The colon functions to absorb water from indigestible food matter and compact waste into feces. Feces move through the colon via segmental contractions that mix contents and peristalsis that propels contents towards the rectum over 1-2 days. Bacteria in the colon aid digestion by producing vitamins and short-chain fatty acids but also gases. The gastrocolic reflex initiates colonic motility after meals. Distension of the rectum by feces triggers defecation reflexes to relax the internal anal sphincter and contract the colon for expulsion, which can be voluntarily controlled by the external anal sphincter.

1. INTERGRATED RESPONSE
TO A MEAL- COLONIC
PHASE
SAMOEI- EGERTON UNIVERSITY, MBChB

2. FUNCTIONAL ANATOMY
 The large intestine or colon is a tube about 6 cm in diameter and 100 cm in length extending from
ileocecal valve up to anus.
 It normally arches around and encloses the coils of small intestine and tends to be more fixed than
the small intestine.
 It is divided into following parts:
 Cecum
 Appendix
 Ascending colon
 Transverse colon
 Descending colon
 Sigmoid colon
 Rectum
 Anal canal

3. Cont…
 Caecum is a blind-ended sac into which opens the lower end of ileum. The Ileocecal junction is
guarded by the ileocaecal valve which allows inflow but prevents backflow of the intestinal
contents.
 Appendix is a worm-shaped tube that arises from the medial side of caecum, which in human being
is a vestigial organ.
 Ascending colon extends upward from the caecum along the right side of abdomen up to the liver.
On reaching the liver it bends to the left, forming the right hepatic flexure.
 Transverse colon extends from the right hepatic flexure to the left splenic flexure.
 Descending colon extends from the left splenic flexure to the pelvic inlet below.
 Sigmoid colon begins at the pelvic inlet as a continuation of the descending colon and joins the
rectum in front of the sacrum.
 Rectum descends in front of the sacrum to leave the pelvis by piercing the pelvic floor. Here it
becomes continuous with anal canal in the perineum.
 Anal canal opens to the exterior through the anus, the opening which is guarded by two sphincters.

4. Ileocaecal valve and sphincter
Structure of ileocaecal valve
 Ileocaecal valve functioning occurs due to the invagination of ileum into the caecum at the ileocaecal
junction and a very small ileal opening (only 2–3 mm in diameter).
Functions.
 The principal function of the ileocaecal valve is to prevent back flow of the faecal matter from the
caecum into ileum. The valvular mechanism works in such a way that when the caecal pressure is
increased the ileocaecal opening is closed.
Structure and role of ileocaecal sphincter.
 Ileocaecal sphincter refers to a thickened band of circular muscle coat of the terminal part of ileum just
above the ileocaecal junction. The rhythmic contractions of ileocaecal sphincter leading to rhythmic
opening and closing occur after every 30 s after a meal.
 During every rhythmic opening, a small jet of ileal fluid (approximately 15 mL) escapes into the caecum.
The ileocaecal sphincter slows down the emptying of ileal contents into the caecum and thus, helps in
the completion of the absorption of nutrients in the ileum.
Control.
 Gastrin produces relaxation and secretin causes contraction of the ileocaecal sphincter.
Note
 It is important to note that these hormones show opposite effects on cardiac sphincter.

5. HISTOLOGY OF COLON
STRUCTURE OF THE WALL OF COLON
 It is formed by four layers of structures like any other part of the gut: Mucous, submucous, muscular
and serous layers.
Mucus layer:
 The crypts of Leiberkühn (simple tubular glands) lined by simple columnar epithelial cells are
present in mucosa of large intestine. The epithelial cells contain no enzyme.
 The crypts of Leiberkühn secrete bicarbonate ions.
 But the villi and plica circulares, which are present in mucus membrane of small intestine, are
absent in the large intestine.
 Only mucus-secreting glands are present in the mucosa of large intestine.
 The epithelium overlying solitary lymphatic follicles (in ascending colon, caecum and appendix)
contains M-cells similar to those seen in the small intestine.
 Microfold (M) cells are specialized epithelial cells overlying the Peyer’s patches.
Submucus layer:
It is not well developed in large intestine

6. Cont…
Muscular layer:
 Smooth muscles of large intestine are distributed in two layers, namely the outer longitudinal layer
and inner circular layer.
 The circular layer of muscle fibers is continuous from the cecum to the anal canal, where it
increases in thickness to form the internal anal sphincter.
 The longitudinal muscle fibers of large intestine are arranged in the form of three long bands called
taenia coli.
 The length of the taenia coli is less when compared to the length of large intestine. Because of this,
the large intestine is made into series of pouches called haustra (haustrations/sacculations).
Serous layer:
 Is formed by peritoneum and is missing over the posterior aspect of the ascending and descending
colon.
 At places small peritoneal bags of fat, called epiploic appendages, project from the colonic serosa.

7. Colonic secretion
 Large intestinal juice is a watery fluid with pH of 8.0.
Composition of large intestinal juice
 Large intestinal juice contains 99.5% of water and 0.5% of solids.
 Digestive enzymes are absent and concentration of bicarbonate is high in large intestinal juice.
 Mucus secreted by the goblet cells.
 HCO₃‾ is secreted by the glands of Lieberkuhn.
 The mucus lubricates the faecal matter and also protects the mucous membrane of the large
intestine by preventing the damage caused by the mechanical injury or chemical substances.
 The alkaline nature (pH 8.0) of the mucoid secretions of the large intestine is due to the
presence of HCO₃‾.
 It serves to neutralize the acids formed by the bacterial action on the faecal matter.
 Large quantities of water and electrolytes are secreted by the mucosa of large intestine only
when it is intensely irritated.

8. Composition of large intestinal juice

9. Colonic bacterial activity
 Bacterial flora. At birth, the colon is sterile, but the colonic bacterial flora becomes established
early in life and includes:
1. Harmless bacteria, such as E. coli and Enterobacter aerogenes and,
2. Potentially dangerous bacteria, such as Bacteroides fragilis, various types of cocci and gas
gangrene bacilli.
 These bacteria can cause serious disease in tissues outside the colon.
Intestinal bacterial activities can be grouped as:
1. Beneficial bacterial activities,
2. Indifferent bacterial activities and
3. Detrimental bacterial activities.
Beneficial bacterial activities include:
 Synthesis of vitamins, such as vitamin C, a number of B-complex vitamins and folic acid.
 Trophic effects on colonic mucosa. Unabsorbed carbohydrates are converted to short-chain fatty
acids by colonic bacteria. Some of the short-chain fatty acids produced have a trophic effect on the
colonic mucosa.
 Play a role in cholesterol metabolism by decreasing plasma cholesterol and LDL levels.

10. Cont…
Indifferent bacterial activities include:
 Production of intestinal gases. The colonic bacteria produce gas in large volumes up to 7–10 L/day,
which contribute towards flatus. The gas is produced chiefly through breakdown of undigested
nutrients that reach the colon. The gases produced by colonic bacteria include carbon dioxide
(CO2), hydrogen sulphide (H2S), hydrogen (H2) and methane (CH4) which contribute to flatus.
Nitrogen gas (N2) derived from the swallowed air accounts for most of the flatus passed through
rectum, or other gases diffuse readily through the intestinal mucosa. Therefore, the volume of
flatus expelled is reduced to about 600 mL/day.
 The absorption of protein antigens (bacterial or viral protein) occurs through the M cells. The M cells
pass on antigens to the lymphoid cells which respond by the secretion of IgA antibodies. Thus
secretory immunity plays an important role in localized protection of the intestinal mucosa.
 Organic acids formed by the colonic bacteria from the carbohydrates are responsible for the slight
acidic reaction of the stools (pH 5–7).
 Substances responsible for the odor of the faeces, such as indole, skatole, mercaptans are
synthesized by colonic bacteria.
 Pigments formed by the colonic bacteria from the bile pigments are responsible for the known
colour of stools.

11. Cont…
Detrimental bacterial activities include:
 Consumption of nutrients like vitamin C, vitamin B12 and choline by some bacteria may lead to
deficiency symptoms, unless these are supplemented in adequate amounts in the diet.
 Production of ammonia. Colonic bacteria also produce ammonia, which is absorbed by blood and is
normally detoxified quickly by the liver. However, in liver dysfunction, hyperammonemia results,
producing neurological symptoms (hepatic encephalopathy).

12. Colonic motility
Functions
1. It increases the efficiency of colon for water and electrolyte absorption.
2. Promotes the excretion of the faecal matter remaining in the colon.
Types of movements
1. Haustral shuttling (Segmental contractions)
 The haustral shuttling or haustral contractions are similar to the segmentation contractions of small
intestine, which vigorously mix the contents of colon and, by exposing more of the contents to mucosa
(facilitate absorption).
 It is predominant in ascending and transverse colon and accounts for most colonic movements at rest.
 Contraction of circular and longitudinal muscles in the large intestine cause haustrations to develop as:
– Contraction of circular muscle produces constriction rings at regular intervals,
– Contraction of longitudinal muscle (taenia coli) causes the unstimulated portion of large intestine in
between the constriction rings to bulge in bag-like sacs called haustration.
– Contraction disappears within 60 s. After a few minutes, haustral contractions are initiated in a
nearby area. The dynamic formation and disappearance of haustrations squeeze the chyme, moving it
back and forth in a manner similar to that described for the segmentation contractions in the small intestine.

13. Cont…
2. Peristalsis
 Peristalsis is a progressive contractile wave preceded by a wave of relaxation. In the colon, the
peristalsis waves are very small pressure waves of prolonged duration.
Function:
 They propel the contents towards rectum very slowly (5 cm/h). It can take up to 48 h for the chyme
to traverse the colon.
3. Mass movements (Mass peristalsis)
 The mass movements are special type of peristaltic contractions which are observed in the colon
only.
 These occur 3–4 times a day generally after meals and each contraction lasts for about 3 min.
 The mass movements force the faecal material rapidly in mass down the colon.
 They also move material into the rectum and rectal distension initiates the defecation reflex.
 A mass movement can be initiated by:
– Gastrocolic or duodenocolic reflexes,
– Intense stimulation of the parasympathetic nerves or
– Overdistension of a segment of colon.

14. Gastrocolic reflex
 Gastrocolic reflex refers to the contraction of colon induced by entry of food into the stomach.
 This reflex results in an urge to defecate after a meal.
 Because of this, defecation after meals is a rule in children. However in adults, the bowel training
suppresses this reflex.
Initiation.
 It has been reported that perhaps this reflex consists of two phases:
 The early or rapid phase (which occurs within 10 min of meals) is initiated by distension of
stomach and is conducted through the extrinsic nerves of the autonomic nervous system. It can
be abolished by anticholinergic drugs.
 The late or slow phase is considered to be mediated by gastrointestinal hormones like gastrin
and cholecystokinin, which are secreted into the blood stream in significant amounts shortly
after a meal.

15. Transit time in the gut
The transit time in various parts of the gut, studied after a test meal is:
 Up to caecum – 4 h
 Up to hepatic flexure – 6 h
 Up to splenic flexure – 8–9 h
 Up to pelvic colon – 12 h
 From pelvic colon to anus, transport is made slower and as much as a quarter (25%) of the residue
of a test meal may still be in the rectum for up to 3 days.
 Complete expulsion of the meal in stool takes more than a week.
 It has been observed that a high residue diet passes more rapidly through the entire gut.
 This is mainly because of its effect on the colonic movements.

16. Factors affecting colonic motility

17. DEFECATION
Anal sphincters play the most significant role in the process of defecation
Functional anatomy of Anal sphincters:
a) Internal or involuntary anal sphincter
 Muscle type: Formed by thickened circular smooth muscle.
 Nerve innervation: Parasympathetic (pelvic splanchnic) nerves and Sympathetic nerves
 Stimulation: Relaxes by reflex in response to stimulation of stretch receptors in the rectum wall
When the rectum is sufficiently distended by faeces, the internal anal sphincter relaxes through
innervation by the pelvic nerve.
b) External or voluntary anal sphincter
 Muscle type: Formed by somatic skeletal muscle
 Nerve innervation: Pudendal nerves which maintain the sphincter in a state of tonic contraction
 Stimulation: Mild to moderate distension of the rectum increases its force of contraction.
Moderately severe distension of the rectum will initiate a reflex which inhibits the discharge of
somatic pudendal nerves to cause sphincter relaxation. Therefore, the sphincter can be voluntarily
relaxed.

18. The act of defecation
 Defecation, the process of excretion of faecal material, involves both voluntary and reflex activity.
The events involved:
Distension of rectum
 Usually, once or twice a day gastrocolic reflex drives the faeces into the rectum, which increase the
intrarectal pressure passively.
Defecation reflexes
 As the rectum starts filling, the resultant rise in the intrarectal pressure stimulates the stretch
receptors, sets up defaecation reflexes and produces an urge to defaecate (when intrarectal pressure
increases to about 18 mm Hg).
 The voluntary external anal sphincter which normally remains tonically contracted further contracts
when there is moderate rise in the rectal pressure.

19. Cont…
Intrinsic reflex
 It is mediated by an intrinsic nerve plexus.
 Distension of rectum with faeces initiates afferent signals that spread through the myenteric plexus and
Initiates peristaltic waves in the descending colon, sigmoid colon and rectum causing the active contraction of
smooth muscles and further raising intrarectal pressure thus forcing the faeces towards the anus.
 Relaxation of internal anal sphincter occurs by inhibitory signals from the myenteric plexus, when the
peristaltic wave approaches the anus.
N/B
 The intrinsic defaecation reflex functioning by itself is relatively weak. To be effective in causing
defaecation, this reflex usually fortified by a spinal cord reflex.
Spinal cord reflex.
 Distension of rectum by faeces causes transmission of afferent impulses through the pelvic nerves to sacral
segments of spinal cord.
 This induces reflex parasympathetic discharge (mainly from S2) and the pelvic splanchnic nerves to cause:
 Intensification of colonic peristaltic contraction further raising the intrarectal pressure,
 When the rectal pressure reaches to about 55 mm Hg there occurs,
 Further relaxation of internal anal sphincter and relaxation of external sphincter as well.

20. Role of voluntary control on defecation
Once the previously described reflex effects are obtained, the voluntary control mechanism depending upon
the convenience may or may not allow the act of defecation to occur:
When defaecation is not allowed,
 The voluntary control mechanism maintains the contraction of external anal sphincter (which is
composed of skeletal muscle innervated by the pudendal nerves).
 Soon, the internal anal sphincter also closes and the rectum relaxes to accommodate the faecal matter
within it.
 Once the defaecation reflex dies out, it recurs after some hours.
When it is convenient to defaecate,
 The external anal sphincter is relaxed voluntarily. Thus both internal and external sphincters are
relaxed.
 The intra-abdominal pressure is increased by the contraction of abdominal and diaphragmatic muscles (a
process of expiring against closed glottis, i.e. Valsalva manoeuvre).
 The smooth muscles of the distal colon and rectum contract forcibly, propelling the faecal matter out of
the body through the anal canal.

21. Cont…
Voluntary initiation of defaecation.
 As per convenience, before the pressure that relaxes the external anal sphincter is reached (i.e.
below 55 mm Hg but above 18 mm Hg), the defaecation can be voluntarily initiated.
 This is done by voluntarily relaxing the external sphincter and contracting the abdominal muscles
(straining); thus aiding the reflex emptying of distended rectum.

22. FAECES
Composition:
 Faeces or the faecal matter is derived mainly from the intestinal secretion and partly from the
undigested material.
 The faecal matter consists of: water, forms the main bulk of faeces (75%) and solids, contribute
25% to total faecal matter weight.
 Solids include inorganic material, mostly calcium and phosphate, undigested plant fibres, epithelial
cells, dead bacteria, constituents of intestinal secretions including bile pigments, fats and proteins.
 It is important to note that:
 Proteins in the stools are not of dietary origin but comes from bacteria and cellular debris.
 Fats in the stools come some from the dietary intake but most of it is also derived from the
desquamated epithelial cells and from the bacterial synthesis.
 On an average intake of (about 100 g/day) fat, only 5–6 g is lost in faeces.
pH of stools:
 Is slightly acidic (5–7) due to the organic acids formed from the carbohydrates by colonic bacteria.

23. Cont…
Brown colour of stools:
 Is due to the pigment urobilin, which is formed from oxidation of urobilinogen which is colourless.
 Urobilinogen is formed from the bile pigments by the intestinal bacteria.
 Oxidation of residual urobilinogen in the stools accounts for the darkening of faeces, which occurs
upon standing in the air.
 When the bile fails to enter the intestine, stools become white (acholic stools), as seen in
obstructive jaundice.
Odor of stools:
 Is due to the presence of substances like indole, skatole, mercaptans and hydrogen sulphide.
 These substances are formed by the action of colonic bacteria on the food.

24. APPLIED ASPECTS OF DEFECATION
Defaecation in infants.
 In infants, defaecation reflex causes automatic emptying of lower bowel without normal
voluntary control on external anal sphincter.
 The voluntary control of the reflex by higher centres is attained by social training as the child
grows.
Defaecation in individuals with spinal cord transection.
 In individuals with spinal cord transection, initially there occurs retention of faeces. But
defaecation reflex returns quickly.
 However, reflex evacuation occurs automatically, without voluntary control, when the rectal
pressure increases to about 55 mm Hg.
Role of dietary fibres.
 Dietary fibres increase bulk of faeces, this plays a role in defaecation reflex by distending the
rectum.

25. DIETARY FIBER (ROUGHAGE)
 Is a group of food particles which pass through stomach and small intestine without being digested and
reach the large intestine unchanged.
 Characteristic feature of dietary fiber is that it is not hydrolyzed by digestive enzymes. So, it escapes
digestion in small intestine and passes to large intestine.
 It provides substrate for microflora of large intestine and increases the bacterial mass.
 The anaerobic bacteria, in turn, degrade the fermentable components of the fiber. Thus, in large
intestine, some of the components of fiber are broken down and absorbed and remaining components
are excreted through feces.
Components of Dietary Fiber
 Major components of dietary fiber are cellulose, hemicelluloses, D-glucans, pectin, lignin and gums.
 Cellulose, hemicelluloses and pectin are partially degradable, while other components are indigestible.
 Dietary fiber also contains minerals, antioxidants and other chemicals that are useful for health.
Sources of Dietary Fiber
 Sources of dietary fiber are fruits, vegetables, cereals, bread and wheat grain (particularly its outer
layer).

26. Roles of dietary fiber
a) Physiological role of dietary fibres on intestinal food transit:
 Add bulk to the faeces, and thus play a role in defaecation reflex by distending the rectum.
b) Role of dietary fibres in prevention of diseases:
Epidemiological evidences indicate that groups of people who consume a diet which contains large
amounts of vegetable fibres have a low incidence of diverticulitis, cancer of colon, diabetes mellitus
and coronary artery disease.
Probably, the dietary fibres might be playing role by their following effects:
 Reduction in absorption of digested foodstuffs is caused by
dietary fibres by forming a mechanical barrier between the
nutrients and absorptive surface. Due to this effect, the dietary
fibres reduce chances of post-prandial hyperglycaemia and are
thus especially useful in diabetics.
 Reduction in blood cholesterol level by dietary fibres
is caused by increasing excretion of bile salts in faeces
as summarized:

27. Cont…
Therefore, dietary fibres are especially useful in the patients with atherosclerosis, obesity,
hypercholesterolemia and diabetes mellitus.
c) Therapeutic role of dietary fibres
The daily recommended intake of dietary fibres is about 25–35 g/day. High-fibre supplements have
therapeutic role in following conditions:
 In constipation, the dietary fibres work as bulk laxatives by providing a larger volume of indigestible
material to the colon. Plantago lanata or isabgol, rich in hemicellulose, is being used since ages as
ancient Indian medicine for constipation.
 In spastic colon and diverticular disease, the dietary fibres are useful by making the stools softer and
thus lowering the intraluminal pressure.
 In diabetes and high cholesterol levels, the role of dietary fibres as mentioned earlier.
 In diarrhoea, complete avoidance of dietary fibres is useful by increasing the transit time, decreasing
the frequency and volume of stools.

28. FUNCTIONS OF LARGE INTESTINE
1. ABSORPTIVE FUNCTION
Large intestine plays an important role in the absorption of various substances such as:
1. Water
2. Electrolytes
3. Organic substances like glucose
4. Alcohol
5. Drugs like anesthetic agents, sedatives and steroids.
2. FORMATION OF FECES
 After the absorption of nutrients, water and other substances, the unwanted substances in the large
intestine form feces. This is excreted out.
3. EXCRETORY FUNCTION
 Large intestine excretes heavy metals like mercury, lead, bismuth and arsenic through feces.
4. SECRETORY FUNCTION
 Large intestine secretes mucin and inorganic substances like chlorides and bicarbonates.
5. SYNTHETIC FUNCTION
 Bacterial flora of large intestine synthesizes folic acid, vitamin B12 and vitamin K. By this function,
large intestine contributes in erythropoietic activity and blood clotting mechanism.

29. CLINICAL NOTES
HIRSCHSPRUNG’S DISEASE (AGANGLIONIC MEGACOLON)
 Refers to the congenital absence of Auerbach’s plexus in the wall of rectosigmoid region.
 This leads to the blockage of both the peristalsis and mass contractions at the aganglionic
segment.
 Therefore, the faeces pass the aganglionic segment with difficulty and accumulate in the large
intestine leading to dilatation of the colon (megacolon).
CONSTIPATION
 Refers to the failure of voiding of faeces which produces discomfort.
 It results from infrequent mass movement in the colon.
 As a result, the faecal matter remains in the colon for longer time, so a large amount of fluid is
absorbed and the faeces become hard and dry.

30. Cont…
CONSTIPATION
 Failure of voiding of feces, which produces discomfort is known as constipation.
 It is due to the lack of movements necessary for defecation.
 Due to the absence of mass movement in colon, feces remain in the large intestine for a long time,
resulting in absorption of fluid. So the feces become hard and dry.
Causes
1. Dietary causes: Lack of fiber or lack of liquids in diet causes constipation.
2. Irregular bowel habit: Irregular bowel habit is most common cause for constipation. It causes
constipation by inhibiting the normal defecation reflexes.
3. Spasm of sigmoid colon: Spasm in the sigmoid colon (spastic colon) prevents its motility, resulting
in constipation.
4. Diseases: Constipation is common in many types of diseases.
5. Dysfunction of myenteric plexus in large intestine – megacolon
6. Drugs: The drugs like diuretics, pain relievers (narcotics), antihypertensive drugs (calcium channel
blockers), anti parkinson drugs, antidepressants and the anticonvulsants cause constipation.

31. Cont…
APPENDICITIS
Inflammation of appendix is known as appendicitis.
Appendix does not have any function in human beings. But, it can create
major problems when diseased.
Appendicitis can develop at any age. However, it is very common between 10
and 30 years of age.
Causes
 The cause for appendicitis is not known.
 It may occur by bacterial or viral infection.
 It also occurs during blockage of connection between appendix and large
intestine by feces, foreign body or tumor.
Features
 Main symptom of appendicitis is the pain, which starts around the
umbilicus and then spreads to the lower right side of the abdomen. It
becomes severe within 6 to 12 hours.

32. Cont…
Other features include:
1. Nausea
2. Vomiting
3. Constipation or diarrhea
4. Difficulty in passing gas
5. Low fever
6. Abdominal swelling
7. Loss of appetite.
N/B
 If not treated immediately, the appendix may rupture and the inflammation will spread to the
whole body, leading to severe complications, sometimes even death.
 Therefore, the treatment of appendicitis is considered as an emergency.
 Usual standard treatment for appendicitis is appendectomy (surgical removal of appendix).

33. Cont…
ULCERATIVE COLITIS
 Is an inflammatory bowel disease (IBD), characterized by the inflammation and ulcerative aberrations in
the wall of the large intestine.
 It is also known as colitis or proctitis. Rectum and lower part of the colon are commonly affected.
Sometimes, the entire colon is affected.
 Ulcerative colitis can occur at any age. More commonly, it affects people in the age group of 15 to 30
years. Rarely it affects 50 to 70 years old people.
Cause
 Exact cause for ulcerative colitis is not known. However, it is believed that the interaction between the
immune system and viral or bacterial infection causes this disease.
Features
1. Abdominal pain
2. Diarrhea with blood in the stools
3. Early fatigue
4. Loss of appetite and weight
5. Arthritis and osteoporosis
6. Eye inflammation
7. Liver diseases like hepatitis, cirrhosis, etc.
8. Skin rashes
9. Anemia.

34. C.O.L.O.N