Physiology of the Large Intestine and Defecation

Question 1

What are the functions of the large intestine?

Answer 1

• absorption of water and electrolytes from chyme to form solid feces (proximal half)
• storage of fecal matter until expulsion (distal half)

Question 2

How long does it take to move chyme from the ileocecal valve, through the entire length of the colon?

Answer 2

8 - 15 hours

Question 3

What are the mixing movements of the large intestine called?

Answer 3

haustrations= large circular constrictins (2.5 cm) combined with longitudinal contractions via tenia coli (3 strips)

Question 4

What are “mass movements”?

Answer 4

propulsive movements that can last for 10-30 minutes 1 - 3 times/day.

Question 5

What happens when a mass of feces is forced into the rectum?

Answer 5

desire to defecate is felt

Question 6

Are there cryps of Lieberkuhn in the large intestine?

Answer 6

YES, but there are NO villi or digestive enzymes

Question 7

What secretions take place in the large intestine?

Answer 7

mucus secretion ONLY from mucus cells. It contains bicarb (pH 8), is important for protection of the intestinal wall, and acts as a medium for holding fecal matter together.

Question 8

What regulates mucus secretion in the large intestine?

Answer 8

tactile stimulation of the epithelial cells and local nervous reflexes (parasympathetic innervation to distal 2/3 via pelvic nerves) to the mucous cells in the crypts of Lieberkuhn

Question 9

How much chyme enters the the large intestine per day?

Answer 9

1.5 L= mostly water and electrolytes absorbed from this

Question 10

How does absorption in the large intestine occur?

Answer 10

like the small intestine, using active transport of Na+ across the epithelial cells (and hence water via osmosis), pulling Cl- ions along with it (allowing HCO3- to be exchanged).

Question 11

Are the tight junctions tighter or looser than in the small intestine?

Answer 11

tighter to prevent back diffusion of ions and water

Question 12

What is the maximum absorption capacity of the large intestine?

Answer 12

5-8 L of fluid and electrolytes per day!

Question 13

What is Diarrhea?

Answer 13

excess secretion of water

Question 14

What is enteritis?

Answer 14

bacterial infection of large intestine. The mucosa secretes extra-large quantities of water and electrolytes plus normal viscid alkaline mucus. This dilution causes rapid movement of feces toward the anus (helping to clear the bacterial infection), but causes severe loss of water.

Question 15

What are the main bacteria in the colon and what do they produce?

Answer 15

bacilli, which digest small amounts of cellulose to provide a few extra calories of nutrition. Other substances formed include:

• vitamin K (maintains blood coagulation)
• vitamin B12
• thiamine (B1)
• riboflavin (B2)
• various gases (contribute to flatus)= CO2, hydrogen gas, and methane

Question 16

What makes of the composition of feces?

Answer 16

• 3/4 water
• 1/4 solid matter= dead bacteria, fat, inorganic matter, protein, undigested roughage, bile pigment, and sloughed epithelial cells

Question 17

What causes the brown color of feces?

Answer 17

stercobilin and urobili (derivatives of bilirubin from heme metabolism via colonic bacteria).

Question 18

What causes the odor of feces?

Answer 18

byproducts of bacterial action (indole, skatole, mercaptans, and hydrogen sulfide)

Question 19

What are the 2 anal sphincters?

Answer 19

1. internal= circular smooth muscle immediately inside anus.
2. external= striated voluntary muscle surrounding internal sphincter.
   * both are normally tonically constricted.

Question 20

What nerve fibers control the external anal sphincter?

Answer 20

pudendal nerve (somatic nervous system). 
*conscious signals inhibit constriction.

Question 21

Is there a weak functional sphincter between the sigmoid colon and rectum?

Answer 21

YES

Question 22

What happens when feces enters the rectum?

Answer 22

stretch receptors initiate impulses to the spinal cord, leading to motor impulses, which cause contraction of the rectal muscles and relaxation of the anal sphincters.

Question 23

What are the 2 defecation reflexes?

Answer 23

1. intrinsic reflex

2. parasympathetic reflex

Question 24

** What is the intrinsic reflex?

Answer 24

• distention of rectum initiates ENTERIC NERVOUS SYSTEM afferent signals that spread through the MYENTERIC PLEXUS, initiating peristaltic waves in descending colon, sigmoid and rectum, forcing feces towards anus.
• as the peristaltic wave approaches the anus the internal sphincter relaxes via inhibitory signals from the myenteric plexus, and defecation will occur with final conscious voluntary relaxation of the external anal sphincter.

Question 25

*** What does the intrinsic reflex require to effectively cause defecation?

Answer 25

fortification from the parasympathetic defecation reflex

Question 26

** What is the parasympathetic defecation reflex?

Answer 26

when nerve endings of the rectum are stimulated, signals are transmitted to sacral segements of the spinal cord, then reflexively back to the descending colon, sigmoid colon, rectum, and anus by parasympathetic nerve fibers in pelvic nerves.
* this greatly intensifies peristaltic waves and relaxes the internal anal sphincter.

Question 27

What are some additional effects that defecation signals initiate?

Answer 27

• taking a deep breath
• closure of glottis
• contraction of abdominal wall muscles
• pelvic floor to relax downward and pull outward on anal ring to evaginate feces.

Question 28

*** What are 3 other autonomic reflexes that affect bowl activity?

Answer 28

1. peritoneointestinal reflex= irritation of peritoneum storngly inhibits excitatory enteric nerves, causing intestinal paralysis.
2. renointestinal reflexe= inhibit intestinal activity due to kidney irritation.
3. vesicointestinal reflex= inhibit intestinal activity due to bladder irritation.