Small bowel and large intestine Flashcards
1
Q
What does the large intestine consist of?
A
Consists of the colon (ascending, transverse, descending and sigmoid), cecum, appendix, rectum and anal canal.
2
Q
What is the cecum?
A
The cecum is a blind pouch just distal to the ileocecal valve- larger in herbivores.
3
Q
What is the appendix?
A
The appendix is a thin, finger-like extension of the cecum- not physiologically relevant in humans.
4
Q
Function of colon? (x2)
A
The principal functions of the colon are the reabsorption of electrolytes and water and the elimination of undigested food and waste.
5
Q
Dimensions of the colon - length and diameter?
A
1.5m long, 6cm diameter.
6
Q
Describe journey of the colon anatomically?
A
Begins on right side. 1. Cecum–>Hepatic flexure (describes the bend at the top of the ascending colon - called hepatic, because bend occurs at liver) 2.Hepatic flexure–>splenic flexure (transverse - splenic fixure is the turn of the colon by the spleen) 3.Splenic flexure–>sigmoid colon (descending) 4. Sigmoid (s-shaped) colon–>rectum.
7
Q
What are the attachments of the transverse colon? (x2)
A
Transverse colon hangs off the stomach, attached by a wide band of tissue called the greater omentum.
Transverse mesocolon attaches the transverse colon to the posterior abdominal wall.
8
Q
Blood supply of colon? (x2) Clinical implication of tissue supplied by both sources?
A
The proximal transverse colon (first two thirds of the transverse colon) is supplied with blood by the middle colic artery (branch of the superior mesenteric artery).
Distal third of transverse colon is perfused by the inferior mesenteric artery.
Regions where inferior and colic artery capillary plexi join, tissue is sensitive to ischaemia e.g. more sensitive to haemorrhages.
9
Q
Name of fatty tags along colon?
A
apendices epicoli
10
Q
Describe muscle structure of colon
A
3 longitudinal bands (taenia coli)
11
Q
The large intestine is pouched in appearance: what are these pouches called?
A
Haustra.
12
Q
Secondary lymphoid tissue of the colon? How does this differ from the small intestine?
A
Solitary nodules.
These are also found in the small intestine, but small intestine also contains Peyer’s patches.
13
Q
Function of fatty tags
A
Might have protective function against infection
14
Q
Tenia coli function? (x2)
A
Contributes to peristalsis.
Because they are shorter than the large intestine, they create the pouch ovoid segments called haustrae (yellow arrow).
15
Q
Describe the reabsorption that takes place in the colon.
A
Colon absorbs electrolytes and water.
More in proximal colon.
Na+ and Cl- absorbed by exchange mechanisms and ion channels. Water follows by osmosis. K+ moves passively into lumen.
16
Q
How much water does the large intestine reabsorb?
A
Up to 4.5L, any more than this, and reach diarrhoea potential
17
Q
What is the rectum? Structure of the anal canal?
A
Dilated distal portion of the alimentary canal.
Histology similar to the colon.
Terminal portion is anal canal. Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters (photo).
18
Q
What histologically distinguishes colon from rectum? (x2)
A
Also distinguished by transverse rectal folds in its submucosa.
And the absence of tenia coli in its muscularis externa.
19
Q
What is the purpose of the transverse rectal folds?
A
For shit to sit on before excretion.
20
Q
Describe control of anal sphincters
A
Internal is unconscious, external is conscious control
21
Q
What are the layers of the colon external -> internal?
A
Serosal, longitudinal muscle, circular muscle, submucosa, muscularis mucosa, lamina propia, epithelal layer.
22
Q
Similarities and differences between LI and SI wall? (x2 and x3)
A
SIMILARITIES: Enterocytes and goblet cells are abundant. Abundant crypts, which contain stem cells.
DIFFERENCES: There’s more goblet cells in LI. Mucosa in large intestine smoother because NO villi because you don’t need as much SA (in the large intestine, enterocytes lack villi; they are still the dominant cells facing the gut lumen in the LI, but mucus-secreting goblet cells dominate the crypts);
Microvilli - shorter and more irregular on the luminal side of the enterocytes in the large intestine.
23
Q
How does enterocyte function differ between the large and small intestine?
A
SMALL: primarily concerned with absorption of carbs and fats.
LARGE: primarily concerned with reabsorption of salts.
24
Q
How is water reabsorbed in the large intestine?
A
(Water is absorbed as it passively follows the electrolytes, resulting in more solid gut contents)
25
Describe goblet cell distribution in large intestine?
Two functions of these goblet cells?
How is goblet cell secretion secreted?
More prevalent in crypts than surface.
Numbers increase distally, to aid passing of faeces because the more distally you go, more water is reabsorbed; so mucus needed to replace water for continued feacal passage.
And covers bacteria and particulate matter so that when faeces emerges, there is reduced infection risk.
Goblet cell secretion stimulated by parasympathetic and enteric nervous systems. Stimulated by Ach.
26
What are Paneth cells and their function?
* Found only in the bases of the crypts of the SMALL INTESTINE.
* Contain large, acidophilic granules containing: antibacterial enzyme lysozyme (protects stem cells); glycoproteins, and zinc (essential trace metal for a no.of enzymes).
* Also engulf some bacteria and protozoa.
* May have a role in regulating intestinal flora.
27
Describe distribution of paneth cells in colon. Why?
No paneth cells, to save gut flora
28
Describe enteroendocrine cells in Colon
Present but far less abundent than in small intestine
29
What is glycolax? Function? (x3)
The surface of the microvilli are covered with glycocalyx.
Glycocalyx:
–rich carbohydrate layer on apical membrane.
–protection from the digestional lumen, yet allows for absorption.
–traps a layer of water & mucous known as the “unstirred layer” which regulates rate of absorption from intestinal lumen.
30
What is the difference between the glycocalyx in the small intestine than colon.
Glycocalyx in colon doesnt contain digestive enzymes.
Small intestine glycocalyx contains digestive enzymes that break down e.g. small peptides into amino acids for absorption - essentially, the final breakdown step.
31
Describe muscle layers of colon
Like the small intestine, muscularis externa consists of an inner circular and outer longitudinal layer.
Circular muscles segmentally thickened.
Longitudinal layer concentrated in three bands- taenia coli.
Between the taenia coli are thin longitudinal layers of muscle is thin.
Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals.
Tinea coli are shorter than the large intestine circular muscle layer - this creates ovoid segments called haustra which can contract individually.
Apart from rectum and anal canal- the longitudinal muscle is substantial and continuous.
Movements of large intestine more complicated than small intestine
32
What is the purpose of colonic contractions?
Kneading process and not really propulsive - only 5-10cm/hr.
More to promote reabsorption.
Anti-propulsive contractions occur in proximal colon, as most of reabsorption occurs here - so contractions aim at retaining the chyme.
33
What do muscle contractions in transverse and descending colon do?
In transverse and descending colon, localised segmental contractions of circular muscle called Haustral contractions cause back and forth mixing.
Short propulsive movements occur every 30 mins. Increase in frequency following a meal to clear colon for incoming food.
34
What is mass movement in the large intestine? What aids this process?
A peristaltic wave 1-3 times daily, Can propel contents 1/3-3/4 of length of large intestine in few seconds. Food that contains fibre (indigestible material) promotes rapid transport through colon).
35
Describe sympathetic, parasympathetic and somatic nervous control of colon. BRIEF role of each.
PARASYMPATHETIC: ascending colon and most of transverse colon (2/3) innervated by vagus nerve. More distal innervated by pelvic nerves. PROMOTION of function.
SYMPATHETIC: lower thoracic and upper lumbar spinal cord. INHIBITION of function e.g. blood supply or motiliy.
External anal sphincter controlled by SOMATIC motor fibres in the pudendal nerves.
36
Describe the presence and name of the enteric nervous system in the gut. Importance?
Present as nerve plexi within wall of the gut.
Myenteric plexus ganglia sits between muscle layers of large intestine wall and are especially concentrated below the taenia coli.
Important within disease ie, IBS, Hirschsprung’s disease (no enteric intramural ganglia).
37
What can stimulate mass movement
food in the stomach
38
What is the function of aldosterone in the large intestine
increases proteins necessary for Na+ reabsorption (and therefore water reabsorption.
39
What controls the defecation reflex?
sacral spinal chord, both reflex and voluntary
40
Describe process of defecation.
What is the process called? What does this mean?
Voluntary control? (x2 aspects)
Reflex to sudden distension of walls of rectum.
Pressure receptors send signals via myenteric plexus to initiate (more) peristaltic waves in descending, sigmoid colon and rectum. Internal anal sphincter inhibited.
WHAT IS THE PROCESS CALLED? This is what we call a weak intrinsic signal augmented by autonomic reflex (meaning that without nerves, this process will still occur but it will just be weaker, and can be augmented (made stronger) by autonomic reflex).
VOLUNTARY CONTROL:
...External anal sphincter under voluntary control.
...Nature of defecation means that if you resist the urge to defecate, the sensation will subside.
41
What is the social part of the rectum. What does this mean?
last few centimetrs
•Can distinguish between solid, liquid and gas. That perceptual ability is important in knowing what can be passed appropriately in what circumstance.
42
How much faeces per day?
150-250g
43
components of faeces?
Two thirds water.
Solids: cellulose, bacteria (lost from gut all the time), cell debris (remember, there's a cycle of losing and replacement of small and large intestinal epithelium all the time), bile pigments, salts (K+).
Bile pigments give colour.
Bacterial fermentation gives odour.
44
Abundance of bacteria in colon?
Relatively high compared to stomach and small intestine. The small intestine and stomach are 'protected'; flora in gut is essential for normal function.
45
Approximate mass of gut flora?
1.5Kg
46
What are the roles of the intestinal flora? (x7)
* Synthesize and excrete vitamins e.g. Vitamin K - germ-free animals can have clotting problems.
* Prevent colonization by pathogens by competing for attachment sites or for essential nutrients.
* Antagonize other bacteria through the production of substances which inhibit or kill non-indigenous species.
* Stimulate the production of cross-reactive antibodies. Antibodies produced against components of the normal flora can cross react with certain related pathogens, and thereby prevent infection or invasion.
* Stimulate the development of certain tissues, including cecum and lymphatic tissues.
* Fibre (indigestible carbohydrate) can be broken down by colonic bacteria.
* Produces 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 or insulin sensitivity directly.
47
What does the short chain fatty acid acetate do (produced from gut flora)?
Don't actually need to know this.
Reduces apetite via central homeostatic mechanism
48
What are the types of gut flora? (x2) Brief function of each.
1. MOST PREVALENT are bacteroides- Gram-negative, anaerobic, non-spore forming bacteria. Implicated in the initiation of colitis and colon cancer.
2. Bifidobacteria are Gram-positive, non-sporeforming, lactic acid bacteria. Have been described as "friendly" bacteria. Thought to prevent colonization by potential pathogens.
49
What are the general functions of the small intestine?
To absorb nutrients, salt AND water.
50
What are the three regions of the small intestine and their lengths?
DUODENUM: 25cm – joins the stomach to the small intestine.
JEJUNUM: 2.5m.
ILEUM: 3.75m – separated from the large intestine by the ileocecal valve.
There is no sudden transition between the regions of the small intestine.
51
What is gut mesentery?
Mesentery is connective tissue which throws the small intestine into folds and supports the blood and nervous supply for the small intestines.
52
How is surface area of the small intestine wall increased?
Internal mucosa arranged into circular folds to increase surface area – look at photo.
Mucosal folds are covered in villi which further increase surface area for absorption.
Invaginations are known as Crypts of Lieberkühn.
53
Where are villi found in the body?
Villi are found in the small intestine only.
54
How are villi innervated?
Have innervation from the submucosal plexuses.
55
What different cell types exist in the small intestine epithelium? (x3 and x2)
MUCOSA LINED WITH: simple COLUMNAR epithelium consisting of primarily ENTEROCYTES (absorptive cells), scattered GOBLET CELLS, and ENTEROENDOCRINE CELLS.
IN THE CRYPTS OF LIEBERKUKN: epithelium includes PANETH and STEM cells.
56
What are the characteristics (x3) and functions (x1) of enterocytes?
CHARACTERISTICS: Have microvilli on the apical surface which make up the ‘brush border’, and a basal nucleus. They have a SHORT LIFESPAN of 1-6 days.
FUNCTIONS: Specialised for absorption of substances.
57
What is the surface of microvilli coated by? Functions? (x2)
Glycocalyx.
(1) Rich carbohydrate layer on apical membrane that serves as protection from the digestional lumen, yet it allows for absorption.
(2) Traps a layer of water and mucous known as the ‘unstirred layer’ which regulates rate of absorption from intestinal layer.
58
What are goblet cells? What is mucous? Function of mucous? Abundance of goblet cells in small intestine?
Mucous containing granules accumulate at the apical end of the cell. Mucous is a large glycoprotein that facilitates the passage of material through the bowel. As you go down the small intestine, the number of goblet cells INCREASE.
59
Where are enteroendocrine cells found in the epithelial layer, and what is their function in the small intestine?
Enteroendocrine cells are columnal epithelial cells found most often in the lower parts of the villi and secrete hormones which have MANY FUNCTIONS, including influence gut motility (movement).
60
Where are Paneth cells found in the Crypts and what are their functions? (x3)
Found in the BASES of Crypts.
1. Contain large acidophilic granules which contain ANTIBACTERIAL ENZYME lysozyme (which protects the stem cells also found in the Crypts), GLYCOPROTEINS and ZINC (essential for a number of enzymes).
2. Engulf some bacteria and protozoa.
3. May have a role in regulating intestinal flora.
61
What is the function of stem cells in the small intestine and where are they found in the Crypt?
Stem cells replace epithelium in the villi by mitosis. The differentiating cells migrate up to the villus tip, replacing older cells that die by apoptosis.
62
Gut epithelium has VERY RAPID turnover: Why is this so? (x3)
1. Enterocytes are the first line defence against GI pathogens and may be directly affected by toxic substances in the diet.
2. Effects of agents which interfere with cell function, metabolic rate etc. will be short-lived because cells are replaced quickly which means function can be restored.
3. And lesions will be short-lived.
63
What are the general differences between the three parts of the small intestine?
DUODENUM: BRUNNER’S GLANDS are present which are submucosal coiled tubular mucous glands secreting alkaline fluid, found in the base of Crypts.
JEJUNUM: characterised by the presence of numerous, large submucosal folds called plicae circulares (or valves of Kerckring) (NB: these are also present in the duodenum and ileum, but plicae in the jejunum tend to be TALLER, THINNER and more FREQUENT.)
ILEUM: shares some features with the large intestine and therefore have a more powerful immunological role. They have a lot of Peyer’s patches which are large clusters of lymph nodules in the submucosa (submucosal layer is connective tissue below the epithelium and above the muscle – look at ‘Oesophagus and stomach’ revision notes).
64
What are Peyer’s patches specialised for?
Prime immune system against intestinal BACTERIA.
65
What complements the function of Peyer’s patches in the ileum?
COMPLEMENTED BY: Paneth cells and rapid cell turnover.
66
Why is the ileum more immunological than other regions of the small intestine?
To prevent bacteria from the COLON migrating up into the small intestine – remember, there is a lot MORE gut flora in the large intestine.
67
What is the function of the alkaline secretions of the Brunner’s gland?
Neutralise acidic chyme from the stomach: (1) protecting the proximal small intestine, and (2) help optimise pH for action of pancreatic digestive enzymes.
68
What are the three functions of small intestine motility?
1. To mix ingested food with digestive SECRETIONS and ENZYMES.
2. To facilitate contact between contents of the intestine and the intestinal mucosa for ABSORPTION.
3. To propel intestinal contents along the alimentary tract.
69
What are the three types of movement in the small intestine? (#1: Where are contractions more frequent? What does this mean for movement of chyme? #2: Length of waves? #3: Purpose? (x2) In the fed state? (x2 points))
SEGMENTATION: mixing function and occurs by stationary contraction of CIRCULAR muscles at INTERVALS. More frequent contractions in duodenum compared to ileum – allow pancreatic enzymes and bile to mix with chyme. Although chyme moves in both directions, there is net movement towards the colon – net direction is partly facilitated by the more frequent contractions in the duodenum than ileum.
PERISTALSIS: involves sequential contraction of adjacent rings of smooth muscle which propels chyme towards the colon. Most waves of peristalsis travel about 10CM (not whole s. intestine length).
Peristalsis and segmentation work together and their functions pretty much link.
MIGRATING MOTOR COMPLEX: in FASTING, cycles of smooth muscle contractions which occur down the small intestine from the stomach. On reaching the terminal ileum, cycle restarts and next contraction starts in the duodenum. PUPROSE: prevents migration of bacteria from the large intestine (colon) into the ileum and prevents accumulation of food in the s. intestine by sweeping it towards the colon. IN FED STATE: this still occurs, but it is less ordered and less frequent.
70
What is the name of the sphincter that separates the stomach from the small intestine? Function?
Pyloric sphincter – contains smooth muscle and controls what gets through the stomach and into the small intestine.
71
How does digestion occur in the duodenum?
1. Occurs in alkaline environment.
2. Digestive enzymes and bile enter the duodenum from the pancreatic duct and bile duct respectively from the pancreas and liver.
3. Duodenal epithelium also produces its own digestive enzymes.
4. Digestion occurs both in the lumen and in contact with the membrane.
72
Does facilitated diffusion require energy?
No.
73
What is the difference between primary and secondary active transport?
PRIMARY: energy is required, coupled with hydrolysis from ATP.
SECONDARY: requires energy but uses energy second-hand from electrochemical gradient.
74
What enzyme in the saliva breaks down carbohydrates?
Salivary alpha-amylase.
75
What is the main enzyme that breaks down carbohydrates in the small intestine? From where? Optimal activity? (x2)
PANCREATIC alpha-amylase secreted in the duodenum and digests starch and glycogen in the small intestine started by salivary alpha-amylase – into smaller carbohydrates.
OPTIMAL ACTIVITY: Needs Cl- AND neutral/slightly alkaline pH which is mediated by the Brunner’s glands in duodenum.
76
Where in relation to the lumen of the small intestines, does digestion of carbohydrates occur? And what enzymes does each location use?
Pancreatic a-amylase action is mainly in the lumen.
Digestion of amylase products and simple carbohydrates occurs at the membrane.
At the brush border, there are embedded enzymes such as maltase, lactase and sucrase which breaks down the amylase products.
77
How are monosaccharides absorbed through enterocytes? (x2 mechanisms)
GLUCOSE AND GALACTOSE: Moves by secondary active transport by exploiting low sodium levels in the cell: Na+ moves down concentration gradient; energy that is released from Na+ movement is used to move glucose/galactose into the enterocytes against concentration gradients. Raising concentration in the enterocytes means that monosaccharides can move into the blood via the basolateral side via facilitated diffusion.
FRUCTOSE: by facilitated diffusion because there is more of it in the lumen than the cell.
78
What carrier proteins are involved in monosaccharide absorption in the enterocytes? (x3)
GLUCOSE AND GALACTOSE: SGLT-1 carrier protein on apical membrane.
FRUCTOSE: GLUT-5 carrier protein.
GLUCOSE, GALACTOSE AND FRUCTOSE on basolateral membrane: GLUT-2 carrier protein.
79
How is the duodenal peptide hormone trypsin activated, and what does trypsin do?
Pancreatic proteases are secreted as precursors. TRYPSIN is activated by enterokinase (an enzyme located on the DUODENAL brush border) from the precursor trypsinogen. Trypsin activates other proteases and more trypsinogens.
80
How are protein broken down and absorbed in the small intestine? Carrier proteins involved in absorption?
(Pepsin breaks down larger peptides in the STOMACH.)
1. BREAK DOWN: Pancreatic peptidases break down larger peptides in the small intestine lumen.
2. BREAK DOWN: Brush border peptidases break down the peptides prior to absorption.
3. ABSORPTION: Amino acids are absorbed by facilitated diffusion and secondary active transport (similar to sugars). Di- and tri-peptides are absorbed using carrier proteins distinct from single amino acids.
4. BREAK DOWN: Cytoplasmic peptides break down most of the di- and tri- peptides before they cross the basolateral membrane.
81
Why are lipids difficult to digest?
Lipids are poorly soluble in water which makes them difficult to digest.
82
What are the four stages of lipid digestion in the small intestine?
1. Secretion of bile (from liver) and lipases (from pancreas).
2. Emulsification from BILE.
3. Enzymatic hydrolysis of ester linkages from LIPASES.
4. Further solubilization of lipolytic products by bile salts --\> micelles.
83
What is the purpose of emulsification? (x2)
Bile salts increase the surface area for digestion and allows pancreatic lipases to split triglycerides by making the fats soluble in the mixture of enzymes.
84
How do bile salts make the fat molecule soluble?
Bile salts surround the fat molecules – bile salts are amphiphilic, so have a hydrophobic face (methyl groups…) to interact with the fat, and hydrophilic end (OH, peptide bonds, COOH groups…) to make it soluble in solution.
85
What do lipases do to triglycerides? How is it secreted?
A triglyceride is broken down into two fatty acids and a 2-monoglyceride (fatty acid attachment on the 2nd carbon) at fat/water interface. Lipase is secreted in its active form.
86
What does lipase use to aid its function?
Colipase secreted in inactive form. Helps shoulder enough of the bile salts out of the way so that lipases can access the fat molecule and break it down.
87
## Footnote
Name and describe the action of two OTHER lipases.
Phospholipase A2 – hydrolyses fatty acids at the 2-carbon position, resulting in lyso-phospholipids and free fatty acids.
Pancreatic cholesterol esterase – hydrolyses cholesterol ester to form free cholesterol and fatty acid.
88
What are mixed micelles?
Have lipid core which contain products of fat synthesis (cholesterol esters and fatty acids), with mono- and di-glycerides, cholesterol, (apoproteins and phospholipids?) surrounding the outside, orientated with hydrophilic heads pointing out.
Structure is stabilised by bile salts, found on the outside which have hydrophilic heads that interact with surrounding solvent and hydrophobic tails that interact with micelle centre.
89
Why are micelles formed?
Micelles can be absorbed much quicker than an emulsion.
90
What happens when micelles are absorbed?
Micelles allow transport across the unstirred layer, and present the fatty acids and monoglycerides to the brush border.
The whole micelle is not absorbed together – bile salts absorbed in the ileum, but lipid absorption is usually complete by the middle of jejunum.
91
What happens to bile salts once they are absorbed? Term to describe this?
Transported back to the liver for recycling. Called ENTEROHEPATIC CIRCULATION.
92
What happens to monoglycerides and free fatty acids once they are absorbed? (x2 pathways)
Resynthesized in cells of the VILLI into triglycerides by different pathways:
1. MONOGLYCERIDE ACYLATION (major pathway – fatty acids bind to apical membrane and fatty acid binding protein (FABP) facilitates the transfer of fatty acids from apical membrane to the SER. In the SER, fatty acids are esterified into di- and tri-glycerides).
2. PHOSPHATIDIC ACID PATHWAY (minor pathway – CoA fatty acids and glycerophosphate used to synthesise a new triglyceride).
93
## Footnote
How are triglycerides transported around the body once synthesised?
CHYLOMICRONS are formed: mostly triglycerides, but contain some phospholipids, cholesterol, protein and carbohydrate. They are transported to the Golgi and secreted across the basement membrane by exocytosis. They are too big to enter blood capillaries of the villi, so enter lacteals (lymph channels) of the villi instead.
94
What separates the ileum from the colon?
The ileocaecal sphincter.
