L11 The Colon Flashcards
What is the structure of the large intestine?
The colon begins as a blind ended sac known as the caecum. This continues as the ascending colon, at the hepatic flexure it becomes the transverse colon. This changes to the descending colon at the splenic flexure. The S shape part of the colon at the end is the sigmoid colon. This is continuous with the rectum which starts at the rectosigmod junction.
What is the histology of the large intestine?
There is an outer peritoneum, followed by a layer of muscle which is in two layer: longitudinal and circular (around the colon). Under the mucosa is the submucosa in which there are vessels and lymphatics. The mucosa is not ciliated or has villi as it doesn’t need a large surface area for absorption. It has only columnar epithelium in which there are mucus cells. As the stool gets solid, mucus is needed to move it along. The main job of the colon is to resorb water and electrolytes.
What is the ileocecal valve?
This is the valve between the ileum and the cecum. In most people it is a one way valve. It is stimulated by distension of the ileum, causing it to contract and pass things through. This occurs due to mechanoreceptors.
What is mass movement?
A wave of motion from the cecum around the bowel to the rectum. This is not in segments around the bowel but one movement.
Haustra briefly disappear as the mass movement sweeps by, then reform. Mass movement is a wave of peristalsis that goes around the bowel. The act of opening the bowel is due to a mass movement starting from the splenic flexure. It is aided by flattening of the diaphragm onto the sigmoid colon.
What is haustration?
Haustral contractions are slow segmenting, uncoordinated movements that occur approximately every 25 minutes.
How is motility of the colon coordinated?
It is smooth muscle. It is controlled by the sympathetic and parasympathetic nervous system. Most control is intrinsic, via the enteric nervous system. Endocrine and neurocrine influences from cells releasing 5-HT and Peptide YY. Gastrocolic and orthocolic triggers can trigger mass movements.
Most of the time it is controlled by the parasympathetic nervous system.
What is digested in the large intestine?
Digestion in the colon is done by bacteria, not by human enzymes. Bacteria digest:
• fibre, to produce short chain fatty acids such as butyric acid, hydrogen, and methane. Fibre is needed solid stool.
• Urea and amino acids, to produce ammonia
• Bilirubin, to produce urobilinogen (gets reabsorbed) and stercobilins
• Cysteine and methionine to produce hydrogen sulphide
• Primary bile acids, to produce secondary bile acids
• Conjugated bile acids, to produce unconjugated bile acids
Fermentation releases about 1L of nitrogen, hydrogen, and CO2 This is anaerobic
How does short fatty acid absorption occur at the colon?
The enterocytes of the colon needed to absorb sodium and water have adapted to rely on short chain fatty acids. Secondary active transport via sodium linked transporter rely on short chain fatty acids which the bacteria produce. Important source of energy for colonocytes. Sodium is moving from the lumen to the blood. At the same time short chain fatty acids are brought in.
What is the role of chloride in the colon?
Colonocytes or enterocytes produce and secrete cAMP and calcium ions out of the cell. This is to do with chloride passing out of the cell. Cholera and E. coli toxins increase cAMP and C. difficle toxin increases calcium ion. Both therefore lead to secretory diarrhoea. This occurs around the entire large bowel.
What is the role of potassium in the diarrhoea?
Potassium can undergo secretion and absorption in the colon (usually there is a net secretion). Passive secretion exceeds active secretion. Balance is determined by plasma potassium concentration, aldosterone and cAMP. When secreted, there are channels that exchange potassium for hydrogen ions. Diarrhoea means that people are likely to lose potassium.
What are the valves of the rectum?
The internal anal sphincter (smooth muscle) - involuntary and accounts for 70% of anal tone
The external anal sphincter(skeletal muscle) - voluntary and accounts for 30%
Dentate (pectinate) Line where epithelium becomes stratified squamous
How does defecation occur?
Sitting or squatting increases the rectosigmoid angle, facilitating the passage of faeces.
Relaxation of the puborectalis muscle also increases the angle. Relaxation of the external sphincter and the pelvic floor muscles opens the way. Rectal peristalsis, sometimes triggering a colonic mass movement, plus raised intraabdominal pressure (Valsalva manoeuvre) provides the motive force.
What is anal sampling?
There is a bend as the sigmoid comes into the rectum. Stool will then sit in the rectum waiting for defecation. There are many nerve endings in the anal canal. This is sampling takes place. The rectum releases a little bit of what is inside in into the tip of the anal canal and the anal canal can tell what it is (whether it is solid, liquid or gas). This happens all the time. Sphincter muscles can override this feeling to go to the toilet.
