Digestive system
The digestive system includes the digestive tract and its accessory organs, which process
food into molecules that can be absorbed and utilized by the cells of the body. Food is
broken down, bit by bit, until the molecules are small enough to be absorbed and the
waste products are eliminated. The digestive tract, also called the alimentary canal or
gastrointestinal (GI) tract, consists of a long continuous tube that extends from the mouth
to the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, and
large intestine. The tongue and teeth are accessory structures located in the mouth. The
salivary glands, liver, gallbladder, and pancreas are major accessory organs that have a
role in digestion. These organs secrete fluids into the digestive tract.
Food undergoes three types of processes in the body:
o
o
o
Digestion
Absorption
Elimination
Digestion and absorption occur in the digestive tract. After the nutrients are absorbed,
they are available to all cells in the body and are utilized by the body cells in metabolism.
The digestive system prepares nutrients for utilization by body cells through six
activities, or functions.
Ingestion
The first activity of the digestive system is to take in food through the mouth. This
process, called ingestion, has to take place before anything else can happen.
Mechanical Digestion
The large pieces of food that are ingested have to be broken into smaller particles that can
be acted upon by various enzymes. This is mechanical digestion, which begins in the
mouth with chewing or mastication and continues with churning and mixing actions in
the stomach.
Chemical Digestion
The complex molecules of carbohydrates, proteins, and fats are transformed by chemical
digestion into smaller molecules that can be absorbed and utilized by the cells. Chemical
digestion, through a process called hydrolysis, uses water and digestive enzymes to break
down the complex molecules. Digestive enzymes speed up the hydrolysis process, which
is otherwise very slow.
Movements
After ingestion and mastication, the food particles move from the mouth into the
pharynx, then into the esophagus. This movement is deglutition, or swallowing. Mixing
movements occur in the stomach as a result of smooth muscle contraction. These
repetitive contractions usually occur in small segments of the digestive tract and mix the
food particles with enzymes and other fluids. The movements that propel the food
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particles through the digestive tract are called peristalsis. These are rhythmic waves of
contractions that move the food particles through the various regions in which
mechanical and chemical digestion takes place.
Absorption
The simple molecules that result from chemical digestion pass through cell membranes of
the lining in the small intestine into the blood or lymph capillaries. This process is called
absorption.
Elimination
The food molecules that cannot be digested or absorbed need to be eliminated from the
body. The removal of indigestible wastes through the anus, in the form of feces, is
defecation or elimination.
Digestive system
 Concerned with prehension, mastication, Digestion, absorption and expulsion of
undigested portion.
Organs making digestive system
• Oral cavity.
• Pharynx.
• Oesophagus.
• Stomach / forestomach.
• Intestine – small & large.
• Accessory glands.
Oral cavity
• includes lips, teeth, tongue, cheek and cheek muscles and major salivary glands (
empties to buccal cavity)
• Lips for prehension.
• Tongue – prehension, mastication, taste, ingestion (swallowing), during chewing,
it moves
Dentition of farm animals.
• Teeth- types
incisors
cutting
canine
tearing flesh
premolar
grinding
molar
grinding
• Herbivores have a more specialized digestive system than that of a carnivore
because it is more difficult to digest vegetation than meat.
• The teeth are flat so that grass and plant material can be ground down, rather than
the sharp teeth of carnivores designed to tear flesh.
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Glands – salivary gland
- parotid (in front of ears) :serous
- Mandibular (below lower jaw): mucous
Sublingual (below tongue): mixed.
- Tongue- taste, prehension.
Pharynx.
• The common passage for the food and air.
• Swallowing & breathing mechanism
Nasal cavity
Oesophagus
Trachea
Mouth
Epiglottis
Pharynx
oesophagus
Nasal cavity
Oral cavity
Epiglottis
Trachea
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Esophagus.
• Is a muscular tube Continuation of pharynx joins stomach or forestomach.
• Separated from stomach by the esophageal sphincter.
• At neck it is seen towards the left side of trachea.
Stomach
Non-ruminant or Monogastric stomach
Are single-stomach animals. They have one stomach (mono=one, gastric=stomach).
Muscular gland lined sac that receives ingesta from the esophagus and conducts both
physical and chemical digestion.
• Stomach lies behind the left side of the diaphragm.
• Is pear shaped.
• Divided into four regions- oesophagal, cardiac, fundic and pyloric.
•
The thick wall of stomach has gastric glands (cardiac, pyloric and which aids in
digestion.
• Presence of folding that receives ingesta from the oesophagus and conducts both
physical and chemical Gastric glands- enzymes (pepsin), HCL and mucus.
Functional and non-functional caecum
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•
•
•
In the stomach, food is mixed with pepsin and HCL to help to break down solid
particles.
Horses is not ruminant as the oesophagus leading to stomach has the sphincter
valve which is very strong, it is almost impossible for horse to vomit.
The caecum in horse has got the similar function like that of ruminant stomach, as
the caecum do contain the micro-organisms which helps in fermentation.
Relation of stomach with oesophagus
Ruminant (or multi-stomached) system
• Rumination - the regurgitation, rechewing and reswallowing of ingested feed
• Cud - mass of regurgitated ingesta; bolus
• Process of rumination
– regurgitate bolus from rumen
– rechew and reinsalivate
– reswallow
– repeat with another bolus
• multiple stomach forestomach,
• cattle, sheep, goat, yak, buffalo
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Four compartments are
–
–
–
–
Rumen
Reticulum
Omasum
Abomasum
largest
stomach
Rumen
•
•
Large muscular sac, subdivided into
Smaller sacs by muscular pillars
Large fermentation vat; also called the "paunch" Extends from the diaphragm to
the pelvis filling the left side of the abdominal cavity.
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•
Ruminants evolved to consume and subsist on roughage - grasses and shrubs built
predominantly of cellulose.
• The rumen is a fermentation vat par excellance, providing an anaerobic
environment, constant temperature (Temperature = 39oC (103oF)) and pH,
and good mixing.
• Its purposes is to store large quantities of feed, keep the feed mixing by strong
contractions, and to provide a suitable environment for micro-organisms.
• The “finger-like” projections lining (papillae) the bottom and sides of rumen wall
absorbs the fatty acids produced by micro-organisms during fermentation.
Functions of Microorganisms
• digest roughages to make Volatile Fatty Acids (waste product, but are the
primarily source of energy for animals).
• make protein (through kerb’s cycle)
• Make vitamins K and B complex
(Very similar to cecum of hind gut fermentors)
• The function of the rumen is to house microorganisms.
Rumen
• saturated with gasses
• constant motion.
Reticulum
• It is the most cranial compartment.
• It is also called as the “honey-comb”
•
•
•
•
It is located immediately behind the diaphragm places it almost in apposition to
the heart.
It aids to help bring bolus of feed back up to the mouth for rechewing.
houses microorganisms
It also serves as a receptacle for heavy foreign objects that animal takes.
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“Hardware Disease”
• When metal object such as wire or a nail is swallowed and punctures the
reticulum wall. This condition may prove lethal for two reasons
– The bacteria and protozoa can contaminate the body cavity resulting in
peritonitis.
– The heart and diaphragm may be punctured by the object causing failure
of these tissues.
Omasum
• Once the feed has been reduced in size by chewing and digestion by bacteria and
protozoa, it passes to spherical-shaped organ the third compartment called the
Omasum.
• It is located to the right of the rumen and reticulum just caudal to the liver.
• It is also called as “many-piles”, “bookstomach”.
• Inside of Omasum is thrown into broad longitudinal folds or leaves. Got full of
folded tissue therefore the name.
• The leaves got small blunt papillae on them which absorbs FA, water and
electrolytes like K and Na beside it assist in grinding roughage before it enters the
next compartment
T/S of ruminant stomach
• A = Abomasum
B = Rumen &
Reticulum
C = Omasum
D = Liver
Abomasum
• It is the true stomach in ruminant, located ventral to the omasum and extends
caudal on the right side of rumen.
• Got at the terminal part of abomasum sphincter (thickening of circular smooth
muscle fibers) at the junction of the stomach and small intestine.
• Similar to the stomach of mono-gastric animals.
• The abomasum contains many folds to increase its surface area.
• The pH coming to abomasum is around 6.0 but it is quickly lowered to about 2.5
by the acid creating the suitable environmental condition for the enzymes.
• The chief enzymes secreted are pepsin.
• It has mucus and HCL that aids in digestion.
• Here actual digestion takes place.
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Summary for ruminant stomach.
Chamber
Function
a) Rumen
(part of forestomach)
b) Reticulum
(part of forestomach)
c) Omasum
(part of forestomach)
d) Abomasum
mechanical and chemical breakdown
of food; breakdown of food by
microbes; production of volatile fatty
acids; absorption of volatile fatty
acids, lactic acid, ammonia, inorganic
ions and water
=enzymatic digestion
Young RuminantsEssentially they are non-ruminants.
• Rumen and reticulum are non-functional
abomasum (secretes rennin for milk to coagulate) is largest part of stomach.
• dry feed stimulates reticulorumen
• Esophygeal groove allows milk consumed to go through rumen and reticulum to
the intestines.
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Ratio of Rumen/Reticulum to Omasum/Abomasum according to age
Age
R/R
O/A
Birth
1
3
6 months 4
8-10
1 year
Rumen Size
Species
1
1
Maximum
Normal Content
1000 lb cow
1000 lb ewe
~55-60 gallons
~5-10 gallons
25-30 gallons
3-5 gallons
Small intestine
• Once the feed has enter disheveled by the enzymes and by the high acid content
of the stomach it is moved along and stopped in the small intestine, which is tiny
and very long.
• The feed is semi-solid.
• As the feed enters the small intestine, it mixes with secretions from pancreas and
liver which elevate the pH of the
• Digesta from 2.5 to between 7-8.
• Enzymes digestion and maximum absorption takes place.
• It is made up of 3 parts viz duodenum, jejunum and ileum.
• It contains finger-like projection called villi (villus) that aid in absorption of
digested food.
Function s of small intestine.
 Digestion of proteins.
 Digestion of carbohydrates.
 Digestion of fats.
 Absorption of the end products of digestion.
 There are three digestive juices are pancreatic juice, bile, and intestinal juice.
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Three parts of small intestine.
• Duodenum – 1st part attached to pylorus, liver (which secretes bile salt and
pigment) and pancreas (which secretes enzymes for the digestion) open. most
digestion occurs here.
• Jejunum – 2nd part, some digestion and some absorption occur.
• Ileum – 3rd part, joins colon. mostly absorption.
•
Digestive tract of the mono-gastric mammals.
Digestive enzymes in intestine.
Enzyme
Function
Source
Trypsin
Chymotrypsin
carboxypeptides
digest proteins
secreted from pancreas
pancreatic amylase
Digest CHO
lipases
Digest lipids.
disaccharides
digests carbohydrates
secreted from small
intestine
dipeptidases
digest peptides
secreted from small
intestine
secreted from pancreas
Bile juice.
• made in liver
• stored in gall bladder
• active in the small intestine
• emulsifies fat to aid in digestion which simply means that bile helps pancreatic
lipase I the breakdown of fats in feed.
• So no enzymes.
Large intestine
• the terminal part of ileum joins the caecum (horse), colon (dog) or caecum and
colon (in pig and ruminant).
• The caecum, colon and rectum makes up the rest digestive system.
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•
•
It is shorter, but larger I diameter than the small intestine.
Its main function is absorption of water and reservoir of waste materials.
Caecum
• Blind sac” junction of small & large intestine.
• It is also referred as “blind gut”.
• I case of horse ad rabbits, caecum is very essential in the digestion of fibrous
feeds.
– Got microbes that breaks down feed that was not digested I the small
intestine.
Colon
•
•
•
Food may reach here in a little ad stay here for appro.60 hours.
Microbial digestion continues, and most of the nutrients made through microbial
digestion are absorbed here.
In addition to the vitamins and FA, water is also absorbed resulting to fecal
formation, which goes to rectum.
Intestine of cattle
Rectum
Transverse colon
Caecum
Descending colon
Small Intestine
Stomach
Ascending colon
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Organ
Function
Lips
Ingestion and fragmentation of food
Teeth
Fragmentation of food
Tongue
Fragmentation and swallowing
Salivary Glands
Fragmentation and moistening of
food; swallowing
Esophagus
Passage of food from oral cavity to
the stomach
Stomach
Completion of fragmentation and
beginning of digestion
Small Intestine - duodenum
Digestion; emulsificaton of fats by
enzymes from the pancreas and bile
from the liver
Completion of digestion and
absorption
Small Intestine - jejunum & ileum
Large Intestine- cecum
Absorption of water from liquid
residue.
Large Intestine - colon
Absorption of water from liquid
residue
Large Intestine - rectum
Storage of feces prior to defecation
Anus
Route for defecation of feces outside
the body
Digestive system of chicken
The central fact of being bird is that everyone wants to eat. Almost every bird is
on predators menu. Therefore they need maximum energy for survival.
Consequently their anatomy reflects adaptations for evasion and escape.
To be able to “eat and run” or “eat and fly” so that for them to “eat now, digest
later”, after they get away from danger.
Oral cavity.
-includes beak, tongue, accessory glands.
The chicken can discern shape, size and colour and can feel material with its tongue
and mouth.
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Mouth/Beak –
The presence of hard beak is most adaptive to birds.
Made of keratin.
Gather and break down feed, along with the takes stones.
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

Tongue



The tongue is heavily keratinized.
They have little sense of taste. Their tongue has a bone running the full
length.
Their distal end of this bone is replaced by with hyaline cartilage.
Teeth
As stated earlier Birds require maximum energy.
There is no tooth.
No teeth (hence the expression “scarce as hen’s teeth”, this means that
they have to swallow whatever food they take in whole. thus fowl cannot
consume particles of feed that are too large enter their oesophagus.
• With the feed they also take tiny rocks which aids in digestion.
At the proximal end of buccal cavity got accessory glands.



Oesophagus






Tube that connects the oral cavity and the stomach.
The muscular walls produce wave-like contractions (peristalsis) that help
propel food from the oral cavity in the stomach.
The oesophagus is larger in them.
At the end of oesophagus there is out-pocketing or a diverticulum called
as “Crop”.
Crop- is a temporary storage of food. And for it is nice example of
specialization of the avian digestive system.
Crop can store food as much as half of their body.
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Stomach
It is divided into two parts.
a) Proventriculus- also called the glandular stomach which is pear-shaped;
receives food from the crop and secretes mucus, HCL and pepsinogen.
They also secretes enzymes but by one cell type (as you can see in
other animals chief and zymogenic cells).
b) Ventriculus – also called as “gizzard”. As noted earlier, birds don’t
have teeth, but they still have to masticate food before it can pass
into the small intestine. It is connected to Proventriculus.
The avian equivalent of teeth
-Very muscular.
-Used primarily to grind & break-up food (such as seeds)
- Chicken picks up gravel and small stones in their food, called grit (to
help grind the food).
-Lined with a tough, abrasive keratin which assisted by grit and
stones deliberately ingested.
Small intestine


Chief organ of digestion ad absorption
Receives bile from liver through bile duct and pancreatic juice
from the pancreas via duct.
Large intestine
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


Everything looks similar but, little variations.
Has out-pocketing called caeca
Aid in digestion of plant material (bacteria in the caeca
help enzymatic digest the material).
Cloaca


Receives waste from the large intestine and material from the urinary and
reproductive systems.
Divided into three sectionsa) Coprodaeum-receives waste from the large intestine.
b) Urodaeum- receives urine from the kidneys (via the uterus) and sperm and
egg from the gonads.
c) Protodaeum- stores (temporary) and ejects material; closed posteriorly by
the muscular anus.
Why bird faces are white?
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References
1) Notes for Diploma (by: Dr. Penjor and Nidup.K).
2) http://training.seer.cancer.gov/module_anatomy/unit10_1_dige_functions.html
3) Test book of Vet. Anatomy (SACK, DYCE, WENSING)
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