The intestines are located between the stomach and the anus and are divided into two major sections: the small intestine and the large intestine. The function of the small intestine is to absorb most ingested food. The large intestine is responsible for absorption of water and excretion of solid waste material.
The intestines are a long,
continuous tube running from the stomach to the anus. Most absorption of
nutrients and water happen in the intestines. The intestines include the small
intestine, large intestine, and rectum.
The small intestine (small bowel) is about 6 metres
long and about 2.5cm in diameter. Its job is to absorb most of the nutrients
from what we eat and drink. Velvety tissue lines the small
intestine, which is divided into the duodenum, jejunum, and ileum.
The large intestine (colon or large bowel) is about 1.5
m long and about 7.6 cm in diameter. The colon absorbs water from wastes,
creating stool. As stool enters the rectum, nerves there create the urge to
defecate.
SMALL INTESTINE STRUCTURE
The small intestine which, though small in diameter, is the longest part
of the gastrointestinal system. “Small Bowel” is one of the commonly used small
intestine names.
The organ is responsible for the absorption of over 90% of nutrients
from the food. It absorbs more than 95% of the ingested proteins and
carbohydrates.
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| Structure of the small intestine. |
The small intestine starts from the pyloric sphincter at the lower end of the stomach and continues to the beginning of large intestine at the ileocecal junction. It is as big as your middle finger. In other words, it measures around 2.5cm in diameter.
It is easy to find small intestine location in the body. Stretching from
the stomach to the cecum of large intestine, the small bowel is located within
the central and lower abdominal area. It coils around several times to fit into
a small space in the lower abdominal area.
In the small intestine diagram, it is visible between the ascending,
transverse and descending part of the colon. It is located more centrally while
the large bowel is more towards the periphery. The latter has the diameter of
around 6.4 centimetres against 2.5 centimetres lumen of the small intestine.
Small Intestine Vs Large Intestine
The intestines are the hollow muscular tubes, running from the stomach
till the anus. As you already know, one of them is called ‘small intestine’ and
the other ‘large intestine’ or ‘colon’.
Moving from the mouth to the anus, the small bowel runs from the pyloric
sphincter at the end of the stomach till the ileocecal junction. On the other
hand, the colon begins at the ileocecal junction and ends at the anus.
The words “small” and “large” attached to the name of intestine can be
the source of confusion. The small bowel is small not in terms of length but
for its narrow lumen. In fact, it forms the longest part of the
gastrointestinal system.
On the other hand, the large bowel is not large in terms of length.
Actually, it has more than twice the diameter of the small bowel.
The main job of small intestine is to absorb nutrients from the digested
food while the colon serves as a temporary storage for the digestive wastes
before their discharge out of the body.
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| The small and large intestines |
Small Intestine Parts
There are three major small intestine parts, namely, duodenum, jejunum
and ileum. Moving from the stomach to the large intestine, they appear in the
following sequence:
Duodenum ⇒ Jejunum ⇒ Ileum
These segments of the bowel also increase in length in the same
sequence. It means duodenum is the smallest and the ileum the largest part.
Overall length, however, may be greater in tall persons. It also increases when
it is empty and after the individual dies.
There is division of labour among different segments. In other words,
each of the small intestine parts must perform a specific assigned task.
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| Parts of the small intestine |
Here follows a brief overview of each of these parts.
Duodenum
The first and shortest segment of the small bowel, duodenum connects the
stomach to the jejunum. This C-shaped hollow tube measures around a foot in
length. Duodenum receives food from the stomach in the form of chyme and mixes
it with bile juice and the pancreatic secretions. It neutralises the partially
digested food with the help of alkaline mucus containing a high concentration
of bicarbonate ions. The mucus is secreted by the Brunner’s glands present only
in the duodenum. Sometimes, it may reach the length of 38 centimetres.
The enzymatic secretions help in the digestion of carbohydrates,
proteins and fats. So, the process of chemical digestion that started in mouth
gets completed here. It also prepares food for absorption in jejunum. You can
distinguish duodenum into four different parts. These are:
a.
The Superior Part
The superior part is the continuation from the pylorus and lies
posterior to all other segments. It lies at the vertebral level of L1. About 2
cm in length, the superior part of duodenum is mobile and ends at the superior
duodenal flexure.
b.
The Descending Part
The descending part starts at the superior duodenal flexure and descends
to reach below L3 level of vertebral body. It ends at the inferior duodenal
flexure. The common bile duct and pancreatic duct are attached to the
descending section of the bowel at the point of major duodenal papilla.
Through the bile and pancreatic ducts, the intestine receives bile juice
and pancreatic secretions.
c.
The Horizontal Part
The third part of duodenum is also called the inferior part or the
horizontal part. Beginning at the inferior duodenal flexure, it passes in front
of the inferior vena cava and the abdominal aorta.
d.
The Ascending Part
The fourth and last part of the duodenum passes upward to reach the
duodenojejunal flexure where it joins with the jejunum. Lying at the vertebral
level L2, it may pass directly on top of the aorta.
Jejunum
Jejunum is the second or middle part of the small intestine. In terms of
length, it is longer than duodenum and shorter than ileum. Measuring around 2.4m,
the duodenum is as many as 8 times longer than the duodenum.
While duodenum carries out and completes the chemical digestion, jejunum
is the principle site for the absorption of nutrients from the digested food.
This is the place where over 90% absorption of the foodstuffs takes place.
Though villi or finger-like projections are present across all segments
of small bowel, they are longer and more active in jejunum. The epithelial
cells, lining the villi, also have microvilli. The folds in the epithelium,
villi and microvilli collectively contribute to incredibly increase the surface
area for absorption.
Nutrients move across the epithelium of jejunum and ileum with the help
of active or passive transport. Small peptides, amino acids, vitamins and
glucose are actively transported. Fructose, on the other hand, moves through
the passive transport.
The pH is between 7 and 9 which means the medium is neutral or slightly
alkaline. The suspension of the jejunum by mesentery in the abdominal area
renders it sufficient mobility. Meanwhile, the circular and longitudinal layers
of smooth muscles in the intestinal wall generate peristaltic movements.
Peristalsis helps in the pushing of foodstuffs towards the posterior end of the
gastrointestinal tract.
Ileum
The third and longest segment of small bowel, ileum runs for the length
of 3.7 metres, accounting for three-fifths the length of the small intestine.
Continuous with the jejunum, it ends at the ileocecal valve which separates it
from the cecum. Mesentery, a fold of the serous membrane, suspends ileum from
the abdominal wall.
The layers of longitudinal and smooth muscles in the ileal wall are
thinner than those of other parts of small intestine. So, the peristaltic
contractions are slower. Also its lining is less permeable than that of the
preceding small intestine parts.
Regarding its role in digestive system, the ileum contains receptors for
absorption of vitamin B12 and bile salts. Meanwhile, it also absorbs the
leftover nutrients from the digested food. Ileum absorbs about 95% of the
conjugated bile salts from the intestinal contents.
Hans Conrad Peyer, a Swiss anatomist, discovered bundles or patches of
lymphatic cells in ileum. These patches are named after him as Peyer’s patches.
You can see them by the human naked eye as elongated thickened areas without
villi. Every individual has 30 to 40 such patches in their intestine.
Small Intestine Functions in
Digestive System
A long, hollow, narrow and folded tube, the small intestine is the
principle organ of the digestive system. It is the site for most digestion and
absorption of food you eat. Apart from digestion and absorption, it performs
several other tasks as well.
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| Small intestine function in digestive system |
Some of the important small intestine functions are discussed below:
Neutralisation
The semi-digested food or chyme, coming from the stomach, is highly
acidic in nature. It needs to be neutralised for the optimum activity of
intestinal enzymes.
How is acidic chyme neutralised in the duodenum of small intestine?
Duodenum, the first segment of small bowel, secretes a hormone, secretin. This
hormone causes the pancreas secrete large amounts of sodium bicarbonate. Sodium
bicarbonate raises the pH of chyme from 2 to 7. Meanwhile, the mucus and bile
juice also has a neutralising effect.
Protection from Acids
The pH of the acidic chyme, expelled from the stomach to the small
intestine, is 2. It can damage the wall of small bowel. Now it is the job of
small intestine to protect itself from such damage. The goblet cells in the
intestinal epithelium secrete mucus which forms a layer to save the intestinal
wall from luminal contents.
Protection from Microbes
There are anti-microbial cells (Paneth cells) in the epithelial wall of
the bowel. The Paneth cells secrete anti-microbial peptides which kill harmful
microbes in the luminal contents.
Emulsification of Fats
As chyme enters the duodenum from stomach, bile is released by
the gallbladder. The bile juice emulsifies and breaks down the fats into
small particles, thus aiding in the mechanical digestion of fats.
Mechanical Digestion
The contractions of circular muscles in the intestinal wall facilitate
the mechanical breakdown of the food. Through these contractions, the
foodstuffs are mixed with secretions from pancreas and gallbladder. Meanwhile,
the large food particles reduce to smaller ones.
Chemical Digestion
Duodenum not only mechanically breaks down the food but also digests it
chemically. The secretions of liver, pancreas and bowel help in the chemical
breakdown of the carbohydrates, lipids and proteins.
Absorption of Nutrients
Small intestine plays a key role in the absorption. In fact, it absorbs
about 95% of all the useful substances from the food, including ions, amino
acids, glucose, small peptides, lipids, water, vitamins, fructose, iron and
bile salts.
Absorption of iron takes place in the duodenum and the terminal ileum
takes up bile salts and vitamin B12. The remaining nutrients are absorbed by
jejunum through either diffusion or active transport.
Bowel Movements
Among other crucial functions, the small intestine also helps in the
bowel movements to expel digestive waste towards the posterior end of the
digestive tract for its ultimate removal out of the body. The coordinated
contractions of circular and longitudinal smooth muscle fibres in the
intestinal wall not only help in the mixing of foodstuffs with secretions but
also facilitate bowel movements along the lumen.
Role in Immunity
The small intestine also supports your body’s immune system. The
probiotic gut flora has a positive and supportive effect on the defence system
of your body. The ileum contains Peyer’s patches which are an important part of
the local immune system of the gastrointestinal system. The lymphocytes present
in the Peyer’s patches are a part of the lymphatic system.
Small Intestine Diseases
Small intestine is the principle digestive organ. It not only serves as
a site for most of the digestion and absorption of food but also has a role in
immunity and performs several other functions. So, a small intestine disease
can adversely affect one or more of the functions of the organ.
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| Small intestine diseases |
Some of the small intestine disorders include cancer, ulcer, celiac
disease, intestinal obstruction, intestinal lymphoma, tropical sprue, lactose
intolerance, and so on. Discussed below are causes, symptoms and treatment of
common intestinal disorders.
Small Bowel Obstruction
Small bowl obstruction refers to the partial or complete blockage of
small bowel. Such a condition impedes the normal passage of luminal contents
through the small intestines. This potentially dangerous condition can
seriously affect the quality of your life.
Causes
Small bowel obstruction may occur due to multiple reasons, for example,
cancer, internal scar tissue and strangulated hernia. A strangulated hernia is
a protruding portion of the small intestine that comes out through a weak spot
in the abdominal wall.
Symptoms
·
Bouts of painful muscle contractions after intervals.
·
Painful abdominal swelling which increases with time.
·
Progressive constipation and inability to pass faeces
or even gas.
·
diarrhoea due to partial obstruction.
·
Nausea leading to vomiting.
·
Low-grade fever and weakness.
Celiac Disease
The celiac disease, also called celiac sprue, is a digestive disorder
that is more common in people of European descent than in others. It may cause diarrhoea,
weight loss, fatigue and pain in bones.
Causes
Celiac disease is a condition that results from gliadin protein found in
gluten. Gluten is a component of wheat that renders smoothness and elasticity
to the flour. The presence of gluten in food produces an immune reaction. Such
an immune reaction causes a damage to the lining of the small intestine.
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