Physiology of the large intestine Flashcards
What is the proximal portion of the large intestine?
1) Cecum
2) Ascending colon
3) Half of the transverse colon
What is the main function of the proximal part of the large intestine?
Absorption of water and electrolytes (as the material coming from the S.I is still liquid)
What are the distal parts of the large intestine?
1) Distal half of the transverse colon
2) Descending colon
3) Sigmoid colon
What is the main function of the distal part of the large intestine?
Storage of the fecal matter
What regulates the ileocecal sphincter?
- 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
Describe the movement of the large intestine
- 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
Describe the mixing movement/Haustration of the L.I
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
Which muscle is responsible for the haustrations?
Teniae coli & the circular muscles
Describe the propulsive of the large intestine
- 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
What will happen if the propulsive movements keeps on occuring?
This will cause disorders like ulcerative colitis
Describe the mass movement of the L.I
- 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
What are the reflexes that controls the intestines?
1) Gastrocolic reflex
2) Duodenocolic reflex
3) Irritation of the colon
4) Peritoneo-intestinal reflex
5) Renointestinal and vesicointestinal reflex
What are the reflexes involved in initiating the mass movement?
1) Gastrocolic reflex
2) Duodenocolic reflex
3) Irritation of the colon (as seen in ulcerative colitis)
What activates the gastrocolic and duodenocolic reflexes?
Distension, due to the food being in the stomach or the duodenum
What regulates the gastrocolic and duodenocolic reflexes?
1) Neural: The autonomic nervous system
2) Hormones: CCK, Gastrin
What is the function of the peritoneo-intestinal reflex?
- Occurs due to the irritation of the peritoneum
- It inhibits the excitatory enteric nerves causing intestinal paralysis
What is the function of the renointestinal and vesicointestinal reflex?
They inhibit the intestinal activity due to the irritation of the kidney or the bladder
Why is the rectum mostly empty?
Due to the existence of the functional sphincter between the sigmoid colon and the rectum and due to the sharp angulation
What prevents the continuous dribbling of fecal matter through the anus?
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
At what rectal pressure do we get the urge to defecate?
18mmHg
- At 55mHg a reflux expulsion of fecal matter will occur
What is the stimulus of the intrinsic defecation reflex?
- Mediated by the Enteric nervous system in the rectal wall
It is stimulated by the feces entering the rectum distending the rectal wall
What are the receptors that senses the distention to initiate the defecation reflex?
Stretch receptors in the rectal wall
What are the nerve fibers that send impulses for the defecation reflex?
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
What are the effector muscles in the defecation reflex?
The smooth muscle cells of the descending colon, sigmoid and rectum
What is the final response of the defecation reflex?
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
What is the parasympathetic defecation reflex?
- 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
What are the other effects that are produced by the defecation signals that enters the spinal cord?
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
What will happen if a Spinal cord injury occurs below S2-S4?
Reflexive defecation is lost (causing fecal incontinence)
What will happen if a Spinal cord injury above S2-S4?
Reflexive defecation persists, but voluntary control is lost
What is meant by voluntary defecation?
- 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
What prevents the back diffusion in the large intestine (makes it impermeable)?
Tight junctions, in the L.I they are tighter than those found in the S.I
What are the electrolytes that gets absorbed in the large intestine?
- 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)
What are the electrolytes that gets secreted in the large intestine?
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”)
In-words describe the absorption in the large intestine
- 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)
What are the structures responsible for the L.I secretions?
The crypts of liberkuhn
What are the contents of the secretory products of the L.I?
1) Mucus (protective and adherent functions) “secreted by the mucus”
2) Moderate amount of bicarbonate “secreted by the non-mucus secreting epithelial cells”
What regulates the secretion of the L.I?
1) Tactile stimuli
2) Local nervous reflexes
3) Parasympathetic nerves
What is the function of the mucus produced?
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
How is chloride secreted?
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
How is HCO3- secreted into the lumen?
- In the mucus
- Excessive bicarbonate secretion in the colon during diarrhea results in metabolic acidosis
What is the bacterial action in the colon?
- 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)
What is the composition of the feces?
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
What is a aganglionic megacolon?
- 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
What are the causes of constipation?
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
What is the treatment of constipation?
1) More fiber in the diet
2) Laxatives
3) Lubiprostone (which enhances the chloride and water secretion)
What is diarrhea?
The rapid movement of fecal material through the large intestine
What are the causes of diarrhea?
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”)
What is the treatment of diarrhea?
1) Fluid replacement
2) Antibiotic if needed
What is meant by ulcerative colitis?
- 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)
What are the possible causes of ulcerative colitis?
- It is unknown but can be due to:
1) Allergic or immune destructive effect
2) Chronic bacterial infection
