SMALL AND LARGE INTESTINE - ENGLISH
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SMALL AND
LARGE INTESTINE-
DIGESTIVE SYSTEM-
• The
digestive system is also known as the gastrointestinal system or the alimentary
canal or GIT. This vast system is
approximately 10 m long. It travels the length of the body from the mouth through
the thoracic, abdominal and pelvic cavities, where it ends at the anus. We will
cover it in multiple lectures. Today we are going to discuss the anatomy and
physiology of Small intestine, large Intestine, rectum and anus.
SMALL INTESTINE=
• The
small intestine is continuous with the stomach at the pyloric sphincter. The
small intestine is a little over 5 metres long and leads into the large
intestine at the ileocaecal valve. It lies in the abdominal cavity
surrounded by the large intestine.
• The
small intestine is composed of three
parts.
• The
duodenum is about 25 cm long and curves around the head of the pancreas.
Secretions from the gall bladder and pancreas merge in a common structure – the
hepatopancreatic ampulla – and enter the duodenum at the duodenal
papilla. The duodenal papilla is guarded by a ring of smooth muscle, the hepatopancreatic
sphincter (Sphincter of Oddi)
• The
jejunum is the middle section of the small intestine and is about 2 metres
long.
• The
ileum, or terminal section, is about 3 metres long and ends at the ileocaecal
valve, which controls the flow of material from the ileum to the caecum, the
first part of the large intestine, and prevents regurgitation (back flow).
STRUCTURE OF SMALL INTESTIVE-
• The
wall of the small intestine is composed of the same four layers that make up
most of the GI tract: mucosa, submucosa, muscularis, and serosa with some
modifications in outer-most layer
and inner-most layer. The outermost layer is a double layered peritoneum called the mesentery
attaches the jejunum and ileum to the posterior abdominal wall. The large
blood vessels and nerves lie on the posterior abdominal wall and the branches
to the small intestine pass between the two layers of the mesentery.
• The
surface area of the small intestine mucosa is greatly increased by permanent
circular folds, villi and microvilli.
• The
permanent circular folds, unlike the rugae of the stomach, are not
smoothed out when the small intestine is distended . They promote mixing of
chime with entric juices, as it passes along.
• The
villi are tiny finger-like projections of the mucosal layer into the
intestinal lumen, about 0.5 to 1 mm long . Their walls consist of columnar
epithelial cells, or enterocytes, with tiny microvilli (1 μm
long) on their free border. Goblet cells that secrete mucus are interspersed between
the enterocytes. These epithelial cells enclose a network of blood and lymph
capillaries. The lymph capillaries are called lacteals because absorbed
fat gives the lymph a milky appearance. Absorption and some final stages of
digestion of nutrients take place in the enterocytes before entering the blood
and lymph capillaries.
• The
intestinal glands are simple tubular glands situated below the surface
between the villi. The cells of the glands migrate upwards to form the walls of
the villi replacing those at the tips as they are rubbed off by the intestinal
contents. The entire epithelium is replaced every 3 to 5 days.
• There
are four types of cell present in the mucosa of the small intestine
• The
absorptive cell produces digestive enzymes and absorbs digested foods.
• Goblet
cells secrete mucus .
• Enteroendocrine
cells produce regulatory hormones such as secretin and CKK. These hormones are
secreted into to initiate release pancreatic juice and bile.
• Paneth
cells produce lysozyme, which protects the small intestine from pathogens that
have survived the acid conditions of the stomach. Peyer’s patches (lymphatic
tissue of the small intestine) also protect the small intestine.
FUNCTIONS OF SMALL
INTESTINE-
• Onward
movement of its contents by peristalsis, which is increased by parasympathetic
stimulation
• Secretion
of intestinal juice, also increased by parasympathetic stimulation
• Completion
of chemical digestion of carbohydrates, protein and fats in the enterocytes of
the villi
• protection
against infection by microbes that have survived the antimicrobial action of
the hydrochloric acid in the stomach, by the solitary lymph follicles and
aggregated lymph follicles
• secretion
of the hormones cholecystokinin (CCK) and secretin
• absorption
of nutrients.
PROCESS OF DIGESTION IN SMALL INTESTINE-
• When
acid chyme passes into the small intestine it is mixed with pancreatic juice,
bile and intestinal juice, and is in contact with the enterocytes
of the villi. In the small intestine digestion of all the nutrients is
completed:
• carbohydrates
are broken down to monosaccharides
• proteins
are broken down to amino acids
• fats
are broken down to fatty acids and glycerol.
• Pancreatic
juice is secreted by the exocrine pancreas
and enters the duodenum at the duodenal papilla. It consists of:
-Water -mineral
salts
-Enzymes ( amylase, lipase & nucleases )
-Inactive enzyme precursors (trypsinogen &
chymotrypsinogen)
• Digestion
of proteins-Trypsinogen and chymotrypsinogen are inactive enzyme precursors
activated by enterokinase, an enzyme in the microvilli, which converts
them into the active proteolytic enzymes trypsin and chymotrypsin.
These enzymes convert polypeptides to tripeptides, dipeptides and amino acids.
It is important that they are produced as inactive precursors and are activated
only upon arrival in the duodenum, otherwise they would digest the pancreas.
• Digestion
of carbohydrates-Pancreatic amylase converts all digestible
polysaccharides (starches) not acted upon by salivary amylase to disaccharides.
• Digestion
of fats-Lipase converts fats to fatty acids and glycerol. To aid the
action of lipase, bile salts emulsify fats & reduce the size of the
globules, increasing their surface area.
• The
principal constituents of intestinal secretions are water, mucus and mineral
salts. Most of the digestive enzymes in the small intestine are contained in
the enterocytes of the walls of the villi. Digestion of carbohydrate, protein
and fat is completed by direct contact between these nutrients and the
microvilli and within the enterocytes. The enzymes that complete chemical
digestion of food at the surface of the enterocytes are:
• Peptidases,
lipase, sucrase, maltase and lactase.
• The
final stage of breakdown of all peptides to amino acids takes place at the
surface of the enterocytes. Lipase completes the digestion of emulsified
fats to fatty acids and glycerol in the intestine. Sucrase,
maltase and lactase complete the digestion of carbohydrates by converting
disaccharides such as sucrose, maltose and lactose to monosaccharides at the
surface of the enterocytes.
ABSORPITON IN SMALL INTESTINE-
• All
the chemical and mechanical phases of digestion from the mouth through the
small intestine are directed toward changing food into forms that can pass
through the absorptive epithelial cells lining the mucosa and into the
underlying blood and lymphatic vessels. These forms are monosaccharides
(glucose, fructose, and galactose) from carbohydrates; single amino
acids, dipeptides, and tripeptides from proteins; and fatty acids,
glycerol, and monoglycerides from fats. Passage of these digested
nutrients from the gastrointestinal tract into the blood or lymph is called absorption.
• Absorption
of materials occurs via diffusion, facilitated diffusion, osmosis, and active
transport. About 90% of all absorption of nutrients occurs in the small
intestine; the other 10% occurs in the stomach and large intestine. Any
undigested or unabsorbed material left in the small intestine passes on to the
large intestine. Monosaccharides and amino acids pass into the capillaries in
the villi. Fatty acids and glycerol enter into the lacteals where they are
transported along lymphatic vessels and enter the circulation at the thoracic
duct
• The
surface area through which absorption takes place in the small intestine is
greatly increased by the circular folds of mucous membrane and by the very
large number of villi and microvilli present . It has been calculated that the
surface area of the small intestine is about five times that of the whole body.
Large amounts of fluid enter the alimentary tract each day . Of this, only
about 1500 ml is not absorbed by the small intestine, and passes into the large
intestine.
LRAGE INTESTINE-
• The
large intestine is about 1.5 metres long, beginning at the caecum in the
right iliac fossa and terminating at the rectum and anal canal deep
in the pelvis. Its lumen is about 6.5 cm in diameter, larger than that of the
small intestine. It forms an arch round the coiled-up small intestine. For
descriptive purposes the large intestine is divided into the caecum, colon, rectum
and anal canal.
• The
caecum- This is the first part of the large intestine . It is a dilated
region which has a blind end inferiorly and is continuous with the ascending
colon superiorly. Just below the junction of the two the ileocaecal valve opens
from the ileum. The vermiform appendix is a fine tube, closed at one
end, which leads from the caecum. It is usually about 8 to 9 cm long and has
the same structure as the walls of the large intestine but contains more
lymphoid tissue.
• The
colon- The colon has four parts which have the same structure and
functions.
• The
ascending colon- This passes upwards from the caecum to the level of the
liver where it curves acutely to the left at the hepatic flexure to
become the transverse colon.
• The
transverse colon-This is a loop of colon that extends across the abdominal
cavity in front of the duodenum and the stomach to the area of the spleen where
it forms the splenic flexure and curves acutely downwards to become the
descending colon.
•
The
descending colon- This passes down
the left side of the abdominal cavity then curves towards the midline. After it
enters the true pelvis it is known as the sigmoid colon.
•
The sigmoid colon- This part describes an S-shaped curve in the pelvis that continues
downwards to become the rectum.
• The
rectum- This is a slightly dilated section of the large intestine about 13
cm long. It leads from the sigmoid colon and terminates in the anal canal.
• The
anal canal- This 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, is under the
control of the autonomic nervous system and the external sphincter,
formed by skeletal muscle, is under voluntary control
STRUCTURE OF LARGE INTESTINE-
• The
four layers of tissue described in the basic structure of the gastrointestinal
tract are present in the caecum, colon,
the rectum and the anal canal. The arrangement of the longitudinal muscle
fibres is modified in the caecum and colon. They do not form a smooth
continuous layer of tissue but are instead collected into three bands, called taeniae
coli. Sac like strucures are formed in large intestine which is known as
haustra.
• In
the mucosal lining of the colon and the upper region of the rectum are large numbers
of mucus secreting goblet cells within simple tubular glands. They are not
present beyond the junction between the rectum and the anal canal. The lining
membrane of the anal canal consists of stratified squamous epithelium
continuous with the mucous membrane lining of the rectum above and which merges
with the skin beyond the external anal sphincter. In the upper section of the
anal canal the mucous membrane is arranged in 6 to 10 vertical folds, the anal
columns. Each column contains a terminal branch of the superior rectal
artery and vein.
FUNCTIONS OF LARGE INTESTINE-
• Absorption-
though a large amount of water has been absorbed in the small intestine. In
the large intestine absorption of water, by osmosis, continues until the
familiar semisolid consistency of faeces is achieved. Mineral salts, vitamins
and some drugs are also absorbed into the blood capillaries from the large
intestine.
• Microbial
activity- The large intestine is heavily colonised by certain types of
bacteria, which synthesise vitamin K and folic acid. They include Escherichia
coli, Enterobacter aerogenes, Streptococcus faecalis and Clostridium
perfringens. These microbes are commensals, i.e. normally harmless, in
humans. However, they may become pathogenic if transferred to another part of
the body, e.g. E. coli may cause cystitis if it gains access to the
urinary bladder.
• Mass
movement- The large intestine does not exhibit peristaltic movement as in
other parts of the digestive tract. Only at fairly long intervals (about twice
an hour) does a wave of strong peristalsis sweep along the transverse colon
forcing its contents into the descending
and sigmoid colons. This is known as mass movement and it is often
precipitated by the entry of food into the stomach. This combination of stimulus
and response is called the gastrocolic reflex.
• Defaecation-
Usually the rectum is empty, but when a mass movement forces the contents
of the sigmoid colon into the rectum the nerve endings in its walls are
stimulated by stretch. In the infant, defaecation occurs by reflex
(involuntary) action. However, during the second or third year of life the
ability to override the defaecation reflex is learned. In practical terms this
acquired voluntary control means that the brain can inhibit the reflex until such
time as it is convenient to defaecate.
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