Esa 3 - Gi - Session 8

Front 1

What happens to chyme in the duodenum.

Back 1

Chyme leaves the stomach as a hypertonic, acidic, partially digested substance and within the duodenum it becomes isotonic, neutral and nearly completely digested.

Front 2

Describe the structure of the small intestine including crypts.

Back 2

The small intestine comprises of the duodenum, the jejunum and the ileum. The mucosa folds into villi which are separated by crypts of lieburkurn. Within the crypts the cells multiply and then migrate to the tip of the villus, maturing as they move. They develop absorption capacity and the micro-villi on the mucosa increase surface area – brush border. They are shed from the villus tip, continually renewing the mucosa lining. This constant shedding of cells is a good physical defense against infection. Inflammatory bowel disease eg Crohn's disease can cause cryptitis, causing malabsorption.

Front 3

Describe formation and purpose of unstirred layer on mucosa of small intestine.

Back 3

Cells in the brush border are very active and secrete many enzymes which remain in between the microvilli to form an unstirred layer on the intestine. Partly digested nutrients diffuse into the unstirred layer and the trapped enzymes complete digestion and release the nutrients for absorption. There for digestion to be complete, there must be effective microvilli lining the small intestine. They are damaged in coliac disease due to a protein in gluten(gliadin) which is deaminated by tissue transaminase and the enzyme-substrate complex is presented by MHC to t cells, causing antibodies against tissue transaminase to be produced. This results in atrophy of villi.

Front 4

Describe the mechanisms of digestion and absorption of sugars in the small intestine.

Back 4

Starch comprises of amyloses with alpha1-4 glycosidic bonds and amylopectins with alpha1-6 glycosidic bonds.
Alpha amylases break down alpha glycosidic bonds to form glucose and maltose from amylases and alpha restricted dextrins from amylopectins.
Alpha amylases are secreted in the saliva and by the pancreas. The unstirred layer of the brush border on small intestines secrete isomaltose which breaks down alpha 1-6 bonds, maltase which converts maltose to glucose and sucrase and lactase.
Glucose is absorbed actively using energy from sodium gradient established by NaKpump on basolateral membrane. It enters mucosal cells via Na+/glucose symporter (SGLT1) which also transports galactose. Glucose then leaves the cell to ECF by facilitated diffusion via GLUT2 transporter on basolateral membrane.
Fructose and lactose enter by facilitated diffusion not linked to Na+

Front 5

Describe the basis of oral rehydration therapy.

Back 5

Glucose and Na are co-transported into the enterocyte. This transport is ‘powered’ by the Na gradient created by the Na/K+ ATPase pump on the basolateral side of the enterocyte. In diseased states if you just put water into the gut, it would simply pass through without being absorbed. If you put water and salt into a solution and ingested this, there would be very limited absorption of Na into the enterocyte and hence very little movement of water across the cell membrane of the enterocyte. If you add glucose to the solution, even in a disase state (Eg resulting in diarrhoea), the gut will absorb this. Remember that Glucose is cotransported with Na (SGLT1) so there is a net movement of both Glucose and Na into the enterocyte. The movement of Na sets up the movement of water and so water moves into the enterocyte. In short the presence of Glucose and Na in a solution will result in greater absorption of water and this is the basis for Oral rehydration therapies.

Front 6

Describe digestion and absorption of amino acids.

Back 6

Proteins are digested to oligopeptides in stomach by Pepsin released in zymogen form from chief cells. Pepsin digests bonds near to aromatic side chains. In the duodenum there are many peptidases from the pancreas. Trypsin cleaves bonds near basic side chains. Chymotrypsin breaks down bonds near aromatic side chains like pepsin and carboxypeptidase likes C terminal amino acids with basic side chain.
Trypsinogen is activated by enterokinase (which is present in the intestinal mucosa) and Trypsin then activates Chymotrysinogen.
Amino acids and small peptides are absorbed and are futher broken down in the unstirred layer.
There are five different Na+/amino acid symporters. Some transport small neutral amino acids, Neutral amino acids, basic amino acids & cystine , Acidic amino acids, Imino-amino acids and b amino acids (mainly taurine). As in glucose the symporter uses secondary active transport, using the Na gradient to power the transport.
Some amino acids are absorbed by facilliated passive diffusion. Dipeptides and tripeptides are absorbed by H+ ions being actively pumped out into the lumen and then H+ ions move back into the cell with the peptide via a symporter.

Front 7

Describe absorption of electrolytes.

Back 7

Sodium diffuses into the cell and is then actively pumped out into the ECF. Chloride follows sodium and this creates an osmotic gradient so water uptake will follow.
Calcium reabsorption is regulated by short term regulation with parathyroid hormone and long term regulation by calcitriol (active vitaminD – 1,25 dihydroxyvitamin D). It enters cell via facilitated passive diffusion and is pumped out in ECF by ca2+ - ATPase.

Front 8

Describe absorption of iron.

Back 8

20mg a day of Fe are consumed, mostly in haem or related pigments as Fe2+. The stomach secretes gastric acid and gastroferrin which is important for Fe absorption as it solubises iron complexes and Fe.
In the unstirred layer, Fe3+ is converted to Fe2+ and is absorbbed. here it either remains in the cell or is attached to apoferritin and released into the circulation.
Iron absorption is a receptor mediated endocytosis. Mucosal cells secrete apotransferrin which binds to 2 Fe2+ in lumen to form transferrin. The complex is binds to transferrin receptors which taken into cells by endocytosis at the coated pits. The vesicle is uncoated and fuses with an acidic endosome (CURL) where the Fe2+ and transferrin are separated. The transferrin receptor and the apotransferin remains attached and are recycled back to the surface membrane, where in the neutral ph, the apotransferin is released.

Front 9

Describe absorption of vitamins and pernicious anaemia.

Back 9

Water soluble vitamins absorbed largely by passive diffusion eg Vitamin C and B vitamins.
Vitamin B12 is absorbed only with a co-factor, intrinsic factor secreted from parietal cells in stomach, in the terminal ileum only.
In Pernicious anaemia vitamin B12 deficiency results due to either stomach damage such as anti parietal cell antibodies or if the terminal ileum is removed. It can result in ineffective erythropoiesis (as a result of thymidine synthase dysfunction) and a megaloblastic anaemia. It is caused by atrophic gastritis, triggered by autoimmune disease with anti parietal cell antibodies. if disease progresses it can lead to tachycardia ( anaemia), confusion, unsteady gait, fever and sensory loses. Treatment is cobalamine, vitamine B12.

Front 10

Describe the patterns of motility of the small intestine.

Back 10

Contents of the small intestine move very slowly ( compared to peristalysis which moves very quickly). The transit time is several hours and gentle agitation is required to expose them to the larger surface area for absorption.
Therefore the contents are not moved by peristalysis but by segmenting. The small intestine is divided into sections each with a pacemaker. The intestinal pacemakers are located at intervals along the length of small intestine with the most lying at the stomach end. This establishes an ‘intestinal gradient’ where the firing of the pacemakers decreases from about 12 per minute in the duodenum to about 8 per minute in the terminal ileum. Each pacemaker drives a short section of intestine causing intermittent contraction of smooth muscle along its length.
These contractions divide the intestines into segments where the muscle is not contracted and so contents are effectively mixed by movement from portions that do contract. After a few seconds the contractions relax and the next pacemaker firing different segments contracts.
Segmenting mixes the contents, whereas it is the intestinal gradient that causes a net movement of contents in a caudal direction as the more rapidly contracting cephalic segment squirts contents into adjacent caudal segment more frequently than that segment does into it.
Most absorption occurs in the small intestine but some occurs in the large intestine so that a semi solid mass of material is formed.

Front 11

Describe the motility in the colon and rectum and their function.

Back 11

The large intestine is divided into segments known as haustra as the circular muscles are more complete than the longitudinal which have been reduced to taenia coli. The taenia coli are about a foot shorter than the colon and so causes the formation of haustra. Haustral shuttling agitates the contents, allowing more reabsorption to form semi solid faeces, and propels them very slowly towards the sigmoid colon. Once or twice a day there is a coordinated peristalsis like movement from the transverse colon towards the rectum known as a mass movement. This forces faeces rapidly into the rectum which is normally empty. The resulting distension of the rectum produces the urge to defaecate.
The colon mainly absorbs water and not many nutrients. The sodium transport in the colon is therefore uncoupled and is mediated by aldosterone.

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Front 12

Describe the mechanisms of defaecation.

Back 12

Mass movement forces faeces rapidly into the rectum which is normally empty. The resulting distension produces the urge to defaecate.
Mass movements are often triggered by eating – the gastro-colic reflex but also often occurs at fixed times of day.
Once the rectum has been filled, stretch receptors are stimulated bringing about the urge to defecate. The defaecation reflexes are activated voluntarily leading to increases in contraction of the rectal smooth muscle, relaxation of the smooth muscle internal anal sphincter and the skeletal muscle external sphincter. It combines with expiration against a closed glottis ( increased parasympathetic activity) and abdominal muscle contraction to increase intra abdominal pressure so expelling the faeces.
If defaecation is not initiated voluntarily sacral reflexes will eventually trigger if involuntarily as rectal pressure rises and overrides inhibition by higher centres.
Feaces contain 40% bacteria, cells and dietary fibre.

Front 13

Describe the gross differences between the small intestine and the large intestine

Back 13

Large intestines are arranged into visible haustra created by the contraction of 3 visible bands of smooth muscle called taeniae coli which arise from longitudinal muscle in the muscularis externa.
- Mesocolic tenia – to which the transverse and sigmoid mesocolons attach
- Omental tenia – to which the omental appendices attach
- Free tenia - neither mesocolons nor omental appendices are attached.
They also have epiploic appendages(omental appendices) which are small fat accumulations on the viscera.
Small intestines have plicae circulares which are permanent folds of the mucosa which increase surface area for absorption. Instead of haustra, the small intestine is divided into smaller segments.
The small intestines main role is for nutrient absorbtion, the large intestins main role is for to reabsorb remaining water.

Front 14

Describe the duodenum.

Back 14

- Duodenum = 25 cm (shortest and widest part of small intestine). Pylorus -> duodenojejunal flexure on the left side at L2 vertebrae, 2-3cm to left of the midline
 First part: duodenal cap is the first 2cm and has a mesentery and is mobile, the other 3 cm and the other parts are retroperitoneal! The hepatoduodenal ligament attaches superiorly and the greater omentum attaches inferiorly.
 Second descending part: curves around the head of the pancreas. The bile and main pancreatic ducts unite to form the hepatopancreatic amupulla which opens on the duodenum at the major duodenal papilla (ampulla of vater controlled by sphincter of oddi)
 Third inferior part: passes over IVC, aorta and L3 vertebra and is crossed by superior mesenteric artery and vein and the root of the mesentery of the jejunum and ileum. Superior is the head of the pancreas and its uncinate process.
 Forth ascending part: runs superiorly along left side of aorta to reach the inferior border of the body of the pancreas. Here it curves anteriorly to join the jejunum at the duodenojejunal flexure ( supported by attachement of a suspensory muscle of the duodenum – contraction widens acute angle of DJ flexure, facilitating movement of the intestinal contents from duodenum into jejunum).

Front 15

Describe the jejunum and the ileum and their characteristic features

Back 15

The jejunum begins at the duodenojejunal flexure where it becomes intraperitoneal. Most of the jejunum lies in the left upper quadrant of the infracolic compartment
There is no clear line of demarcation between the jejunum and the ileum. The ileum ends at the ileocecal junction where the terminal ileum and cecum unite. Most of the ileum lies in the right lower quadrant.
Together the jejunum and ileum are 6-7m long, the jejunum occupying 2/5ths of the small intestine.
The mesentery extends from DJ flexure on the left side of L2 to the ileocolic junction to the right sacroiliac joint and is 15m long.

Front 16

Describe the distinguishing characteristics of the jejunum and ileum.

Back 16

Jejunum – deeper red, 2-4cm wide, thick and heavy wall, greater vascularity, longer vasa recta, a few large arterial arcade loops, less fat in mesentery, large tall and closed packed plicae circulars and a few peyers patches.
Ileum – paler pink, 2-3cm wide, thin and light wall, lesser vascularity, shorter vasa recta, many small arterial arcade loops, more fat in mesentery, low and sparse plicae circulars which are absent in terminal ileum and many peyer’s patches.

Front 17

Describe the blood supply ( superior mesenteric artery and vein) and its distribution to the small intestine.

Back 17

Duodenum:
- Celiac trunk -> hepatic artery -> gastroduodenal artery -> superior pancreaticoduodenal artery proximal to entry of the bile duct into the 2nd part.
- superior mesenteric aretery -> inferior pancreaticoduodenal artery distal to duodenal papilla.
- Veins of duodenum follow arteries and drain into the hepatic portal vein ( joining of superior mesenteric vein and splenic vein posterior to pancreas).
Jejunum and ileum
- The superior mesenteric artery runs between the layers of the mesentery and sends 15-18 branches to the jejunum and ileum via jejunal and ileal arteries. The arteries unite to from loops called arterial arcades which give rise to straight arteries called vasa recta.
- The superior mesenteric vein drains the jejunum and the ileum and lies anterior and to the right of the superior mesenteric artery. It ends posterior to the neck of the pancreas where it unites with splenic vein to form the portal vein

Front 18

Describe the nerve supply to the small intestine.

Back 18

Nerve supply:
The sympathetic fibres in the nerves to the small intestine originate in T8-T10 segments of the spinal cord. The fibres leave via the anterior root, by pass the spinal ganglion and unlike somatic fibres they quickly separate out through white rami communicantes which connect to either the paravertebral (near vertebral column) or prevertebral (near bifurcation of aorta) ganglia alongside the spinal column. The fibres enter the abdominopelvic splanchnic nerve and then Fibres to the head of the duodenum synapse at the celiac ganglion in celiac plexus and those for the rest of the small intestine synapse at the superior mesenteric ganglion. The postsynaptic neurones then surround the superior mesenteric arteries and their branches via periarteriolar nerve plexuses.
The parasympathetic nerves derive from the posterior vagal trunks from the brainstem. The presynaptic fibres synapase with postsynaptic fibres in the myenteric and submucosal plexuses in the interstinal wall.
The intestine is insensitive to most pain stimuli including cutting or burning but it is sensitive to distension that is perceived as colic – spasmodic abdominal pains or intestinal cramps.

Front 19

Describe the parts, appearance and blood supply of the large intestine
The larger intestine is supplied by the superior and inferior mesenteric artery.

Back 19

The superior mesentery emerges anteriorly from the abdominal aorta at L1 and forms the ileocolic branch, the right colic branch (ascending colon) and the middle colic branch ( transverse colon).
The inferior mesentery emerges anteriorly from the abdominal aorta at L3 and forms the left colic branch which supplies the descending colon, the sigmoid arteries and continues as the superior rectal artery.
The branches of the superior and inferior mesenteric arteries all contribute to form an anastomosing loop called the marginal artery which extends the length of the colon on the mesenteric border.

Front 20

Describe the location of the paracolic gutters.

Back 20

The paracolic gutters lie between the lateral ascending and descending colons and the adjacent medial parietal peritoneum.

Front 21

Describe the recto vesicle and recto uterine pouches.

Back 21

The recto- uterine pouch is the lowest part of the peritoneum in a female and is between the posterior uterus and the anterior rectum. It is an area for spread of bacteria and abscess formation in peritonitis and is the site for entry of peritoneal dialysis.
The recto-vesicle pouch is an area between the posterior rectum and anterior bladder and is the comparable pouch in males that is also found in females.

Front 22

How does segmentation differ from peristalsis?

Back 22

Peristalsis is the coordinated contraction of the smooth muscle of the gut with the aim of propelling luminal contents in one direction. Movement is the primary purpose. Segmentation as described above has its purpose more in the agitation and mixing of luminal contents. Due to the intestinal pacemakers firing more rapidly at their proximal locations, segmentation does create a slow caudal progression of intestinal contents.

Front 23

What is the Gastro-colic reflex?

Back 23

This is a reflex whereby the stretching of the stomach following a meal initiates peristalsis within the gut. This can include mass movements and hence the urge to defecate can be stimulated by eating.

Front 24

Diverticulosis of the colon is a common disease. Is there an anatomical explanation for the sites of development of this problem?

Back 24

The condition is more commonly seen in the sigmoid colon. The diverticula are out-pouchings of the mucosa through the muscle wall of the colon. The anatomy of the wall of the colon predisposes it to the occurrence of diverticula. The outer longitudinal muscle is incomplete (i.e. it is concentrated into the three bands – taenia coli) and thus presents areas of weakness. The branches of the blood vessels supplying the colon also appear to enter the gut wall at these sites, thus further weakening the sites.

Front 25

What factors lead to the relaxation of the internal anal sphincter?

Back 25

Internal sphincter relaxes involuntarily in consequence of reflexes set up by the presence of material in the rectum. Relaxation of internal sphincter is transient because the receptors accommodate - the sphincter regains its tone & sensation of need to defecate passes.

Front 26

What is meant by rectal accommodation?

Back 26

Stretch receptors of the rectum adapt to stretch - so that the rectum can become quite distended. The accommodation is accompanied by a reflex relaxation.

Front 27

What is the importance of "dietary fibre? Consider transit time, water holding and cation binding.

Back 27

Dietary fibres are non-starch, plant carbohydrates (usually cellulose) they are not digested and adds to bulk of the intestinal contents. They are helpful in the treatment of constipation, by giving gut walls something to work against and by providing an osmotic force keeping water in the. gut.
Some elements of fibre (hemicellulose etc) form a gel in the gut which bind monosaccharides and slow down their absorption - such a slowing may be important for diabetics. Gel may also trap bacteria and eliminate them.
Fibres provide substrates for bacterial metabolism. (Also increases gas production - flatus)

Front 28

What factors influence the mucus secretions of the colon?

Back 28

Surface epithelium of colon has large number of goblet cells. These produce an alkaline mucus (see HCO3 above). This buffers luminal pH against bacterial fermentation.
Mucus secretion is stimulated by contents of gut (pH and volume) but interestingly is increased by sympathetic stimulation - getting nervous makes you go the lavatory.
Some bacteria can digest this mucus so that bacterial overload my decrease the protection afforded by the mucus.