Question

1. Name, describe, and locate the structures (macroscopic and microscopic) of the alimentary canal; discuss anatomy and function through each structure.

Answer 1

Alimentary canal

Digestive system is made up of gastrointestinal tract
(GI tract) or alimentary canal and accessory organs,
which help in the process of digestion and absorption
. GI tract is a tubular structure extending from
the mouth up to end with a length of about 30 feet. It
opens to the external environment on both ends.
GI tract is formed by two types of organs:
1. Primary digestive organs.
2. Accessory digestive organs.
1. Primary Digestive Organs
Primary digestive organs are the organs where actual
digestion takes place.
Primary digestive organs are:
i. Mouth
ii. Pharynx
iii. Esophagus
iv. Stomach.   
  Digestive system is made up of gastrointestinal tract
(GI tract) or alimentary canal and accessory organs,
which help in the process of digestion and absorption
. GI tract is a tubular structure extending from
the mouth up to end, with a length of about 30 feet. It
opens to the external environment on both ends.
GI tract is formed by two types of organs:
1. Primary digestive organs.
2. Accessory digestive organs.
1. Primary Digestive Organs
Primary digestive organs are the organs where actual
digestion takes place.
Primary digestive organs are:
i. Mouth
ii. Pharynx
iii. Esophagus
iv. Stomach.   
v. Small intestine
vi. Large intestine.
2. Accessory Digestive Organs
Accessory digestive organs are those which help prim-
ary digestive organs in the process of digestion.
Accessory digestive organs are:
i. Teeth
ii. Tongue
iii. Salivary glands
iv. Exocrine part of pancreas
v. Liver
vi. Gallbladder.
WALL OF GASTROINTESTINAL TRACT
In general, wall of the GI tract is formed by four layers
which are from inside out:
1. Mucus layer
2. Submucus layer
3. Muscular layer
4. Serous or fibrous layer

.
NERVE SUPPLY TO GASTROINTESTINAL TRACT
GI tract has two types of nerve supply:
I. Intrinsic nerve supply.
     1. Auerbach plexus
2. Meissner plexus.
II. Extrinsic nerve supply.   
     Extrinsic nerves that control the enteric nervous system
are from autonomic nervous system. Both sympathetic and parasympathetic divisions of autonomic nervous
system innervate the GI tract .

FUNCTIONAL ANATOMY OF MOUTH
Mouth is otherwise known as oral cavity or buccal
cavity. It is formed by cheeks, lips and palate. It encloses
the teeth, tongue and salivary glands. Mouth opens
anteriorly to the exterior through lips and posteriorly
through fauces into the pharynx.
Digestive juice present in the mouth is saliva, which
is secreted by the salivary glands.
FUNCTIONS OF MOUTH
Primary function of mouth is eating and it has few other
important functions also.
Functions of mouth include:
1. Ingestion of food materials
2. Chewing the food and mixing it with saliva
3. Appreciation of taste of the food
4. Transfer of food (bolus) to the esophagus by
swallowing
5. Role in speech
6. Social functions such as smiling and other
expressions.
SALIVARY GLANDS
In humans, the saliva is secreted by three pairs of major
(larger) salivary glands and some minor (small) salivary
glands.

MAJOR SALIVARY GLANDS
Major glands are:
1. Parotid glands
2. Submaxillary or submandibular glands
3. Sublingual glands.

MINOR SALIVARY GLANDS
1. Lingual Mucus Glands
2. Lingual Serous Glands
3. Buccal Glands
4. Labial Glands

FUNCTIONAL ANATOMY OF STOMACH
Stomach is a hollow organ situated just below the
diaphragm on the left side in the abdominal cavity.
Volume of empty stomach is 50 mL. Under normal
conditions, it can expand to accommodate 1 L to 1.5 L of
solids and liquids. However, it is capable of expanding
still further up to 4 L.
PARTS OF STOMACH
In humans, stomach has four parts:
1. Cardiac region
2. Fundus
3. Body or corpus
4. Pyloric region.

FUNCTIONS OF STOMACH
1. MECHANICAL FUNCTION
i. Storage Function
Food is stored in the stomach for a long period, i.e.
for 3 to 4 hours and emptied into the intestine slowly.
The maximum capacity of stomach is up to 1.5 L. Slow
emptying of stomach provides enough time for proper
digestion and absorption of food substances in the small
intestine.
ii. Formation of Chyme
Peristaltic movements of stomach mix the bolus with
gastric juice and convert it into the semisolid material
known as chyme.
2. DIGESTIVE FUNCTION.
3. PROTECTIVE FUNCTION.
4. HEMOPOIETIC FUNCTION.
5. EXCRETORY FUNCTION
Many substances like toxins, alkaloids and metals are
excreted through gastric juice.

SMALL INTESTINE FUNCTIONAL ANATOMY
Small intestine is the part of gastrointestinal (GI) tract,
extending between the pyloric sphincter of stomach
and ileocecal valve, which opens into large intestine. It
is called small intestine because of its small diameter,
compared to that of the large intestine. But it is longer
than large intestine. Its length is about 6 meter.
Important function of small intestine is absorption.
Maximum absorption of digested food products takes
place in small intestine.
Small intestine consists of three portions:
1. Proximal part known as duodenum
2. Middle part known as jejunum
3. Distal part known as ileum.
Wall of the small intestine has all the four layers as
in stomach .

FUNCTIONS OF SMALL INTESTINE
1. MECHANICAL FUNCTION
Mixing movements of small intestine help in the thorough
mixing of chyme with the digestive juices like succus
entericus, pancreatic juice and bile.
2. SECRETORY FUNCTION
Small intestine secretes succus entericus, enterokinase
and the GI hormones.
3. HORMONAL FUNCTION
Small intestine secretes many GI hormones such as
secretin, cholecystokinin, etc. These hormones regulate
the movement of GI tract and secretory activities of small
intestine and pancreas .
4. DIGESTIVE FUNCTION
5. ACTIVATOR FUNCTION.
6. HEMOPOIETIC FUNCTION
7. HYDROLYTIC FUNCTION

FUNCTIONAL ANATOMY OF
LARGE INTESTINE
Large intestine or colon extends from ileocecal valve up
to end .
PARTS OF LARGE INTESTINE
Large intestine is made up of the following parts:
1. Cecum with appendix
2. Ascending colon
3. Transverse colon
4. Descending colon
5. Sigmoid colon or pelvic colon
6. Rectum
7. Anal canal.

FUNCTIONS OF LARGE INTESTINE
1. ABSORPTIVE FUNCTION
Large intestine plays an important role in the absorption
of various substances such as:
i. Water
ii. Electrolytes
iii. Organic substances like glucose
iv. Alcohol
v. Drugs like anesthetic agents, sedatives and
steroids.
2. FORMATION OF FECES
After the absorption of nutrients, water and other substances, the unwanted substances in the large
intestine form feces. This is excreted out.
3. EXCRETORY FUNCTION
Large intestine excretes heavy metals like mercury, lead, bismuth and arsenic through feces.
4. SECRETORY FUNCTION
Large intestine secretes mucin and inorganic substances
like chlorides and bicarbonates.
5. SYNTHETIC FUNCTION
Bacterial flora of large intestine synthesizes folic acid,
vitamin B12 and vitamin K. By this function, large
intestine contributes in erythropoietic activity and blood
clotting mechanism.

FUNCTIONAL ANATOMY OF LIVER AND BILIARY SYSTEM
Liver is a dual organ having both secretory and excretory
functions. It is the largest gland in the body, weighing
about 1.5 kg in man. It is located in the upper and right
side of the abdominal cavity, immediately beneath
diaphragm.

FUNCTIONS OF BILE
Most of the functions of bile are due to the bile salts.
1. DIGESTIVE FUNCTION.
2. ABSORPTIVE FUNCTIONS
3. EXCRETORY FUNCTIONS
Bile pigments are the major excretory products of the
bile. Other substances excreted in bile are:
i. Heavy metals like copper and iron
ii. Some bacteria like typhoid bacteria
iii. Some toxins
iv. Cholesterol
v. Lecithin
vi. Alkaline phosphatase.
4. LAXATIVE ACTION
Bile salts act as laxatives .
5. ANTISEPTIC ACTION
Bile inhibits the growth of certain bacteria in the lumen
of intestine by its natural detergent action.
6. CHOLERETIC ACTION
Bile salts have the choleretic action .
7. MAINTENANCE OF pH IN GASTROINTESTINAL TRACT
As bile is highly alkaline, it neutralizes the acid chyme
which enters the intestine from stomach. Thus, an
optimum pH is maintained for the action of digestive
enzymes.
8. PREVENTION OF GALLSTONE FORMATION.
9. LUBRICATION FUNCTION
The mucin in bile acts as a lubricant for the chyme in
intestine.
10. CHOLAGOGUE ACTION
Bile salts act as cholagogues .
FUNCTIONS OF LIVER
Liver is the largest gland and one of the vital organs of the
body. It performs many vital metabolic and homeostatic
functions, which are summarized below.
1. METABOLIC FUNCTION
Liver is the organ where maximum metabolic reactions
such as metabolism of carbohydrates, proteins, fats,
vitamins and many hormones are carried out.
2. STORAGE FUNCTION
Many substances like glycogen, amino acids, iron, folic
acid and vitamins A, B12 and D are stored in liver.
3. SYNTHETIC FUNCTION
Liver produces glucose by gluconeogenesis. It synthe­
sizes all the plasma proteins and other proteins (except
immunoglobulins) such as clotting factors, complement
factors and hormone­binding proteins. It also synthesizes
steroids, somatomedin and heparin.
4. SECRETION OF BILE
Liver secretes bile which contains bile salts, bile
pigments, cholesterol, fatty acids and lecithin.
The functions of bile are mainly due to bile salts. Bile
salts are required for digestion and absorption of fats in
the intestine. Bile helps to carry away waste products
and breakdown fats, which are excreted through feces
or urine.
5. EXCRETORY FUNCTION
Liver excretes cholesterol, bile pigments, heavy metals
(like lead, arsenic and bismuth), toxins, bacteria and
virus (like that of yellow fever) through bile.
6. HEAT PRODUCTION
Enormous amount of heat is produced in the liver
because of metabolic reactions. Liver is the organ where
maximum heat is produced.
7. HEMOPOIETIC FUNCTION
In fetus (hepatic stage), liver produces the blood
cells . It stores vitamin B12 necessary
for erythropoiesis and iron necessary for synthesis of hemoglobin. Liver produces thrombopoietin that
promotes production of thrombocytes.
8. HEMOLYTIC FUNCTION
The senile RBCs after a lifespan of 120 days are
destroyed by reticuloendothelial cells (Kupffer cells) of
liver.
9. INACTIVATION OF HORMONES
AND DRUGS
Liver catabolizes the hormones such as growth hormone,
parathormone, cortisol, insulin, glucagon and estrogen.
It also inactivates the drugs, particularly the fat­soluble
drugs. The fat­soluble drugs are converted into water­
soluble substances, which are excreted through bile or
urine.
10. DEFENSIVE AND DETOXIFICATION
FUNCTIONS.

FUNCTIONS OF GALLBLADDER
Major functions of gallbladder are the storage and
concentration of bile.
1.   Storage   of   Bile
Bile is continuously secreted from liver. But it is released
into intestine only intermittently and most of the bile is
stored in gallbladder till it is required.
2.   Concentration   of   Bile
Bile is concentrated while it is stored in gallbladder.
The mucosa of gallbladder rapidly reabsorbs water and
electrolytes, except calcium and potassium. But the
bile salts, bile pigments, cholesterol and lecithin are not
reabsorbed. So, the concentration of these substances
in bile increases 5 to 10 times .
3.   Alteration   of   pH   of   Bile
The pH of bile decreases from 8 – 8.6 to 7 – 7.6 and it
becomes less alkaline when it is stored in gallbladder.
4.   Secretion   of   Mucin
Gallbladder secretes mucin and adds it to bile. When bile
is released into the intestine, mucin acts as a lubricant
for movement of chyme in the intestine.
5.   Maintenance   of   Pressure   in   Biliary   System
Due to the concentrating capacity, gallbladder maintains
a pressure of about 7 cm H2O in biliary system. This
pressure in the biliary system is essential for the release
of bile into the intestine.