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Guided
Notes for
the
Digestive
System
Objective: List and
describe the functions of
the digestive system. List
the organs of the
alimentary canal
(gastrointestinal tract) and
describe their general
functions.
Functions of the Digestive System
1) Ingestion – taking food in, occurs in the mouth.
Digestion – breaking down food into smaller particles.
2) Mechanical digestion – physical process that breaks food down into smaller
pieces but does not change the chemical nature of the food. Converting food into smaller
particles increases surface area and mobility. Examples:
- mastication : commonly known as chewing is the process by which food is crushed and
ground by teeth.
- churning : pattern of contractions of the muscularis externa in the stomach that
breaks apart food and mixes it with gastric juices.
- segmentation : Alternating contractions of muscularis externa in the small intestines
that breaks apart digestive material and mixes it with mucous.
3) ____Chemical___ digestion – the process of breaking covalent bonds of molecules in food
to produce smaller molecules. Digestive organs secrete enzymes that assist with digestion.
4) Propulsion – movement of food along the digestive tract.
- Deglutition - the process of moving food from the mouth to the stomach commonly
known as swallowing.
- Peristalsis - Involuntary waves of contraction and relaxation of muscularis externa
that propels digestive material forward through the digestive tract.
5) _Absorption – transport of nutrients from the lumen of the GI tract into the blood.
6) Defecation – elimination of indigestible substances in the form of feces.
Organs of the Alimentary Canal: also known as the gastrointestinal (G.I.) tract, is a tube
extending from the proximal opening of the mouth to the distal opening of the anus, contents of
lumen are considered outside of the body.
7) _Oral______ cavity (Mouth) - location for ingestion, mastication, food is mixed with saliva
and forms a bolus (rounded
mass), early stages of
chemical digestion for
carbohydrates and fats,
voluntary phase of deglutition.
8) Pharynx__ - commonly
known as the throat this
muscular passageway is the
location for the second phase
of deglutition.
9) Esophagus__ - a muscular
tube that carries a bolus from
the cervical region, through
the thoracic cavity and into
the stomach during the final
phase of deglutition.
10) Stomach__ - location for
churning (muscular
contractions), secretion of
acidic gastric juice that is
antimicrobial and performs
early stages of chemical
digestion for proteins,
formation of chyme (semi-
fluid composed of partially
digested food) from a bolus.
11) Small_ intestine-
primary location of chemical
digestion and nutrient
absorption.
12) Large intestine_-
absorbs most of remaining
water and some nutrients, forms feces, eliminates feces in the process of defecation.
Accessory organs – produce secretions that assist with digestion.
13) Salivary_ glands- produce saliva that lubricates food and contains the enzymes
(salivary amylase) that initiate chemical digestion of carbohydrates. Also contains enzymes
and antibodies that aid with barrier defenses to protect against infection.
14) Pancreas_- produces digestive enzymes and bicarbonate.
15) Liver_- produces bile which emulsifies lipids to aid with chemical digestion
and absorption of lipid soluble nutrients.
16) Gall bladder- stores concentrates and releases bile.
Objective: Describe the gross structure and histological organization of the esophagus, stomach,
duodenum, jejunum, ileum, liver, pancreas, and colon.
Histological Layers of the Alimentary Canal
27) ___ Mucosa ____- deepest layer. Contains epithelial tissue lining the lumen.
- Lamina propria- thin layer of loose areolar connective tissue.
- Muscularis mucosae- thin layers of smooth muscle, pulls the mucosa into undulating
folds that increase the surface area available for digestion and absorption.
28) ___ Submucosa ____- layer of dense irregular connective tissue that contains blood vessels,
lymphatic vessels, glands, and a plexus of nervous tissue.
29) ___ Muscularis ____- layer of muscle tissue. For most of the alimentary canal this layer
contains smooth muscle organized into an inner circular muscle layer and an outer
longitudinal muscle layer. Between these layers of muscle is a layer of nervous tissue called
the myenteric plexus.
30) ___ Serosa ____ ( visceral peritoneum )- layer of serous membrane covering the superficial
surface and helping to anchor the alimentary canal to the walls of the abdominopelvic cavity.
31) ___ Stratified Squamous ______ epithelial tissue in the mucosa forms a protective lining.
- Muscularis- contains skeletal muscle in the proximal third and transitions to smooth
muscle at the distal end.
o Sphincters- circular muscle that constricts to stop the passage of substances through
the lumen. Must relax to allow a bolus through during deglutition.
▪ Upper esophageal sphincter- controls the entrance from pharynx.
▪ Lower esophageal (cardiac) sphincter- controls the entrance to the stomach.
32) _Adventitia- dense irregular connective tissue that anchors the esophagus to surroundings.
Stomach - left upper quadrant of the
abdomen inferior to the diaphragm
and medial to spleen.
33) ______ Cardia ________- The region
where the esophagus empties into the
stomach is the cardia. A bolus enters
through the lower esophageal
(cardiac) sphincter.
34) _____ Fundus _________- The
dome-shaped top of the stomach.
35) _____ Body ___________- The largest portion of the stomach.
36) _____ Pylorus _________-. The terminal portion of the stomach is the “gatekeeper”, which
abuts the first portion of the small intestine. Contains a sphincter that controls the flow of
materials between the stomach and the small intestine.
Muscularis: three layers of smooth muscle tissue enable churning. In addition to the circular
layer and longitudinal layers, the stomach contains an oblique layer adjacent to the serosa.
37) _____ Gastric ____ pits - indentations of the surface of the epithelium, connecting to glands,
which secretes a complex digestive fluid referred to as gastric juice.
Small Intestine - the mucosa is extensively folded
to increase surface area for nutrient absorption.
- Plicae circularis are large circular folds.
- Villi are smaller folds of the mucosa.
- Microvilli are folds of the cellular
membranes of the epithelial cells.
38) ___ Duodenum __- short proximal segment
(~20-25cm) that receives chyme from stomach
and exocrine secretions from the pancreas and
liver/gall bladder. Responsible for most
chemical digestion and some nutrient
absorption. Contains submucosal glands that
secrete alkaline mucous.
39) ___ Jejunum___- long middle segment (2-3 m) responsible for most nutrient absorption.
Surface is highly folded with large circular folds called plicae circulares covered in
smaller folds called villi.
40) ___ Ilium_____- longest and most distal segment (2.5-3.5 m) responsible for some nutrient
absorption, drains into the large intestine through a sphincter called the ileocecal valve.
Contains numerous clusters of lymphoid tissue called Peyer’s patches.
Large Intestine
41) ___ Cecum_____- The first part of the large intestine, a sac-like structure that is suspended
inferior to the ileocecal valve. Receives the contents of the ileum.
- Appendix - small, twisted tube that attaches to the cecum has a closed end. Functions as a
reservoir of bacteria for the large intestine. Contains mucosa associated lymphoid tissue.
Enteroendocrine cells:
- G cells produce gastrin
- S cells produce secretin
- I cells produce CCK
Objective: Describe the secretions
produced by gastric pits (glands).
Describe the function of the autonomic nervous system and the hormones gastrin,
secretin, and CCK in the regulation of the stomach.
45) ____ _Parietal____ cells —Located primarily in the middle region of the gastric pits.
- Produce both hydrochloric acid (HCl) and intrinsic factor.
o HCl is responsible for the high acidity (pH 1.5 to 3.5) of gastric juice and is needed to
activate the protein-digesting enzyme, pepsin. The acidity also kills much of the
bacteria you ingest with food and helps to denature proteins, making them more
available for enzymatic digestion.
o Intrinsic factor -glycoprotein necessary for the absorption of vitamin B12 in the
small intestine
46) _Chief cells - Located primarily in the basal regions of gastric pits.
- Secrete pepsinogen , the inactive precursor (proenzyme) of the protease pepsin.
- Regulation of the Stomach: secretion of gastric juice, churning (gastric motility), and
relaxation of the pyloric sphincter to allow gastric emptying are regulated by extrinsic
and intrinsic control mechanisms.
47) _____Extrinsic____ control mechanisms:
Autonomic Nervous System: Control centers in the hypothalamus and brain stem regulate
gastric secretion through:
- Parasympathetic fibers release ACh to stimulate churning and secretion.
- Sympathetic fibers decrease churning and secretion.
48) Intestinal gastrin secreted by G cells of the small intestine stimulates churning
and secretion.
49) Cholecystokinin (CCK) secreted by I cells of the small intestine inhibits churning and
secretion.
50) Secretin secreted by S cells of the small intestine inhibits churning and secretion.
Intrinsic control mechanisms:
- Enteric neurons release ACh to stimulate churning and secretion.
51) Gastrin hormone produced by G cells in gastric pits stimulates churning and secretion.
52) Cephalic phase of gastric secretions: extrinsic mechanisms stimulated by brain
when thinking about food.
- Parasympathetic efferent fibers of vagus nerve (CNX) stimulate gastric secretion.
53) Gastric_ phase of gastric secretions: intrinsic mechanisms stimulated when a
bolus enters the stomach.
- Enteric neurons release Ach and G cells release Gastrin to stimulate gastric secretion.
54) Intestinal_ phase of gastric secretions: extrinsic mechanism stimulated when
chyme enter duodenum.
- Positive feedback mechanism- G cells in duodenum release intestinal gastrin to
stimulate secretion.
- Negative feedback mechanisms :
o Enterogastric reflex - decreases release of Ach by parasympathetic fibers to decrease
secretion. CCK and Secretin also inhibit secretion of gastric juice.
Objective: Describe the mechanisms regulating the secretion of saliva.
Salivary Glands: Three pairs of salivary glands located outside of the oral cavity secrete
saliva through ducts into the oral cavity.
55) ___Parotid______ glands: The largest salivary glands located lateral to the masseter.
- Produce thin watery saliva containing enzymes that initiates chemical digestion in the
oral cavity.
56) Sublingual glands: The most anterior salivary glands located in the floor of the
mouth inferior to the tongue.
- Produce thick, slippery mucous saliva.
57) Submandibular glands: Located posterior to the sublingual glands in the floor of the
mouth. Normally produce most of the saliva volume.
- Produce thick, slippery mucous saliva containing some digestive enzymes.
Regulation of Salivation: Salivatory Nuclei in the brain stem regulate autonomic efferent fibers
travelling to the salivary glands to regulate the volume of saliva production.
58) _Parasympathetic____ division of ANS – acetylcholine stimulates larger volume of saliva.
59) __ __Sympathetic_____ division of ANS – postganglionic fibers release norepinephrine
stimulating secretion of a smaller volume of mucous rich saliva to lubricate the mouth.
Salivary secretion is
regulated by two reflexes :
60) ____Simple______ reflex :
ingested food stimulates the
gustatory receptors in the
mouth which relay
information to the salivary
center leading to activation
of the parasympathetic
efferent fibers that release
acetylcholine in the salivary
glands.
61) _Conditioned reflex :
the thought of food
(processing in the cerebral
cortex) triggers the salivary
center leading to activation
of the parasympathetic efferent fibers that release acetylcholine in the salivary glands.
transport mechanism to cotransport sodium and glucose (or other water-soluble
nutrients) into cells. Transports glucose and galactose into enterocytes across
the apical surface.
- Monosaccharides leave enterocytes cells via facilitated diffusion and
enter the capillaries through intercellular clefts.
67) ___Glut______ (also known as SLC solute carrier proteins)- a family of membrane transport
proteins that enable facilitated diffusion of glucose and other water-soluble nutrients.
Objective: Describe the digestion and absorption of proteins.
68) ______Pepsin_ - enzyme that initiates chemical digestion
of proteins in the stomach by catalyzing the hydrolysis of
peptide binds within polypeptides to produce shorter
peptides.
- Produced by chief cells in gastric pits of the stomach
as the inactive precursor pepsinogen that is activated
by the low pH of gastric juice.
Pancreatic Proteases: Produced by acinar cells in the
pancreas. Secreted as inactive precursors that become
activated in the duodenum.
- Trypsinogen is activated in the duodenum by a brush
border enzyme producing Trypsin. Trypsin activates
other several other pancreatic protease precursors.
Absorption of Protein: Active transport mechanisms, primarily
in the duodenum and jejunum, absorb most proteins as their
breakdown products, small peptides and amino acids. The type
of carrier that transports an amino acid varies. Most carriers are
linked to the active transport of sodium.
Objective: Describe the mechanisms regulating secretion of
bile. Describe the enterohepatic circulation of bile salts. Describe the
function of bile salts and lipase. Describe the digestion and absorption of
lipids.
Production of Bile by the Liver: Bile salts are amphiphilic molecules
produced from cholesterol.
Cholic Acid 69) ____Hepatocytes___ -epithelial
cells of the liver constantly produce
bile acids (such as cholic acid) and
secrete them into bile canaliculi
that empty into bile ducts. Bile salts
are formed when bile acids react
with a base such as NaHCO 3.
Function of Bile: Bile is secreted into
the duodenum to emulsify fats: large
fat droplets are broken down into small
droplets called micelles with a
hydrophilic outer coating that enables
lipids to mix with an aqueous solution.
The hepatopancreatic sphincter (of
Oddi) at the base of the common bile
duct prevents bile from entering the
duodenum between meals. Bile moves
through the cystic duct into the gall
bladder where it is concentrated and
stored until it is needed.
Regulation of Bile Release:
- Acidic, fatty chyme entering the
small intestine stimulates release
of CCK to stimulate contraction
of the gallbladder and relaxation
of the sphincter to release bile
into the duodenum.
Recycling of Bile Salts: bile salts are reclaimed by the enterohepatic circulation: Most bile
salts are reabsorbed back into the blood in the ileum are returned to the liver by the hepatic
portal system. About 5% of bile salts are lost in feces.
Digestion of Fat:
70) _ Lipase_ - enzyme that catalyzes the hydrolysis of triglycerides to produce fatty acids and
monoglycerides.
Absorption of Fat: Lipid soluble nutrients are absorbed by simple diffusion. Fatty acids and
monoglycerides are reincorporated into triglycerides within the intestinal epithelial cells.
71) Chylomicrons - lipoprotein particles that are released into the lamina propria of
the mucosa and then enter the lymphatic lacteals.
