Large Intestine Physiology Flashcards
How long is the large intestine?
Approximately 1.5m long, 6cm diameter
What does the large intestine comprise of?
Cecum
Appendix
Colon
Rectum
Anal canal
What volume of chyme does the large intestine receive per day? How?
500ml
Gatroileal reflex
What are the different sections of the colon?
Ascending
Transverse
Descending
Sigmoid
Which intestine is longer?
Small intestine, 4.5 to 7.0m
What is the difference between the width of the intestines?
Small intestine –> narrow
Large intestine –> comparatively broader
How many parts do the intestines have?
Small –> 3 (duodenum, jejunum, ileum)
Large –> 4 (ceacum, colon, rectum, anal canal)
Are there villi in the intestines?
Small –> Yes
Large –> No
Is digestion part of the intestines’ functions?
Small –> Yes but it is complicated
Large –> No role in digestion, completed prior to reaching large intestine
Do the intestines secrete hormones?
Small –> Yes
Large –> No
What is the role of absorption of the intestines?
Small –> Absorbs digested nutrients
Large –> Absorption of water and electrolytes from indigestible residue
What is the motility like of the intestines?
Small –> small movement in the abdominal cavity
Large –> It is fixed, motility is limited
Where are the bacteria of the small intestine mainly found?
In the ileum
Why are there no bacteria in the large intestine?
Due to the acid present, the ileocecal valve prevents bacteria from going from the small intestine to the large (infection prevention)
What are the functions of the large intestine? (6)
- Absorption of water and electrolytes
- Formation and elimination of feces
- Bacterial fermentation
- Synthesis of vitamins
- Immune function
- Storage
What is the absorption of water and electrolytes function of the large intestine?
It is the primary function of the large intestine, absorb water from undigested food material
What are the electrolytes that also get absorbed in the large intestine?
Na+, K+ and Cl-
What is the formation and elimination of feces function of the large intestine?
After absorption of water, the remaining water material becomes more solid and is formed into feaces
What is the bacterial fermentation function of the large intestine?
It houses diverse microbiota, they ferment un-absorbed carbohydares and produce SCFA which can be used as an energy source
What is the synthesis of vitamins function in the large intestine?
The bacteria synthesize certain vitamins that are beneficial like Vitamin K and B7
What is the immune function of the large intestine?
The intestinal walls contain lymph nodes that produce and house immune cells, defend against pathogenic bacteria
What is the storage function of the large intestine?
Rectum serves as a temporary storage for feces until they can be eliminated
How much water is taken in/ secreted daily?
A total of about 9L/day
How is the water absorbed?
Small intestine –> 7L/day
Colon –> 2L/day
What is the most important electrolyte absorbed in the large intestine?
Na+
Why do the mechanisms of absorption of Na+ vary between the proximal and distal colons?
The concentration of Na+ decreases as it moves to the distal colon, so it requires a stronger mechanism in order to absorb it
What other movement does Na+ absorption methods affect?
Affects water movement and helps form solid feces
What is the mechanism of absorption in the proximal colon?
Na+ transport is mostly through Na+-uni-transport (electrogenic)
What is the mechanism of absorption in the distal colon?
Mostly through coupled Na+ - H+ anti-port
What facilitates the absorption of Na+ in the distal colon?
Aldosterone facilitates absorption through the Na+/K+ channel
What is the absorption of Cl- like?
It is in exchange for bicarbonate
The movement of sodium into the plasma produces an electrochemical gradient to allow absorption of chloride
How is water absorbed in the large intestine?
The absorption of electrolytes creates an osmotic gradient to allow further absorption of water
Where is the movement of Na+ like?
From the lumen of the small intestine or colon to the interstitial fluid
What is the absorption in the GIT regulated by?
Neuro-endocrine mechanisms
What are the different endocrine mechanisms?
Aldosterone, gluccocorticoids and somatostatin
How do the endocrine mechanisms affect absorption?
Increases the net absorption of water and electrolytes by stimulating the basolateral Na+-K+ ATPase.
This increases the electrochemical gradient and driving force for sodium absorption
It also increases the transcription of epithelial sodium channels
What is the effect of enteric nervous system on absorption?
Parasympathetic innervation promotes net secretion from intestines
Sympathetic promotes net absorption from the intestines (inhibits secretions)
What are the bacterial digestion function kinds of the large intestine?
Fermentation of Carbohydartes
Digestion of Proteins
Synthesis of Vitamins
What is the fermentation of carbohydrates like in the large intestine?
Undigested carbohydrates that reach the large intestine are fermented by the colonic bacteria
What does the fermentation process produce?
Several gases
SCFA
What gases are produced from the fermentation process?
CO2, methane and hydrogen
What are SCFA produced from the fermentation process?
Acetate, propionate, butyrate
What is the digestion of proteins like in the large intestine?
Bacteria can also digest proteins that were not completely digested in the small intestine
What is the result of bacterial protein digestion ?
Formation of simpler compounds, like amines and ammonia
What happens to the ammonia?
Most of it is absorbed back into the liver, and part of it will be used by the bacteria for energy.
What vitamins are synthesized by the large intestine?
Vitamin K,
Biotin,
Vitamin B7
What is the function of the proximal part of the large intestine?
Absorption
What is the function of the distal part of the large intestine?
Storage and slight absorption
What is the movement of the proximal part of the large intestine like?
Segmentation movement, the most common type of movement in the colon
How often do haustral contractions occur?
Every 30 minutes
What are segmentation movements?
Mixing movements that help expose fecal material to the colon’s surface, aiding absorption
What are the contraction segments of the large intestine called?
Haustra
What is the movement of the distal part of the large intestine like?
Mass movement (propulsive movement)
What are mass movements like?
Long, slow-moving, propulsive contractions that occur 2 to 3 times a day
What is the function of mass movements?
Help move fecal matter towards the rectum
Which parts of the large intestine use mass movements?
Descending and Sigmoid colons
When can mass movements occur and why?
Can occur after meals, in order to empty the large intestine for the new chyme to come
Gastro-colic reflex
Duodeno-colic reflex
What is the effect of castor oil?
Cause diarrhea because it initiates mass movement
Which irritants can initiate mass movement?
Castor oil, threatening agents (parasites and enterotoxins)
What is the gastrocolic reflex?
Food in stomach –> distension
Increase of motility of colon
Increase of mass movement in large intestine
Which factors can increase motility of colon?
CCK & Gastrin
Which factors affect the activity of the colon?
PNS activity (parasympathetic nervous system)
What are the gastrocolic and duodenocolic reflexes?
Reflexes that enhance colic motility in response to stomach and duodenal stretching, respectively.
They might also be responsible for the urge to defecate following a meal
What is the regulation of transit?
The sphincter helps modulate the rate at which digested food enters the large intestine, ensuring optimal absorption of nutrients and water
What is the prevention of cecal backflow?
Prevents bacteria-laden contents of the colon from returning into the small intestine
What happens if there is ileal distension?
Increases sphincter relaxation, allowing chyme to pass into the large intestine
What happens if there is cecal distension?
Tightens the sphincter, inhibiting backward flow, acts as a valve
What is the protective function of the ileocecal sphincter?
Prevents the migration of bacteria from the large intestine to the semi-sterile environment of the ileum
What are the mechanisms of control of the ileocecal sphincter?
Neural control
Hormonal control
What is the neural control of the ileocecal sphincter?
Enteric nervous system, which is local
What is the enteric nervous system primarily influenced by?
Reflexes that respond to the distension in ileum or cecum
What is the hormonal control of the ileocecal sphincter like?
Gatsrin
Motilin
CCK
Where is gastrin secreted from, why and what is its effect?
Secreted by the G cells in the stomach in response to food intake and stomach stretching.
It promotes gastro-intestinal motility and indirectly influences the ileocecal sphincter by relaxing it, allowing chyme to move into the cecum
When is motilin secreted?
During fasting
What is the function of motilin?
Stimulates migrating motor complexes, which also help regulate the sphincter, ensuring periodic relaxation
What is the function of migrating motor complexes?
To push the residual
To push bacteria
What is the function of CCK?
To stimulate gallbladder contraction and pancreatic secretion, it can also influence the motility of the GIT, including the sphincter
What is the clinical relevance of sphincter dysfunction?
Issues with the ileocecal sphincter can result in conditions such as ileocecal valve syndrome or contribute to the small intestine bacterial overgrowth
What is the function of the ascending colon?
Processes chyme by absorbing water and salts from the indigestible food matter
What is the role of the ascending colon?
Water absorption, turning chyme into semi-solid feces
What is the duration of the chyme in the ascending colon?
87 minutes
What is the motility of the ascending colon like?
Slow haustral contractions which aid in mixing and absorptiom
Limited peristalsis compared to small intetsine
What is the function of the transverse colon?
Specialised for storage and dehydration of feces
What is the transverse colon the primary site of?
The removal of water and electrolytes
The storage of feces
What is the descending colon?
A conduit between transverse and sigmoid colon
What is special about the descending colon?
It is the region that has the neural program for power propulsion
What is the recto-sigmoid region?
The junction between the sigmoid colon and rectum
What is the recto-sigmoidal region’s function?
Fecal material is stored in this region until defecation
Sensory function
Muscular contraction
What is the sensory function of the recto-sigmoidal region?
Stretch receptors in the rectum detect fullness –> signal the need for defecation
What is the muscular contraction function of recto-sigmoidal region?
The smooth muscles in this region help retain fecal material by contracting and relaxing as needed
What is fecal continence?
The ability to control the passage of stool and gas
what maintais fecal continence?
Recto-sigmoid region
Anal canal
Pelvic floor muscles
Which is the pelvic floor muscle specific to continence?
Puborectalis, skeletal muscle
It contracts and prevents release of gas and feaces
How does the puborectalis help keep feces in the rectum?
The muscle maintains an angle between the rectum and the anus
What is the mechanism of fecal continence?
At rest, the internal sphincter remains contracted, and the puborectalis muscle maintains the anorectal angle
The external anal sphincter stays contracted, providing extra control
What are the factors affecting continence?
Age-related weakness
Neurological Disorders
Diet and Fibers
What happens to pelvic floor muscles with age?
They weaken
How do neurological disorders affect continence?
Conditions like spinal cord injuries can impair the control of sphincter and pelvic muscles
How does diet and fiber affect continence?
A diet high in fiber helps in forming bulkier stools, aiding continence
What initiates the spinal reflex?
What is it aided by?
Distention of the rectum
Aided by voluntary contractions of the diaphragm and abdominal muscle
What are the two reflexes involved in defecation?
Intrinsic defecation reflex
Parasympathetic defecation reflex
What is he intrinsic defecation reflex?
Mediated by the local enteric nervous system, when the rectum is stretched, it initiates peristaltic wave pushing feces towards anus
What is the parasympathetic defecation reflex?
Intensifies the intrinsic reflex, it is initiated by stretch receptors in the rectum sending signals to the spinal cord, which amplify the peristaltic waves and relax the internal anal sphincter
What happens to people with neural injury (spinal)?
The stretch receptors from the rectum cannot send strong signals to the spinal cord, which leads to the accumulation of feces and thus constipation
What is the involuntary control of defecation? (parasympathetic)
Parasympathetic;
1. Stretch receptors in the rectal walls activated
2. Receptors send signals via afferent nerves to spinal cord
3. From spinal cord, efferent fibers travel to smooth muscles of colon and rectum
4. Contraction of the rectal muscles and relaxation of the internal anal sphincter
What is the involuntary control of defecation (sympathetic)?
Maintains tone in the internal anal sphincter, keeping it contracted
During defecation, there is inhibition of this tone, aiding relaxation
What is the voluntary control of defecation (somatic nervous system)?
External anal sphincter –> skeletal muscle
Once the internal sphincter relaxes, a person can choose to voluntarily contract the external anal sphincter, thereby postponing defecation if it isn’t a suitable time
The opposite can also happen
Why does the large intestine not secrete enzymes?
Digestion is complete before chyme reaches large intestine (colon)
What are the colonic secretion?
Alkaline mucus solution
What is the role of the colonic secretion?
Protects mucosa from mechanical and chemical injury
Lubrication for stool passage
Alkaline secretion neutralizes acid
Where does the acid in the large intestine come from?
- The chyme is acidic
- Due to local bacterial fermentation
