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Overview
Of
Digestion
Chapter 14
The Digestive System and
Body Metabolism
20 Cool Things
You Don’t Know
About the
Digestive System
Pill Cam
Digestive System
 Takes
in food, breaks it down
into nutrient molecules and
absorbs them into the
bloodstream, then rids the
body of indigestible remains
Anatomy of Digestive System
 Alimentary canal –
digests
food and absorbs digested
fragments through its lining into the
blood
GI, tract - continuous hollow tube:
mouth, pharynx, esophagus, stomach,
small and large intestine
 Accessory digestive organs
assist: teeth, tongue, glands
-
Mouth (Oral Cavity)
 Food enters
Mucus membrane-lined cavity
lips, cheeks, hard palate (anterior roof), soft
palate (posterior roof)
uvula - fingerlike projection of soft palate
Mouth
Tongue - attached to hyoid bone and styloid
processes of skull
papillae containing taste buds on surface
 Frenulum - secures tongue and limits its
posterior movements
 As food enters, it is mixed w/ saliva by tongue
and chewed and swallowed

Taste
Salivary Glands - 3 pair
 Parotid
glands - anterior to ears
 mumps is inflammation of parotid
glands
 Submandibular and sublingual glands empty secretions into mouth through
ducts
Saliva
 Product
of salivary glands, mixture of
mucus and serous fluids
 mucus moistens and binds food
together into a mass (bolus)
 serous part contains salivary amylase
(enzyme for starch digestion)
Polls
Everywhere
Teeth
Masticate (chew)
 Deciduous (baby or milk) teeth - first set;
formed from 6 months to 2 years
 Permanent teeth - cause baby teeth to fall
out b/t 6 and 12
 32 permanent teeth
 3rd molars (wisdom teeth) form b/t 17 and
25; sometimes absent or impacted in
jawbone and must be surgically removed

Teeth by shape/function
Incisors - chisel-shaped, cutting
 Canines - fanglike, tearing/piercing
 Premolars (bicuspids)
 Molars - broad crowns w/ rounded tips, grinding

Pharynx
 Nasopharynx
(respiratory),
oropharynx (potesterior to oral
cavity), and laryngopharynx
(continuous w/ esophagus)
 Peristalsis: Alternating
contraction of muscles propel
food into esophagus
Peristalsis
Esophagus


Conducts food from pharynx through diaphragm
to stomach
25 cm long
Walls of Alimentary Canal
Mucosa - innermost layer; moist membrane
 Submucosa - blood vessels, nerve endings,
lymph
 Muscularis externa - inner circular, outer
longitudinal smooth muscle
 Serosa - outermost layer

Stomach





Peristalsis
C-shaped, left side, nearly hidden by liver and
diaphragm
cardioesophageal sphincter - food enters from
esophagus
fundus - expanded part
body – midportion
3rd oblique layer in muscularis externa to move,
churn, mix, and pummel food
 chemically breakdown proteins
Stomach
Pylorus - funnel-shaped, terminal
 Pyloric sphincter - goes to small intestine
 25 cm long
 when full, holds 4 liters of food
 empty - collapses into folds (rugae)

Rugae on Dog Stomach
Stomach
Mucosa has gastric pits which lead into gastric
glands that secrete gastric juice
 chief cells - produce protein-digesting
enzymes (pepsionogen)
 parietal cells - produce HCl
 Chyme is produced

Stomach - Food Breakdown
Secretion of gastric juice by sight, smell, and
taste of food
 presence of food and falling pH stimulate
release of hormone gastrin that makes stomach
produce enzymes, mucus, & HCl
 2-3 liters gastric juice per day

Stomach: Food Propulsion
Peristalsis in lower half, and contractions squirt
3 ml of chyme into small intestine
 takes 4 hrs for stomach to empty
 Irritation (food poisoning) may activate vomiting

Small Intestine
Major digestive organ
 Muscular tube extending from pyloric
sphincter to ileocecal valve
 average length: 2 m (6 feet)
 Hangs from coils suspended by mesentery
 Large intestine encircles and frames it

Small Intestine - 3 subdivisions
Duodenum - curves around head of
pancreas
 Jejunum - extends from duodenum to ileum
 ileum - terminal part that joins large intestine
at ileocecal valve

Small Intestine
Chemical digestion begins
 Small amount of food processed at a time controlled by pyloric sphincter
 Pancreatic enzymes from pancreatic duct
and bile from bile duct enter duodenum

Small Intestine - 3 structures
that increase absorption
Microvilli - tiny projections that give fuzzy
look (brush border)
 Villi - fingerlike projections that give velvety
appearance
 Circular folds - deep folds of both mucosa
and submucosa

Small Intestine: Food
Breakdown and Absorption
Takes 3-6 hours
 By end, digestion is complete and most
absorption has occurred
 Microvilli have brush border enzymes to break
down sugars and complete protein digestion

Food Breakdown and
Absorption
Pancreatic juice digests starch, proteins, fats,
and nucleic acids; contains bicarbonate to
neutralize chyme
 When chyme enters, it stimulates hormones
secretin and cholecystokinin to release bile

Food Breakdown and
Absorption
Bile is necessary for absorption of fats and fatsoluble vitamins K,D,A
 At end, all that remains is water, indigestible
food and bacteria which enters large intestine
 Food propulsion - peristalsis

Large Intestine
 Larger
in diameter, shorter
in length (1.5 m)
 Extends from ileocecal
valve to anus
 Dries out indigestible food
by absorbing water,
eliminates residue as feces
Large Intestine Subdivisions
Cecum - saclike, first part
 Appendix - wormlike structure
hanging from cecum; ideal
bacteria location - appendicitis
 Colon - ascending, transverse,
descending, and sigmoid
 Rectum
 Anal canal - ends in anus which
opens to exterior; has external
voluntary sphincter and internal
involuntary sphincter

Large Intestine
 Lots
of goblet cells to produce
mucus to act as lubricant to
ease passage of feces
Large Intestine: Food
Breakdown and Absorption
Residue is there 12-24 hours
 Bacteria metabolize nutrients and release
gases (methane, hydrogen sulfide)
 500 ml of gas produced each day

Food Breakdown and
Absorption
Absorption limited to vitamins, some ions, and
most of remaining water
 Feces - solid product delivered to rectum;
undigested food residue, mucus, bacteria, and
some water


Large Intestine: Propulsion and
Defecation
Peristalsis and mass movements (long, slowmoving, powerful contractile waves that move
over colon 3-4 times daily to push contents
toward rectum)
occur after eating; fiber increases strength of
contraction
Propulsion and Defecation
When feces in rectum, defecation reflex causes
rectum to contract and anal sphincters to relax
 Diarrhea - food rushes through before water is
absorbed, can result in dehydration and
electrolyte imbalance

How fast food
Travels thorough.
Propulsion and Defecation

Constipation - food residue remains too
long and too much water is absorbed; due
to lack of fiber, poor bowel habits, or
laxative abuse
A constipated body
Other Accessory Digestive
Organs –
Pancreas
Soft, pink, triangular gland extending from
spleen to duodenum
 produces enzymes that break down food and
neutralize acidic chyme from stomach,
produces hormones insulin, glucagon

Liver

Liver - largest gland in body; under
diaphragm on right
4 lobes
produces bile which leaves liver
through common hepatic duct
Gallbladder


Small, thin-walled green sac in the inferior surface
of liver
When digestion is not occurring, bile is stored and
concentrated by removal of water
 bile stored too long, it crystallizes forming
gallstones
 Yellow-green, watery solution of bile salts, bile
pigments (bilirubin), cholesterol, phospholipids,
and electrolytes
 bile salts emulsify fats to provide more surface
area
Disease: Jaundice
 Bile
pigments enter bloodstream
 Can result from hepatitis (inflammation of
liver from viral infection of contaminated
water or blood transfusion) or cirrhosis
(severe damage from drinking excess
alcoholic beverages)
Nutrition and Metabolism
 Most
foods used as metabolic
fuels (transformed into ATP);
some nutrients build cellular
molecules
 Energy value measured in
kilocalories (kcal) or Calories (C)
Nutrition
 Nutrient
- substance in food used
to promote normal growth,
maintenance and repair
 Carbohydrates, lipids, proteins bulk of food; vitamins, and
minerals in minute amounts
.
 Water
- 60% of volume of food
 Most foods are combination of
nutrients from 5 food groups
(grains, fruits, vegetables,
meats, and milk products)
Obesity Rates
in the US.
Dietary Sources of Major
Nutrients - Carbohydrates
 From
plants except lactose
and glycogen in meats
 sugar - fruit, sugar cane, milk
 starch - grains, legumes, root
vegetables
 cellulose - most vegetables
Lipids
 Neutral
fats: saturated in
animal products, unsaturated
in seeds, nuts, vegetable oils
 Cholesterol - egg yolk, meats,
and milk
 Phospholipids
Proteins
 Animal
products, eggs, milk
 Amino acid polymers
 legumes, nuts, and cereals are
low in one or more essential
amino acids
Vitamins
 Organic
nutrients, small amounts
 No one food contains all required
vitamins, need balanced diet
 Most function as coenzymes: act
w/ enzymes for task
Minerals
 Requires
adequate supplies of 7:
Ca, P, K, S, Na, Cl, and Mg; trace
amounts of others
 Fats/sugars have none, cereals
and grains poor sources
 In veggies, legumes, milk, meats
Metabolism
 All chemical reactions necessary
to maintain life
 Catabolism - substances broken
down, energy released and
captured to make ATP
 Anabolism - larger molecules built
from smaller ones
Metabolism
 Carbohydrates
(glucose) broken down to make ATP
 Fats - build cell membranes,
myelin sheaths, insulate, ATP
 Proteins - structural materials
hoarded by body cells
Carbohydrate Metabolism
 Carbs - preferred fuel to produce
ATP from glucose (blood sugar):
energy from bonds broken binds
phosphate to ADP to make ATP
 Carbon atoms leave as CO2 and
hydrogen combines w/ oxygen to
make water
Carbohydrate Metabolism Homeostasis of blood glucose
 Hyperglycemia
- high levels;
excess stored as glycogen and
converted to fats
 Hypoglycemia - low levels; liver
breaks down stored glycogen
and releases glucose to blood
Fat Metabolism
 Liver - make ATP, synthesize
lipoproteins, clotting protein and
cholesterol for membranes or
steroid hormones
 Form myelin sheaths and fatty
cushions around organs
 Most concentrated form of enegy
Fat Metabolism
 To be used for ATP synthesis, it
must be broken down into acetic
acid; when not enough glucose,
acetone accumulates in blood
making it acidic (acidosis/ketosis)
no carb diets, diabetes, and
starvation
Protein Metabolism
 Proteins - bulk of cell structures;
broken down into amino acids for
enzymes, membranes
 Cells use ATP to actively transport
amino acids (8 of the 20 are not
made by cells - essential amino
acids)
Protein Metabolism
 Amino
acids make ATP when
protein is in excess or no fats or
carbs
amine groups are removed as
ammonia which is toxic so it
combines w/ CO2 to form urea
Central Role of Liver
 Manufactures bile, detoxifies
drugs and alcohol, degrades
hormones, makes substances
vital to body, metabolism
 We have more liver tissue than
needed, so if damaged, it
regenerates rapidly and easily
General Metabolic Functions
 Liver
maintains blood glucose
levels
 After high carb meal, glucose is
removed from blood and
converted to glycogen
(glycogenesis) and stored in liver
General Metabolic Functions
 As
body cells remove glucose
from blood, liver breaks down
stored glycogen
(glycogenolysis)
 gluconeogenesis - make
glucose from fat and protein
General Metabolic Functions
 Hormones insulin, thyroxin, and
glucagon control blood sugar
 Fats are oxidized for energy,
broken down into acetic acid or
stored as fat reserves
 Makes cholesterol and secretes
its breakdown products as bile
General Metabolic Function
 Albumin - most abundant protein;
holds fluids in bloodstream
insufficient albumin causes fluid
to go from blood to tissues
(edema)
Synthesize amino acids and
detoxify ammonia
Cholesterol Metabolism and
Transport
 Cholesterol - structural base for
steroid hormones and vitamin D;
major building block of plasma
membranes
 15% from diet, 85% made by liver
 Broken down and secreted in bile
salts, which leave as feces
Cholesterol Metabolism
 Insoluble in water, so transported
bound to lipid-protein complexes lipoproteins
 Low-density lipoproteins (LDLs) transport cholesterol/lipids to body
cells; if too much deposited on
arteries - “bad lipoproteins”
Cholesterol Metabolism
 High-density lipoproteins (HDLs) transport cholesterol from tissue
cells to liver for disposal in bile;
high levels are good
 Both types necessary; ratio
determines whether cholesterol
will go to arterial walls
Body Energy Balance
 When
fuel is burned, it consumes
O2 and generates heat
 Energy intake = total energy
output (heat + work + energy
storage)
Body Energy Balance
 Energy
intake - liberated
during food oxidation
 Energy output - lost as heat
(60%) + used to do work
(driven by ATP) + stored as fat
or glycogen
Regulation of Food Intake
 When energy intake and energy
outflow are balanced, body weight
is stable
 Food intake controlled by rising or
falling blood levels of nutrients,
hormones, body temp. and
psychological factors
Basal Metabolic Rate
 Carbs & proteins yield 4 kcal/g,
fats yield 9 kcal/g
 Basal metabolic rate (BMR) amount of heat produced by body
per unit time at rest; energy supply
for breathing, heartbeat, and
kidney function
Basal Metabolic Rate
 Avg. adult has BMR of 60-72
kcal/hr; influenced by surface
area, gender, age, and thyroxin
production (more thyroxin
produced by thyroid gland, higher
O2 consumption and ATP use and
metabolic rate)
Basal Metabolic Rate
 Hyperthyroidism - excessive rate,
lose weight despite increased
hunger and food intake, bones
and muscles weaken
 Hypothyroidism - slow rate,
obesity, diminished thought
process
Total Metabolic Rate
 When
active, more glucose
must be oxidized to provide
more energy for activities
 Total Metabolic Rate (TMR) total amount of kcal body must
consume to fuel all activities
Total Metabolic Rate
 When total calories = TMR,
weight is constant
 If eat more, excess calories
appear as fat deposits
 If active w/o enough food, break
down fat reserves and even
tissues to satisfy TMR
Body Temp Regulation
 Heat warms tissues and
blood keeping them at
homeostatic temps
 Reflects balance b/t heat
production and heat loss,
controlled by hypothalamus,
regulated b/t 96-100ºF
Heat-Promoting Mechanisms
 When cold, heat is conserved
by vasoconstriction of blood
vessels and shivering makes
blood route to vital body organs
causing temp of skin to drop
if extended, skin cells w/o O2
die leading to frostbite
Heat-Promoting Mechanisms
 Hypothermia
- extremely low body
temp from prolonged exposure to
cold; vital signs decrease, person
becomes drowsy and can
progress to coma and death as
metabolic rate stops
Heat Loss Mechanisms
 Most loss occurs through skin by
radiation (when body temp
increases, blood vessels dilate
and heat radiates off surface) or
evaporation (too hot - perspiration
off skin’s surface; effective unless
humid)
Heat Loss Mechanisms
 Hyperthermia
(elevated body
temp) depresses hypothalamus
and positive-feedback cycle
occurs: soaring body temp
increases metabolic rate, which
increases heat production
Heat Loss Mechanisms
 Heat stroke - skin hot and dry fatal unless immersed in cool
water and given fluids
 Heat exhaustion - collapse during
vigorous activity due to excessive
loss of fluids (dehydration),
causing low blood pressure, fast
heart rate and cool, clammy skin
Heat Loss Mechanisms
 Fever - controlled hyperthermia results from infection, cancer, or
allergies; pyrogens are released to
hypothalamus that set thermostat
at higher level
chills - reset lower - sweat
too high - protein denatures
Developmental Aspects
 5th week - alimentary canal forms
 cleft palate/lip - child unable to
suck properly
 tracheoesophageal fistula connection b/t esophagus and
trachea - causes drool, cyanosis
during feedings
Development Aspect
 Cystic fibrosis - blockage of
pancreatic ducts so that fats and
fat-soluble vitamins are not
digested or absorbed
 PKU - inability of tissue cells to
use phenylalanine (amino acid)
causes brain damage
Developmental Aspects
 Newborn: rooting & sucking reflex
 Appetite decreases in elementary
age and increases in adolescence
 Gastroenteritis - inflammation of
GI tract due to contaminated food
 Appendicitis - common in teens
Developmental Aspects
 Middle
age - metabolic rate
decreases 5-8% every 10 yrs
ulcers & gallbladder problems
 Old age - activity of GI tract
declines, taste and smell decrease
cancer of stomach and colon