Small bowel Flashcards
What is the function of the small bowel?
Absorb nutrients, salt and water
What is the structure of the small bowel?
Approx 6m long and 3.5cm in diameter
Duodenum: 25cm
Jejunum: 2.5m
Ileum: 3.75m
No sudden transition between each part, all have the same basic histiological organisation
What is the mesentery?
Suspends small and large bowel from posterior abdominal wall
Anchors them in place whilst still allowing some movement
Provides a conduit for blood vessels, lymphatics and nerves
Blood vessels include superior messnteric artery which supplies: jejunal and Ilial arteries, ileocolic artery, right colic artery, middle colic artery
What is the digestive epithelium of the small bowel?
What Outer covering is the serosa
Underneath is the longitudinal muscle
Underneath that is the circular muscle (important for motility)
On this is the plica circularis, which has villi on it
Structure of villus: apex at top. Lots of microvilli Arteries veins and lymphatics at the centre. Crypt of lieberkuhn at the bottom of each villi
What are some facts about villi?
Only occur in small intestine
Motile
Have rich blood supply and lymph drainage for absorption of digested nutrients
Have good innervation from the submucosal plexus
Have simple epithelium: 1 cell thick, dominated by enetetocytes (columnar absorbative cells)
What cell types are found in the small bowel?
Villi lined with:
Mostly enterocytes (absorbative)
Scattered goblet cells
Enteroendocrine cells
Crypts of lieberkuhn:
Paneth cells
Stem cells
What are enterocytes?
Most abundant cells in small bowel
Tall columnar cells with microvilli and a basal nucleus
Specialise for absorption and transport of substances
Short lifespan of 1 to 6 days
Surface area:
Cylindrical internal surface area of small-bowel is 0.4 m²
Folds, villi and microvilli increase surface area to around 200 m² (at least a 500 fold increase) (size of a tennis court)
What are microvilli?
They make up the brush border
Several thousand microvilli Per cell
Surface of microvilli covered with glycocalyx
Glycocalyx:
Rich carbohydrate layer on apical membrane
Serves as protection from digestional lumen
Yet allows for absorption
Traps a layer of water and mucus known as “unstirred layer”
This regulates the rate of absorption from intestinal lumen
What are goblet cells?
Second most abundant epithelial cell type
Mucus containing granules accumulate at apical end of cell, causing goblet shape
Mucous -> large glycoprotein that facilitates passage of material through bowel
Increasing abundance of goblet cells along entire length of bowel (lower in duodenum , higher in colon)
What are the enteroendocrine cells of the small bowel?
Columnar epithelial cells
Scattered among enterocytes
Most often found in lower part of crypts
Hormone secreting (eg. To influence gut motility)
(May be referred to at chromaffin cells)
What are paneth cells?
Found only in the bases of crypts
Contain large, acidophilic granules
Granules contain: antibacterial enzyme lysozyme (pretexts stem cells). Glycoproteins and zinc (essential for a number of enzymes)
Also engulf some bacteria and Protozoa
May have a role in regulating intestinal flora
What are stem cells?
Undifferentiated cells which remain capable of cell division to replace cells which die
Epithelial stem cells are essential in the GI tract to continually replenish the surface epithelium
Continually divide by mitosis
Migrate up to tip of villus, replacing all the cells that die by apoptosis (these are digested and reabsorbed)
Differentiate into various cell types (pluripotent)
What is the duodenum?
Distinguished by presence of brunners glands
Submucosal coiled tubular mucous glands secrete img alkaline fluid
Open into the base of the crypts
Alkaline secretion: neutralises acidic chyme from stomach, protecting proximal small bowel. Helps optimise pH for action of pancreatic digestive enzymes
(It goes duodenum, jejunum, ileum, large bowel)
What are the differences between the jejunum and ileum?
Jejunum is wider and thicker walled, and redder than the ileum
As they have larger, and more numerous plica circularis
The jejunal mesentery is above and to the left of the aorta, the ileal mesentery is attached below and to the right of the aorta
Ileum also had peyers patches (lymphoid tissue) on anterior mesenteric border of lower ileum. Involved in gut immunity
What are the functions of the small bowel motility?
To mix ingested food with digestive secretions and enzymes
To facilitate contact between contents of intestine and the intestinal mucosa
To propel intestinal contents along alimentary tract
What are the three different types of movement in small bowel motility?
- Segmentation (mixing)
Mixes contents of lumen
Occurs by stationary contraction of circular muscles at intervals (more frequent in duodenum)
Allows pancreatic enzymes and bile to mix with chyme (net movement of chyme is to the colon)
- Peristalsis (propelling)
Sequential contraction of adjacent rings of smooth muscle (most waves only travel about 10cm)
Propels chyme towards colon
- Migrating motor complex
cycles of smooth muscle contractions sweeping through gut
Begin in stomach -> small intestine -> colon -> next wave starts in duodenum
Prevents migration of colonic bacteria to ileum
What are the digestive and absorbative mechanisms in the duodenum?
Digestion:
Occurs in an alkaline environment
Pancreatic digestive enzymes and bile enter duodenum from major pancreatic duct and common bile duct
Duodenal epithelium also produces its own digestive enzymes
Digestion occurs in lumen and in contact with the membrane
What happens in the digestion of carbohydrates?
These contain around 50% of ingested calories in western diet
Digestion begins in the mouth but salivary alpha amylase (destroyed in the stomach by acid pH)
Most carbohydrate digestion occurs in the small intestine
Monosaccharides - (glucose and fructose). disaccharides (sucrose and maltose). Complex carbohydrates (starch, cellulose, pectins)
Pancreatic a amylase : secreted into duodenum in response to a meal
Continues digestion of starch and glycogen in small bowel
Needs Cl- and neutral/alkaline pH for optimum activity
Acts mainly in lumen
Digestion of amylase products and simple carbohydrates occurs at the brush border (enzymes such as sucrase, isomaltase, lactase)
How does the absorption of digested carbs work?
Absorption of glucose and galactose by secondary active transport - carrier protein SGLT-1 on apical membrane
Absorption of fructose is by facilitated diffusion - carrier protein GLUT-5 on apical membrane
GLUT-2 facilitates exit at basolateral membrane
Human small intestine can absorb 10kg of simple sugars a day
How does the digestion of proteins work?
Begins in lumen of stomach by pepsin (which is deactivated in the alkaline duodenum)
5 pancreatic proteases are secreted as precursors (eg. Trypsinogen). These travel to the lumen of the small bowel
Trypsin is activated by enterokinase (which is located on the brush border)
Trypsin -> activates other proteases
This hydrolysed proteins into single amino acids and oligopeptides
How are digested proteins absorbed?
Action of luminal, brush border and cytosolic peptidases
A variety of peptidases at brush borders of enterocytes progressively hydrolyse oligopeptides to amino acids
Enterocytes directly absorb some small oligopeptides via the action of H+/oligopeptide cotransporter PepT1
These small oligopeptides are digested to amino acids by peotidases in the cytoplasm of enterocytes
How are lipids digested?
Lipids are poorly soluble in water so more difficult to digest
4 stage process in small bowel:
- secretion of bile salts and pancreatic lipases
- emulsification increases surface are
- enzymatic hydrolysis of ester linkages (colipase complexes with lipase - prevents bile salts displacing lipase from fat droplet)
- solubilisation of lypolytic products in bile salt micelles
How are lipids absorbed?
Fatty acids and monoycerides leave micelles and enter enterocytes
Fatty acids and monoglycerides resynthesised into triglycerides by 2 pathways: monoglyceride acylation (major) and phosphatidic acid pathway (minor)
Chylomicrons - lipoprotein particles synthesised as an emulsion in Golgi apparatus
Chylomicrons secreted across basement membrane by exocytosis
Chylomicrons enter lacteal (lymph) -> lymph transports them away from bowel
What is the ileocaecal valve?
Separates the ileum from the colon
Relaxation and contraction controls passage of material into colon
Also prevents back flow of bacteria into colon
