PBL 4- Large Intestine Flashcards
where does the large intestine extend from
Extends from the caecum to the anal canal
what is the function of large intestine
receives digested food from the small intestine, from which it absorbs water +electrolytes to form faeces
what is large intestine’s total length
150 cm
what are the parts of the large intestine
Ascending colon
Transverse colon- intraperitoneal, enclosed by transverse mesocolon
Descending colon
Sigmoid colon- attached to thepelvic wall by the sigmoid mesocolon
what is the hepatic flexure
the start of transverse mesocolon
what is the splenic flexure
attached to spleen and start of descending colon
colon attaches here to the diaphragm by the phrenicolic ligament
what are the paracolic gutters
two spaces between ascending and descending colon. allows passage for infectious fluids from different compartments of the abdomen
what are the omental/epiploic appendices
small pouches of peritoneum, filled with fat
what are the teniae coli
3 strips of muscle that run longitudinally along the surface of the large intestine
what are haustra
Teniae coli contract to shorten the wall of large intestine, producing sacculations known as haustra
what is the blood supply of the large intestine
- (midgut) ascending colon + proximal 2/3 of the transverse colon- superior mesenteric artery
- (hindgut) distal 1/3 of transverse colon + descending colon + sigmoid colon- inferior mesenteric artery
what is the caecum
1st part of the LI, connects to ileum through the ileocaecal valve
what is the vermiform appendix/ appendix
worm like tube which opens into the caecum and position is variable
what is the rectum
Downward continuation of the sigmoid canal, continues to anal canal
what is the anal canal and what muscles does it have
Downward continuation of the rectum that ends in the anus. has external and internal sphincters.
External- skeletal muscle and under voluntary control
Internal- smooth muscle and under involuntary control
what type of cells are found in the large intestine
- absorptive cells (enterocytes)- responsible for absorption of mainly water and electrolytes
- goblet cells- synthesise alkaline mucous that lines mucosal wall and protects it from acidic enzymes
what are the walls of the LI lined with
lined with simple columnar epithelium with invaginations. The invaginations are called the intestinal glands or colonic crypts.
does LI have villi
no villi
Large intestine has ___ typical pacemaker activity
no
how does the LI mix material
since it functions to absorb water, it mixes material without propulsion
what is the purpose of haustrations
segmentation movements that mix the contents of the adjacent haustra. The purpose of haustration seems to be to squeeze and roll the faecal material around so that every portion of it is exposed to the absorptive surfaces of the colonic mucosa, thus aiding the absorption of water and electrolytes
Since LI does not have a typical pacemaker activity, what does it have then and what controls it?
mixture of short + long duration contractions controlled by parasympathetic activity
what are the other roles of colon (other than water absorption)
Act as a storage site
Cause aboral movement of content.
Expel faeces
After a meal, what is the transit time from caecum to rectum
takes 1-2 days
which sex has a shorter transit time
shorter duration in men than women so men have increased faecal weight
which part of LI has the slowest transit time
transit slowest through the caecum
how much fluid is ingested each day and where does it come from
Approximately 2 litres of fluid are ingested each day. Salivary secretions, gastric secretions, bile, pancreatic secretions and intestinal secretions all add up to about 9000 mL of total fluid in the gut.
how much fluid is absorbed in SI, LI and how much is lost in faeces
Small intestine- Most absorbed (roughly 8000mL, with Large Intestine- 1250mL,
Faeces- loses about 100-150mL
describe sodium absorption into the enterocytes and into the blood.
INTO THE ENTEROCYTES: Sodium can passively flow through an ion channel into the enterocytes, and since water follows sodium, it helps the absorption of the majority of water throughout the intestines.
INTO THE BLOOD: There is a Na/K pump on the basolateral side which actively pumps Na out of the enterocytes and into the blood, thus keeping a constant gradient for sodium to passively flow through at the apical end.
Describe Sodiums role as a cotransporter
Helps to transport other things like amino acids, peptides, bile salts and vitamins into enterocytes via active transport.
The energy for active transport is provided by the sodium gradient.
Describe sodiums role as an antiporter
antiporter with H+ ions
when Na moves into the entercoytes, H+ ions leave the entercoytes
Describe Glucose transport (including sodiums role)
SGLT1 transporter- on the apical side, Na required for glucose transport into the enterocytes
GLUT2 Transporter- on the basolateral side, glucose enters the blood through these transporters
The colon is responsible for Potassium __________ and __________
Absorption
Secretion
Where does potassium absorption and secretion occur in the large intestine
the proximal colon performs net K+ secretion
while net K+ absorption is observed in the distal colon
how does K+ absorption occur
Potassium is absorbed by active transport and is conducted by the H+/K+ATPase
How does K+ secretion occur
The mechanism of K+ secretion follows the general principle for many other types of ion secretion (e.g. Cl−)
what is the role of CFTR transporter in secretory diarrhoea
- Chloride secretion
This is due to the presence of ENDOTOXINS like bacteria (E. Coli and Vibrio cholera) which release heat stable enterotoxin that activate an intracellular cascade (mainly cAMP/PKA) to signal the CFTR on the apical plasma membrane.
What way is Cl- absorbed into enterocytes
Chloride is also absorbed into the cell via a Chloride-Bicarbonate antiporter.
Chloride in and bicarbonate out
Why does the upper gut (small intestine) has very low populations of bacteria
due to a range of factors including gastric acidity, propulsive motility, Bile acids and pancreatic enzymes.
What properties of the large intestine allows a large and complex and bacterial ecosystem to develop?
- The large intestine has a very stagnant motility with retropulsive contractions keeping the contents in the proximal colon for long periods.
- The pH of the colon is buffered by bicarbonate secretion.
what type of bacteria are found in the LI
up to 400 different species of bacteria and 99% are strict anaerobes
what are the functions of gut flora
- Digests resistant carbohydrates and lipids through fermentation into short chain fatty acids which are then utilised for energy
- synthesises Vitamin K and B
- Combats aggression from other microorganisms, maintaining wholeness of the intestinal mucosa
- Plays an important role in the immune system- performing a barrier effect
why do most colonic diseases occur in distal colon
as proximal colon is more acidic
what factors control the composition of gut flora
Physiochemical factors- pH, nutrients supply, O2 tension, O2 reduction potential
Host-bacteria interactions- saliva, bile, gastric+pancreatic secretions and immune system of host
Microbe-microbe interactions- bacteriophages, bacteriocines, toxic metabolites
give examples of bacteria that may reside in gut flora
Clostridium perfringens and Bacteroides fragilis
what is the definition of diarrhoea
3 or more loose/ watery stools per day
what are the types of diarrhoea
Secretory diarrhoea
Inflammatory diarrhoea
Osmotic diarrhoea
describe secretory diarrhoea
↑ Active secretions and ↓ Absorption
This is due to bacterial enterotoxins which are released by E.coli etc.
- The toxins stimulate the release of anions, especially chloride ions through CFTR transporter. Therefore, to maintain a charge balance in the lumen, sodium is carried with it, along with water.
- high volume diarrhoea
How does vibro cholera cause secretory diarrhoea
The Cholera bacterium (vibro cholera) produces a heat labile toxin, which acts on the crypt cells (secretory) and hence causes increased secretion of chloride, sodium and water. This toxin also loosens the tight junctions between enterocytes (leakage of water) and cause intestinal vasodilatation.
what infections cause secretory diarrhoea
rotavirus e.coli shigella campylobacter salmonella Vibro cholera laxatives Drugs- asthma, cardiac, antidepressants carcinoid syndrome zollinger-ellison syndrome tumours
describe inflammatory diarrhoea
widespread destruction of absorptive epithelium due to inflammation results in insufficient water absorption –> diarrhoea (low volume)
what diseases cause inflammatory diarrhoea
Inflammatory bowel disease:
- Crohns disease
- Ulcerative colitis
Infectious disease:
- Shigella
- Salmonella
Irritable colon
describe osmotic diarrhoea
Osmotic diarrhoea occurs when too much water is drawn into the bowels. If a person has excessive salt in the diet which isn’t fully absorbed, these can draw water from the body into the bowels, causing osmotic diarrhoea.
This could be due to malabsorption or some sort of malabsorptive disease e.g. Coeliac disease. It can also be caused by pancreatic insufficiency, short bowel syndrome and inflammatory disease.
this type of diarrhoea stop when patient fasts
what are the 2 types of intravenous solutions given to patients with diarrhoea
- Colloids - these are molecules with a relatively large molecular weight (nanograms) e.g. Albumin • - Crystalloids - these are water + electrolytes (much smaller) e.g. Saline & Dextrose.
describe saline
Saline 0.9% (Normal Saline)- is 9g of NaCl in 1000mL of water. It is used to replace fluids and electrolytes. It is isoosmotic with the normal blood concentrations.
Therefore it is given to increase the amount of fluid in the blood vessels without changing the balance of electrolytes in the body.
describe dextrose
Dextrose 5%- contains half amounts of NaCl as Saline 0.9% + the dextrose sugar. This sugar can be absorbed and utilised by the body. Once its absorbed, there is just fluid remaining, so it is hypo-osmotic (initially iso-osmotic)
describe sodium bicarbonate
sodium bicarbonate (8.4%)- is only used in emergencies because it is hyper-osmotic. It is used to manage cardiac arrest, metabolic acidosis, hyperkalaemia.
what are the factors taken into consideration before fluid replacement
Rate of fluid replacement Factors taken into consideration before fluid replacement: age, cardiovascular status, renal function, severity of dehydration and How much time it took for dehydration to develop
how do you control the spread of enteric infections
Aseptic Technique Hand washing Cleaning & Disinfection of wards Regular cleaning of toilets Waste disposal Educate patients Patient isolation
