Large Bowel Flashcards
What is the main function of the large bowel?
Reabsorption of electrolytes and water and elimination of undigested food and waste
What is the length and diameter of the large bowel?
1.5m long
6cm diameter
What are the parts of the large bowel?
Consists of:
- Appendix
- Caecum
- Colon(large intestine) -Ascending colon, Transverse colon, descending colon and sigmoid colon
- Rectum
- Anal canal
Starts from the ileoceacal valve/Caecum following on from the end of the small intestine(ilium) part of the small bowel
What is the appendix?
Thin finger like projection from the caecum that is not clinically relevant in humans
Where do each parts of the colon run to and from?
Ascending Colon-right side of the abdomen up from Caecum to hepatic flexture (the turn of the colon near the liver)
Transverse Colon-runs from the hepatic flexure to splenic flexure(turn of the colon near the spleen)
Descending Colon- runs from splenic flexure to the sigmoid colon
Sigmoid colon(s-shaped)-runs fromthe descending colon to the rectum
Describe the blood supply of the bowel?
- Then proximal transverse colon is supplied with blood form the middle COLIC ARTERY(branch of the superior mensenteric artery)
- The distal third of the transverse colon is supplied by the INFERIOR MENSENTERIC ARTERY and supplies the rest of the colon
The region between the midgut and the hindgut is snesitive to Ischemia as there is less blood supply here
What does the foregut, midgut and hindgut consist of?
Foregut -gives rise to the esophagus, stomach, liver, gallbladder, bile ducts, pancreas and proximal duodenum
Midgut-distal half of the duodenum, jejunum, ileum, cecum, ascending colon, and the proximal half of the transverse colon
Hindgut-distal 1/3 of the transverse colon, descending colon, sigmoid colon and the upper anal canal.
What is the peritoneum and its structure and functions?
Serous membrane forming the lining of the abdominal cavity and covers most of the organs in the abdominal cavity including the colon
- Has FATTY TAGS (appendices epiploicae) -function unknown thought to be protective
- has muscle coat has 3 thick LONGITUDINAL BANDS( taeniae coli)-necessary for large intestine mobility
- These cause a pouched a ppearence of the cut wall(HAUSTRA)
-Lymphoid tissue nodulaes are common in small intestine(Peyers Patches) and large intestine(Solitary noduales)
What are the main functions of the colon?
- absorbs electrolytes and water
- Most things done in the proximal colon
- Na+ and Cl- absorbed by exchandge mechanisms and ion channels
- water follows by osmosis
- K+ moves passively into the lumen
How much water can the large intestine absorb?
4.5 L but normallly 1.5L is reabsorbed
What is the difference between the rectum and the rest of the colon?
- Has TRANSVERSE RECTAL FOLDS in its SUBMUCOSA
- absence of TAENIA COLI in its muscular exterior
What are the key structures of the Anal Canal?
-surrounded by internal circular muscle and external striated muscle
Describe the mucosal structure of the Large Bowel?
From Lumen to ouside:
MUCOSA-Epithelium, lamina propria, Muscularis Mucosae
SUBMUCOSA- submucosa space which has mucous secreting glands, Submucosal Plexus (MEISSNERS PLEXUS) nerves that sit on top of the submuscosal layer
MUSCULARIS- longitudinal muscel and circular muscle, Myentric Plexus (AUERBACH’S PLEXUS)
SEROSA- secures everything beneath it and has the vein and arteries going through it
What are the intestinal glands in the large intestine made up of and what are there functions?
- Enterocytes have lots of microvilli to absorb water
- Crypts dominated by Goblet cells and increase of goblet cells towards the rectum(as more mucus needed to help flow of increasingly solid contents)
- Stem cells are found in the crypts
What stimulates Goblet cells?
ACh -parasympathetic of goblet cell secretion
Describe the muscle layers of the large bowel?
Inner circular muscles-thickened
Outer longitudinal layers concentrated into 3 bands called the TAENIA COLI
These Taenia coli penatrate the circular layer at irregular intervals whIch help form the HAESTRA(not in rectum or anal cannal) which can contract INDIVIDUALY
Describe the motility of the large Bowel?
- Colon contractions (neading process) 5-10cm/hour at most
- Slow =more time for the absorption of electolytes water as more time being exposed to the mucosa
PROXIMAL COLON- ANTIOPROPULSIVE PATTERNS (where there is forward and backwards) whcih keeps the chyme in the area so more time for absorption
TRANSVERSE and DESCENDING COLON-localised segmental contractions of circular muscles called HAUSTRAL CONTRACTIONS causing back and forth mixing
-Short propulsive movements evewry 30mins to keep tings moving
What are the features opf mass movement out of the rectum occur?
1-3 times a day
can propel contents 1/3 -3/4 length of the large intestine
-Food that contains fibre promotes rapid transport through the colon
Describe the Autonomic (involuntary) and Somatic(voluntary) nervous control of the large bowel?
AUTONOMIC:
PARASYMPATHETIC:
- the ascending colon and most of the transverse colon is innervated by the VAGUS NERVE
- distal part of the transverse colon and descending colon is innervated by the PELVIC NERVE
- SYMPATHETIC:
- goes form T8 to L2
- stops motility
SOMATIC:
-somatic motor fibres of the PUDENDAL NERVES from S2,3, 4
Which neurons detect pressure in the large bowel?
AFFERENT SENSORY NEURONS -especially detect in rectum
Describe enteric nervous control of the Large Bowel?
In submucosal and musculari layer
MYENTERIC PLEXUS GANGLIA concentrated below the TAENIA COLI
Enteric Nervous control innervates Interstitual cells of Cajal make up the internal pacemaker of the large bowel=causes motility-patients without this can develop HIRSHSPRUNGS DISEASE(no bowel movement =contipation as infant)
Give an example of hormonal control in the large bowel ?
ALDESTERONE- promaotes sodium and therefore water reabsorption
Describe defecation of the large bowel?
- Rectum filled with faeces by mass movement in the sigmoid colon
- store stool unitl convenient to void
What controls the defacation reflex?
SACRAL SPINAL CORD- S2, 3, 4
Both reflex and voluntary control: Internal sphincter is a reflex but the external sphincter is voluntary control
Pressure receptors send signals via the MYENTERIC PLEXUS to initiate PERISTALTIC WAVES in the descending, sigmoid colon and rectum. This inhibits the internal anal sphincter
-weak intrinsic signal augmetned by autonomic reflex
Why are we able to pass waste appropriately?
Rectum can distinguish between Solid, liquid and Gas
What gives faeces its colour and odour?
- Bile pigments give colour
- Bacterial fermentation gives odour
What do faeces consist of?
- 2/3rds water
- sloids: cellulose, bacteria, cell debri, bile pigments, salt, K+
- 150g/ per day
How much microbiome do we have ?
1.5kg i adult of active bacteria
What are the 7 main roles of intestinal flora/microbiome?
- synthesis and excretion of VITAMINS e.g Vit K
- Prevent colonisation by pathogens by COMPETING for attatchment sites or for essential nutrients
- Antagonise other bacteria through the production of SUBSTANCES which inhibit/kill non -indigenous substances
- stimualte the production of CROSS REACTIVE ANTIBODIES, which are produced against certain components of the flora and therefore can prevent infection/invasion of related pathogens
- FIBRE can be broken down by colonic bacteria
- Produce short chain fatty acids which can regulate GUT HORMONE RELEASE or be absorbed to be used as an ENERGY SOURCE or to influence functions such as FOOD INTAKE(can reduce appetite) or INSULIN SENSITIVITY directly
- stimulate the development of certain TISSUES including caecum and lymphatic tissues
What is the most common flora we have?
BACTERIOCIDES- GRAM -NEGATIVE anaerobic, non spore forming bacteria
BIFIDOBACTERIA-gram postive, non spore forming, lactic acid bacteria, described as freindly bacteria, thought to prevent colonisation of pathogens
What can be caused by too much Bacterioides?
Implicated in the initiation of COLON CANCER and COLITIS
What 5 main problems are gut bacteria sometimes assosiated with?
- Drug Metabolism
- Insulin resistance
- Bile acid metabolism
- Lipid Metabolism
- Obesity
(GI disorders)
