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Digestive Systems
•Campbell, 6th edition, Chapter 41
•Animal Nutrition
DIGESTIVE SYSTEMS
I. Digestion
II. Food Types
III. Feeding Mechanisms
IV. Compartmentalization
V. Stages of Food Processing
VI. Types of Digestive Systems
VII.Vertebrate Digestive System &
Regulation
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I. Digestion
Any nutritionally adequate diet satisfies
three needs:
• 1) fuel (chemical energy)
• 2) raw organic materials (biosynthesis)
• 3) essential nutrients
• Homeostatic mechanisms manage these
resources.
• Know difference between undernourished and
malnourished.
Essential Nutrients
• 8 Amino Acids:
tyrptophan methionine
valine
threonine phenylalanine leucine
isoleucine lysine
(histidine is essential for infants)
• Fatty acids: membrane structure
unsaturated; linoleic acid
• 13 Vitamins: coenzymes
Fat-soluble: vitamins A, D, E, & K
Water-soluble: B complex,Vitamin C
• Minerals: building materials, cofactors
Ca, P, S, K, Cl, Fe, Mg, Zn, I, Na,
Essential amino acids from a vegetarian diet.
I. Digestion
•Chemical and mechanical breakdown
of organic molecules into units small
enough for the body to absorb.
•These molecules provide:
1) Energy resources
2) Essential chemical elements
3) Raw materials for anabolism
II. Food Types
• Most animals are opportunistic
feeders:
1) HERVIVORES – feed on autotrophs
2) CARNIVORES – eat other animals
3) OMNIVORES – both
III. Feeding Mechanisms
Four major groups:
• suspension-feeders
Baleen whale, clam, oyster
• substrate-feeders
Leaf miner, earthworm
• fluid-feeders
Mosquito, leech
• bulk-feeders
Python, lion, bear
Suspension-feeding:
a baleen whale
Substrate-feeding: leaf miner
Fluid-feeding: a mosquito
Bulk-feeding:
a python
IV. Digestion is compartmentalized
• Intracellular – digestive enzymes are
secreted by cells & food is digested by
enzymes within the cell
– sponges (choanocytes)
• Extracellular - digestive enzymes are
secreted by cells into a digestive cavity
– cnidarians (both intracellular & extracellular)
Intracellular digestion in Paramecium
Extracellular digestion in a gastrovascular cavity.
V. Stages of Food Processing
• Ingestion
– the act of eating or ingesting
• Digestion
– process of breaking food into small
enough molecules for the body to absorb
• Absorption
– process of absorbing small molecules
from the digestive compartment into
bloodstream
• Egestion
– act of eliminating undigested materials
from the digestive compartment
VI. Types of Digestive Systems
• Channel network
– Porifera
• Incomplete (one-hole sac or
gastrovascular cavity)
– Cnidarians  Platyhelminthes
• Complete (two-hole sac or
alimentary canal)
– Nematoda  Chordata
VI. Types of Digestive Systems
• Incomplete Digestive Tract or
gastrovascular cavity
– functions in digestion (gastro)
& distribution (vascular)
– dual role: mouth = anus
– Cnidaria  Platyhelminthes
VI. Types of Digestive Systems
• Complete Digestive Tract
• Alimentary canal
– food moves in one direction
– tube is organized into specialized
regions
– Nematoda  Chordata
Alimentary Canal
Complete digestive tract of a mollusk
Earthworm
Digestive
Tract
The Human
Digestive System
VII. Vertebrate Digestive System
Oral cavity: mouth, tongue, teeth
• digestion begins here
• mastication= mechanical grinding
action of teeth
• food soften with saliva from salivary
glands
• bolus = moistened ball of food
Accessory Organs: Salivary Glands
Saliva
• contains mucus for lubrication
and swallowing (1 - 1.5 L per day)
• contains salivary amylasehydrolysis of amylose
• contains mucin, buffers,
antibacterial agents
• venom- secreted by the salivary
glands of some vertebrates
• Parotid, submaxillary, sublingual
salivary glands
Pharynx (throat region)
• swallowing is accomplished by the
pharynx
• an intersection leads to both
esophagus and trachea; cartilage flap,
epiglottis, covers the glottis &
prevents choking (Figure 41.12)
• passes bolus from mouth to esophagus
• originates from a groove in the floor
of the lungs
• acts as a muscular pump in some
worms (proboscis w/ pharynx)
From mouth to stomach: the swalling
reflex and esophageal peristalsis
Esophagus
• 25 cm long
• muscularized passageway to
stomach
• peristalsis begins here
rhythmic waves of contraction
by smooth muscles in the wall of
the canal
• ruminants (cud-chewers),
ruminating pouches, chambers of
esophagus where fermentation
occurs (cows produce 60 L saliva
per day & burp 2 L gas/minute)
Layers of the Digestive Tube
From esophagus to anus:
• 1) mucosa – lines tube; glandular
epithelium; villi; contains some
smooth muscle; produces mucus
• 2) submucosa – connective
tissue, nerves, blood vessels,
lymph
• 3) muscularis externa – inner
circular muscle & outer
longitudinal muscle
• 4) serosa – outer fibrous coating
or visceral peritoneum
Cardiac sphincter
• ringlike valve of smooth
muscle that functions like a
drawstring
• controls entrance of food
into stomach from
esophagus
Stomach
• collapsible muscular bag
• suspended in abdominal cavity
by folds of peritoneum called
mesentery
• functions in mechanical mixing
of food with HCl and enzymes
• fully distended, human stomach
holds 2-4 liters of food
3 regions of the Stomach:
• 1) cardiac – upper
• 2) fundus – deep, storage
• 3) pylorus – lower, empties
into small intestine
Stomach
• glucose and alcohol are absorbed
in stomach
• takes about 4 hours to empty
stomach
•
chyme – semi-liquid mass; may
back up in gastric pits and cause
ulcers
Stomach
• Rugae – folds of stomach with deep
pockets, or gastric pits, contain
• 1) mucous cells – secrete mucus for
protection
• 2) parietal cells – secrete HCl (pH 1.5-2.5)
HCL kills most bacteria & living cells;
erodes plant materials; initiates change of
pepsinogen to pepsin
• 3) chief cells – secrete pepsinogen
(inactive) which is converted by HCL into
active pepsin
Chemical Digestion
Digestion in Stomach
Digestion is regulated by hormones and the
Autonomic Nervous System
• Stomach hormone: Gastrin
• produced in the presence of proteincontaining food in the stomach
• stimulates the release of gastric juices
and muscular contractions of stomach &
intestine
• Blood sugar is regulated by pancreatic
hormones insulin and glucagon
Accessory Digestive Organs
Ruminant digestion
Enzymatic
Digestion in the human digestive system
Digestion in Stomach
Stimulation of epithelial cells of
stomach mucosa increases secretion
of gastric juice:
•
•
•
•
•
1)
2)
3)
4)
5)
mucous
HCl
pepsinogen
renin (hydrolyzes milk)
water
Pyloric sphincter
• sphincter separating the stomach
and small intestine
• regulates the passage of material
from stomach to small intestine
Small Intestine
• digestion is completed here
• most enzymatic hydrolysis and
adsorption occurs here
• surface area of small intestine is
300 m2; about the size of a double
tennis court
• mucosa has fingerlike projections,
villi, which extend into the lumen
•
the villi have microvilli (cytoplasmic
projections on the surface of
epithelial cells) Figure 41.15
Structure of the small intestine
Small Intestine
• Small intestine is 20-23’ long:
– 1) duodenum 8-10”
– 2) jejunum 8’
– 3) ileum 12’
• duodenum – most active in digestion
• jejunum & ileum – absorption
• study Figure 41.13: Enzymatic digestion
Digestion in Small Intestine
Digestion is regulated by hormones and the Autonomic
Nervous System
• Duodenal hormones:
• 1) secretin – stimulates pancreas & liver to
secrete alkaline fluids
• 2) cholecystokinin – triggers release of enzymes
from pancreas and gall bladder (amylase, lipase,
deoxyribonuclease, protease, etc.)
Activation of protein-digesting enzymes in
the small intestine
Accessory Organs: Liver
• largest internal organ = 3 lb chemical factory
• processes food by the Hepatic Portal Vein
delivered from digestive tract
• variable nutrient levels in HPV while level in
systemic circulation remains constant
• though the liver performs many functions, cells
of the liver function without division of labor
Liver Functions:
• 1) maintenance of a constant glucose level in
bloodstream
• 2) detoxification of drugs & alcohol
• 3) production of bile
• 4) destruction old RBC & converts hemoglobin
to bilirubin
• 5) production the plasma protein prothrombin
• 6) regulation of cholesterol & other fats
Bile
• produced by liver
• enters small intestine via duct from gall
bladder
• fat emulsifier
• contains bilirubin (breakdown hemoglobin)
• jaundice occurs when bilirubin is not
removed
• neutralizes stomach acid, contains sodium
bicarbonate
Accessory Organs:
Pancreas
• clusters of cells in pancreas called
Islets of Langerhans secrete pancreatic
hormones:
• 1) Glucagon – increases blood sugar by
converting stored liver glycogen to glucose
• 2) Insulin – decreases blood sugar level by
converting glucose to glycogen for storage
• produces alkaline fluid to neutralize
stomach acid
Homeostatic
regulation of
cellular fuel
Accessory Organs: Gall Bladder
• stores bile produced by the liver
• underneath side of liver
• balloon-like structure
Ileo-cecal valve
• valve between small and large
intestine
• regulates the passage of
material from small to large
intestine
Large Intestine or colon
• main function is reabsorption of
H2O, sodium, and minerals,
vitamins
• Escherichia coli
symbiotic bacteria that escape
digestion & absorption in the
small intestine; chief source of
vitamin K
• 7 L of water per day enter
stomach and large intestine
Histology of intestine
Large Intestine
• fecal material – unusable material left in
the digestive tract
• mostly water, dead bacterial cells,
cellulose fibers, other indigestible
materials, lubricated by mucus
• elimination through the anus after brief
storage in the rectum
• elimination of feces = egestion or
defecation; NOT an excretory
process since fecal material was in
the intestine, but never in the body
proper.
Appendix/ Cecum
•
•
•
•
•
•
blind pouch off large intestine
evolutionary remnant
may become inflamed, irritated, infected
plays no role in human digestion
may be involved in the immune response
CECUM – well developed in herbivorus
vertebrates; contains microbes that
breakdown cellulose