Organs of Digestive System

Mouth through esophagus
Stomach
Liver, gallbladder and pancreas
Small intestine
Large intestine

Five Functions of Digestive System

1.Ingestion: selective intake of food
2.Digestion: mechanical and chemical breakdown of food into a form usable by the body
3.Absorption: uptake of nutrient molecules into the epithelial cells of the digestive tract and then into the blood and lymph
4.Compaction: absorbing water and consolidating the indigestible residue into feces
5.Defecation: elimination of feces

Stages of Digestion

Mechanical digestion:
-physical breakdown of food into smaller particles
-teeth and churning action of stomach and intestines
Chemical digestion :
-series of hydrolysis reactions that break macromolecules into their monomers
-enzymes from saliva, stomach, pancreas and intestines
-results
polysaccharides into monosaccharides
proteins into amino acids
fats into glycerol and fatty acids

Tissue Layers of GI Tract

Mucosa: -epithelium -lamina propria -muscularis mucosae
Submucosa
Muscularis externa: -inner circular layer -outer longitudinal layer
Adventitia or Serosa : -areolar tissue or mesothelium

Tissue Layers of GI Tract

GI Relationship to Peritoneum

-Only duodenum, pancreas and parts of large intestine are retroperitoneal
( peritoneum on their anterior side only)
-Dorsal mesentery suspends GI tract and forms serosa (visceral peritoneum) of stomach and intestines
-Ventral mesentery forms lesser and greater momentum
=lacy layer of connective tissue that contains lymph nodes, lymphatic vessels, blood vessels
-Mesocolon anchors colon to posterior body wall

Lesser and Greater Omentum

Lesser - attaches stomach to liver
Greater - covers small intestines like an apron

Enteric Nervous Control

-Able to function independently of CNS
-Composed of two nerve networks
1.Submucosal plexus =
-controls glandular secretion of mucosa
-contractions of muscularis mucosae
2. myenteric plexus =
-controls peristalsis
-contractions of muscular externa
-stimulates muscularis mucosae

Regulation of digestive activities pic

Regulation of the Digestive Tract

-Motility and secretion of the digestive tract are controlled by neural, hormonal, and paracrine mechanisms
-Neural control
1. Short (myenteric) reflexes: stretch or chemical stimulation acts through myenteric plexus
>Stimulates parastaltic contractions of swallowing
2.Long (vagovagal) reflexes: parasympathetic stimulation of digestive motility and secretion
-Hormones: Chemical messengers secreted into bloodstream, and stimulate distant parts of the digestive tract
>Gastrin and secretin
-Paracrine secretions: Chemical messengers that diffuse through the tissue fluids to stimulate nearby target cells = histamine and prostaglandins

Anatomy of Oral Cavity

Anatomy of tongue

The muscles of the tongue

-Intrinsic: produce tongue movements for speech
-Extrinsic: produce movement of food manipulation
1.genioglossus 2.hyoglossus 3.palatoglossus 4.styloglossus

Teeth anatomy

Alveolus—tooth socket in bone
Gomphosis joint formed between tooth and bone

32 adult teeth; 20 deciduous (baby) teeth
From midline to the rear of each jaw:
2 incisors—chisel-like cutting teeth used to bite off a piece of food
1 canine—pointed and act to puncture and shred food
2 premolars—broad surface for crushing and grinding
3 molars—even broader surface for crushing and grinding

Tooth Structure

Periodontal ligament is modified periosteum
anchors into alveolus
Cementum and dentin are living tissue
Enamel is noncellular secretion formed during development
Root canal leads into pulp cavity
nerves and blood vessels
Gingiva or gums

Label parts of tooth structure

Chewing and Mastication

Breaks food into smaller pieces to be swallowed
- inc. surface area exposed to digestive enzymes

Contact of food with sensory receptors triggers chewing reflex
-tongue, buccinator and orbicularis oris manipulate food
-masseter and temporalis elevate the teeth to crush food
-medial and lateral pterygoids swing teeth in side-to-side grinding action of molars

Composition and function of Saliva

Functions of saliva: moisten, begin starch and fat digestion, cleanse teeth, inhibit bacteria, bind food together into bolus

Hypotonic solution of 99.5% water and solutes
-salivary amylase, begins starch digestion
-lingual lipase, digests fat activated by stomach acid
-mucus, aids in swallowing
-lysozyme, enzyme kills bacteria
-immunoglobulin A, inhibits bacterial growth
pH of 6.8 to 7.0

Salivary Glands

Small intrinsic glands found under mucous membrane of mouth, lips, cheeks and tongue - secrete at constant rate
3 pairs extrinsic glands connected to oral cavity by ducts
parotid, submandibular and sublingual

Histology of Salivary Glands

Compound tubuloacinar glands
Composed of :
-Mucous cells secrete mucus
-Serous cells secrete thin fluid rich in amylase
-Mixed acinus has both

Salivation

~Cells filter water from blood and add other substances
~Food stimulates receptors that signal salivatory nuclei in medulla and pons
1.parasympathetic stimulation >salivary glands produce thin saliva, rich in enzymes
2.sympathetic stimulation > produce less abundant, thicker saliva, with more mucus
~Higher brain centers stimulate salivatory nuclei so sight, smell and thought of food cause salivation

Pharynx

~funnel that connects oral cavity to esophagus
~composed of skeletal muscle :
deep layer – longitudinal orientation
superficial layer – circular orientation
-superior, middle and inferior pharyngeal constrictors

Esophagus

~Straight muscular tube 25-30 cm long
nonkeratinized stratified squamous epithelium
esophageal glands in submucosa
skeletal muscle in upper part and smooth in bottom
~Extends from pharynx to cardiac stomach passing through esophageal hiatus in diaphragm
>inferior pharyngeal constrictor excludes air from it
~Lower esophageal sphincter closes orifice to reflux

Swallowing (Deglutition)

~Series of muscular contractions coordinated by center in medulla and pons
~Buccal phase:
tongue collects food and pushes it back into oropharynx
~Pharyngeal-esophageal phase
soft palate rises and blocks nasopharynx
pharyngeal constrictors push bolus down esophagus
>liquids in 2 seconds -- food bolus may take 8 seconds
>lower esophageal sphincter relaxes

Stomach Function

~Mechanically breaks up food, liquifies food and begins chemical digestion of protein and fat
>resulting soupy mixture is called chyme
~Does not absorb significant amount of nutrients
>absorbs aspirin and some lipid-soluble drugs

Gross Anatomy of Stomach

~Muscular sac (internal volume from 50ml to 4L)
~ J - shaped organ with lesser and greater curvatures
regional differences:
1.cardiac region just inside cardiac orifice
2.fundus - domed portion superior to esophageal opening
3.body - main portion of organ
4.pyloric region - narrow inferior end
>antrum and pyloric canal
~Pylorus - opening to duodenum
=thick ring of smooth muscle forms a sphincter

Unique features of stomach wall

Mucosa
-simple columnar glandular epithelium
-lamina propria is filled with tubular glands (gastric pits)
Muscularis externa has 3 layers
-outer longitudinal, middle circular and inner oblique layers

Cells of Gastric Glands in Stomach wall

1.Mucous cells: secrete mucus
2.Regenerative cells: divide rapidly to produce new cells that migrate to surface
3.Parietal cells =secrete HCl acid and intrinsic factor
4.Chief cells =secrete pepsinogen, chymosin and lipase in infancy
5.Enteroendocrine cells =secrete hormones and paracrine messengers

Gastric Cell Secretions

2 to 3 L of gastric juice/day =H2O, HCl and pepsin
Parietal cells contain carbonic anhydrase (CAH) to make HCL
CAH
CO2 + H2O -> H2CO3 -> HCO3- + H+
>co2 diffuses from blood into parietal cell
~H+ is pumped into stomach lumen by H+K+ATPase
~HCO3- goes into blood and exchanged for Cl- (chloride shift)
~inc HCO3- in blood causes alkaline tide (blood pH inc)

Functions of Hydrochloric Acid

~Activates pepsin and lingual lipase
~Breaks up connective tissues and plant cell walls
>liquefies food to form chyme
~Destroys ingested bacteria and pathogens

Gastric Enzymes and Intrinsic Factor

Intrinsic factor – Parietal cells
-essential for B12 absorption by small intestine
-RBC production (lack causes pernicious anemia)
Pepsin - protein digestion – Chief cells
-secreted as pepsinogen (inactive)
-HCl converts it to pepsin (active)
Gastric lipase – Chief cells
-lipase digests butterfat of milk in infant

Chemical Messengers in Stomach

Many produced by enteroendocrine cells
~hormones enter blood ->distant cells
~paracrine secretions -> neighboring cells
Gut-brain peptides
~Several are peptides produced in both the digestive tract and the central nervous system
>Substance P, vasoactive intestinal peptide (VIP), secretin, gastric inhibitory peptide (GIP), cholecystokinin (CCK), and neuropeptide Y (NPY)

Roles of gastrin, GIP, Secretin, CCK, VIP

~food in stomach stimulates Gastrin -> acid production and gastric motility
~Chyme in duodenum stimulates
-GIP >release of insulin from pancreas and inhibits gastrin
>>facilitates nutrient utilization by all tissues
- Secretin and CCK >release of pancreatic enzymes and buffers
& bile secretion and ejection of bile from gallbladder
-VIP > dilation of intestinal capillaries >> facilitates nutrient absorption

Mode of Gastric Motility

~Swallowing center signals stomach to relax
~food stretches stomach activating receptive relaxation response
>resists stretching briefly, but relaxes to hold more food
~Rhythm or peristalsis is controlled by pacemaker cells in longitudinal mm
>gentle ripple of contraction every 20 seconds -churns food and juice
> strong contraction of pyloric region -ejects 3 ml
> typical meal emptied in 4 hrs

Regulation of Secretion (phases 1 & 2)

Cephalic Phase
-sight, smell, taste, or thought of food; vagus nerve stimulates gastric
secretion and motility
Gastric Phase
-Food stretches the stomach and activates short myenteric and long
vagovagl reflexes.
-Secretion stimulated by: 1.Ach (from parasympathetic fibers), 2.Histamine
(from gastric enteroendrocrine cells) and 3.gastrin (from pyloric G cells)
>receptors on parietal and chief cells

Regulation of gastric secretion
Phase 3: Intestinal phase

=duodenum regulates gastric activity through hormones and nervous
reflexes
1. at stomach activity increases - and when the duodenum is stretched or
there are amino acids in the chyme gastrin is released (stimulator)
2. then enterogastric reflex inhibit gastric secretion and motility
>allows the duodenum to process the chyme already in it
3. the chyme stimulates duodenal cells to release secretin, GIP, and
cholecystokinin (CCK), >>suppress gastric secretion and motility
-sympathetic nerves further suppress gastric activity, while the vagal parasympathetic nerves have been inhibited.

What causes the enterogastric reflex?

-caused by acid and semi digested fats in duodenum

Feedback control of Gastric Secretion

Positive feedback
1. ingested food buffers stomach acid elevating PH -> stimulating G cells
-> release gastrin -> stimulating chief and parietal cells
-chief cells secrete pepsinogen and parietal cells secrete HCL
->forms pepsin -> digests dietary protein
-the digested protein: 1. directly stimulate G cells of the pyloric glad
2.buffer stomach acids which elevate PH ->stimulating G cells
- G cells secrete gastrin stimulating the chief cells and parietal cells

Liver Gallbladder and Pancreas

Gross Anatomy of Liver

-3 lb. organ located inferior to the diaphragm
-4 lobes - right, left, quadrate and caudate
-falciform ligament separates left and right
-round ligament, remnant of umbilical vein
-Gallbladder adheres to ventral surface between right and quadrate lobes

Microscopic anatomy of Liver

consists of several tiny cylinders called Hepatic Lobules:
-Central Vein surrounded by sheets of hepatocytes separated by Hepatic
sinusoids lined with fenestrated epithelium
>each plate of hepatocytes is an epithelium 1-2 cels thick
-Blood is filtered by hepatocytes on way to central vein

Function of Hepatocytes

-After a meal hepatocytes absorb from blood: glucose, amino acids, iron,
vitamins, and nutrients for metabolism or storage
- removes and degrades: toxins, hormones, bile, and drugs
- Secretes into blood: albumin. lipoproteins, clotting factors,
angiotenisinogen
-Between meals: hepatocytes break down stored glycogen and release
glucose into blood

Histolgy of Liver

3 Structures can be found in corner between lobules:
-hepatic portal vein and artery bring blood to liver
-bile duct collects bile from bile canaliculi, between the sheets of hepatocytes, to be secreted into hepatic ducts

Name ducts of Gallbladder, liver, and pancreas

-L and R Hepatic ducts from liver become common hepatic duct
->joins cystic duct from gallbladder and becomes bile duct
-> bile duct travels to the hepatopancreatic sphincter
- the pancreatic duct travels from tail to head and ends at hepatopancreatic
sphincter
- An accessory pancreatic duct bypasses the sphincter an goes into
duodenum at minor duodenal papilla
- the hapatopancreatic sphincter feeds into major duodenal papilla

components of bile

Yellow-green fluid containing minerals, bile acids, cholesterol and bile pigment
-bilirubin pigment from hemoglobin breakdown
> intestinal bacteria convert to urobilinogen= brown
- bile acid (salts) emulsify fats and aid in their digestion

Gross Anatomy of Pancreas

-Retroperitoneal gland (peritoneum on their anterior side only) post. stomach
-tail, body, head
-Endocrine: secretes insulin and glucagon into blood
-Exocrine: secrets 1500 ml of pancreatic juice into duodenum
> water, enzymes, zymogens, and sodium bicarb
-Pancreatic duct runs length of glad to open at sphincter of Oddi
>accessory duct opens independently on duodenum

Pancreatic Acinar Cells

-contain zymogens: trypsinogen, chymotrypsinogen, procarboxypeptidase
>hydrolyze proteins
-Secretes other enzymes: amylase, lipase, ribonuclease and deoxyribonuclease

Activation of Zymogens in pancreas

-trypsinogen,chymotypisnogen, and procarboxypepidase are released from pancreatic duct into duodenum
-trypsinogen is converted to trypsin by enterokinase in the intestinal epithelium
-trypsin then converts the other 2 into chymotrypsin and carboxypeptidase

Stimulating Hormones released from duodenum

1.CCK: released in response to arrival of acid and fat
-causes contraction of gallbladder, secretion of pancreatic enzymes, and relaxation of hepatopancreatic sphincter
2.Secretin: released in response to acidic chyme
-stimulates all ducts to secrete bicarbonate
3.Gastrin: weakly released from duodenum (mostly from stomach)
-stimulates gallbladder contraction and pancreatic enzyme secretion

Parts of Small Intestine

1.Duodenum: curves around head of pancreas
-retroperitoneal
-neutralizes stomach acids, emulsifies fat, pepsin inactivated by ph increase, stimulates secretion of pancreatic enzymes
2.Jejunum: in upper abdomen
-has large tall circular fold, thick walls, muscular
-most digestion and nutrient absorption**
3.Ileum: lower abdomen
-has peyers patches =clusters of lymph nodules
-ends at ileocecal valve with large intestine

what contributes to large surface area at SI

1.large circular folds, plicae circulares ; part of mucosa and submucosa
2.villi: fingerlike projections, contain BVs and lacteals =lymph vessels ; part of mucosa
3.Microvilli: cover surface of villi, brush border on cells
-have brush border enzymes for final stages of digestion

Microscopic anatomy of Villi

-a villus is covered with two kinds of cells:
1.enterocytes = absorptive cells, have a fuzzy brush border of microvilli, and contain brush border enzymes that carry out the final stages of digestion
>they enzymes are not secreted out, but must come into contact with the chyme = contact digestion ->important for the intestinal contractions to churn chyme to ensure contact with the mucosa
2.goblet cells =mucus secreting cells

Intestinal Crypts

Located in small intestine
-pores between villi that open to intestinal crypts
-upper half composed of enterocytes and goblet cells, and at the base rapidly dividing cells and Paneth cells
-secrete 1-2 L of intestinal juice a day
>mucus and water ph 7.8
Paneth cells =antibacterial secretions
Rapidly dividing cells = life span of 3-6 days until pushed to the top where digested

Brunners Glands

aka duodenal glands
-in submucosa
-unique to duodenum in the submucosa
-secrete bicarbonate
->neutralizes the acid chyme produced by the stomach so that the ph is suitable for the digestive enzymes of the small intestine

Intestinal Motility: function and mechanism

Contractions of the small intestine serve three functions:
1.to mix chyme with intestinal juice, pancreatic juice, and bile, to allow neutralization of the acid
2.churn chyme to bring in contact with mucosa- enterocytes
3. to move residue toward large intestine
Two mechanisms:
A. Segmentation = movement in which stationary ringlike constrictions appear several places along intestine and then relax and pacemaker cells is muscularis externa set the rhythm
-then when most nutrients have been absorbed by the enterocytes and only undigested residue remains peristalsis begins
B. Peristalsis = the wave begins in duodenum

Function of segmentation in SI

-purpose of segmentation is to mix and churn (not move) material along

Peristalsis

-Gradual movement of contents toward colon
-Begins after absorption occurs
-migrating motor complex controls waves of contraction
>second wave begins distal to first wave

Digestion in Oral Cavity

1. Salivary amylase - carbs/starch
2.Lingual Lipase -lipids

Digestion in Stomach

stimulus: anticipation of arrival of food
Hormone: gastrin -from g cells
Enzymes released: pepsinogen released by chief cells and activated to pepsin by HCL released from parietal cells ->1.breaks down proteins into peptides
2. acid, pepsin, and the churning denatures the amylase in the stomach (ph lower than 4.5) so only 50% of starch is digested before reaching SI
3. lipids are not digested in stomach

Digestion in Small Intestine

-Stimulus: arrival of chyme in duodenum
-Hormones: CCK =causes the release of digestive enzymes and bile from the pancreas and gallbladder and suppresses hunger
-Enzymes released:
1.Enteropeptidase -> activates Typsin -> activates chymotrypsin, and carboxypeptidase to digest Peptides into amino acids
>carboxypeptidase removes amino acids from carboxyl end
2.Pancreatic Lipase to digest lipids with help of bile salts -> converts to fatty acids and monoglycerides
3.Pancreatic alpha amylase to digest carbs into disaccharides

Absorption of starch derivatives in Intestinal Mucosa

-lactase, maltase, and sucrase at brush border facilitate diffusion of disacharides into cell body and convert to monosacharides
> the plasma membrane of enterocytes then has transport proteins that absorb the monosacharides as soon as the enzymes release them
>Glucose and galactose are absorbed by SGLT symport
>Glucose is also absorbed along with water via "solvent drag" between cells
>Fructose is absorbed via facilitated diffusion and converted to glucose in cell
->>the monosacharides pass the basal membrane by facilitated diffusion to be absorbed in blood capillaries of the villus

Absorption of protein derivatives in intestinal mucosa

-carboxypeptidase at bush border continues to remove amino acids
-aminopeptidase at brush border removes amino acids from NH2 end
-Dipeptidase splits dipeptides
>sodium cotransporters absorb the amino acids
>> then they leave the basal membrane via facilitated diffusion into blood caps of villus and off to hepatic portal circulation
**infants can absorb large proteins (maternal igA) by pinocytosis

Absorption of Lipids

-fat enters duodenum as large glubules -> it is then broken into small emulsification droplets by lecithin and bile acids which coat the drops
->emulsification droplets are acted upon by pancreatic lipase into 2 free fatty acids and a monoglyceride
->>Micelles in the bile pass to the small intestine and pick up the lipid derivatives
->>Enterocytes absorb lipids from the micelles, resynthesize them back into triglycerides and package a triglyceride, a phospholipid, and a cholesterol into a protein coated Chylomicron
->>>Golgi complex packages chylomicrons into secretory vesicles and release them from basal membrane by exocytosis -> to enter a lacteal of the villus and into the lymphatic system
** do not reach the blood stream until enter thoracic duct and then left subclavian vein

Absorption of Nucleic Acids, Vitamins, and Minerals

Nucleic Acids:
-nucleosidases and phosphatases of brush border split them into phosphate ions, ribose or deoxyribose sugar, and nitrogenous bases to be decomposed
Vitamins: absorbed unchanged
-Fat soluble: A,D,E,K are absorbed with other lipids
-Water Soluble: C and B diffuse
-B12 has to be bound to an intrinsic factor from the stomach
Minerals: absorbed all along the small intestine
-Na contransported with sugars and amino acids
-Cl exchanged for bicrarbonate in stomach
-Iron and Ca absorbed as needed

What happens when body needs Ca

-Parathyroid hormone is secreted in response to a drop in blood calcium levels
->stimulates kidney to synthesize vitamin D from precursors made by epidermis and liver
->Vitamin D affects the absorbitive cells of the duodenum in 3 waysL
1. inc # of Ca Channels in apical membrane
2. Inc amount of calbindin in cytoplasm
3. Inc # of Ca-ATPase pumps at basal membrane
Thus, parathyroid inc absorption of ca and raises blood calcium

Water Balance in Digestive System

-Digestive tract receives 9L of water/day
>a little from food and drink and then majority from secretions
>8L is absorbed by the SI and 0.8L by Large intestine
-Water is absorbed via osmosis following the absorption of salts and organic nutrients that create an osmotic gradient
*Diarrhea occurs when to little water is absorbed in Large Intestine
>feces pass through too quickly if irritated due to infection
>Occurs when feces contain high concentrations of solute (lactose)

Anatomy of Large Intestine

Microscopic anatomy of Large Intestine

1.Mucosa - simple columnar epithelium
-anal canal has stratified squamous epithelium
*No circular folds or villi
3.Intestinal crypts (glands sunken into lamina propria) produce mucus only
4.Muscularis externa
-muscle tone in longitudinal muscle fibers, concentrated in the taeniae coli
> when contacted form bulges called Haustra on both sides of taeniae coli
5. The transverse and sigmond colon have a serosa, the rest are retroperitoneal

Bacterial Flora

-Populate the Large intestine
-800 species
-digest cellulose and other undigested carbs for body to absorb
-help synthesis of vitamins B and K

Intestinal Gas

=Flatus
-avg person produces 500 ml of flatus
-most is swallowed air, but hydrogen sulfide, indole, and skatole produce the odor
-hydrogen gas can cause explosion during surgery which is why you are not allowed to eat before

Absorbtion and Motility in LI

-Transit time thru is 12-24 hours > reabsorbs water and electrolytes
-Feces consists of water and solids
-Haustras contract every 30 minutes -stimulated by distension of haustrum
-Mass movements are triggered by gastrocolic and duodenum reflexes
>filling of the stomach and duodenum stimulates motility
> moves residue for several cm with each contraction

-Anal

-Anal canal is 3 cm in length
-anal columns are longitudinal ridges separated by mucus secreting anal sinuses
-inter anal sphincter involuntary mm and external anal sphincter voluntary mm -> when you pass gas you don’t need to use the valves
-hemorrhoids are permanently distended veins

Neural Control of Large Intestine

1. Filling the rectum -stimulates stretch receptors which send signals to spinal cord
2.Spinal Reflex stimulates contraction of rectum and relaxation of internal anal sphincter
3. Impulse from brain prevent untimely defecation by keeping the external anal sphincter contracted --defecation only occurs when this relaxes