Structures
After going through this lesson, students are
expected to:
1. identify the organs that make up the digestive system;
2. give the function of each organ;
3. describe how some accessory organs and glands help the body in
the digestive process; and
4. Explain ingestion, digestion, absorption, assimilation, and excretion.
Why do we eat? Is eating necessary in keeping us alive? Where do we
get the energy that enables us to carry out the many activities that we do
each day? How do we obtain materials needed for the growth and repair of
body parts?
The food that we eat plays a central role in the survival of species. It
provides the energy that enables us to carry out the many activities that we
do each day such as breathing, walking, studying, and cooking. Food also
provides the substances needed for growth and repair of body parts.
The cells in the body need food for energy used for growth and
repair. Food must be broken down into a form that these microscopic
cells can use. The body changes food into a usable form by means of a
group of organs referred to as the DIGESTIVE SYSTEM.
The functions of the digestive system basically
involve the following processes:
1. Ingestion – it is the taking in of food or any substance into the body
through the mouth
2. Digestion – the process that involves break down of large food molecules
into smaller molecules for easy absorption of the cells.
3. Absorption – the process of passing the soluble food molecules in the
wall of the small intestine through the villi.
4. Assimilation – the movement of digested food nutrients into the blood
vessels of the small intestine through diffusion and use of nutrients into
the body cells through the microvilli
5. Egestion – it is the release of undigested food collected in the rectum
called feces and pushed out of the body through the anus by defecation
Digestion
Digestion refers to the breakdown of food into smaller components
that can be absorbed into the bloodstream. This digestion or
catabolism is divided into two types, the mechanical digestion that
occurs in the mouth when food is physically broken up into smaller
pieces and the chemical digestion that takes place in the
gastrointestinal tract when the digestive enzymes break down the food
into small molecules (Mandal, 2019).
MOUTH
Each species of organisms has its own way of digesting food.
Digestion in human starts in the mouth where food is chewed and
broken down to smaller pieces for easier digestion. This is so called
mechanical digestion. Initial chemical digestion starts also in the
mouth. This is carried out by molecules that speed up chemical
reactions called enzymes. Amylase, an enzyme present in saliva
helps break down carbohydrates, which are large complex molecules,
into simple ones that can be used by the body.
THE PHARYNX
The pharynx muscular walls function in the
process of swallowing, and it serves as a
pathway for the movement of food from the
mouth to the esophagus. The constrictive
circular muscles of the pharynx’s outer layer
play a big role in peristalsis. A series of
contractions will help propel ingested food
and drink down the intestinal tract safely. The
inner layer’s longitudinal muscles, on the
other hand, will widen the pharynx laterally
and lift it upward, thus allowing the
swallowing of ingested food and drink.
THE ESOPHAGUS
Approximately, the esophagus is 20 cm
long. Its functions include secreting
mucus and moving food from the throat
to the stomach using muscle movement
called peristalsis.
THE STOMACH
The stomach is a J-shaped muscular bag that
stores the food and breaks it down into tiny
pieces. It mixes food with gastric juices that
contain enzymes to break down proteins and
lipids. Hydrochloric acid in the stomach kills
bacteria. Food found in the stomach is called
chyme.
THE SMALL INTESTINE
The small intestine is a little over 5
meters long and lies in the abdominal
cavity surrounded by the large intestine.
In the small intestine, the chemical
digestion of food is completed and most
of the absorption of nutrients takes
place. Lining of intestine walls has
fingerlike projections called villi, to
increase surface area.
The small intestine comprises
three main sections continuous with
each other:
Small Intestine - Duodenum
• The duodenum is the first part of the
small intestine. It is located between
the stomach part of the small intestine,
or jejunum. After foods mix with
stomach acid, they move into the
duodenum, where they mix with bile
from the gallbladder and digestive
juices from the pancreas. The
absorption of vitamins, minerals, and
other nutrients begins in the
duodenum
Small Intestine - Jejunum
• . It is the middle section of the small
intestine and is about 2 meters long.
The main function of the jejunum is
absorption of important nutrients such
as sugars, fatty acids, and amino acids.
Small Intestine - Ileum
• The ileum is the last part of the small
intestine. It connects to the cecum
(first part of the large intestine) and
helps to further digest food coming
from the stomach and other parts of
the small intestine. In addition, ileum
absorbs nutrients (vitamins, minerals,
carbohydrates, fats, proteins) and
water from food so they can be used by
the body.
THE LARGE INTESTINE
In the large intestine absorption of water continues
until the familiar semisolid consistency of feces is
achieved. Mineral salts, vitamins, and some drugs are
also absorbed into the blood capillaries from the large
intestine. The entire colon is about 5 feet (150 cm)
long, and is divided into five major segments. The
rectum is the last anatomic segment before the anus.
The ascending and descending colon are supported
by peritoneal folds called mesentery. The right colon
consists of the cecum, ascending colon, hepatic
flexure and the right half of the transverse colon. The
left colon consists of the left half of thetransverse
colon, splenic flexure, descending colon, and sigmoid.
THE RECTUM AND THE ANAL CANAL
The rectum is a straight, 8-inch chamber that connects the colon to the
anus. The rectum's job is to receive stool from the colon, let you know
that there is stool to be evacuated (pooped out) and to hold the stool
until evacuation happens. When anything (gas or stool) comes into the
rectum, sensors send a message to the brain. The brain then decides if
the rectal contents can be released or not. The anus is the last part of
the digestive tract. It is a 2-inch long canal consisting of the pelvic floor
muscles and the two anal sphincters (internal and external). The lining
of the upper anus is able to detect rectal contents. It lets you know
whether the contents are liquid, gas or solid.
Accessory Organs of The Human Digestive
System
The Teeth
The teeth are responsible for the mechanical
digestion of food in the mouth – a process called
mastication. Our teeth are classified into four
types and each performs specific functions. The
chisel-shaped frontal teeth are called incisors
which is specialized in cutting. The fanglike teeth
are called canine and are best for piercing and
tearing. The premolars and molars are the teeth
responsible for crushing and grinding.
The Salivary Glands
Salivary glands are present in the oral cavity and
pour their secretions into the mouth. 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. Salivary glands and saliva work as chemical
digestion of polysaccharides. Saliva contains the
enzyme amylase that begins the breakdown of
complex sugars, reducing them to the disaccharide
maltose. Dry food entering the mouth is moistened
and lubricated by saliva before it can be made into a
bolus ready for swallowing.
The Pancreas
A healthy pancreas produces the correct chemicals in the proper
quantities, at the right times, to digest the foods we eat. As part of the
exocrine system, the pancreas secretes enzymes that work in tandem
with bile from the liver and gallbladder to help break down substances
for proper digestion and absorption. These enzymes include trypsin
and chymotrypsin to digest proteins; amylase for the digestion of
carbohydrates; and lipase to break down fats.
The Liver
The liver is the largest gland in the body, weighing between 1 and 2.3
kg. It is situated in the upper part of the abdominal cavity. The liver
directly affects digestion by producing bile. Bile helps digest fat.
Gall bladder
The gall bladder is a small, thin-walled sac located beneath the liver.
This organ stores and concentrates the bile produced in the liver which
will be then used in the digestion of lipids.
HOW DIGESTION WORKS
Digestion is the process by which the gastrointestinal system retrieves
important nutrients for the body and chemically changes the unused food
into waste. The first step of digestion is mastication or the chewing of food
in the mouth. Saliva initiates digestion and changes the chewed food into a
soft mass called bolus. Bolus is made slippery by saliva, making it easier to
be swallowed and slip down the back of the throat and esophagus. Before it
enters the stomach, the bolus passes through the esophageal sphincter.
Hydrochloric acid is released inside the stomach that causes to break down
large food molecules into smaller ones and liquefies the bolus.
The liquefied bolus, now called chime, then passes through the pyloric
sphincter and enters the first section of the small intestine, the duodenum.
It is here that enzymes released from the pancreas, liver and gallbladder
further breakdown chime into elements that can be easily absorbed and
used by the body. The small intestine is lined with a heavily folded inner
mucosa and small fingerlike projections called villi. The villi permit digested
food to enter the bloodstream. All nutrients and vitamins are absorbed here
in the small intestine. Chime can travel through up to 20 feet of small
intestine before it passes through the ileocecal valve to enter the large
intestine. A very little digestion occurs in the large intestine.
Waste is formed from the deposits of undigested food in the large
intestine. As the waste passes through the large intestine, it becomes
more and more solid because of the water that is continuously being
reabsorbed from the waste. Waste deposits in the rectum, or end of
the large intestine, until the brain signals for it to be expelled from the
body (Ahsan, 2015).