Involves:

Ingestion

Mechanical Processing (mechanical digestion)

Digestion (chemical digestion)

Secretion

Absorption

Excretion

The Four Major Layers of the Digestive Tract:

1) THE MUCOSA:

consists of...... a) a mucous membrane (an outer layer of epithelium)

and a layer of connective tissue (lamina propia)

b) muscularis mucosa

arranged in....... folds and villi

contains.......... mucosal glands (in lamina propia)

2) THE SUBMUCOSA:

consists of...... a single layer of connective tissue

contains....... a)nerve endings (submucosal plexus)

b) blood vessels

c) lymphatics

3) THE MUSCULARIS EXTERNA:

consists of..... a) two layers of smooth muscle (an inner layer of

circular  muscle and an outer layer of longitudinal

muscle)

b) myenteric plexus(parasympathetic and sympathetic

innervation

4) THE SEROSA:

consists of....... a serous membrane ( the visceral portion of the

peritoneum) covering the muscularis externa)

found in....... stomach and intestine only

(oral cavity, pharynx, esophagus, rectum attached by

the adventitia)

Movement of Digestive Materials:

1) PERISTALSIS:

consists of... a) muscular contractions of the muscularis

externa

occurs when.... contraction of circular muscles occur behind a

food mass followed by contraction of the

longitudinal muscles

results in..... propelling food forward (peristalsis)

2) SEGMENTATION:

consists of..... muscular contractions (circular muscles only)

results in.... mixing and fragmentation of food

The Oral Cavity:

FUNCTIONS:

1) analyzes material before swallowing

2) mechanical processing (teeth, tongue, the palate)

3) lubricates material (saliva)

4) chemical digestion: carbohydrates by the enzyme

salivary amylase

ORGANS OF THE ORAL CAVITY:

1) cheeks 2) labia (lips) 3) gingivae (gums)

4) hard palate

5) soft palate

6) tongue

7) teeth

8) linguinal frenulum

SUBDIVISIONS OF THE ORAL CAVITY:

vestibule (space between the cheeks,lips and teeth)

THE TONGUE:

functions in.... a) mechanical processing: compressing, abrasion,

distortion

b) aids in chewing; moves food toward the teeth

c) sensory analysis (touch, temperature, taste)

d) contains the linquinal tonsils at its base

THE TEETH:

functions in... a)mastication (chewing)

structure............ a) three parts: crown, neck, root

b) layers: enamel (covers crown and neck)

b) types:

 central incisors

lateral incisors

 cuspids

bicuspids(premolars) 

molars

THE SALIVARY GLANDS:

3 pairs (exocrine glands.... 

a) parotid glands (below zygomatic arch)

(parotid duct empties into the vestibule at

level of the second upper molar)

b) sublingual glands (below the floor of

the mouth--behind the teeth on each side of

the linguinal frenulum)

c) submandibular glands (floor of the mouth

(ducts open behind the teeth on each side of the lingual frenulum)

SALIVA:

a) 99.4% water

b) .6 % ions, buffers, waste products, enzymes

c) mucous-like mixture coats the food (aids in

swallowing)

d) enzyme - salivary amylase chemically digests

starch into maltose

e) salivary immunoglobins (IgA) and lysozyme control

population of oral bacteria

f) mandibular and sublingual glands contain buffers

and mucous (little enzyme released)

g) 1-1.5 liters of saliva produced per day (50% by the

parotid gland

h) pH shifts from 6.7 to 7.5

THE PHARYNX:

initiates swallowing of food from the mouth

(food mass in mouth, pharynx and esophagus called

the BOLUS)

THE ESOPHAGUS:

a) 1 foot long

b) carries food from pharynx to the stomach by

peristalsis through the esophageal (cardiac)

sphincter

THE STOMACH:

four major functions:

1) temporary storage of food

2) mechanical processing (churning)

3) secretion: a) enzyme (pepsin) for the digestion of protein

b) hydrochloric acid for the denaturation of protein and to destroy bacteria

4) production of an intrinsic factor (compound needed

for the absorption of vitamin B₁₂

***soupy mixture of acid, enzymes and food is called chyme

FOUR REGIONS OF THE STOMACH:

1) cardia 2) fundus 3) body 4) pylorus

a) has a three layered muscularis externus: longitudinal,

circular and an additional INNER OBLIQUE LAYER

(which adds strength and another motion for churning)

b) inner lining of "relaxed" stomach has ridges called

RUGAE

c) visceral peritoneum (serosa) continuous with the greater

and the lesser omentum

THE GASTRIC WALL:

a) epithelium dominated by mucous cells (mucus protects

stomach from acid and enzymes

b) gastric pits contain gastric glands (secretes gastric juice

1500 ml per day

c) the gastric glands:

---parietal cells secrete: INTRINSIC FACTOR (needed for the absorption of vitamin B₁₂ ) and HYDROCHLORIC ACID

---chief cells secrete: PEPSINOGEN (converted to pepsin by hydrochloric acid,

PEPSIN (protolytic enzyme that breaks down proteins), and RENNIN and GASTRIC

LIPASE (newborn infants only---milk digestion)

c) endocrine cells release the hormone GASTRIN

GASTRIC SECRETION:

1) CEPHALIC PHASE-

sight,smell,taste,or thought of food

2) GASTRIC PHASE- 

a) arrival (presence) of all food in the stomach-stretch receptors stimulated by gastrin(released into bloodstream and accelerates gastric secretions

3) INTESTINAL PHASE-

THE SMALL INTESTINE:

a) 6 meter long (20 feet)

b) three divisions:

---DUODENUM: 1 ft long, begins at the pyloric sphincter, and site where pancreatic and hepatic duct empty---

---JEJUNUM: 8 ft long, most chemical

digestion and absorption occur here

---ILEUM: 11 ft long and ends at the ileocecal valve

THE INTESTINAL WALL:

a) intestinal lining (contains folds called PLICAE CIRCULARIS)

and finger-like projections called VILLI)

b) each villus in turn contains MICROVILLI(in

epithelial layer) along with capillaries, nerve endings,

and a **lacteal (a terminal lymphatic)

***lacteals absorb fatty acids and combine with proteins to form protein-lipid packages called CHYLOMICRONS which reach the circulation through the lymphatic system

c) intestinal glands at the base of the villi secrete

intestinal juice ( in duodenum an alkaline mucus is

secreted which buffers the acid chyme), also contain

endocrine cells which secrete intestinal hormones

INTESTINAL MOVEMENTS:

a) SEGMENTATION (contraction of circular muscles only)

b) PERISTALSIS ( movement of food controlled by

local reflex action:

1) gastroenteric reflex: initiated by the

distension of the stomach which increases intestinal

secretions, and peristalsis in the small intestine ` 2) gastroileal reflex: release of gastrin

relaxes ileocecal valve allowing increased peristalsis to

push food out of ilium

INTESTINAL JUICE:

a) 1.8 liters per day

b) moistens intestinal contents and buffers hydro-

chloric acid from the stomach

** sympathetic activation inhibits activation of glands (brought about by stress can lead to duodenal ulcers)

Hormones and  Hormonelike Products That Act in Digestion (see table 23.1; page 905)

THE PANCREAS:

1) consists of exocrine and endocrine glands (endocrine

glands make up only 1% of the population)

2) pancreatic acini (pouches) secrete enzymes and buffers

from acinar cells into small ducts which combine to form the pancreatic duct

3) PANCREATIC JUICE:

consists of.......... a) pancreatic lipase

b) pancreatic amylase

c) protolytic enzymes: trypsin, chymotrypsin,

carboxypeptidase (these form 70% of the

pancreatic juice) (these are the inactive forms-

activated by other enzymes ie trypsin is the

inactive form of trypsinogen)

THE LIVER:

structure and composition

a) the largest visceral organ

b) four lobes separated by ligaments (**falliciform ligament

thickened portion is the remnant of the fetal umbilical vein)

c) gall bladder under the right lobe (stores bile)

d) liver lobules-basic functional unit of the liver (100,000

lobules)

e) hepatocytes - liver cells

f) sinusoids - capillaries between adjacent plates of hepatocytes

g) phagocytic kupffer cells

circulation....

Blood enters the sinusoids from branches of the hepatic portal vein and hepatic artery (oxygen is absorbed by liver cells), as blood flows past the liver cells absorb and secrete materials into the bloodstream. Blood then leaves the sinusoids and enters the central vein of the lobule. All central veins merge to form the hepatic veins that empty into the inferior vena cava.

release of bile ( 1 liter per day)....

released into the canaliculi from the central vein, which combine to form the common hepatic duct, which then separates into the cystic duct (going to the gall bladder to be stored), or to the common bile duct to be secreted directly into the duodenum (sphincter in common bile duct stays open during eating) (if closed bile has to be stored in the gall bladder)

LIVER FUNCTIONS (OVER 200)

1) absorb nutrients or toxins from the blood before it

reaches the rest of the body

2) monitors and adjusts levels of glucose in the blood

3) stores fat soluble vitamins (A,D,K,E)

4) phagocytic cells in liver remove aged or damaged

RBCs, pathogens and debris

5) liver cells synthesize "plasma proteins"

----albumins (determine osmotic pressure of blood)

----globulins (carry nutrients)

----fibrinogen (blood clotting)

----synthesizes and secretes bile ( water, ions, bilirubin,

cholesterol, and bile salts)

THE LARGE INTESTINE:

functions:

 1) reabsorption of water and compaction of feces

2) absorption of vitamins freed by bacterial action are

Vitamin K (a fat soluble vitamin needed by the liver

to synthesize four clotting factors) Biotin (a water

soluble vitamin important in glucose metabolism)

Vitamin B₅ (water soluble vitamin required in the

manufacture of steroid hormones and that of some

neurotransmitters)

3) storage of fecal matter before defecation

structure:

1) the cecum

a) compaction begins

b) vermiform appendix attached to it

c) rounded sac, regulation of wastes controlled

by ileoceacal valve

2) the colon

a) contains pouches called HAUSTRA that permit distention and elongation

b) TAENIAE COLI --longitudinal bands of

muscle -- visible on the outer surface of the colon

just beneath the serosa (muscle tone within these

bands produce the haustra)

c) ascending, transverse, descending and sigmoid colon

3) the rectum

a) anorectal canal , last portion called the

anorectal canal contains longitudinal folds

joined by transverse folds(marks boundary between

columnar epithelium of the rectum and a

stratified squamous epithelium.

b) circular muscle layer forms the internal anal

sphincter(involuntary control), external anal

sphincter guarding entrance into the anorectal

canal (consists of skeletal muscle-under voluntary

control

a) defecation reflex involves two positive feedback

loops:

---stretch receptors in the rectal walls order a

series of peristalic contractions in the colon and

rectum---

---the sacral parasympathetic system activated

by stretch receptors also stimulates peristalsis---

b) movement of feces through the anorectal canal

requires relaxation of the internal anal sphincter; when

this happens the external anal sphincter clamps shut

c) if commands do not go to external anal sphincter

at once then peristalic contracts cease until another

reflex is produced a second time

d) anal sphincter can be opened voluntarily,

defecation can be expedited by tensing abdominal

muscles and by making expiratory motions by closing

glottis (these pressures also force blood into the

network of veins in anorectal canal causing them to

stretch and become permanently distended causing

hemorrhoids