Flashcards in The intestines Deck (47):
When the inferior mesenteric artery (IMA) enters the pelvis it is renamed. What is it then called?
Superior rectal artery
What does the superior rectal artery supply?
Blood to the rectum
What branch of the IMA broadly supplies the descending colon?
Left colic artery
What branch of the IMA supplies the sigmoid colon?
Several sigmoid arteries
Where is the appendix located?
Most commonly on the caecum
What is the caecum and where it it located?
The caecum is a large bulge in the GI tract that follows on from the terminal ileum. It is the start of the large intestine
What distinguishes sites of high absorption in the GI tract from sites of low absorption?
Large surface area, though good blood supply is essential too
What is the most common site for peptic ulceration in the GI tract?
Describe the structure of the duodenum
1st part - duodenal cap (superior)
2nd part - descending portion (descending)
3rd part - horizontal portion (inferior)
4th part - ascending portion (ascending)
What is the main function of the colon?
Water absorption, although most water has already been absorbed in the small intestine and the colon removes almost all of the rest.
It does not play a major role in nutrient absoprtion
What is the marginal artery?
A blood vessel that anastomoses (connects) the inferior mesenteric artery (IMA) with the superior mesenteric artery (SMA)
What are arcades of blood vessels?
Loops of arteries around the ileum and jejunum
What branch of the superior mesenteric artery (SMA) delivers the most direct blood supply to the caecum?
What does the middle colic artery supply?
Mostly the transverse colon
What does the right colic artery supply?
What does the term 'mass movement' describe?
The rapid peristalsis of colonic material forcing it into the rectum which is normally empty. This stretches the rectum producing the urge to defaecate.
A mass movement is often triggered by a gastro-colic reflex. What is a gastro-colic reflex?
An increase in the motility of the colon in response to the stretching of the stomach or the presence of food break-down products in the small intestine following a meal
In the digestive process carbohydrates such as starch are broken down to monosaccharides which can be absorbed by the gut. Which transporter moves glucose across the basolateral membrane of the enterocyte?
What is the role of the GLUT-5 transporter in enterocytes?
Moves fructose across the apical membrane of the enterocyte (from the gut lumen into the enterocyte)
What is the role of the SGLT-1 transporter in enterocytes?
It is a sodium glucose linked transporter that co-transports sodium and glucose from the gut into the enterocyte
Where do the epithelial cells of the small intestine arise from?
Rapid division in the crypt between the villi and then they migrate towards the tips from which they are shed
What three structures help give the small intestine its large surface area?
1. Plicae circulares - permanent folds of mucous membrane
2. Villi - finger-like projections of the epithelium
3. Microvilli - cell membrane protrusions on each epithelial cell
What is the 'unstirred layer'?
The brushborder of the intestinal epitherlium where nutrients meet and react with enzymes secreted by the enterocytes, completing digestion prior to absorption
What is the name of enzymes which break the branch points of amylopectins?
How is glucose absorbed by enterocytes?
1. Na+ is actively pumped (requires ATP) out of the enterocyte basolateral membrane using Na+/K+ ATPase which decreases the intracellular concentration of Na+
2. Na+ travels down its concentration gradient from the intestinal lumen to inside the cell through SGLT-1 transporter which binds Na+ and transports glucose/galactose at the same time (facilitated diffusion)
3. Glucose then leaves the cell via facilitated diffusion across the basolateral membrane using the GLUT-2 receptor down its concentration gradient
4. The more glucose and Na+ that enter the cell from the intestinal lumen the more Na+ that is pumped across the basolateral membrane
What is the basis of oral rehydration fluid?
Oral hydration fluid is a mixture of gluocose and salt. Glucose has the effect of increasing the active pumping of Na+ across the basolateral membrane of enterocytes, so more glucose and Na+ are transported into the cells by facilitated diffusion. Therefore oral hydration fluid drives maximal absorption of osmotically active solute, causing water to follow.
How is fructose absorbed by enterocytes?
Facilitated diffusion into the cell by GLUT-5 transporter protein. Once in the enterocyte fructose can be transported out of the basolateral membrane of the cell by the same transporter as glucose, GLUT-2 (down its concentration gradient).
In the stomach, how is pepsinogen converted to pepsin?
By acid which alters the charge of amino acid groups on the protein, breaking some non-covalent interactions so part of the protein hinges off and the active form pepsin is formed.
What is the role of trypsinogen in protein digestion?
Converted to trypsin by an enteropeptidase in the brush border. Trypsin then proteolytically cleaves other protease zymogens into their active forms
What major proteases are released by the pancreas?
Exopeptidase (hydrolyse from C-terminal end):
Carboxypeptidase (A and B)
How are most amino acids absorbed?
Mostly by active transport linked, like glucose, to the inward movement of Na+ ions. Dipeptides and tripeptides can be absorbed by mechanisms linked to the expulsion of H+ ions.
The process by which amino acids are absorbed is specific to different classes of amino acids
How are sodium, chloride and other electrolytes absorbed?
Mechanisms related to the Na+ pump
How is Ca2+ absorbed?
Ca2+ enters cells by facilitated diffusion, and is then expelled across the basolateral membrane by active transport. Both processes are dependent upon Vitamin D (Calcitriol) and stimulated by PTH
How is iron absorbed?
Iron is absorbed mainly as Fe2+ bound to transferrin which is secreted by the enterocytes. Once in the cell the Fe2+ is liberated and exported to the blood where it is bound to transferrin again
What is the role of intrinsic factor in vitamin B12 absorption?
Intrinsic factor is a carrier molecule. Vitamin B12, in the terminal ileum, must be attached to intrinsic factor for it to be absorbed.
If there is damage to or removal of either B12 of intrinsic factor pernicious anaemia occurs as B12 is essential for erythropoiesis
What is the pattern of motility seen in the small intestine called 'segmenting'?
This is very different to the peristalsis seen in the oesophagus and stomach.
The small intestine is divided into sections, each with a pacemaker. The frequency of the pacemaker gets less from the duodenum to the terminal ileum (intestinal gradient).
The firing of the pacemaker sends activity through the nerve plexuses which cause contraction of the smooth muscle at intervals along its length. Therefore contents in areas which are not contracted are mixed by movements from adjacent areas which do contract. Next time a different pacemaker fires causing different areas to contract.
What is the phenomenon known as "intestinal gradient"?
How the frequency of the pacemaker of each intestinal segment decreases from the duodenum (12 times/min) to the terminal ileum (8 times/min).
What is the effect of segmenting and the intestinal gradient?
Segmenting in itself does not move intestinal contents in a cranial to caudal direction, it just mixes them, but the intestinal gradient does - although at a slow rate
What are 'Haustra'?
Natural segments of the large intestine caused by the large intestine's circular muscle being more complete than their longitudinal - which have been reduced to taenia coli.
What is the name of the process that agitates the contents in the large intestine and causes them to slowly move towards the sigmoid colon?
What is a 'mass movement'?
A co-ordinated peristalsis-like movement from the transverse colon towards the rectum which forces faeces rapidly into the rectum (which is normally empty). The resulting distension produces the urge to defaecate
What occurs to trigger defaecation once the rectum is full?
Defaecation is activated voluntarily causing enhanced contraction of the rectal smooth muscle, relaxation of the smooth muscle internal anal sphincter and the skeletal muscle external sphincter. This is combined with expiration against a closed glottis and abdominal muscle contraction to increase intraabdominal pressure, so expelling the faeces.
What happens if defaecation is not triggered voluntarily?
Eventually reflexes take over which trigger it involuntarily as rectal pressure rises
Describe two factors that encourage absorption in the intestines
Mucosa folded into villi
The villi themselves have micro-villi (brush border)
What are intestinal crypts (intestinal glands)?
These are glands lining the intestinal epithelia which usually sit at the base of the villi. They secrete
various enzymes and play a vital role in cell turnover in the gut. The base of these glands have cells
that multiply and then migrate up to the tips of the villi where they are shed. As the cells migrate
they mature and develop the ability to absorb. This process allows the mucosa to be continually
renewed. These glands are affected by inflammatory bowel disease which can produce so called
What does amylase catalyse?
Breakdown of amylose -> glucose and maltose (glucose-glucose dissacharide)