Small Intestine Flashcards
What are the different cell types present in the small intestinal mucosal layer?
Differentiated villus cells:
- absorptive enterocytes
- enteroendocrine goblet cells
Proliferating progenitors:
Crypt intestinal stem cells
Crypt paneth cells:
Pericryptal fibroblasts
What are some of the reasons why the small intestine is a normally sterile environment?
Bile Proteolytic enzymes Lack of nutrients Anaerobic environment Shedding of epithelial cells Rapid transit in peristalsis
How is the mucosa in the small intestine renewed?
Crypt intestinal stem cells multiply and migrate towards the tips of the villi
Acquire the capacity to absorb as they move upwards
Shed from villus tips once degenerated
What are the enzymes in the small intestine brush border which digest carbohydrates?
Isomaltase: act at alpha-1,6 bonds (alpha-limit dextrins —> glucose)
Maltase: maltose —> glucose + glucose
Sucrase: sucrose —> glucose + fructose
Lactase: lactose —> glucose + galactose
How is glucose absorbed in the small intestine?
SGLT1 on apical membrane (lumen): glucose absorbed actively against sodium gradient
Uptake of sodium generates osmotic gradient (water follows)
GLUT2 on basal membrane (ECF): glucose (& fructose/galactose) enters blood via facilitated diffusion
Sodium (and chloride) uptake stimulated by glucose uptake, and osmotic gradient generated (water follows)
What are the components of oral rehydration fluid?
Na+ & glucose
Increased water absorption in small intestine:
- sodium uptake drives glucose movement into cell via SGLT1
- glucose uptake into blood drives sodium & chloride movement via GLUT2
Movement of sodium & glucose generates osmotic gradient
How are amino acids digested in the small intestine?
Peptidases synthesised in pancreas enter the duodenum
- trypsin cleaves basic side chains
- chymotrypsin cleaves aromatic side chains
- carboxypeptidase cleaves C-terminal basic side chains
Amino acids/small peptides absorbed by active & passive processes
Active = 5+ Na+/amino acid co-transporters, H+ pumped into lumen is returned with peptide by co-transport
How does the small intestine mix intestinal contents?
SEGMENTING (not peristalsis!)
Intestinal pacemakers drive intermittent contraction of circular smooth muscle along different sections:
- mixes/agitates intestinal contents
- gradual caudal progression of contents into large intestine
note: nerves only transmit AP to circular muscle at certain intervals (which change with each AP) - so that segments contract at different times and mix the intestinal contents
What is the relationship between the duodenum and the peritoneum?
Not entirely retroperitoneal
If an ulcer erodes posteriorly through the first part of the duodenum, what structure might be damaged?
Gastro-duodenal artery
This can cause massive haematemesis & blood passed per rectum
How is the duodenum divided into sections? What are some significant features of each section?
1st (superior) (duodenal ampulla/cap) = before major duodenal papilla —> prone to ulceration as acid has not yet been neutralised
2nd (descending) = major duodenal papilla which delivers bile & digestive enzymes (+ minor duodenal papilla - accessory pancreatic duct)
3rd (inferior) = longest section; crosses aorta & IVC & vertebral column; crossed by SMA
4th (ascending) = passes aorta; terminates at duodenaljejunal flexure at ~L2
What is Meckel’s diverticulum?
Remnant of part of yolk stalk pouching out of the small intestine that may be gastric or pancreatic tissue
Rule of 2’s:
- 2ft proximal of ileocaecal valve
- 2 inches long
- 2% of population
Can present as appendicitis/volvulus but usually asymptomatic
What factors detected in the duodenum slow the rate of gastric emptying?
Presence of lipids
Low pH
Hypertonicity
All indicate slowed rate of digestion/absorption by the small intestine
