AS Lecture 7 - The Large Intestine Flashcards
What is the cecum?
Blind pouch just distal to ileocecal valve - larger in herbivores
What is the appendix?
Appendix is thin finger like extension of caecum - not physiologically relevant in humans
What are the functions of the colon?
Reabsorption of electrolytes and water Elimination of undigested food and waste
What is the structure of the colon?
1.5m long and 6cm in diameter Ascending: RHS, from cecum to hepatic flexure Transverse: hepatic flexure to splenic flexure, hanging off stomach from greater omentum Descending: from splenic flexure to sigmoid colon Sigmoid colon from descending colon to rectum
Where does the colon’s blood supply come from and what is a complication that can arise?
Proximal transverse colon uses middle colic artery
Distal third of transverse perfused by inferior mesenteric artery
Region between these two is sensitive to ischemia
What is the layout and features of the colon?
Peritoneum carries fatty tags and the muscle coat has 3 thick longitudinal bands (taeniae coli), the gut wall is pouched in appearance (haustra) Many nodules of lymphoid tissue
What is the function of Fatty tags/appendices epiploica?
May have protective function against intraabdominal infections
What is the function of the taenia coli?
Necessary for large intestine motility
What does the colon absorb and where?
Colon absorbs electrolytes and water, more in proximal colon Large intestine can reabsorb around 4.5 l
What is the rectum and what is its histology?
Dilated distal portion of alimentary canal Has transverse rectal folds in its submucosal and absence of taenia coli in its muscularis externa
What is the function of the transverse rectal folds?
Store the faeces until allowed to remove it - with sphincter under conscious control
Fill in the blanks of the general plan of the gut tube
How is the colon similar to the small intestine?
Enterocytes and goblet cells are abundant, as well as crypts Stem cells are also found in the crypts
How is the large intestine mucosal layer organised?
Mucosa is smooth as it has no villi (smaller SA) Enterocytes have Microvilli (short, irregular) and are concerned with reabsorption of salts Water is absorbed passively following the electrolytes
What are the levels of goblet cells like in the crypts of the large intestine?
Higher number of goblet cells, which increases the further down the gut, as it facilitates the passage of increasingly solid faeces Dominated by goblet cells
What is the abundance of Paneth cells and enteroendocrine cells in the colon?
None in colon, fewer enteroendocrine cells than in SI
What are the muscle layers of the large intestine?
Muscularis externa - consists of inner circular and outer longitudinal layer Cicular muscles segmentally thickened Longitudinal layer in taenia coli (in between layer is thin) Bundles of muscle from taeniae coli penetrate the circular layer at irregular intervals
How are the haustra distributed in the large intestine?
Shorter than circular muscle layers, Haustra can contract individually Apart from rectum and anal canal, they are substantial and continuous
What are the colonic contractions of the large intestine?
Colonic contractions - kneading process (minimally propulsive) helps to promote absorption of electrolytes and water Proximal colon: Anti propulsive patterns dominate to retain chyme Transverse/descending: Haustral contractions cause back and forth mixing
What is mass movement (colon)?
Resembles peristaltic wave, can propel contents 1/3-3/4 of length of large intestine in a few seconds - food containing fibre promotes rapid transport through colon
What is the parasympathetic and sympathetic innervation of the colon?
Parasympathetic innervation: vagus nerve for ascending and most of transverse; pelvic nerves for distal Sympathetic: lower thoracic and upper lumbar spinal cord
How does defecation occur?
Rectum filled with faeces by mass movement in sigmoid colon, which stores it until convenient to void - defecation reflex controlled primarily by spinal cord
What is the process of defecation?
Reflex to distension of wall of rectum, pressure receptors send signals via myenteric plexus to initiate peristaltic waves in descending, sigmoid colon and rectum, internal anal sphincter inhibited Weak intrinsic signal augmented by autonomic reflex BUT since external anal sphincter is under voluntary control if the urge is resisted then sensation subsides
What is important about the ‘social part’ of the rectum?
Can distinguish between solid, liquid and gas - that perceptual ability is important in knowing what can be passed appropriately in what circumstances NB: oil and gas are detected as the same thing
How much faeces do we produce in one day?
150g/day
Where is the gut flora more commonly present?
All mammals have symbiotic relationships with their gut microbial community Stomach and small intestine have few as it is protected Large intestine contains many bacteria
What is the flora and how much does it weigh?
Diverse, highly metabolically active community The microbiome is around 1.5 kg
What are some of the roles of intestinal flora?
Synthesise and excrete vitamins Prevent colonisation by pathogen by competing for attachment sites and nutrients Antagonise other bacteria Stimulate production of cross reactive antibodies which can prevent invasion/infection Stimulate development of certain tissues including caecum and lymph
What happens when gut bacteria break down fibre?
Fibre can be broken down by colonic bacteria, produces scfa which can be released as hormones
What are the types of normal flora present in the gut?
Most are bacteroides - GNB, anaerobic, non spore forming bacteria which may be implicated in colon cancer/colitis Bifidobacteria are GPB non spore forming, lactic acid bacteria - described as friendly and are thought to prevent colonisation by potential pathogens
What does the large intestine consist of?
Colon, caecum, appendix, rectum and anal canal
What are haustra and how are they formed?
Because the Taenia Coli are shorter than those in the small intestine, it causes formation of pouched ovoid
By which mechanisms are Na, Cl, water and K absorbed into the large intestine?
Na and Cl absorbed by exchange mechanisms and ion channels Water follows by osmosis K moves passively into lumen
What is the terminal portion of the rectum?
The anal canal which is surrounded by internal (circular muscle) and external (striated muscle) anal sphincters
What does the anal canal look like? - FITB
What stimulates goblet cell secretion in the colon?
Acetylcholine (parasympathetic and enteric NS)
Which cells dominate the crypts in the colon and what is the difference in the glycocalyx?
Crypts dominated by goblet cells Glycocalyx doesn’t contain any digestive enzymes
When do colonic contractions increase?
Following a meal
What are haustral contractions?
Localised segmental contractions of circular muscle
How many times a day does mass movement occur?
1-3 times daily
How many times a day do colonic contractions occur?
Every 30 minutes
What are 4 types of control of the large intestine’s contractions?
Enteric NS Myenteric plexus ganglia concentrated below taenia coli Presence of food in stomach can stimulate mass movement Hormonal/paracrine control - aldosterone promotes Na and H2O absorption
What type of innervation controls the external anal sphincter?
External anal sphincter controlled by somatic motor fibres in pudendal nerves
What are the components of faeces?
Majority is water Solids: cellulose, bacteria, cell debris, bile pigments, salts Bile pigments give the colour and bacterial fermentation gives sme
What 5 things has researched linked gut bacteria to?
Drug metabolism Insulin resistance Bile acid metabolism Lipid metabolism Obesity
