THE ANATOMY OF DIGESTIVE SYSTEM
The human digestive system, (also known as the GI tract,
or the alimentary canal) is a series of connected organs leading from the
mouth to the anus. The digestive system allows us to break down the food we eat
to obtain energy and nourishment.
The alimentary canal begins at the mouth, passes through the
thorax, abdomen, and pelvis and ends at the anus. It is thus a long tube
through which food passes.
The digestive system -- which can be up to 30 feet in length
in adults -- is usually divided into eight parts: The parts include:
The Oral cavity or mouth, the esophagus,
the stomach, the small intestine and the large intestine
with the salivary gland, liver, pancreas, and gallbladder
adding secretions to help digestion. These organs combine to perform six
tasks: ingestion, secretion, propulsion, digestion,
absorption, and defecation.
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Now, let’s see the structure in detail.
Oral Cavity
The mouth is the first part of the upper GI tract and is equipped with several
structures that begin the first process of digestion. These include salivary
glands, teeth, tongue hard palate and the soft palate.
The Teeth: The teeth are embedded in the
sockets of the alveolar ridges of the mandible and the maxilla. Teeth are used
for mastication. Each individual has two sets, the temporary and the permanent
teeth. There are 20 temporary teeth, 10 in each jaw. They begin to erupt when
the child is about 6 months old, and the permanent teeth begin to replace the Temporary
teeth in the 6th year of age and this dentition, consisting of 32 teeth, is
usually complete by the 24th year.
On each side of each dental arch there are 2 incisors, 1
canine, 2 premolar and 3 molar, for a total of 32. The teeth are named after
their particular roles in the process of mastication—incisors are used
for cutting or biting off pieces of food; canines, are used
for tearing, premolars and molars are used
for chewing and grinding.
Teeth are made of a bone-like material called dentin, which is
covered by the hardest tissue in the body—enamel.
The Tongue: The
tongue is a fleshy and muscular sensory organ,
The tongue is attached to the floor of the
mouth by a ligamentous band called the frenulum
and this gives it great mobility for the manipulation of food (and speech);
The tongue plays an important part in:
- mastication (chewing)
- deglutition (swallowing)
- speech
- taste
The roof of the mouth is
termed the palate
and it separates the oral cavity from the nasal cavity. The palate is hard at
the front of the mouth since the overlying mucosa is covering a plate of bone; it is softer and more
pliable at the back being made of muscle and connective tissue, and it can move
to swallow food and liquids. The soft palate
ends at the uvula.
The Pharynx
The pharynx is a part of the conducting zone of the respiratory system and also a part of the digestive system. It is the part of the throat
immediately behind the nasal cavity at the back of the mouth and above
the esophagus and larynx. The pharynx is
divided for descriptive purposes into three parts, the nasopharynx,
oropharynx, and laryngopharynx. The nasopharynx is important in respiration.
The oropharynx and laryngopharynx are passages common to both the respiratory
and the digestive systems. For the digestive system, the muscular walls
of pharynx function in the process of swallowing, and it serves as a pathway
for the movement of food from the mouth to the esophagus.
The Esophagus
The esophagus, commonly known as the food pipe,
consists of a muscular tube through which food passes from the pharynx to the
stomach. The esophagus is about 25 cm long and about 2 cm in diameter. The upper and lower ends of the
esophagus are closed by sphincter muscles. The upper cricopharyngeal sphincter
prevents air from passing into the esophagus during inspiration and the
aspiration of esophageal contents. The
cardiac or lower esophageal sphincter prevents the reflux of acid gastric
contents into the esophagus.
The Stomach
The stomach is a
J-shaped dilated portion of the alimentary tract. The stomach acts as a temporary “storage tank” for food as well as a
site for food breakdown. The stomach is divided into three regions: the
fundus, the body, and the antrum. Stomach size varies with the volume of food
it contains, which may be 1.5 liters or more in an adult.
The Small Intestine
The small intestine
is a muscular tube extending from the pyloric sphincter to the large intestine.
It is the longest section of the alimentary tube, with an average length of 2.5
to 7 m (8 to 20 feet) in a living person. In the small intestine, the
chemical digestion of food is completed and most of the absorption of nutrients
takes place.
The small intestine
has three subdivisions: the duodenum, the jejunum, and the ileum,
The duodenum: It is about 25 cm long and curves around the head of the pancreas. Secretions
from the gall bladder and pancreas are released into the duodenum through a
common structure, the hepatopancreatic ampulla.
The jejunum:
It is the middle section of the small intestine and is about 2 meters long.
The ileum, or terminal section, is
about 3 meters long and ends at the ileocaecal valve, which controls the flow
of material from the ileum to the caecum.
The Large Intestine
The large intestine is much larger in
diameter than the small intestine but shorter in length. It is About 1.5 m
(5 feet) long, it extends from the ileocecal valve to the anus. Its major
functions are to dry out indigestible food residue by absorbing water and to
eliminate these residues from the body as feces. It frames the small intestines
on three sides and has the following subdivisions: cecum, appendix, colon, rectum, and anal canal.
The caecum: The saclike
cecum is the first part of the large intestine. It is usually about 13 cm long
and has the same structure as the walls of the colon but contains more lymphoid
tissue.
The ascending colon : The
ascending colon travels up the right side of the abdominal cavity and makes a
turn, the right colic
(or hepatic) flexure, to travel across the abdominal cavity.
The transverse colon:
The
ascending colon makes a turn and continuous to be the transverse colon as it
travels across the abdominal cavity.
Descending colon. It then
turns again at the left colic (or splenic) flexure,
and continues down the left side as the descending colon.
The sigmoid colon: This
part describes an S-shaped curve in the pelvis then continues downwards to
become the rectum
The Rectum and the Anal Canal
It
is a slightly dilated section of the colon about 13 cm long. It leads from the
sigmoid colon and terminates in the anal canal.
The
anal canal is a short passage about 3.8 cm long in the adult and leads from the
rectum to the exterior.
Two
sphincter muscles control the anus; the internal sphincter, consisting of
smooth muscle fibers, is under the control of the autonomic nervous system and
the external sphincter, formed by skeletal muscle, is under voluntary control.
Accessory Organs of the Human Digestive System
Various
secretions are poured into the alimentary tract, some by glands in the lining
membrane of the organs and some by glands situated outside the tract. The
latter are the accessory organs of digestion and their secretions pass through
ducts to enter the tract. They consist of:
1.
3 pairs of salivary glands
2.
Pancreas
3.
Liver and the biliary tract.
Salivary
Gland: Salivary glands are accessory
exocrine gland. The salivary gland includes parotid gland, Submandibular gland,
and sublingual gland. These glands secrets saliva into the oral cavity through
salivary ducts.
The
parotid gland is largest among all salivary gland and situated on the side of
the face immediately below and in front of external ear.
The
sublingual gland is the smallest one and this almond shaped gland situated
beneath the mucous membrane of the floor of the mouth.
The
walnut size submandibular or submaxillary are located in the floor of the mouth.
The
product of the salivary glands, is saliva and Saliva is the combined
secretions from the salivary glands and the small mucus-secreting glands of the
lining of the oral cavity. About 1.5 liters of saliva is produced daily
Pancreas:
The pancreas is a pale grey gland
and about 12 to 15 cm long and is situated in the
epigastric and left hypochondriac regions of the abdominal cavity. The pancreas is both an exocrine and an endocrine
gland. The function of the exocrine pancreas is to produce pancreatic juice
containing enzymes that digest carbohydrates, proteins, and fats and the
function of the endocrine pancreas is to secrete the hormones insulin and
glucagon, which are principally concerned with control of blood glucose levels.
Liver: The liver is the largest gland in the
body, weighing between 1 and 2.3 kg. It is Located under the diaphragm, more to
the right side of the body.
The liver
has four lobes. The two most obvious are the large right lobe and the smaller,
wedge-shaped, left lobe. The other two, the caudate and quadrate lobes, are
areas on the posterior surface. The liver’s digestive function is to produce
bile. Bile is a yellow-to-green, watery solution containing bile salts, bile pigments, cholesterol,
phospholipids, and a variety of electrolytes.
Gallbladder: The gallbladder is a small, thin-walled green sac that
snuggles in a shallow fossa in the inferior surface of the liver. When food
digestion is not occurring, bile backs up the cystic duct and enters the
gallbladder to be stored.
