HUMAN DIGESTIVE SYSTEM
The digestive system of human beings consists of gastrointestinal track
accessory organs and gastric secretion. The long coiled tube of the
gastrointestinal track is called the alimentary canal, and it extends from
the mouth to the anus and associated digestive gland. The alimentary canal
is 7.5 to 9 m in length in an adult and consists of the buccal cavity, the
pharynx, esophagus, stomach, small intestine, large intestine, and
anus.
The buccal Cavity or Oral Cavity
The oral cavity is the site of the entrance of food into the digestive
system. It consists of the tongue, teeth, and palate, and the opening of
salivary glands.
Function of oral cavity
It performs the following functions:
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Food Selection
The oral cavity helps in the selection of food by the senses of taste,
smell, and sight. If the digested food has a bad taste or smells unpleasant,
it is rejected. The tongue plays an important role in food selection via
taste buds. The tongue also helps in food swallowing.
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Food Grinding and Lubrication
After selection, the food is subjected to grinding by means of moral teeth
into smaller pieces. The grinding of food is important because small pieces
of food can pass easily through the esophagus. Small pieces have more
surface area for the action of the enzymes. Saliva contains mucus and water,
which lubricates and moisturizes food immediately to make it easy for
grinding, digestion, and swallowing.
Tongue and test bud
The tongue is the first organ of the gastrointestinal track. It is a
fleshy, strong muscular sense organ and receives sensory information via
the taste buds in the papillae present on its upper surface. The sense of
test is mainly due to the presence of test buds. These buds send the
signal to the brain via cranial nerves. The brain then distinguishes
between the chemical properties of food and can recognize the test as
bitter, sour, and sweet both mechanical and chemical digestion occur in
the oral cavity by the teeth and enzymes of the saliva respectively.
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| Fig 1: Labeled diagram of oral cavity. |
Secretion of saliva
Saliva is secreted by three pairs of salivary glands. The
Parotid gland: it is present in front of the ears.
Sublingual glands: situated below the tongue.
Sub maxillary gland present in the angle of lower jaw.
Saliva Volume
1500 ml of salvia are secreted daily. Saliva is colorless. Its three main
ingredients are: Water and mucus combine to form a slimy liquid that serves
to moisten and lubricate foods, allowing them to be chewed and swallowed
more easily. Sodium bicarbonate and other salts are also present in saliva.
These salts are highly antiseptic, but their main function is to stabilize
the pH of the food. Amylase, or ptyalin, is also an ingredient of saliva. It
is a carbohydrate digesting enzyme that digests starch and glycogen to
disaccharide (maltose). Saliva also contains lysozyme, which hydrolyses the
cell walls of many species of bacteria. It also keeps the mouth clean. As a
result of this mechanical and chemical digestion, food is converted into a
partially digested, slimy food mass called a bolus.
Pharynx
The bolus is pushed into the back of the mouth by the action of the tongue
and muscles of the pharynx. During swallowing, the tongue moves upwards and
backward against the roof of the mouth, forcing the bolus to the back of the
mouth cavity. The background of the tongue pushes the soft palate up and
closes the nasal opening at the back. At the same time, the tongue forces
the epiglottis into a more or less horizontal position, thus closing the
opening of the windpipe. The cartilage of the larynx at the top of the
windpipe moves upwards under the back of the tongue. The glottis is
partially closed by the construction of four rings of muscle. The food does
not enter the partially open glottis because the epiglottis diverts the food
mass to one side of the opening and safely down the esophagus. The beginning
of the swallowing is voluntary, but once the food reaches the back of the
mouth, swallowing becomes automatic. The food is then forced down the
esophagus by a wave of muscular contraction called peristalsis.
Esophagus
An esophagus is a food pipe is a muscular tube extending from the pharynx
to the stomach, passing through the diaphragm. The length of the esophagus
is about 18 inches or 25 centimeters. It runs through the neck and thorax
between the trachea and the vertebral column. It lies above the trachea and
below the vertebral column.
Digestion
There is no digestive juice secreted in the esophagus. The amylase of
saliva keeps on digesting food in the pharynx and esophagus. The food passes
through the esophagus by a process called peristalsis.
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| Fig 2: Labeled diagram of human digestive system. |
Peristalsis
It is a series of rhythmical involuntary waves like contractions of the
muscular wall of the elementary canal. Food moves through the whole
alimentary canal by means of peristalsis. It starts just behind the mass of
the food from the buccal cavity along the esophagus to the stomach and then
along the whole of the elementary canal. A hunger contraction is a
peristaltic contraction that increases as blood sugar levels fall and is
strong enough to cause an unpleasant sensation known as a hunger
pang.
Antiperistalsis
Sometimes the wave of peristalsis is reversed, which causes the food to
pass from the intestine back into the stomach and even into the mouth. This
reverse peristalsis is called antiperistalsis, and leads to vomiting.
Stomach
Stomach is an elastic muscular thick walled J-shaped bag. It is widest part
of the digestive system. The length of stomach is about 10 to 12 inches. It
is situated below the diaphragm on left side of the abdominal
cavity.
Cardiac region: The upper broad region of the stomach near the
heart called the cardiac region. Where ring like muscle called the cardiac
sphincter, which when relax allow the food to enter into the stomach and
when contract closes the opening of the stomach from esophagus. Cardiac
region contain mucus secreting gland called the cardiac gland.
Fundus: It is the largest and middle part of the stomach, that hold
food and gases. This portion of stomach lies below the cardiac notch.
Fundus contain the gastric or fundic glands.
Pylorus end:
The narrow part near the small intestine is called pylorus region at the
junction of pylorus region and duodenum
lies a muscular ring called pylorus sphincter, which allow the food to
passes from stomach into the duodenum. It contains pyloric gland which
secretes number of hormones and mucus.
Composition of stomach wall
Stomach wall is composed of 4 layers. Serosa: it is an outer layer
of connective tissue. Muscularis: the middle layer of muscles consist
of an outer longitudinal and inner circular muscles, it lies next to serosa.
Submucosa: a middle layer of connective tissue laying next to mucosa
and have many glands, blood vessels, and nerve cells. Mucosa: an
inner most layer of smooth muscle. These muscular layers
helps in churning and mixing of the food with the gastric secretion. The
mucosa of stomach possess many tubular gastric glands. Gastric glands are
composed of three kind of cells producing different secretion.
Mucoid cell or pylorus cell : secrete gastric mucous which form the
inner coating and prevent the stomach wall from digestion by enzymes.
Parietal or oxyntic cells: secret hydrochloric acid HCL, HCL provides
an acidic medium for pepsinogen, it act as an antiseptic to kill
microorganism, and also soften the food.
zymogenic cells: secrete pepsinogen, which is an inactive form of
pepsin, pepsinogen is than converted to pepsin when expose to the acidic
medium. Pepsin is the protein digesting enzyme which hydrolysis
protein into smaller peptones and smaller polypeptides.
All the three secretion are collectively called as gastric juices, the
volume of the gastric juices is about 2500ml per day. The gastric juices of
an infants contain an enzyme called rennin which convert milk into curd thus
increasing the stay time of food in stomach. The curd is than digested by
pepsin. As a result of mechanical and chemical digestion food is
converted into a homogeneous semifluid called chyme.
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| Fig 3: Labeled diagram of layers of stomach. |
Small intestine
The stomach open into small intestine. The small intestine is named for its
small diameter. Small intestine is an elongated narrow and coiled tube. It
is longest and most important part of digestive system.
Part of small intestine
Small intestine is divided into three parts:
1:Duodenum 2: Jejunum 3: Ileum
Digestion in Duodenum
The duodenum is the first part of small intestine and it is about 12 inches
in length. Duodenum receives chyme from the stomach. As acidic chyme enter
into the duodenum, it stimulates duodenum wall to secrete hormone called
secretin, which stimulates pancreas to secretes its secretion into the
duodenum.
Both liver and pancreas play an important role in the digestion of food in
the duodenum.
Role of liver in digestion
Liver is the largest gland of the body and is greatest chemical factory. It
is reddish brown in colour and lies under the diaphragm. Liver consists of
two lobes each one is further divided into many smaller lobes. Liver
secretes bile which is stored in the gall bladder and is released into the
duodenum through the bile duct.
Bile
Bile is, alkaline watery fluid and contains no enzymes. Its greenish color
is due to the bile pigment, which are formed from the breakdown of
hemoglobin in the liver.
Composition of bile
Bile contains water, sodium bicarbonate, excess of calcium and the
breakdown product of haemoglobin. Bile contains salts which emulsified the
fats, it is antiseptic and kill germs, it is alkaline and neutralize
the acidic chyme.
Pancreas
The pancreas is a long, many-lobed gland that lies below the stomach. The
exocrine tissue of the pancreas secretes pancreatic juices that flow through
the pancreatic duct into the duodenum.
The nature and composition of pancreatic juice
The pancreatic juices are alkaline, thus neutralizing acidic chyme. It
contains sodium bicarbonate and water, as well as enzymes that digest
all of the food's major components, namely carbohydrates, fats, and
protein. These enzymes are: Amylase: it is the carbohydrate digesting enzyme which digests starch into
maltose. It is also known as an amylopsin. Lipase: is a fat
digesting enzyme that hydrolyzes a small percentage of the fats into
fatty acids and glycerol. Trypsin: it is secreted in the inactive
form trypsinogen. Trypsinogen: is activated into trypsin by an enzyme of the duodenum known as
enterokinse. It digests proteins into small polypeptides and
peptones.
Digestion in Jejunum and Ileum
The jejunum is the middle part of small intestine. It is about 2.4 m
in length, comprising about 2/5 of the small intestine. The ileum is
about 2.60m in length, which makes up about 3/5 of the small
intestine. The food that escapes undigested from the duodenum is
completely digested in the jejunum and ileum by a group of enzymes
contained in the intestinal juices. These enzymes are:
Aminopeptidase: it converts peptones and other smaller
peptides into dipeptides. Erepsin: It is a dipeptisase, which
converts dipeptides into amino acids. Lipase: it converts
fats into fatty acids and glycerol. Maltase: it acts on
maltose and converts it into glucose. Lactase: it
converts lactose into glucose and galactose. Sucrase: it
converts sucrose into glucose and fructose.
Absorption of Food
Absorption is the passage of glucose, fatty acids, glycerol,
amino acids, vitamins, minerals, and water into the circulatory
system. Absorption occurs chiefly in the small intestine,
particularly in the ileum. The internal surface of Ileum has many
folds. It exhibits a velvety appearance due to the presence of
numerous finger-like projections called villi. Villi increases the
surface area of absorption.
Structure and Function of Villi
Each villi consists of three parts.
1: The outer layer of epithelial cells 2: Blood Capillaries 3:
A small lymphatic or lacteal vessel:
Each villa consists of thousands of small microscopic
projections called micro villi. Different parts of the villi
absorb the secretion selectively, e.g. simple sugar, amino
acids, vitamins, minerals, and water enter the blood capillaries
in the villi. These capillaries join up to form hepatic portal
veins, which carry nutrients to the liver. Smaller fatty acids
and glycerol are absorbed into blood capillaries while large
fatty acids and glycerol are absorbed into the lymph vessels or
lacteal. The ileum absorbs the digestive food efficiently, and
the absorption occurs by diffusion and active
transport.
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Fig 4: typical structure of villi. |
Large Intestine
The large intestine is about 6 feet long and consists of
the three regions.Cecum he cecum is about 5 to 8 cm in
length and lies between the small intestine and the
ascending portion of the colon. The cecum has a small
finger-like projection called the appendix. It is a
vestigial organ in man but has a minor role in
immunity.
Colon
The colon is the largest part of the large intestine
and it consists of four parts.
The ascending part: it goes up on the right
side of the body to the level of the lever.
The transverse part: it rosses the abdominal
cavity just below the liver and stomach.
The descending part: it passes down on the left
side of the body.
The sigmoid part: it enters the
rectum.
Function
The large intestine performs the following
function:
Reabsorption of water and salt
The chyme reaches the large intestine, which contains a
lot of water, salt, and undigested material. The water
is reabsorbed into the blood while undigested material
is expelled out of the body as feces.
Synthesis of vitamin K
The large intestine contains a bacterial population
which can synthesize some vitamins, especially vitamin K
which is absorbed into the blood.
Feces
Feces material contains a high concentration of
bacteria, plant fiber, sloughed mucosal cells, mucus,
cholesterol, bile pigment, and other
substances.
Rectum
It is the largest part of the large intestine. It is
about 21–23 cm long. It stores feces temporarily and
ejects them through an anus at intervals. When the
rectum is filled with feces, it stimulates the wall of
the rectum to give rise to the defecation reflex. This
reflex can be consciously inhibited in individuals, but
not in infants. The undigested residue may spend from 12
to 24 hours in the large intestine.
Anus
The anus is the opening through which feces are expelled
out of the body. It is guarded by a sphincter muscle. The
brown color of the feces is due to the bacterial breakdown
of the bilirubin. The bad smell of feces is due to the
decomposition of bacteria, their byproducts, and the
choice of the food you eat. Putrefaction sets up in the
last portion of the large intestine of a microorganism.
Putrefaction also produces various gases.
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