Physiology of the large intestine

Question 1

What is the proximal portion of the large intestine?

Answer 1

1) Cecum

2) Ascending colon

3) Half of the transverse colon

Question 2

What is the main function of the proximal part of the large intestine?

Answer 2

Absorption of water and electrolytes (as the material coming from the S.I is still liquid)

Question 3

What are the distal parts of the large intestine?

Answer 3

1) Distal half of the transverse colon

2) Descending colon

3) Sigmoid colon

Question 4

What is the main function of the distal part of the large intestine?

Answer 4

Storage of the fecal matter

Question 5

What regulates the ileocecal sphincter?

Answer 5

• Pressure changes in the ileum and the cecum
• If pressure in the cecum increases it will close, and if the pressure in the ileum increases it will open

Question 6

Describe the movement of the large intestine

Answer 6

• Similar to the S.I (it is sluggish in nature “as the intense colon movements are not required”)
• We have two types of movement:

1) Mixing movement (AKA: haustrations “Remember that in the S.I it was called segmentation”)

2) Propulsive movement

3) Mass movement

Question 7

Describe the mixing movement/Haustration of the L.I

Answer 7

1) Sac-like

2) Peaks in 30 seconds and disappears for a minute

3) Haustration occurs due to the large circular constrictions caused by the contraction of the circular muscle occurring simultaneously at the same time as the three longitudinal strips of muscle (teniae coli), which causes the budging of the unstimulated part forming a sac-like structure

• The contents are slowly dug and rolled into the next segment
• Occurs mainly in the ascending and transverse colon

Question 8

Which muscle is responsible for the haustrations?

Answer 8

Teniae coli & the circular muscles

Question 9

Describe the propulsive of the large intestine

Answer 9

• Occurs 1-3 times per day (as they are very strong), moving the contents of the L.I for long distances
• They occur for 10-30 minutes, then stops and returns after 12 hrs
• Once the feces reaches the rectum (it will activate the defecation reflex) their will be a desire to defecate
• Mostly seen in the distal portion of the large intestine

Question 10

What will happen if the propulsive movements keeps on occuring?

Answer 10

This will cause disorders like ulcerative colitis

Question 11

Describe the mass movement of the L.I

Answer 11

• A strong, forceful wave of contraction that pushes feces toward the rectum (Modified peristalsis)
• It mainly occur in the transverse and descending colon
• A constriction ring occurs due to the distention of the colon, where the colon distal to the ring loses its haustrations and contract as a unit
• The contraction develops for about 30 seconds and relaxes for the next 2-3 minutes

Question 12

What are the reflexes that controls the intestines?

Answer 12

1) Gastrocolic reflex

2) Duodenocolic reflex

3) Irritation of the colon

4) Peritoneo-intestinal reflex

5) Renointestinal and vesicointestinal reflex

Question 13

What are the reflexes involved in initiating the mass movement?

Answer 13

1) Gastrocolic reflex

2) Duodenocolic reflex

3) Irritation of the colon (as seen in ulcerative colitis)

Question 14

What activates the gastrocolic and duodenocolic reflexes?

Answer 14

Distension, due to the food being in the stomach or the duodenum

Question 15

What regulates the gastrocolic and duodenocolic reflexes?

Answer 15

1) Neural: The autonomic nervous system

2) Hormones: CCK, Gastrin

Question 16

What is the function of the peritoneo-intestinal reflex?

Answer 16

• Occurs due to the irritation of the peritoneum
• It inhibits the excitatory enteric nerves causing intestinal paralysis

Question 17

What is the function of the renointestinal and vesicointestinal reflex?

Answer 17

They inhibit the intestinal activity due to the irritation of the kidney or the bladder

Question 18

Why is the rectum mostly empty?

Answer 18

Due to the existence of the functional sphincter between the sigmoid colon and the rectum and due to the sharp angulation

Question 19

What prevents the continuous dribbling of fecal matter through the anus?

Answer 19

The tonic constrictions of the 1. Internal anal sphincter (thickening of the circular smooth muscle) and 2. External anal sphincter controlled voluntarily by the pudendal nerve

Question 20

At what rectal pressure do we get the urge to defecate?

Answer 20

18mmHg

• At 55mHg a reflux expulsion of fecal matter will occur

Question 21

What is the stimulus of the intrinsic defecation reflex?

Answer 21

• Mediated by the Enteric nervous system in the rectal wall

It is stimulated by the feces entering the rectum distending the rectal wall

Question 22

What are the receptors that senses the distention to initiate the defecation reflex?

Answer 22

Stretch receptors in the rectal wall

Question 23

What are the nerve fibers that send impulses for the defecation reflex?

Answer 23

1) Afferent Sensory fibers terminates in the myenteric plexus

2) The myenteric plexus acts as the center

3) Efferent motor signals are sent to the smooth muscles

Question 24

What are the effector muscles in the defecation reflex?

Answer 24

The smooth muscle cells of the descending colon, sigmoid and rectum

Question 25

What is the final response of the defecation reflex?

Answer 25

1) Peristaltic wave forcing the feces towards the anus 2) Internal anal sphincter gets relaxes by the inhibition from the myenteric plexus - Once the external anal sphincter relaxes voluntarily defecation occurs

Question 26

What is the parasympathetic defecation reflex?

Answer 26

- Myenteric defecation reflex is weak, the parasympathetic defecation strengthens it - It occurs in the following order: 1) The nerve endings in the rectum gets stimulated 2) Signals are then sent to the spinal cord 3) Via the parasympathetic nerves in the pelvic nerves the rexlex goes back to the descending colon, sigmoid, rectum and anus 4) The peristalsis are then intensified and the internal sphincter relaxes 5) The intrinsic myenteric reflex then strengthens

Question 27

What are the other effects that are produced by the defecation signals that enters the spinal cord?

Answer 27

1) Taking a deep breath 2) Closure of the glottis 3) Contraction of the abdominal wall muscles - In newborn babies and people with a transected spinal cord the defecation reflex causes an automatic emptying due to the lack of conscious control

Question 28

What will happen if a Spinal cord injury occurs below S2-S4?

Answer 28

Reflexive defecation is lost (causing fecal incontinence)

Question 29

What will happen if a Spinal cord injury above S2-S4?

Answer 29

Reflexive defecation persists, but voluntary control is lost

Question 30

What is meant by voluntary defecation?

Answer 30

- Defecation is a spinal reflex which can be voluntarily inhibited or facilitated 1) It is Initiated by: - Straining (where the pelvic floor is lowered by 1-3cm) - Relaxation of the puborectalis muscle - The anorectal angleincreases to 120-140 - Relaxation of the external anal sphincter

Question 31

What prevents the back diffusion in the large intestine (makes it impermeable)?

Answer 31

Tight junctions, in the L.I they are tighter than those found in the S.I

Question 32

What are the electrolytes that gets absorbed in the large intestine?

Answer 32

- Their is no brush border instead we have a apical or luminal surface 1) Sodium chloride (Mainly via the sodium/potassium/2-chloride cotransporter "NKCC1") 2) Water 3) Potassium (depending on its level in the body)

Question 33

What are the electrolytes that gets secreted in the large intestine?

Answer 33

1) Bicarbonate (Mainly, secreted in-exchange for chloride) 2) Potassium (depending on its levels "secreted as a component of the mucus") 3) Chloride (exits via the cystic fibrosis transmembrane conductance regulator "CFTR")

Question 34

In-words describe the absorption in the large intestine

Answer 34

- Cl- absorption follows Na+ absorption (via the electrical potential gradient) - Wates moves down the gradient (absorbed in the colon, water permeability is less than in the small intestine) - L.I has a maximum absorption capacity of 5-8L of fluid and electrolytes - During diarrhea K+ secretion by the colon increases due to the increase in the flow rate which will lead to (hypokalemia)

Question 35

What are the structures responsible for the L.I secretions?

Answer 35

The crypts of liberkuhn

Question 36

What are the contents of the secretory products of the L.I?

Answer 36

1) Mucus (protective and adherent functions) "secreted by the mucus" 2) Moderate amount of bicarbonate "secreted by the non-mucus secreting epithelial cells"

Question 37

What regulates the secretion of the L.I?

Answer 37

1) Tactile stimuli 2) Local nervous reflexes 3) Parasympathetic nerves

Question 38

What is the function of the mucus produced?

Answer 38

1) Protects the intestinal wall from abrasion 2) Acts as an adherent medium which holds the fecal matter together 3) Protects the intestinal wall from the bacterial activity + the alkalinity of the secretion protects us from the acids formed by the feces

Question 39

How is chloride secreted?

Answer 39

1) The Sodium, Potassium, 2-chloride cotransporter "NKCC1" brings sodium inside 2) Chloride levels increases inside the cell 3) The CTFR channels will then transport the Cl- to the lumen

Question 40

How is HCO3- secreted into the lumen?

Answer 40

- In the mucus - Excessive bicarbonate secretion in the colon during diarrhea results in metabolic acidosis

Question 41

What is the bacterial action in the colon?

Answer 41

- Colon bacilli are present in the absorbing colon (normally) - They are capable of absorbing small amounts of cellulose - They produce vitamin K, B12, thiamine, riboflavin and various gases (CO2, methane)

Question 42

What is the composition of the feces?

Answer 42

1) 3/4 is water 2) 1/4 solid matter (30% dead bacteria, 10 to 20% fat, 10 to 20 % inorganic matter, 2 to 3% protein, 30% undigested roughage) - Brown color is due to the stercobilin, and urobilin (derivates of bilirubin) - Odor in the stool is due to indole, skatole, mercaptans, and hydrogen sulfide

Question 43

What is a aganglionic megacolon?

Answer 43

- Hirschsprung's disease (large portion of the sigmoid colon does not have functional ganglionic cells - It is genetically determined (congenital absence of ganglionic cells in the myenteric plexus, due to the failure of the normal cranial-to-caudal migration of the neural crest cells during development) - It is clinically known to cause abdominal distension, anorexia, and lassitude, and typically bowel movements occur once in several days - Surgery is the treatment

Question 44

What are the causes of constipation?

Answer 44

1) Pathological - Decrease in bowel movements - Tumors - Adhesions 2) Spasm of the small segment of the sigmoid colon (very common due to the slow movement) 3) Alteration in the balance between colonic secretion and absorption 4) Irregular bowel habits which inhibits the normal defecation reflex

Question 45

What is the treatment of constipation?

Answer 45

1) More fiber in the diet 2) Laxatives 3) Lubiprostone (which enhances the chloride and water secretion)

Question 46

What is diarrhea?

Answer 46

The rapid movement of fecal material through the large intestine

Question 47

What are the causes of diarrhea?

Answer 47

1) Prescence of infection 2) Mucosal irritation 3) Enteritis (which is an inflammation caused by a bacteria or virus "mostly in the large intestine and distal ileum")

Question 48

What is the treatment of diarrhea?

Answer 48

1) Fluid replacement 2) Antibiotic if needed

Question 49

What is meant by ulcerative colitis?

Answer 49

- It is when their is extensive inflammation and ulceration of the walls of the large intestine - Motility of the ulcerated colon is increased which can lead to diarrhea - If the ulcer does not heal ileostomy is performed (surgical removal of the entire colon)

Question 50

What are the possible causes of ulcerative colitis?

Answer 50

- It is unknown but can be due to: 1) Allergic or immune destructive effect 2) Chronic bacterial infection