Cellular Resp

Divided into two parts:
Alimentary tract
o     Or Gastrointestinal tract
o     Hollow tube from mouth to anus
o     Food material inside the tract is considered to be outside the body b/c the canal is open to the external environment at both ends
o     It digests food and absorbs the digested fragments
o     Organs include
Small intestine
Large intestine
Accessory digestive organs
o     Organs include
Gall bladder
Salivary glands

o     The teeth and tongue are in the oral cavity while digestive glands and gallbladder lie outside the GI tract and connect to it by ducts
o     Most of the accessory digestive organs are in the abdomino-pelvic cavity
Covered by a serous membrane called the visceral peritoneum
Parietal peritoneum covering the abdominal walls of the abdomino-pelvic cavity
B/w these two layers is a thin layer of fluid produced by the serous membranes
Lubricates b/c these organs are moving
Organs are held in place, and anchored to the abdominal wall (b/c they are moving) by mesentery
A double layer of peritoneum
o     Sheet of two serous membranes fused back to back that extends to the digestive organs from the body wall
Also a fat storage area
Provide routes for blood vessels, lymphatics and nerves to reach the digestive viscera

GI tract

Hollow tube w/ 4 distinct layers
o      Central is the hollow portion called the lumen
o     Mucosa
Layer closest to the lumen
Wet epithelial membrane (simple columnar epithelium)
Often w/ a lot of goblet cells to secret mucous
Mucous lubricates food to help it pass through the canal
Mucous lining protects the wall of the GI tract from digestive enzymes (in the stomach and small intestine) and HCl from the stomachPROTECTIVE
Epithelial layer rests on a thin layer of loose connective tissue called lamina propria
Has a lot of blood vessels to get broken down nutrients into the blood so they can be distributed to tissues
Blood vessels also nourish the epithelium
Beyond the lamina propria is a thin layer of smooth muscle called the muscularis mucosae
Contraction of this layer moves the epithelium around so food trapped in the mucosa is dislodged and proceeds through the tract
In the small intestine this layer throws the mucosa into a series of small folds that increases the surface area greatly

o     Submucosa
Layer of dense connective tissue just external to the mucosa
Contains lots of blood vessels, lymphatic vessels, lymphoid follicles and nerve fibers
Has a rich supply of elastic fibers that enables the stomach to regain its normal shape after temporarily storing a large meal
The vascular network supplies the surrounding tissues of the GI tract wall
o     Muscularis externa
Muscular layer
Responsible for segmentation and peristalsis
In most of the GI tract it is 2 layers of smooth muscle
Inner circular layer
Outer longitudinal layer
In several places along the tract, the circular layer thickens, forming sphincters that act as valves to prevent the backflow and control food passage from one organ to the next
o     Visceral peritoneum or Serosa
Protective outermost layer
Formed by areolar connective tissue covered w/ mesothelium (a single layer of squamous epithelial cells)


Oral cavity (mouth)
o     Extends form the lips anteriorly to the oropharynx
o     Lined w/ stratified squamous epithelium
On gums, hard palate and dorsum of the tongue the epithelium is keratinized to give extra protection against abrasion when eating
o     Lips and cheeks
Lips extend from the bottom of the nose to indentation on the chin
What we call lips (where lipstick is applied) is called the red margin
This area is red b/c it is poorly keratinized
No sweat glands, no sebaceous glands

o     Causes dryness and cracking due to the elements
Lips and cheeks help to keep food b/w the teeth when we chew and play a small role in speech
Lips formed by the obicularis oris muscle
Cheeks are formed largely by the buccinators
o     The palate
Hard anteriorly
Is the rigid surface that the tongue forces food against when chewing
Soft posteriorly
Mobile fold formed mostly of skeletal muscle
Uvula projects downward from it

o     Uvula and soft palate rise to close off the nasopharynx when we swallow
o     The tongue
Occupies the floor of the mouth and fills most of the oral cavity when the mouth is closed
Skeletal muscle covered w/ epithelium
During chewing, it grips food and constantly repositions it b/w the teeth
Also mixes the food w/ saliva forming a compact mass called the bolus and pushes it back into the oropharynx
Before the oropharynx, usage of the tongue is voluntary
After the oropharynx, movement is involuntary and controlled by the medulla and pons
Has both intrinsic and extrinsic skeletal muscle fibers

o     Confined in the tongue and not attached to bone
o     Muscle fibers running in several planes allow the tongue to change shape (but not position) as necessary for speech and swallowing
o     Extend the tongue from their points of origin on the bones of the skull or the soft palate
o     Protrude, retract and move the tongue from side to side
Lingual frenulum is a fold of mucosa that secures the tongue to the floor of the mouth so that it cannot be swallowed
Papillae are rough peg-like projections of the tongue

o     Conical in shape
o     Give the tongue surface its rough texture that aids in licking and provides friction for manipulating food
o     Mushroom-shaped
o     Scattered widely on the tongue surface
o     Each has a vascular core, giving them a reddish tint
o     Some are on the esophagus
o     10-12 arranged in a V, located at the back of the tongue
o     Resemble fungiform but have a surrounding groove
Tastebuds are located on the fungiform and circumvallate papillae
o     Pick up tastes that are sweet, salty, sour and bitter

o     Japanese suggest another tastebud, the umami, which means delicious
Makes you crave protein in red meat
Beyond the cirumvallate is the sulcus terminalis, a groove that distinguishes the anterior two thirds of the tongue in the oral cavity from the posterior 1/3 in the oropharynx
o     Salivary glands
Extrinsic salivary glands produce most of the saliva
Parotid glands
o     2 masses, 1 in front of each ear
o     Secretion is mostly a watery fluid from serous cells

o     Become inflamed w/ mumps
Submanibular glands
o     Secretes a watery fluid (serous cells) plus mucous (mucous cells) called seromucous
o     Size of a walnut located along the medial aspect of the mandibular body
Sublingual glands
o     Anterior to the submandibular gland under the tongue

o     These are mucous glands
Intrinsic salivary glands also called buccal glands are scattered through the oral cavity mucosa
97-99.5% water
Lysozyme, an antibacterial enzyme
Salivary amylase
o     Begins CHO digestion
o     Why when you continue to chew bread it begins to taste sweetb/c you are breaking down the sugars (CHO)
o     Cleanses mouth
o     Dissolves food chemicals so they can be tasted
o     Moistens the food and aids in compacting it into a bolus

o     Contains enzymes that begin the chemical breakdown of starchy foods
Control of salivation
Under control of the parasympathetic nervous system (facial VII, and glossopharyngeal IX nerves)
o     Teeth
32 permanent teeth
Talk of them in quadrants
In each jaw (upper and lower) you have
2 incisorsused for biting
2 caninefanglike teeth used to tear, piece food (rip apart)
2 pre-molars: grinding or crushing
3 molars: grinding and crushing
o     Last of these is the wisdom teeth

     Tooth structure
Top part, what you see, is the crown
Part you cant see is the root
Gum is called gingival
o     Inflammation of the gum is called gingivitis
Crown of the tooth is covered w/ enamel
o     Hardest substance in the body, containing mostly Ca
o     Acellular material
o     Cells that produce it die, so cannot be replaced
Root is covered w/ a calcified connective tissue called cementum
o     Cementum connects the tooth to the periodontal ligament which anchors the tooth in an immovable joint (gompyhsis joint)
Dentin is a bonelike material, underlying the enamel cap forming the bulk of the tooth
o     Produced by odontoblasts
Dentin surrounds the pulp cavity
o     Contains a number of soft tissue structures (connective tissue, blood vessels and nerve fibers) that are collectively called pulp

The pharynx

o      From the mouth, food passes posteriorly into the oropharynx and then the laryngopharynx
Common passageways for food, fluids and air
Stratified squamous epithelium supplied w/ mucous producing glands
Contractions of the pharyngeal constrictor muscles propel food into the esophagus below
The esophagus
o     Muscular tube that is collapsed when not involved in food propulsion
o     After food goes through the laryngopharynx it is routed to the esophagus posteriorly as the epiglottis closes off the larynx to food entry
o     Lined w/ stratified squamous epithelium
o     Slightly thickened portion of the esophageal wall at the junction of the stomach is called the gastroesophageal (cardiac) sphincter
Supposed to close off the esophagus from the stomach, but does not do that great of a job
Diaphragm, which surround the sphincter, does a better job of closing off the esophagus
Hiatal hernia, when the stomach protrudes slightly above the diaphragm so diaphragm cannot reinforce the cardiac sphincter
o     Allows gastric juices to flow into the esophagus particularly when lying down


o     Left side of the abdominal cavity and below the esophagus
o     When empty is collapses
o     Can stretch to hold up to a gallon of food
o     Stomach is not a vital organ to life
Only thing of importance is the intrinsic factor which can be artificially injected
o     Has four regions


     Small region surrounding the cardiac orifice where food enters the stomach from the esophagus
Dome shaped portion tucked beneath the diaphragm
Curves downward
Midportion of the stomach, continuous w/
Funnel shaped region continuous w/ the duodenum
Pyloric sphincter controls passage from stomach to the duodenum of the small intestine; controls stomach emptying
o     Main purpose of the stomach is to store the food and pass it to the small intestine for digestion
o     Anchored by 2 membranes (mesenteries)
Lesser omentum
Runs b/w the liver and the lesser curvature and becomes continuous w/ the visceral peritoneum
Greater omentum
From the greater curvature down to cover the small intestine and wrapping the transverse portion of the transcending colon
Riddled w/ fat deposits
Passageway for nerves, blood vessels
Large collections of lymph nodes
o     Microscopic Anatomy
Lined w/ simple columnar epithelium that secretes an alkaline mucous
Inside of stomach is acidic (lots of HCl secreted) so alkaline mucous protects the lining of the stomach, especially the epithelial cells
W/in the epithelial lining are holes called gastric pits
Openings leading to gastric glands which produce gastric juices that enter the stomach through the gastric pits

     Gastric juice

o     Contents vary depending on the region of the stomach
o     In the pyloric and cardiac regions is composed mostly of mucous
o     Gastric glands of the fundus and body have 4 types of cells (have them everywhere, but highly concentrated in the fundus and body)
Parietal cells
Release HCl into the stomach
PH in stomach is 1.5-3.5 = VERY ACIDIC
HCl kills bacteria and helps to denature proteins
Produce intrinsic factor which is needed for B-12 to be absorbed
o     B-12 needed for healthy RBCs
Chief cells

     Produces pepsinogen, the inactive form of pepsin (protein digesting enzyme)
o     Activated by contact w/ HCl
Pepsin only works well in acidic environment, the stomach
Only protein is digested in the stomach
Enteroendocrine cells
Produce hormones concerned only w/ digestion
These hormones get into the blood and only affect the digestive organs
Gastrin-target cell is the parietal cells
o     If the pH of the stomach rises, become alkaline, gastrin tells the parietal cells to secrete more HCl
Mucous neck cells
Produce an acidic mucous
Function is unknown
o     The products of all these cells, minus hormones, composes gastric juice

Mucosal barrier

     Alkaline mucous secreted by epithelial cells that prevents destruction of the stomach by HCl and pepsin
Created by 4 factors
o     Thick coating of bicarbonate rich mucous
o     Epithelial cells joined together by tight junctions preventing gastric juice from leaking into the underlying tissue layers
o     Deep in the gastric glands the external faces of the plasma membrane are impermeable to HCl
o     Damaged epithelial mucousal cells are shed and replaced quickly by the division of undifferentiated stem cells.
Stomach surface epithelium is replaced every 3-6 days
o     Only digests protein but some things do get into the blood



o     When you eat, see or think of food, signal are sent to the brain stimulating the vagus nerve and stimulating glands of the stomach to secrete more gastric juices
When food enters the stomach, the expansion of the stomach causes more gastric juices to be released
When you start swallowing food and it travels down the esophagus, the parasympathetic nervous system tells muscles to relax and the stomach to expand
The ability of the visceral smooth muscle to be stretched w/o increasing its tension or contracting expulsively is called plasticity

     Allows the stomach to serve as a temporary reservoir for food

o     Food enters the stomach and sends waves of contractions called peristaltic waves
Start at the cardiac region and continue to the pyloric region producing rippling movements
Become stronger in the pyloric region where the musculature is thicker
Happens 3 times a minute
Helps to mix the stomach contents w/ gastric juices
Helps churn material to break it into smaller and smaller pieces
In stomach, the muscularis externa has a third layer, an oblique smooth muscle layer
Allows churning of stomach contents
Particles need to be broken down small enough to get through the pyloric sphincter
Pyloric sphincter can hold 30 ml of chyme (fluid and small particles produced by churning in the stomach)
W/ each peristaltic wave, 3 ml or less of chyme spits into the small intestine
After a usual meal, the stomach empties in approximately 4 hours
If the meal is high in fat (digested in the small intestine) it will take longer b/c the small intestine has so much fat to digest, which takes a long time
When empty, the stomach collapses into many folds called rugae
Small intestine
o     Diameter is one inch
o     In the living person, it is 8-13 feet long, coiled and folded
o     It is held in place by the mesentery
o     Anatomy
Is the tube extending from the pyloric sphincter to the ileocecal valve
Three subdivisions
Duodenum curves around the head of the pancreas and is just beyond the stomach
o     Is about 10 inches long
o     Secretions from the liver (bile) and pancreas (digestive enzymes) enter here through ducts
Secretions to the duodenum controlled by the hepatopancreatic sphincter
o     About 8 feet long
o     Extends from the duodenum to the ileum
o     12 feet in length
o     Joins the large intestine at the ileocecal valve
o     The majority of digestion takes place here
Nucleic acids
o     Absorption is in the small intestine too
Specialized for absorption b/c
Long length increase the surface area over which nutrients are absorbed
Folds plicae circulares in the mucosa and submucosa force chyme to spiral through the lumen
o     This slows the movement allowing time for absorption
Villi are everywhere on the surface of the small intestine
o     Finger like projections that are more than a mm high
o     Epithelial cells of the villi are columnar cells, highly absorptive cells
o     Increases the surface area increasing absorption
o     Lacteal at the core of each villus
Lymph capillary that picks up digested lipids
o     Core also has a dense capillary bed to pick up absorbed material
o     Lots of goblet cells to secrete mucous
o     Tiny projections on the plasma membrane of absorptive cells (columnar epithelium) only seen w/ electron microscope
o     Function to increase surface area
o     Give the mucosa a fuzzy appearance called the brush border
Plasma membranes of microvilli contain enzymes referred to as brush border enzymes
Complete the digestion of CHO and proteins
o     Lined w/ simple columnar epithelium
Lots of goblet cells producing mucous to help food slide through
o     Lining has pits that lead into intestinal crypts or crypts of Lieberduhn
Decrease in number along the length of the intestine
Epithelial cells that line the crypts secrete intestinal juice
Watery mixture containing an alkaline mucous
Crypts secrete lysozyme an antibacterial enzyme
Epithelial cells arise from rapidly dividing stem cells located at the base of the crypts
Every 3-6 days the epithelial lining is replaced
Treatments for cancer (chemotherapy and radiation) target rapidly dividing cells
o     Kills cancer cells but also nearly obliterates the GI tract epithelium causing nausea, vomiting and diarrhea after treatment
o     A lot of lymph nodules in the small intestine b/c there is lots of bacteria
They increase in size from the duodenum to the ileum
In the ileum they are called Peyer’s patches
White patches
o     In the duodenum there are glands called duodenal (Brunner’s) glands
Make an alkaline mucous that protects the small intestine from the highly acidic chyme from the stomach
Also raises the interior of the small intestine to the alkaline range needed for optimal enzymatic action (pH above 7 is necessary)
When the secretion of alkaline mucous is insufficient, get duodenal ulcers

The Liver
o     Organ that is essential for lifeno machine can replace it
o     Large organ
o     Three pounds in the adult
o     Located just below the diaphragm, mostly to the right
o     Has four lobes
o     Function
Removes nutrients from the blood and stores them
Produces bile needed for fat breakdown
o     Microscopic anatomy
The liver is composed of sesame seed-sized structural and functional units called liver lobules
Each lobule is roughly hexagonal (six sided) consisting of liver cells called hepatocytes that are arranged in plates
These hepatocyte plates radiate out form a Central vein like the spokes on a bicycle wheel
In a rough model, open a paperback book so that the covers touch
o     The pages represent the plates of hepatocytes
o     The hollow cylinder made by the rolled spine is the central vein
Each liver lobule is surrounded by connective tissue that has blood vessels to supply the hepatocytes
At each of the six corners of the lobule is a portal triad
o     Each has a
Branch of the hepatic artery
Supplying oxygen rich arterial blood to the liver
Branch of the hepatic portal vein
Carrying venous blood laden w/ nutrients from the digestive system
Bile duct
In between the plates are liver sinusoids, leaky permeable capillaries
o     Blood from the hepatic portal vein and hepatic artery percolate through sinusoids to the central vein
The structure of the liver lobule is essential to the filtering process
Nutrient rich blood from the small intestine comes to the liver trough the hepatic portal vein and oxygen rich blood comes through the hepatic artery
This blood flows b/w the plates of hepatocytes through sinusoids
o     Macrophages, Kupffer cells, that line the sinusoid walls pull out bacteria, cell debris, and worn out RBCs
o     At the same time, hepatocytes pull out glucose, string it together as glycogen and store it until the blood sugar drops
Also pull out amino acids needed for making albumin and clotting proteins
Fat soluble vitamins (ADEK) are pulled out and stored
Some toxic substances and drugs are pulled out too
Eventually blood flows through the sinusoids to the central vein
o     Empties to the hepatic vein
o     Empties to the inferior vena cava
o     Goes back to the heart
Structure of liver lobule is also essential to digestion of lipid
Hepatocytes make bile
Bile flows through tiny canals called bile canaliculi that run b/w adjacent hepatocytes toward the bile duct in the portal triad
o     Run opposite of blood flow
All the bile ducts in the portal triads merge together to two major bile ducts (right and left)
Right and left bile duct merge to the common hepatic duct
Bile leaves the liver through the common hepatic duct and is brought straight down to the small intestine
Empties into the small intestine at the hepato-pancreatic sphincter
o     Unless you eat fat and the appropriate hormones are present, the sphincter is closed
o     Bile backs up if it is closed into the cystic duct of the gallbladder
Gallbladder stores bile and concentrates it by pulling out water
Stored for when you do eat fat and the appropriate hormones are present.
Composition of Bile
Necessary for fat digestion
Chyme from the stomach, liquids and food you take in are high in water
o     Lipids are hydrophobic to they get together in larges masses (globules) such that only the lipids on the outside have contact w/ water
o     PROBLEM is that enzymes have a tough time getting to the lipids in the middle that are buried b/c not a lot of surface area to work on
Bile has bile salts made from cholesterol
o     Allow emulsification
o     Pull apart the glob of lipid and encircle the pieces of the lipid to prevent them from adhering together again
o     Now it is easier for enzymes to get at lipids for digestion
Bile salts enter the duodenum of the small intestine by the hepatopancreatic sphincter and are reclaimed at the end of the small intestine
o     Re-used over and over
Also has cholesterol which goes to the small intestine and is secreted in feces
o     Too much forms crystals in the biliary calculi of the gallbladder causing gall stones
Pigment of bile is bilirubin
o     Breakdown product of hemoglobin
Hgb breaks to
Amino acidsreused
Feused in new Hgb
Heme portion breaks to bilirubin which goes to liver is added to bile and breaks to urobilinogen (by bacteria) giving feces its red-brown color
If you have a problem getting bile to the small intestine, feces are gray b/c there is no bilirubin in them and no fat breakdown (steatorrhea)
Lipids are digested from the beginning to the end of the small intestine

o     Secretes pancreatic juice to the small intestine
o     Released through the pancreatic duct that fuses w/ the bile duct as it enters the duodenum
o     Has enzymes that can digest
Nucleic acids
o     Enzymes are inactivated till in the small intestine so that they cannot destroy the pancreas
In small intestine, enzymes clip off a piece of the pancreatic enzyme to active it
Others may need a vitamin to be activated
o     Bicarbonate ions are also in pancreatic juice
To neutralize HCl in duodenum from stomach
Bring pH to the alkaline range

Digestive Process in the Small intestine

Chyme moves through the small intestine in 3-6 hours by segmentation
o     Back and forth movement
o     Contraction that pushes contents forward, then another contraction pushes the contents backward, then forward again
o     Functions to further break down food by mixing it w/ pancreatic juices and bile
After most of the food is digested and nutrients are absorbed, start to have peristaltic waves in the small intestine
o     Begin at the duodenum and move to the ileum
o     Clears out what is left after absorption to the large intestine
Gastrin relaxes the sphincter b/w the small intestine and the large intestine (ileocecal sphincter)
o     Is a hormone released w/ peristaltic waves

Large Intestine
o     4.5 feet long
o     Diameter = 7 meters
o     Some water is absorbed here
o     Real function is to move the residue of digestion out as feces
o     Subdivisions
Just below the ileocecal sphincter
Vermiform appendix
Attached to the posteromedial surface of the cecum
Lymphoid tissue that prevents bacteria from getting to blood
Its twisted structure, however, is a ideal location for enteric bacteria to accumulate and multiply
Has several distinct regions
o     Up right side of body is ascending
o     Turns and goes across body transverse
o     Turns down on left side descending
o     Across again as the sigmoid
o     Down again as the rectum
o     Last segment is the anal canal
o     Passageway to outside is the anus
The anal canal has two sphincters
o     Interior anal sphincter
Smooth muscle that you have no control over
o     Exterior anal sphincter
Skeletal muscle that you have control over
o     Large intestine is lines w/ simple columnar epithelium w/ lots of goblet cells
o     Anal canal is stratified squamous epithelium b/c most of the water has been absorbed and material is dry and abrasive
The anal canal needs more protection from friction
o     No digestive enzymes in the large intestine
o     No specializations for absorption (no villi or microvilli)
Only water and vitamins are absorbed
o     Unusual things
Longitudinal muscle layer of muscularis is only three strips of smooth muscle outside the circular layer and is called the teniae coli
Not in the rectum (terminal end)
Present in the
o     Ascending
o     Descending
o     Transverse
o     Sigmoid
Contraction of teniae coli cause the intestine to pucker into pocketlike sacs called haustra
Epiploic appendages are small fat filled pouches of visceral peritoneum hanging from the surface of the large intestine
No one knows why they are there
o     Bacterial flora in the large intestine
Make B-complex vitamin
Make most of the vitamin-K the liver needs to synthesize clotting proteins
o     Digestive function
Harvests vitamins made by the bacterial flora
Reclaims most of the remaining water
Reclaims some electrolytes; particularly Na and Cl
o     Major function is to remove waste from the body
Does this by mass movements
Three or four times a day peristaltic waves push material to the rectum
As more is pushed the walls of the sigmoid and rectum stretch causing the defecation reflex
o     Parasympathetic reflex that causes the sigmoid colon and rectum to contract and the anal sphincters to relax.
o     As feces are forced into the anal canal, messages reach the brain allowing us to decide whether the external anal sphincter should remain open or be constricted to stop feces passage
o     If defecation is delayed, the reflex contractions end w/in a few seconds and rectal walls relax
o     W/ next mass movement the defecation reflex is initiated again.


CHO: breaks to monosaccharides (glucose, fructose)
o     In the mouth by salivary amylase secreted by the salivary glands
o     In the small intestine by
Pancreatic enzymes
Brush border enzymes
o     Absorbed in by active transport in the capillaries of the small intestine

Proteins: break to amino acids

o     In the stomach by pepsin from the chief cells
o     In the small intestine by
Pancreatic enzymes
Brush border enzymes
o     Absorbed by active transport by the capillaries of the small intestine

o     In the small intestine by pancreatic enzymes (lipases)
o     Break to
Not used until in the blood
Fatty acids
Go to the epithelial lining and packaged into chylomicrons
o     Lipid, protein, cholesterol
Goes to lymphatic system
o     Absorbed by the lacteals of the small intestine

o     Absorbed in the small intestine
o     Absorbed in the large intestine
B and K

o     Absorbed mostly in the small intestine
o     Small amounts absorbed in the large intestine