys

• Esophagus (abdominal portion)

• Rectum

Greater omentum

An apron-like structure that lies superficial to the small intestine and transverse colon; a site of fat deposition in people who are overweight

Falciform ligament

Anchors the liver to the anterior abdominal wall and the inferior border of the diaphragm

Lesser omentum

Suspends the stomach from the inferior border of the liver; it provides a pathway for structures connecting to the liver

Mesentery

Vertical band of tissue anterior to the lumbar vertebrae and anchoring all of the small intestine except the initial portion (duodenum)

Mesocolon

Attaches two portions of the large intestine( transverse and sigmoid colon) to the posterior abdominal wall

Upper GI tract organs

oral cavity and salivary glands

pharynx

esophagus

stomach

duodenum

Mouth regions

• vestibule

• oral cavity proper

Mouth functions

• ingests food

• chews and mixes food

• begins chemical breakdown of carbohydrates

• moves food into the pharynx

• Begins breakdown of lipids via lingual lipase (activated in stomach)

-other: moistens and dissolves food, allowing you to taste it, cleans and lubricates the teeth/oral cavity, and has antimicrobial activity

Parasympathetic stimulation of salivary glands

increases salivation

Sympathetic stimulation of salivary glands

decreases salivation→ dry mouth

Humans produce

1.0-1.5L /day

Saliva is…

• slightly acidic (6.4-6.8)

• water (99.5%) electrolytes

• digestive enzymes: salivary amylase(extrinsic salivary glands), lingual lipase(instrinsic salivary glands)

• proteins: mucin(becomes mucous w/water), lysozymes (enzyme, kills bacteria), defensins, IgA

hardest substance in the body

enamel

Primary (deciduous) teeth

20

Secondary (permanent) teeth

32

incisor teeth

tearing

canine/cuspid teeth

shredding/ tearing

premolar/molar teeth

grinding

• premolars are missing in primary teeth

Molecule that breaks down starch

salivary amylase

• produced by salivary glands

tongues extrinsic muscles

• move tongue sideways (in and out)

• shape food into bolus

tongue intrinsic muscles

• manipulate food for swallowing

lingual glands

• activated in stomach

• breakdown triglycerides into fatty acids and diglycerides

Pharynx

• funnel-shaped muscular tube lined with mucous mebrane

• behind nose, mouth and larynx

• Extends from the base of the skull to the C6 vertebrae

• 3 parts: nasopharynx, oropharynx, laryngopharynx

Pharynx

• propels food from oral cavity to esophagus

• lubricates food and passage ways

Esophagus

• collapsible, muscular tube

• posterior to trachea

• connects pharynx to trachea: travels through mediastinum and pierces diaphragm

• Secretes mucus, transports food: NO enzymes, NO absorption

Upper esophageal sphincter (skeletal muscle) (VOLUNTARY)

allows the bolus to move from the laryngopharynx to the esophagus

lower esophageal sphincter relaxation (smooth muscle) (INVOLUNTARY)

allows the bolus to move from the esophagus into the stomach and prevents chyme from entering the esophagus

Mucus secretion in esophagus

lubricates it, allowing easy passage of bolus

Esophagus histology (inner → outer)

• mucosa

• submucosa

• muscularis (divided into 3rds)

• upper and lower esophageal sphincters

• Adventitia

Esophagus mucosa (innermost)

• nonkeratinized stratified squamous epithelium

• lamina propia

• epithelium

Muscularis esophagus layer

circular layer

Muscularis esophagus layer

longitudinal layer

Deglutition/Swallowing

The movement of food from the mouth to stomach

Deglutition/Swallowing (3phases)

• Voluntary phase: tongue pushes food to back of oral cavity

• Pharyngeal phase(involuntary): food is in pharynx & airway is closed off, so it doesn’t go the wrong way

• Esophageal phase (involuntary): food is going through esophagus into the stomach

Stomach major regions

• cardia

• fundus

• body

• pylorus

• addition of inner oblique smooth muscle layer: gives muscularis ability to churn and mix food

Pyloric sphincter

leads to duodenum

Cardiac sphincter

connects esophagus to stomach

Gastric juices

• HCI

• pepsin (pepsinogen)

• gastric lipase

• mucus

• electrolytes

• intrinsic factor

Intrinsic factor

• secreted by stomach

• needed for vitamin b12 absorption in small intestine (required for RBC production)

Stomach function

• mixes and churns food with gastric juices to form chyme

• begins chemical breakdown of proteins

• releases food into the duodenum as chyme

• absorbs some fat-soluble substances (alcohol, aspirin)

• possesses antimicrobial functions

Stomach epithelium replaced…

x3-5 days

• made of simple columnar cells

Surface mucous cells

secrete mucin in an alkaline fluid

• neutralizes acid

Mucous neck cells

secrete mucin in an acidic fluid

parietal cells

secrete HCI & intrinsic factor

• HCI: denatures proteins, kills bacteria, activates pepsinogen→ pepsin

chief cells

• make pepsinogen (inactive pepsin) and lipase

G cells/enteroendocrine cells

secrete gastrin

Lower GI tract

• digestion + absorption continued

• elimination of indigestible + unabsorbale materials

• small intestine :DJI

• large intestine: A,T, D colon

• acessory glands

Small intestine

• 20-25 feet long; 1” diameter (longer when dead as it relaxes)

• majority of digestion and absorption occurs

• duodenum: mixing bowl

• Jejunum: a bulk of chemical digestion and absorption

Small intestine functions

• mixes chyme with digestive juices

• propels food at a rate slow enough for digestion and absorption

• absorbs and breaks down products of carbohydrates, proteins, lipids, and nucleic acids, along with vitamins, minerals, and water

• performs physical digestion via segmentation

• additional: provides optimal medium for enzymatic digestion

Small intestine histology

• mucosa, submucosa, muscularis, serosa (adventitia in duodenum)

• circular folds

• villi

• microvilli

Absorptive cells

digestion and absorption of nutrients in chyme

Goblet cells

secretion of mucus

Paneth cells

secretion of bactericidal enzyme lysozyme; phagocytosis

G cells

secretion of gastrin

I cells

secretion of cholecystokinin

• stimulates release of pancreatic juices and bile

S cells

secretion of hormone secretin

Large intestine anatomy (colon)

• 5 foot long, 2.5 inch diameter

• Ileocecal sphincter

• 4 major regions

• -Cecum (appendix)

• -Colon: Ascending, transverse, descending, sigmoid

• -Rectum

• -Anal canal: internal/external anal sphincter

Ascending and Descending colon

Retroperitoneal (outside of the peritoneal cavity)

• vertical

Internal anal sphincter

• involuntary

• smooth muscle

external anal sphincter

• voluntary

• skeletal muscle

Colon histology

Mucosa

• simple columnar epithelium

• Lamina propria-lymphatic nodules

• Microvilli

• Intestinal glands

Submucosa

Muscularis

• circular layer (normal)

• longitudinal layer(taeniae coil→ haustra)

Serosa/Adventitia

Colon functions (large intestine)

• further breaks down food residues

• absorbs most residual water, electrolytes, and vitamins produced by enteric bacteria

• propels feces towards rectum

• eliminates feces

Chemical digestion in colon

The last stages of chemical digestion occur through bacterial action

no enzymes secreted

bacteria further break down substances and produce vitamins

• ferment remaining/complex carbohydrates (gases, CO2 and CH 4)

• Break down remaining proteins into simple substances (simple substances (indole)→ odor)

• convert bilirubin into stercobilin (brown color)

• produce vitamins (B and K)

30-50% of much feces is made up of microorganisms

Mechanical digestion in colon

Drive contents of transverse colon into rectum

• haustral churning

• Peristalsis

• Mass peristalsis

Haustral churning (colon)

distension reaches a certain point, and the walls of the haustra contract to squeeze contents outwards

Peristalsis (colon)

propulsive contractions (3-12 contractions/ min)

• gastroileal reflex: causes peristalsis in the ileum → ileocecal valve opens

Mass peristalsis (colon)

a strong peristaltic wave that moves contents quickly from transverse colon into the rectum

• gastrocolic reflex: causes mass peristalsis in the lower GI tract→ urge to defecate

Defecation Reflex

Series of impulses that trigger the release of feces

• cerebral cortex→ voluntary motor nerve→ external anal sphincter

• sensory nerve fibers→ involuntary motor nerve → internal anal sphincter

Accessory organs of digestion

• gallbladder

• liver

• pancreas

Liver

• largest internal organ

• RUQ of abdomen

• wrapped in tough fibrous tissue

• 4 superficial lobes

• right lobe( biggest)

• Caudate lobe(tail)

Histology of liver

Hepatic lobules

• structural and functional unit of the liver

Hepatocytes

• secrete bile

Hepatic sinusoids

• kpuffer cells (mixed microphages)

Portal triads

• at the corners of each hepatic lobule( have a portal venule/arteriole and bile duct)

Liver blood flow

• hepatic artery: gives oxygenated blood

• hepatic portal vein: gives nutrient rich deoxygenated blood

Liver functions

• Carbohydrate, lipid and protein metabolism

• Processing of drugs and hormones (detoxifies blood)

• Removes waste product (eg bilirubin)

• Produces bile

• Stores (fat soluble vitamins- A,D,E,K , iron and copper)

• Phagocytosis

• Vitamin D activation ((1st step of synthesis)

• Regeneration- Only organ that can, even if 75% gone

Pancreas

• posterior to the stomach

• Exocrine: digestive enzymes

• Endocrine: insulin and glycogen

• Digestive enzymes: carbs, proteins, fats, and nucleic acid

• Produces sodium bicarbonate: buffers stomach acid

• Empties contents into duodenum: hepatopancreatic ampulla

Pancreatic islet cells

secrete hormones

Acinar cells

secrete digestive enzymes

Gallbladder

• stores and concentrates bile and releases it into the two-way cystic duct when the small intestine needs it (stimulates by CCK hormone)

• Bile components- water, bicarbonate ions, cholesterol, bile salts, bile pigments (bilirubin), lecithin, mucin

• Emulsification (function of bile)

Liver

makes bile (important in the emulsification of fats)

Gallbladder

stores bile until needed

Digestions of carbohydrates

Broken down into their monomers

1. Starch glycogen broken down by salivary amylase( inactivated by stomach acid)

2. Pancreatic amylase

• (short branched polyssacharides) limit dextrins + alpha Dextrinase → glucose (monosaccharide)

• (diassacharides) maltose+ maltase→ 2 glucose , surcose+ surcrase → 1 glucose 1 fructose ,lactose+ lactase→ 1 glucose 1 galactose

bold=brush border enzymes

Digestion of proteins

begins in stomach→ small intestine

Protein → large poly peptides

• in stomach

• by pepsin- pepsinogen (inactive) -secreted by chief cells

Large poly peptides→ Short peptides and amino acids

• in pancreas

• by trypsin, chymotrypsin, carboxypeptidase, and elastase

Short peptides and amino acids→ amino acids

• in small intestine

• by aminopeptidase or dipeptidase

Absorbable food substances (by small intestine)

carbohydrates→ monosaccharides: glucose, galactose, and fructose

proteins→ single amino acids, dipeptides, and tripeptides

triglycerides→ monoacylglycerides, glycerol, and free fatty acids

nucleic acids→ pentose sugars, phosphates, and nitrogenous bases

Gastrin (G-cells -stomach and duodenum)

• stimulates gastric acid secretion

• increases stomach motility