Module 6 Section 6

Question 1

large intestine

Answer 1

• made up of the cecum, appendix, and colon
• unlike the small intestine, the large intestine is not coiled and s made of 3 parts: ascending colon, transverse colon, and descending colon
• the end part of desnecing colon forms the sigmoid colon that straightens out to form the rectum
• each day between 500 and 1500mL of chyme enters the large intestine
• because the majority of absorption occurred in the small intestine, this chyme consists of indigestible food elements, such as cellulose, unabsorbed biliary secretions, adn fluid
• the upper half of the colon is able to extract a little more salt and water but majroity of it moves to the lower half of the colon for storage before being eliminated as feces

Question 2

haustral contractions

Answer 2

• the colon has both circular and longitudinal smooth muscle layers
• the longitudial layers, do no completely surround the large intestine but ar eorganized into 3 distinct bands called the taenia coli
• the length of the taenia is also much shorter than the underlying smooth muscle and mucosal layers (if these layers were stretched out flat) so it causes them to bunhc up and form sacs called haustra
• the haustra are not fixed in position and can change location as a result of contraction of circular smooth muscle layer

Question 3

the colon’s motility

Answer 3

• the colons main motility is haustral contractions, which are slow and non-propulsive
• they are generated bu autonomous rhythmicity of colonic smooth muscle cells and are naalogoous to segmentation
• the frequency though is a lot lower
• for the duodenum, segmentation occurs about 12/minute
• haustral contractions can be as few as 2/hour and are controlled by locally mediated relfexes involving the intrinsic plexus

Question 4

differences between motility of small intestine and the colon

Answer 4

• motility in the small intestine consists of weak persitaltic waves that are sustained for onyl short distances before dying out
• they start at the stomach and slowly move their way down the intestine
• thier purpose is to move any remnants of the previous meal, any mucosal debris, and any bacteria forward towards the colon
• when peristaltic waves reach the end of the ileum, they start again at the beginning of duodenum
• the colons main motility is through hautral contractions which are slow and non-repulsive
• they are generated by autonomous rythmicity of colonic smooth muscle cells and are analogous to segmentation
• the frequency is a lot lower (2/hour)

Question 5

mass movements

Answer 5

• several times a day, usually following meals, there is a marked increase in the haustral contractions that cna move the feces 1/3-3/4 the length of the colon in a few seconds
• these are called mass movemetns and are driven by the gastrocolic reflex
• it is mediated from the stomach by the release of gastrin and by the extrinsic autonomic nerves
• when a new meal enters the stomach, the gastroileal relfex moves the contents of the small intestine to the large intestine, and the gastrocolic reflex moves the contents of the large intestine to the rectum and trigger the defecartion relfex

Question 6

steps of the defecation reflex

Answer 6

1. when fecal matter fills the rectum, stretch receptors send impulses to the spinal cord, which sends a reflex signal via the parasympathetic system to the distal colon
2. this causes ht einternal anal sphincter (smooth muscle, involuntary) to relac and cause contractions in the sigmoidal colon and rectum
3. if the external and sphincter (skeletal msucle, voluntary) is relaxed then defecation will occur. if the timing isnt approproate, voluntary control of external anal sphincter keeps it closed

Question 7

defecation

Answer 7

• when defecation does occur, contraction of the abdorminal msucles and a forcible expiration against the closed glottis increases the abdominal pressure to aid in expelling the feces
• the rectum gradually calms its activity and the urge to defecate subsides
• at the next mass movemet, more feces enters the rectum and the defecation reflex repeats

Question 8

large intestine secretions

Answer 8

• no digestion occurs in the large intestine so no enzymes are secreted
• colonic secretions consists of an alkaline (NaHCO3) mucus to lubricate and protect the colon by neutralizing acids produced from bacterial fermentation
• in contrast to the other digestve organs, the colon does not secrete antibacterial agents
• its slow motility acutally favours bacterial growth so any bactria that do reach the colon cna thrive
• there are more than 10,000 different species of bacteria that comprise the microbiome in the colon and they serve beneficial functions

Question 9

roles of the microbiome in the gut

Answer 9

• enhance intestinal immunity by comepting with potentially pathogenic microbes for nutrients and space
• promote colonic motility
• make nutritional contributions as a result of their metabolism
• help to maintain the integrity of the colon mucosa

Question 10

gut microbiome

Answer 10

• has the largest numbers of bacteria and the largest number of species of bacteria as comapred to the other areas of the body
• by the age of 2, the gut flora has usually been establshed
• by this time ht eintestinal epithelium and intestial mucosal barrier secreted by the epithelium have co-developed in a way which not only provides a barrier to pathogenic organisms, but also is supportive of the gut flora

Question 11

gut microbiome research

Answer 11

• over the past several years, research on the topic of the microbiome has dramatically reshaped our understanding of human biology
• findings from this research has suggested that the composition of our microbiome may mediate digestion as well as disease processes

Question 12

large intestine absorption

Answer 12

• the luminal surface of the colon is smooth so it has a elatively low surface area for absorption
• it lacks the specialized transporters that are found in the small intestine so it typically only absorbs salt and water
• sodium is actively absorbed, while chloride and water follow

Question 13

how a firm fecal mass is formed through the absorption of salt and water

Answer 13

Question 14

irritable bowel syndrome

Answer 14

• a common disorder that affects the large intestine is IBS
• often this disorder does not have severe signs or symptoms
• however the sings and symptoms include crammping, abdominal pain, bloating, gas, diahhroea or constipation or both
• IBS is a chronic disorder and needs to be managed