Biology 2402 AP II

Biology 2402 AP II Lecture Notes
Digestive System
Dr. Weis

Digestive System ::

A. Alimentary Canal --> Gastrointestinal Track (G.I. track)

hollow tube, for digestion and absorption of food

organs : mouth, pharynx, esophagus, stomach, small intestine (s.i.), large intestine (l.i.), anus

B. Accessory Digestive Organs --> teeth, tongue, gallbladder,

glands :: salivary, pancreas, liver

Digestive Processes :: Digestion and Absorption

Activities ->

Ingestion --> take in food (by mouth)

Propulsion --> peristalsis :: wave contraction/relaxation

Mechanical Digestion --> chewing, segmentation

Chemical Digestion --> molecules broken down into chemical components

Absorption --> in Small intestine, from GI track to blood or lymph

Defecation --> elimination of feces

Regulatory Mechanisms ::

Mechanoreceptors -- stretch receptors

Chemoreceptors -- sensitive to pH

Osmoreceptors -- sensitive to solute concentration

Intrinsic -- local hormones that cause increase/decrease in motility

Nerve Plexus -- ANS :: parasympathetic -- incr. gi activity, Vagus N., pelvic nerves

sympathetic -- decrease gi activity; branches T5-L2

local nervse::

myenteric........between muscularis layers

for tone, rhythm, velocity, sphincters

submucosal.......in submucosa for secretions and infolding of mucosa

Reflexes :: Pathways involve

1. Local .... within gi track in small area or at a distance called Gastroenteric reflex

2. CNS....... spinal cord, brain stem

Location and Structure ::

Ventral body cavity....abdominopelvic division

lined by serous membrane called the peritoneum

two parts --> parietal peritoneum... lines walls of the abdomen

visceral peritoneum... on organs

between linings is the peritoneal cavity, that contains serous fluid called peritoneal fluid to help decrease friction when organs move.

special double layer of parietal peritoneum called the MESENTERY

FXN : stores fat, keeps abdominal organs in place, route for blood vessels, lymphatics, and nerves to the gi track.

Known as the Apoliceman of the abdomen@

Special name given to mesentery on stomach - OMENTUM

Inflamation in this area......Peritonitis

Blood supply :: Celiac trunk -- left gastric a., common hepatic a., splenic a.

Mesenteric A. B Superior.... upper gi and the Inferior.... lower gi

Hepatic Portal System

Blood flow to each area of the gi track will be directly related to local activity.

Increased blood flow caused by -->

Vasodilators such as CCK, VIP, Gastrin, Secretin

Decreases in O2

Decreased blood flow caused by --> sympathetic stimulation to cause vasoconstriction

HISTOLOGY :: 4 layers (tunics) -->

Mucosa --<>--

1. Epithelial membrane that lines the lumen, will also contain glands

Fxn :: secretion, absorption, protection, e.g. Simple columnar epithelium with goblet cells

2. Lamina propria is the connective tissue basement membrane, remember that the epithelium with its basement membrane form the mucous membrane

3. Muscularis mucosa...two layers of smooth muscle that create folds or villi in the mucosa to increase surface area.

Submucosa --<>--

dense connective tissue with blood vessels/lymph/nerve/and elastic fibers.

Has intrinsic nerve supply :: Submucosal plexus to regulate gland secretion and the muscularis mucosa smooth muscle to help form folds, such as plicae.

Muscularis --<>--

Smooth muscle in 2 layers :: inner circular,outer longitudinal

smooth muscle --> single nucleus, individual fibers, nonstriated, has actin/myosin NOT

arranged in rigid structures, slow electrical activity

Fxn : GI movements --> mixing/segmentation, peristalsis

Nerve fibers control gi motility... ANS and myenteric plexus

Serosa --<>--

outermost, made of visceral peritoneum.......simple squamous cells with connective tissue

subserous plexus for ANS innervation

ADVENTITIA.... fibrous C.T. on organs that are retroperitoneal

ANATOMY :: Oral Cavity --> Mouth, Pharynx, Esophagus

A. Mouth : oral cavity continuous with the oropharynx lined with stratified squamous epithelium

lips/cheeks -- skeletal muscle such as the orbicularis oris, buccinator, risoris etc.

areas -- vestibule -> between cheeks and gums

oral cavity proper -> within teeth

red margin --> "lips", poorly keritinized so see capillary vessels

labial frenulum -> median fold joining lips to the gums (upper and lower)

Palate

Hard palate......palatine, palatine process of the

maxillary bones, may have mucosal folds

Soft palate...... no bone, skeletal muscle

projection -->uvula to aid in closing the nasopharynx

anchored by arches ::

palatoglossal arch........to tongue

palatopharyngeal arch.....to pharynx

palatine tonsils located in this region

Tongue.... body and root

fxn :: to mix food and create a bolus, speech

has glands, taste buds for gustation, lingual tonsil

skeletal muscle ::

intrinsic.... in three planes within the tongue

to change the shape --> thick, thin

extrinsic... from tongue to origin points

names __________glossal m.

to protrude, retract, move side to side, lingual frenulum attaches tongue to floor

papillae... three types ::

the parotid is a serous salivary gland, & the other extrinsic glands are mixed, (have both cells), though the sublingual secretes more mucus

Saliva :: primarily water with electrolytes

such as K+, HCO3-, Na+, Cl-, proteins, IgA

enzymes such as lysozyme and amylase

Fxn: lubricate, protect, moisten, gustation, also aid in Chemical Digestion

Controlled by ANS

parasympathetic :: serous, CN VII, CN IX

sympathetic :: mucous, change blood vessel, and therefore inhibits release.

emergence of teeth at the gum --> eruption

reference terms for teeth :: surfaces

buccal (at cheek), lingual (tongue),

mesial (between teeth), occlusal (counter arch)

Dental formula : 2 arch, multiple by 2

deciduous upper 2I 1C 2 M permanent 2I 1C 2PM 3M

---------------- ----------------- x2 ------------- ------------------- x 2

lower 2I 1C 2 M 2I 1C 2PM 3M

total deciduous, 20 total permanent, 32

Structure :: 2 regions --> crown....visible part

root.....embedded in jaw, number of roots varies with each tooth

The root outer surface has cementum, which is avascular

avascular CT and will attach the tooth to the

periodontal ligament to form a fibrous joint (gomphosis).

crown and root connected at a constricted region called the neck

associated structures ... Gingiva -- gum, forms the gingival sulcus

Enamel.... avascular, acellular material made primarily of calcium phosphate salts. There is no repair, since cells die @ birth

Tooth is primarily dentin, an avascular bonelike material secreted by odontoblasts.

Within the dentin is a cavity containing blood vessels/connective tissue/ and nerves in the crown this cavity is called the pulp cavity and will extend into the root called the root canal and open into the apical foramen

Problems :: Dental caries (cavities) due to demineralization of enamel/dentin.

Starts as plaque and then forms tartar/calculus

Gingivitis, Periodontal disease, Nerve death

PHARYNX ::

common passageway for food/air. Three regions --> nasopharynx, oropharynx, and the laryngopharynx.

Histology :: mucosa .... stratified squamous with mucus glands.

Skeletal muscle in 2 layers in the oropharynx and laryngopharynx to contract and help propel food to esophagus.

Problems :: hiatal hernia, esophagitis due to reflux and can create esophageal ulcers.

//////////// DIGESTIVE PROCESSES FOR THE ORAL CAVITY ////////////

Mouth :: mechanical digestion --> chewing, muscles of mastication (CN V)

chemical digestion --> salivary amylase for polysaccharides

Swallowing :: 3 phases

1. Voluntary phase --> initial stage, tongue maneuvers food to back of pharynx

2. Pharyngeal phase --> soft palate closes, larynx moves upward while the epiglottis

closes over glottis, esophagus is open, peristaltic wave

3. Esophageal phase --> relaxation of upper esophageal sphincter to allow passage of food bolus to stomach, peristaltic wave.

CN involved :: V, IX, X, XII

STOMACH :: location in upper left quadrant of abdomen

fxn : storage, mechanical breakdown, chemical breakdown, protein digestion, Food converted to CHYME

Gross --> Volume change without tension change

Internal lining when relaxed into folds --> RUGAE

Regions >>>

2. Submucosa :: connective tissue

3. Muscularis :: three layers of smooth muscle. The third layer is innermost @ an oblique angle, then circular, then longitudinal

4. Serosa :: connective tissue

Secretory Glands of the stomach -->

a. Mucous neck glands........secrete mucus

b. Enteroendocrine glands....secrete hormones....

GASTRIN, histamine, serotonin, CCK, somatostatin

Gastrin will stimulate parietal and chief cells.

c. Chief cells ...... secrete pepsinogen

d. Parietal Cells....located at neck regions, secrete

1. Hydrochloric acid

Most of the HCL will be stimulated by these three chemicals ::

ACH from parasympathetic nerves, Gastrin, and histamine

HCL comes from the following formula ::

CO2 in the cell combining with water to form carbonic acid and then Hydrogen ion and bicarbonate (CO2 + H20 <-----> H2CO3 <--------> H+ + HCO3- )

The hydrogen is pumped from the parietal cell into the stomach lumen. Cl- from the plasma is pumped into the lumen also to form HCL. Na+ in the plasma

will combine with the HCO3- for buffering. This sodium bicarb in the plasma has a basic pH or alkaline pH. So the blood leaving the stomach is alkaline and this phenomenum is called the alkaline tide. Meanwhile, the stomach HCL has a pH of 2, a very acidic pH.

HCL functions to activate pepsinogen (to pepsin) to digest proteins and also has bactericidal properties.

2. Intrinsic factor from the parietal cells necessary for B12 absorption in the intestine.

Mucosal Barrier :: protect the stomach wall

Gastric Motility and Emptying ::

Stomach filling... relaxed, stretch with minimal incr tension

Contraction of muscularis smooth muscle

Mixing to aid in mechanical and chemical digestion

Pacemaker cells in between the muscularis sm. m. set up a slow wave rate for mixing.

Peristalsis wave for movement.. starts at the gastroesophageal sphincter and moves toward the pylorus.

Allows for small amount of chyme through the sphincter, and the rest of the chyme is remixed.

Gastric emptying takes approx. 4 hours, fluids will move faster, solids stay until they become a liquid state.

Gastric emptying also depends on amount of contents in the duodenum.

Problems :: Vomiting (emesis)......sensory impulses from stretching, irritants, toxins, drugs are sent

to emetic center of medulla.

Small Intestine :: Digestion and Most All Absorption (80%)

Gross... Starts at the pyloric sphincter and goes to ileocecal valve.

Three subdivisions : duodenum, jejunum, and ileum (not ilium).

1. Duodenum ... the first 12 inches of the SI. Has an opening for the combined ducts of the pancreas
and common bile duct that open onto the duodenal papilla that is controlled by a sphincter, the sphincter of Odi.

2. Jejunum ... from the duodenum to the ileum, 8ft long

3. Ileum ... 12 ft long, from jejunum to Large Intestine

Small Intestine is suspended by MESENTERY (a double fold of parietal peritoneum).

The root of the mesentery is at the upper lumbar vertebrae.

Histology ...

a. Mucosa... epithelium :: simple columnar cells with microvilli projecting from the apical surface of the membrane.
All the microvilli of the SI form the Brush Border. Will contain enzymes for chemical digestion.

Site for absorption

Goblet cells for mucus

Mucosa will be thrown into folds by the connective tissue of the submucosa

These folds are called VILLI. At the base of the villi, called the intestinal crypts are openings that lead to intestinal glands that secrete intestinal juice.

STEM CELLS are also located at the base of the intestinal crypts and will replace the villus epithelial cells approx. every 5 days.

Lamina Propria :: connective tissue with capillaries and lymphatics called the lacteals.

Muscularis Mucosa :: smooth muscle, contraction will aid in absorption and movement of lymph.

b. Submucosa... connective tissue folds to form plica.

may also contain duodenal glands in the duodenum and these secretions help neutralize the acid chyme or lymph nodules in the ileum for protection

c. Muscularis.... two layers of smooth muscle

d. Serosa ... visceral peritoneum except on the duodenum which is retroperitoneal, so it will have adventitia

The intestinal juice from the glands in the mucosa will be primarily water and mucus with a pH of 7 (+/- .5) and provide a fluid to dilute and neutralize chyme pH. Will usually not contain any enzymes for digestion.

Accessory Glands of the Small Intestine ::

I. Liver :

FXN :: digestive --> secrete bile, a fat emulsifier

Anatomy... largest organ, and largest gland

Four Lobes : Rt, Lf, Caudate, Quadrate

Anchored by the Lesser Omentum that runs from the stomach to the liver.

Falciform ligament that runs from the liver to the diaphragm & anterior abdominal wall.

Round ligament of the liver, was the fetal umbilical vein

Usually nonpatent (closes). Provided a shunt to bypass the fetal liver.

Blood supply :: Hepatic A., Portal Vein, Hepatic V.

Gall Bladder is recessed on the inferior side of the right lobe

Bile from the hepatocytes will drain into the bile cannaliculi --> bile duct -->

Hepatic Duct that leaves the liver to join with the cystic duct of the gall bladder to form the Common Bile duct.

Histology ... Divided into functional units called

Hepatic (Liver) Lobules that are hexagonal (6 sided) in shape.

Contain rows (plates) of single file hepatocytes that "radiate" from the central vein.

Portal Triads at each of the six corners.

Will consist of a hepatic a, portal v., and a bile duct

Sinusoids are between the hepatocytes through which the hepatic a., and portal v. will run and connect with the central vein.
The central veins from each of the hepatic lobules will interconnect by inter-lobular veins and form the hepatic veins

The sinusoids also contain monocytes that are called Kupffer cells and their function is to remove debris and foreign matter.

Lymphatics will run between the hepatocytes and the sinusoids.

Bile from the hepatocytes will flow into the bile cannaliculi that run between the hepatocytes.

** Therefore, blood and bile flow in OPPOSITE directions

Other liver functions ::

Problems :: hepatitis, cirrhosis (fibrosis) from drugs or chronic inflammation

portal hypertension

Gall bladder :: thin walled sac...smooth muscle with connective tissue and simple columnar epithelial lining on ventral surface of right liver lobe

FXN : stores and concentrates bile (removes H2O, NaCl)

Cystic duct for bile excretion, responsive to hormone CCK (cholecystakinin)

Bile :: solution that emulsifies fat, aids in the transport of fat, and excretion of waste products =>

such as bilirubin and cholesterol.

Composition -->

1. Bile salts :: derivative of cholesterol, cholic acid and chenodeoxycolic acid will emulsify fats (detergent effect) to form smaller globules.
Will be RECYCLED by reabsorption in the ileum, put into the hepatic portal system and returned to the liver and resecreted in the bile.
This is called the enterohepatic circulation.
Bile salts also help in absorption by forming micelles and mixing with lipids.

2. Bile pigments :: bilirubin (yellow-green) waste product of heme from the hemoglobin of RBC.
Will be metabolized to urobilinogen by the small intestine bacteria and cause the feces to be brown in color

3. Other components :: cholesterol, neutral fats, water, phospholipids, electrolytes, (Na+, HCO3-)
Bile secretion by the liver is increased by secretin, & will mainly be an increase in HCO3- concentration.

Problems :: gall stones due to an increase in free cholesterol and decrease formation of bile salts.

The cholesterol will precipitate out. May also have Ca+ deposits within the stones.

Cholelithiasis --> gall stone condition.

inflammation

obstruction (cholestasis) --> will obstruct bile flow bile pigments will accumulate in the blood and be deposited in the tissues to create a yellow tint called jaundice.

This particular condition, due to the obstructed flow is called Obstructive Jaundice.

Pancreas :: beneath the stomach and encircled by the duodenum

Gland with head, body, tail

Functional parts :: Exocrine and Endocrine

Digestive Function -->

Exocrine Pancreas.... will produce pancreatic juice that drains from the pancreatic duct.

This juice will contain enzymes for digesting protein, fats, and carbohydrates, as well as HCO3- to neutralize the acid pH of the chyme.

Histology.... acinar cells that contain zymogen granules that have the enzymes

islets for the endocrine pancreas (alpha, beta, delta cells, F cells)

Pancreatic Juice -->(Most pancreatic digestive enzymes fall into either Proteases or Peptidases)

primarily water with inactivated (pro) enzymes

such as :: amylase for CH2O digestion

lipase for fat digestion

trypsinogen for protein digestion, will be activated to trypsin

& can cause activation of other pancreatic enzymes

Small Intestine Digestive Processes ::

Digestive activity depends on

1. Enzymes from liver, pancreas

2. Slow delivery from the stomach

Motility :: slow mixing/segmentation....pacemaker cells in the muscularis allow for slow steady movement for absorption as the smooth muscle contracts and relaxes

Propulsive contraction... peristalsis will start after nutrients have been absorbed.
Wave like motion in response to gastroenteric reflexes and hormones ileocecal sphincter (valve) will relax and is under feedback control.
Hormone : Gastrin, also neural stimulus from the gastroileal reflex to cause relaxation of sphincter and allow movement of material into the colon.
Sphincter fxn: to prevent backflow of fecal contents from colon to SI.

Problems :: small bowel diarrhea, malabsorption, maldigestion

*Large Intestine ::

Fxn > H2O absorption, Vit B, K production, storage and elimination of feces

Gross : subdivisions -->

Cecum....junction of ileum at the ileocecal valve

sac like, expanded chamber

Appendix..blind sac off the cecum, lymphatic tissue

Colon... regions :: ascending (up right side), turns at the right colic (hepatic) flexure, transverse colon (from right to left), turns at the left colic (splenic) flexure, descending colon (down), S curve at the sigmoid colon.

Rectum with internal transverse folds called rectal valve

Anal Canal ... 3cm long, with opening called anus has 2 sphincters ::

internal anal and external anal sphincter

Histology of the Large Intestine:

1. Mucosa : simple columnar epithelium

Mucosa in the anal canal will change to stratified squamous and have folds called the anal columns.
Anal sinuses between folds secrete mucus

2. Submucosa : connective tissue

3. Muscularis : inner circular

*** outer longitudinal layer is incomplete and decreased to two bands called the TAENIAE COLI (colon ribbons) that will cause the intestinal wall to pucker into sacs called HAUSTRA. The outer layer is complete in the rectum

4. Serosa : connective tissue

Bacterial flora of the large intestine will colonize and ferment carbohydrates (CH2O) and release gas. Also synthesize Vit B, and Vit K.

Mucus in the large intestine will protect the wall from bacterial activity in the feces and hold the fecal material together.

Large Intestine Digestive Processes ::

Motility :

1. Slow, segmented, distention of haustra when sm. m. contracts to create mixing movements called HAUSTRATIONS that aid in H2O absorption

2. Peristalsis waves called Mass Movements occur 3-4 times daily.

Will start at the transverse colon and initiated by the gastrocolic reflex and causes movement of material into the rectum

Defication reflex :: stretching of the rectal wall by the forcing of feces into the rectum by the mass movement. Parasympathetic (pelvic nerves) will cause relaxation of the internal anal sphincter, contract the sigmoid colon and rectal wall.

Voluntary control of the external anal sphincter can stop the defecation reflex if the sphincter remains closed. Contractions will stop, until the next mass movement or forced defecation.

Problems :: Large bowel diarrhea, Constipation, Colon cancer, Rectal Polyps

Physiology of Chemical Digestion and Absorption

Chemical digestion :: enzymes that will break molecules into functional units by HYDROLYSIS (adding water)

1. CH2O (carbohydrates) : monosaccharides (glucose, fructose, galactose)

disaccharides (sucrose, lactose, maltose)

polysaccharides (glycogen, starch, cellulose)

Digested by : amylase (salivary, pancreatic) AND brush border enzymes of SI

2. Proteins broken down into Amino Acids (AA)

digested by : stomach pepsin, pancreatic trypsin, chymotripsin, carboxypeptidase, and SI brush border enzymes

3. Lipids : triglycerides (neutral fats) into Fatty Acids (FA) and glycerol

enzymes : pancreatic lipase

bile salts to emulsify

4. Nucleic acids : into nucleotides by pancreatic nucleases

ABSORPTION ::

Small Intestine 80%

1. CHO.. facilitated diffusion, cotransport with Na+ increased by insulin from beta islet cells of pancreas

2. Proteins..carrier, cotransport with Na+

3. Nucleic acids.. carrier transport across epithelium

4. Lipids...absorbed in the ILEUM

Here's what happens....

Pancreatic lipases break lipids into FA and glycerol mixed with bile salts to form MICELLES and then cholesterol and fat soluble vitamins are added. Carried down small intestine to ileum.

At brush border of the columnar cells, the FA and glycerol are released from the MICELLES and enter into the cell. The rest of the micelle enters the cell and will go into the bloodstream

The fatty acids and glycerol will recombine into triglycerides in the smooth endoplasmic reticulum of the epithelial cell. There cholesterol, free fatty acids and phospholipids will be added. The whole lipid structure is sent to the Golgi and is coated with a protein. This is then called a CHYLOMICRON.

These chylomicrons leave the epithelial cells and are too big to enter the capillaries, so the small intestinal lymphatics (called lacteals) will pick them up. The lacteals drain into the venous system. Within the capillaries, are enzymes that remove FA and glycerol from the chylomicrons, where they can pass through cell walls to be used by the tissues ! (Whew)

** Remember *** MICELLES and CHYLOMICRONS

5. Vitamins

Water soluble (B,C) ...diffusion

B12 needs intrinsic factor from the stomach to be absorbed

Fat Soluble (A,D,E,K) by micelles

Vitamin source....diet, large intestinal bacteria (B,K)