“disassembly” line 

Nutrients become more available to the body in each step

ingestion

mechanical processing

chemical digestion

secretion

absorption

defecation or excreti

 is anchored in place

organs maintain position in the ventral cavity and outside of that cavity

during pregnancy they get crowded

main membrane (PERITONEAL) is two layered…

between these two layers

Lubricates digestive organs 

Allows them to slide across one another

special folds 

double layer of peritoneum that provides:

Vascular and nerve supplies to the viscera

Hold digestive organs in place and store fat

– organs outside the peritoneum

Ex. parts of colon, pancreas, duodenum

mucosa, 

submucosa, 

muscularis externa, 

serosa

From esophagus to the anal canal the walls of the GI tract have the same four layers

From the lumen outward they are the…

Epithelial

Lining Simple columnar epithelium and mucus-secreting goblet cells

Protect digestive organs from digesting themselves

Ease food along the tract

Enzyme-secreting cells 

Hormone-secreting cells (making them endocrine and digestive organs)

Stomach and small intestine mucosa  ALSO contain:

outside or serous layer – covering

usually part of visceral peritoneum

- changes in response to food

pH, types of foods, stretched organs

– many neurons are parasympathetic

but there are also sensory receptors

part of the autonomic nervous system

extensive network of neurons and networks

found in walls of digestive system

influenced by sympathetic and parasympathetic

but many of its coordinated reflexes are initiated locally – no instruction from CNS

control activity in one region of the tract as necessary

not yet well understood

respond to:

Stretch, osmolarity, and pH

Presence of substrate, and end products of digestion

They initiate reflexes that:

Activate or inhibit digestive glands 

start or stop – like car wash

Mix lumen contents and move them along

Oral or buccal cavity:

Is bounded by lips, cheeks, palate, and tongue 

Has the oral orifice as its anterior opening

Is continuous with the oropharynx posteriorly

The mouth is lined with stratified squamous epithelium 

The gums, hard palate, and dorsum of the tongue are slightly keratinized

– underlain by palatine bones and palatine processes of the maxillae

Assists the tongue in chewing

Slightly corrugated on either side of the raphe (midline ridge)

– mobile fold formed mostly of skeletal muscle

Closes off the nasopharynx during swallowing

Uvula projects downward from its free edge

Occupies the floor of the mouth and fills the oral cavity when mouth is closed

Functions include:

Repositioning food 

Mixing food with saliva 

Initiation of swallowing

Speech

secures the tongue to the floor of the mouth

Filiform 

Fungiform 

Circumvallate 

Produce and secrete saliva that: 

Cleanses the mouth

Moistens and dissolves food chemicals 

Aids in bolus formation

Contains enzymes that break down starch

Secreted from serous and mucous cells of salivary glands

97-99.5% water, very slightly acidic solution containing

Electrolytes – Na+, K+, Cl–, PO42–, HCO3–

Digestive enzyme – salivary amylase

Proteins – mucin, lysozyme, defensins, and IgA

Metabolic wastes – urea and uric acid

Secreted from serous and mucous cells of salivary glands

97-99.5% water, very slightly acidic solution containing

Electrolytes – Na+, K+, Cl–, PO42–, HCO3–

Digestive enzyme – salivary amylase

Proteins – mucin, lysozyme, defensins, and IgA

Metabolic wastes – urea and uric acid

– enlarge and develop causing the root of deciduous teeth to be resorbed and fall out between the ages of 6 and 12 years

All but the third molars have erupted by the end of adolescence

Usually 32 permanent teeth

incisors 

cuspids 

molars 

bicuspids 

exposed part of the tooth above the gingiva

Enamel – acellular, brittle material 

portion of the tooth embedded in the mandible or maxilla

cementum covers

Periodontal ligament

Anchors the tooth in the alveolus of the jaw 

Forms the fibrous joint 

as plaque accumulates, it calcifies and forms calculus, or tartar

Accumulation of calculus:

Disrupts the seal between the gingiva and the teeth 

Puts the gums at risk for infection 

– serious gum disease resulting from an immune response

Immune system attacks intruders as well as body tissues, carving pockets around the teeth and dissolving bone

Food is ingested

Mechanical digestion begins (chewing)

Salivary amylase begins chemical breakdown of starch

The pharynx and esophagus serve as conduits to pass food from the mouth to the stomach

From the mouth, the oro- and laryngopharynx allow passage of:

Food and fluids to the esophagus

Air to the trachea

Lined with stratified squamous epithelium and mucus glands

Has two skeletal muscle layers

Muscular tube going from the laryngopharynx to the stomach

Travels through the mediastinum and pierces the diaphragm 

Joins the stomach below the diaphragm

Coordinated activity of the tongue, soft palate, pharynx, esophagus, and 22 separate muscle groups

– bolus is forced into the oropharynx 

Chemical breakdown of proteins begins and food is converted to chyme

Various regions include = Pyloric region – at bottom

The pylorus is continuous with the duodenum through the pyloric sphincter

Allows the stomach to churn, mix, and pummel food physically 

Breaks down food into smaller fragments

is composed of:

Goblet cells that produce a coat of alkaline mucus

The mucous surface layer traps a bicarbonate-rich fluid beneath it

Gastric pits contain gastric glands that secrete gastric juice, mucus, and gastrin

Mucous neck cells – secrete mucus

Parietal cells – secrete HCl and intrinsic factor

Gastric glands have a variety of secretory cells

pepsinogen 

Pepsinogen is activated to pepsin by:

HCl in the stom

A thick coat of bicarbonate-rich mucus on the stomach wall

Epithelial cells that are joined by tight junctions

Gastric glands that have cells impermeable to HCl

Damaged epithelial cells are quickly replac

To keep from digesting itself, the stomach has a mucosal barrier with:

Holds ingested food

Degrades this food both physically and chemically

Delivers chyme to the small intestine

Enzymatically digests proteins with pepsin

Secretes intrinsic factor required for absorption of vitamin B12  - later

FYI – aspirin and alcohol are absorbed here

approaches 2 

kills many microorganisms

inactivates most of enzymes already present in the food you ate

helps break down plant cell walls and connective tissue in meat

activates pepsin from the chief cells

pepsinogen gets changed to pepsin

so food gets to the stomach and triggers pH lowering, beginning digestion

but the size of your meal and its contents need to be managed

Stomach activities are adjusted by food arriving at duodenum

Cephalic (reflex) phase:  prior to food entry

Gastric phase: once food enters the stomach

Intestinal phase: as partially digested food enters the duodenum

now duodenum responds to slow food movement and get stomach to continue to do its job 

Events occur in three phases

The neural reflex

pyloric sphincter tightens, stretch receptors activated by incoming food to duodenum

Hormonal mechanisms – to slow digestion

gastric juice produced continuously

just the rate varies

ACh is released from nerve ending

ACh stimulates gastric juice production

also released is GASTRIN

causes the glands to produce more juice

causes histamine release = more gastric juice

as pH goes down the gastrin is finally turned off

as food reaches the small intestine hormones will also turn down gastrin production 

gastric juice produced continuously

just the rate varies

ACh is released from nerve ending

ACh stimulates gastric juice production

also released is GASTRIN

causes the glands to produce more juice

causes histamine release = more gastric juice

as pH goes down the gastrin is finally turned off

as food reaches the small intestine hormones will also turn down gastrin production 

Most vigorous peristalsis and mixing occurs near the pylorus

Chyme is either:

Delivered in small amounts to the duodenum or

Forced backward into the stomach for further mixing

intestinal cells are stimulated by the acidic chyme to release the hormone CCK (also by fats in intestine)

goes to stomach to slow gastric juice

goes to pancreas = digestive enzymes

stimulates gall bladder to contract

Carbohydrate-rich chyme quickly moves through the duodenum 

Fat-laden chyme is digested more slowly causing food to remain in the stomach longer

some drugs can be absorbed by the stomach

most digestion is done later – stomach begins protein digestion

Carbohydrate-rich chyme quickly moves through the duodenum 

Fat-laden chyme is digested more slowly causing food to remain in the stomach longer

some drugs can be absorbed by the stomach

most digestion is done later – stomach begins protein digestion

this is where most digestion and absorption takes place

proteins are completely broken down to amino acids

carbohydrates are broken down into monosaccharides

fats are broken down into fatty acids and glycerol

vitamins and mineral absorbed

water absorbed

Runs from pyloric sphincter to the ileocecal valve 

Has three subdivisions: duodenum, jejunum, and ileum-longest segment

duodenum 

Are controlled by a sphincter 

The bile duct and main pancreatic duct:

Join the

varying lengths of small intestine may affect growth rate of animals

longer intestine = more time to absorb nutrients for growth

varying lengths of small intestine may affect growth rate of animals

longer intestine = more time to absorb nutrients for growth

is next section 

most of the digestive action takes place here

about 8 feet long

final section is the longest

Plicae circulares

Structural modifications of the small intestine wall increase surface area

deep circular folds of the mucosa and submucosa

these do not stretch out

finger-like projections

inside are arterioles, capillaries, venules, and a lacteal

finger-like projections

inside are arterioles, capillaries, venules, and a lacteal

Cells of intestinal crypts eventually move up and shed enzymes into lumen

prevention of materials getting through the epithelium to the underlying tissues or blood

regional specializations

various area of small intestine have more or less of the previous structures

since most digestion and absorption takes place in the jejunum – most of villi and plicae circulares

ileum has Peyer’s patches

lymphoid tissue

regional specializations

various area of small intestine have more or less of the previous structures

since most digestion and absorption takes place in the jejunum – most of villi and plicae circulares

ileum has Peyer’s patches

lymphoid tissue

hydrolysis

they are lurking in the microvilli of the intestinal cells

these enzymes, along with bile and pancreatic enzymes do most digestion

digestion is a breakdown process

coming from pancreas 

enzymes to break down all types of food

called pancreatic juice

also bicarbonate

intestinal juice

mostly water

brush border enzymes to break foods down into monomers

Plus bile from liver

segmentation

It is initiated by intrinsic pacemaker cells 

Moves contents steadily toward the ileocecal valve

95% of water is absorbed in the small intestines by osmosis

Water moves in both directions across intestinal mucosa

Ex. water follows glucose uptake

Ex. water enters lumen of thick chyme

So water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients

95% of water is absorbed in the small intestines by osmosis

Water moves in both directions across intestinal mucosa

Ex. water follows glucose uptake

Ex. water enters lumen of thick chyme

So water uptake is coupled with solute uptake, and as water moves into mucosal cells, substances follow along their concentration gradients

After nutrients have been absorbed:

Peristalsis begins with each wave starting distal to the previous 

Meal remnants, bacteria, mucosal cells, and debris are moved into the large intestine

After nutrients have been absorbed:

Peristalsis begins with each wave starting distal to the previous 

Meal remnants, bacteria, mucosal cells, and debris are moved into the large intestine

Location

Lies deep to the greater curvature of the stomach

The head is encircled by the duodenum and the tail abuts the spleen

Exocrine function

Secretes pancreatic juice which breaks down all categories of foodstuff

Acini (clusters of secretory cells)

Pancreas also has an endocrine function – release of insulin and glucagon

Water solution of enzymes and electrolytes (primarily HCO3–)

Neutralizes acid chyme

Provides optimal environment for pancreatic enzymes

Enzymes that act on proteins are released in inactive form and activated in the duodenum

Others for starch and fat and nucleic acids are secreted in active form

Note three varieties of digestive enzymes for proteins.

These are released and activated in small intestine.

KEEPS PANCREAS FROM SELF-DIGESTION

Secretin and CCK are released when fatty or acidic chyme enters the duodenum

CCK and secretin enter the bloodstream

Upon reaching the pancreas:

CCK induces the secretion of enzyme-rich pancreatic juice

Secretin causes secretion of bicarbonate-rich pancreatic juice 

Vagal stimulation also causes release of pancreatic juice

Secretin and CCK are released when fatty or acidic chyme enters the duodenum

CCK and secretin enter the bloodstream

Upon reaching the pancreas:

CCK induces the secretion of enzyme-rich pancreatic juice

Secretin causes secretion of bicarbonate-rich pancreatic juice 

Vagal stimulation also causes release of pancreatic juice

The largest gland in the body

The largest visceral organ

Superficially has four lobes –

Separates the right and left lobes anteriorly

Suspends the liver from the diaphragm and anterior abdominal 

Recall this is a set of vessels

begins in capillaries, 

becomes a vein,

which empties in more capillaries

spleen, stomach, pancreas, intestines empty into various veins

they merge into the large HEPATIC PORTAL VEIN

Collects nutrient-rich venous blood from the digestive viscera

Delivers this blood to the liver for metabolic processing and storage

Bile leaves the liver via:

Ducts, which fuse into the common hepatic duct 

The common hepatic duct, which fuses with the cystic duct

These two ducts form the bile duct

liver

Composed of hepatocyte (liver cell) plates radiating outward from a central vein

Production of bile

Processing bloodborne nutrients

Storage of fat-soluble vitamins

Detoxification

regulates composition of blood

hepatic portal vein from digestive system

stabilizes glucose levels – blood sugar levels

makes glucose

regulates levels of lipids

removes extra amino acids

removes toxins

Ex. ETOH, ammonia to urea

stores vitamins, minerals

macrophages remove old cells

make plasma proteins

transport, osmotic, clotting, complement

removes hormones, antibodies

makes and secretes bile

one of the jobs of the liver is to make globular proteins called albumin

this is about 60% of plasma protein

used to shuttle certain molecules

is a buffer in the blood

major protein that contributes to plasma osmotic pressure

attracts water back INTO the blood

 a damaged liver does not make enough of this protein……..

Thin-walled, green muscular sac on the ventral surface of the liver

Stores and concentrates bile by absorbing its water and ions

Releases bile via the cystic duct, which flows into the bile duct

A yellow-green, alkaline solution containing bile salts, bile pigments, cholesterol, lecithin, phospholipids, and electrolytes

Bile salts are cholesterol derivatives that:

Emulsify fat

Make it easier for lipase to breakdown fats

Help solubilize cholesterol

The chief bile pigment is bilirubin, a waste product of heme

You make about one quart / da

Emulsify fat

Make it easier for lipase to breakdown fats

Help solubilize cholesterol

bilirubin, a waste product of heme

You make about one quart / day

Acidic, fatty chyme causes the duodenum to release:

Cholecystokinin (CCK) and secretin into the bloodstream

CCK causes gall bladder to contract (and sphincter to relax)

NEEDS - pretreatment with bile salts

then lipase from pancreas breaks them down small enough to enter cells

DNA and RNA are dismantled to nucleotides and then fragments of nucleotides

fat-filled pouches of visceral peritoneum

‘BANDS, POUCHES, FAT 

The sac-like cecum:

Lies below the ileocecal valve

Contains a wormlike appendix

Has distinct regions:  ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, and sigmoid colon

The sigmoid colon joins the rectum 

The anal canal, the last segment of the large intestine, opens to the exterior at the anus

sphincters:

Internal anal sphincter composed of smooth muscle

External anal sphincter composed of skeletal muscle

These sphincters are closed except during defecation

The anus has two sphincters:

Colon mucosa is simple columnar epithelium except in the anal canal

Has numerous deep crypts lined with goblet cells

Anal canal mucosa is stratified squamous epithelium 

Inflammation of veins results in itchy varicosities called hemorrhoids

Bacteria surviving the small intestine that enter the cecum and 

Those entering via the anus

These bacteria: 

Colonize the colon

Ferment indigestible carbohydrates

Release irritating acids and gases (flatus)

Synthesize B complex vitamins and vitamin K

famous one = E. coli

The bacterial flora of the large intestine consist of:

Other than digestion of enteric bacteria, no further digestion takes place

Vitamins, water, and electrolytes are reclaimed

Its major function is propulsion of fecal material toward the anus

Though essential for comfort, the colon is not essential for life

Haustral contractions

Slow segmenting movements that move the contents of the colon

Haustra sequentially contract as they are stimulated by distension

Activates the gastrocolic reflex

Ex. baby eats and then releases feces

Initiates peristalsis that forces contents toward the rectum

bilirubin is a breakdown product of heme

bacteria convert this to urobilinogens and stercobilinogens in colon

some go to blood and are urinated out

rest of those are converted– give feces characteristic color

bacteria also breakdown peptides = ammonia, indole, skatole, hydrogen sulfide

some of these wastes enter hepatic portal circulation and are converted by the liver, released to be urinated out

Distension of rectal walls caused by feces:

Stimulates contraction of the rectal walls

Relaxes the internal anal sphincter

Voluntary signals stimulate relaxation of the external anal sphincter and defecation occurs

most nutrients are absorbed across the intestinal cells by ACTIVE TRANSPORT

this takes energy

they pass out the back of those cells into capillary blood in the villus, and then to hepatic portal vein

many lipids just diffuse across the intestinal cells and then enter the waiting lacteal in the villus

fat-soluble vitamins move across the intestinal cell membrane by passive diffusion

the colon makes some K, also some B

most of the water-soluble vitamins either diffuse or are actively transported across

B12 is the exception – a large charged molecule

needs intrinsic factor picks it up and they bind to certain receptors in the ileum for uptake

most nutrients just get absorbed as they arrive

iron is actively transported into the intestinal cells and then bound to ferritin – keep there as needed

vitamin D is a cofactor to allow active calcium absorption

WHERE - found in crypts of duodenum

WHEN – arrival of chyme with lipids and proteins

targets WHO – pancreas, gallbladder, duodenum, stomach, CNS

WHAT – pancreatic enzymes, contraction of gallbladder, relaxation of sphincter from liver/pancreas, inhibits gastric secretion, may reduce hunger

WHERE – duodenum

WHEN – arrival of chyme in duodenum

targets WHO – pancreas, stomach, liver

WHAT – stimulates alkaline buffers, inhibits gastric secretion, increases rate of bile secretion

WHERE – stomach, duodenum

WHEN – vagus nerve stimulates or arrival of food in stomach and arrival of chyme with large quantities of proteins

targets WHO – stomach

WHAT – stimulates acids and enzymes, increases motility

for carbs

maltase, sucrase, lactase, amylase (saliva and pancreas)

for protein

carboxypeptidase, chymotrypsin, pepsin, trypsin, elastase, enterokinase, rennin (infants),peptidases (from pancreas, small intestine, stomach)

for lipids

lipase (tongue glands, and pancreas

for nucleic acids

nucleases from pancreas

CELIAC TRUNK

serve = spleen, liver, and stomach