Physiology of Digestive System Overview

**Main Functions of Digestive System**

Process the food, extract nutrients and eliminate the residues

Stages of digestive system

* Ingestion: intake of food
* Digestion: mechanical and chemical breakdown of food
* Absorption: uptake of the nutrients to the blood and lymph through epithelial cells in GI wall
* Compaction: absorbing water and forming feces from residues
* Defecation: elimination of feces

Components of digestive system

* alimentary canal
* accessory digestive organs

alimentary canal consists of

* Mouth
* pharynx
* esophagus
* stomach
* small intestine
* large intestine

accessory digestive organs

* Teeth
* tongue
* salivary glands
* liver
* pancreas
* gallbladder

Layers of the wall of the gut

* mucosa
* submucosa
* muscularis externa
* serosa and visceral peritoneum

mucosa of the wall of the gut

* epithelium
* lamina propria: areolar connective tissue
* lymphatic tissue
* muscularis mucosa: smooth muscle

submucosa of the wall of the gut

* connective tissue including blood
* lymphatics
* secretory glands
* neurons: submucosal plexus

muscularis externa of the wall of the gut

* circular and longitudinal smooth muscle
* neurons: myenteric plexus

digestive processes

* motility
* secretion
* absorption
* elimination

movements in motility

* mechanical digestion
* propulsion

mechanical digestion includes

cutting and mixing of food with digestive enzymes

stages of mechanical digestion

* chewing in mouth
* churning in stomach
* segmentation in intestines

stages of propulsion

* swallowing in mouth and esophagus
* peristalsis in esophagus and anus

motility movements are regulated by

the autonomic nervous system and local nervous control

what is peristalsis

Wave-like muscular contractions of adjacent segments

how does peristalsis occur

Circular smooth muscle contract behind, relaxes in front of the bolus, followed by longitudinal contraction (shortening) of smooth muscle

rate of peristalsis

2-4 cm/sec

where does peristalsis occur

throughout GIT

Peristalsis vs. Segmentation

Peristalsis is Adjacent segments of the intestine alternately contract and relax, which moves food along the tract distally while segmentation is Nonadjacent segments of the intestine alternately contract and relax

types of secretions in digestive process

* saliva
* gastric secretion
* pancreatic secretion
* bile

Composition of saliva

* increased bicarbonate
* increased potassium
* alpha amylase
* lingual lipase

saliva is stimulated by:

autonomic nervous system

composition of gastric secretion

* HCL
* pepsinogen
* intrinsic factor

gastric secretion is stimulated by:

* gastrin
* parasympathetic nervous system
* histamine

composition of pancreatic secretion

* increased Bicarbonate
* Pancreatic lipase
* amylase
* proteases

pancreatic secretion is stimulated by

* Secretin
* CCK
* Parasympathetic nervous system

bile composition

* Bile salts
* Bilirubin
* Phospholipids
* Cholesterol

bile secretion is stimulated by

* CCK
* Parasympathetic nervous system

Most of the fluid and electrolytes are absorbed by

small intestine

in small intestine 80 % of the digested food elements will be absorbed to the _____________ except ____________

blood, the fatty products will go to lymphatics

absorption of H2O in small intestine occurs

passively / osmosis

Absorption of H2O in small intestine occurs as a result of ________ created by ___________

the osmotic gradient, active transport

Large intestine Absorbs about

90% of the remaining volume

absorption of H2O in large intestine intestine occurs

passively / osmosis

Absorption of H2O in small intestine occurs as a result of ________ created by ___________

osmotic gradient, active transport of Na+ and CL-

Defecation

eliminates the product of digested (and indigested) substances from the body via the anus in the form of feces

what type of parasympathetic reflex is defecation

spinal cord mediated parasympathetic reflex

functions of mouth

* Ingestion
* Tasting
* Mechanical breakdown of food
* Secretion of salivary glands

Esophagus

A straight muscular tube that which connects the mouth with the stomach

length of esophagus

about 10 inches (25 cm) long

function of cardiac sphincter

relaxes to allow food into the stomach

rugae

inner folds that increase the surface area of the

stomach

stomach function

* Churns and grinds together the bolus into smaller pieces
* Food is mixed with gastric juices (hydrochloric acid and enzymes) secreted by the stomach walls

duodenum

first part of small intestine

jejunum

long coiled mid section

mechanical digestion in small intestine is aided by

segmentation

chemical digestion in small intestine is aided by

pancreatic secretions and bile

villi

folded inner walls covered with fingerlike projections in small intestine

microvilli

tinier projections on each villus that absorb

digested food

Villi and microvilli function

increase the surface area of the small intestine for greater absorption

Peristalsis moves the undigested food to the

large intestine

Cecum

first part of the large intestine

Appendix is connected to the

cecum

parts of the large intestine

* cecum
* appendix
* ascending, transverse, descending and sigmoid colon
* rectum
* anus
* haustral segmentation

Mass peristalsis

strong contractions called mass movements occur one to three times a day, last about 15 minutes each

liver is located in

the upper right quadrant of the abdomen

liver serves as

the metabolic factory of the body

functions of the liver

* Produces most of the body’s circulating plasma proteins such as albumins, lipoproteins, glycoproteins, and prothrombin, etc.
* Stores And Converts Several Vitamins And Iron (e.g.Vit.A,D,K)
* Degrades Drugs And Toxins (e.g.alcohol)
* Involved In Carbohydrate Metabolism (e.g.glycogenolysis)
* Bile Production (the exocrine function of the liver) which is critical for the lipids digestion and absorption
* Modifies The Action Of Hormones (e.g.thyroxine,growth hormone)

liver has a dual blood supply consisting of

* a venous (portal) supply via hepatic portal vein
* arterial supply via hepatic artery

Gall bladder

Sac-like organ attached to the inferior surface of the liver that stores and concentrates bile

gallbladder secretes bile to the _______ through the _______ during mealtime

duodenum, bile duct

bile color

yellow green

bile contains

* minerals
* cholesterol
* neutral fats
* phospholipids
* bile pigments
* bile acids

bile salts aid in

digestion of fats

what type of gland is the pancreas

mixed endocrine and exocrine

pancreatic islets of langerhans secrete

insulin and glucagon

pancreatic acini secrete

pancreatic juice

pancreatic acini are activated at

intestinal brush border

Regulation of the digestive system is done via

* paracrine secretion
* hormonal secretion

paracrine secretion examples

Prostaglandins, histamine, and other chemicals

paracrine secretion is released into

interstitial fluid

paracrine secretion affects

adjacent cells within small segment of digestive tract

hormonal secretion is

peptides produced by enteroendocrine cells in digestive tract

hormonal secretion affects

target organs after distribution in bloodstream

hormonal secretion examples

* gastrin


* secretin
* vasoactive intestinal polypeptide (VIP)

Neural regulation of digestive system

* autonomic nervous system
* enteric division

Sympathetic nerves of ANS

decrease GI secretions and motility

parasympathetic nerves of ANS

increase GI secretion and motility

enteric division of neural regulation

* myenteric / auerbach plexus
* submucosal / meissner plexus

enteric division is linked to ______ by ________

central nervous system, autonomic nervous system