The small bowel Flashcards
What is the role of the small bowel?
To absorb nutrients, salt and water
How large is the small bowel?
6m long and 3.5cm in diameter Duodenum: 25cm Jejunum: 2.5m Ileum: 3.75m No sudden transition between them
What is the function of the mesentery?
Suspends small and large bowel from posterior abdominal wall, anchoring them in place and still allowing some movement
It also provides a conduit for blood vessels, nerves and lymphatic vessels
What are the different blood vessels in the mesentery?
Middle colic artery: supplies transverse flexure, hepatic flexure and proximal transverse colon
Right colic artery: supplies ascending colon
Ileocolic artery: supplies terminal ilieum and ascending colon and secum
Superior mesenteric artery: supplies all of jejunum and all of ileum
Jejunal and ileal arteries: supplies all of jejunum and all of ileum
What is significant about villi?
Only occur in small intestines
They’re motile
Have a 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 enterocytes
What are villi lined with?
Simple columnar epithelium consisting of:
- primary enterocytes
- scattered goblet cells
- enteroendocrine cells
What cells can be found in the crypts?
Crypt of Lieberkuhn- epithelium includes:
- Paneth cells
- Stem cells
- Progenitor cells
What are enterocytes?
Most abundant cell in small bowel
Tall columnar cells with microvilli and a basal nucleus
Specialised for absorption and transport of substances
Lifespan of 1-6 days
How do enterocytes play a role in increasing surface area?
Cylindrical internal S.A of small bowel is 0.4m^2
Folds, microvilli and villi increase the surface area to 200m^2
That’s at least a 500 fold increase
What are microvilli?
Make up the brush border
Several thousands per cell
Surface of microvilli covered in glycocalyx
What is the surface of microvilli covered with?
Glycocalyx
This is rich in carbohydrates
Serves as a protection from digestional lumen yet allows absorption
Traps a layer of water and mucous known as “unstirred layer”
Regulates rate of absorption from intestinal lumen
What is a goblet cell?
2nd most abundant cell in the small bowel
Mucous containing granules accumulate at apical end of cell causing goblet shape
There is a smaller amount of goblet cells in the duodenum compared to in the colon
What is mucous?
Large glycoprotein that facilitates passage of material through the bowel
What are enteroendocrine cells?
Columnar epithelial cells
They’re scattered amongst enterocytes
Theyre most often found in the lower part of crypts
What is the role of enteroendocrine cells?
Secrete hormones to influence gut motility
What are enteroendocrine cells also known as?
Chromaffin cells
What are paneth cells?
Found only in the base of crypts
Contain large acidophilic granules
Engulf some bacteria and protozoa
May have a role in regulating intestinal flora
What do paneth cell granules contain?
Antibacterial enzyme lysozyme (protects stem cells)
Glycoproteins and zinc (essential trace metal for a number of enzymes)
What are stem cells?
Undifferentiated cells which remain capable of cell division to replace dead cells
Epithelial stem cells are essential in GI tract to continually replenish surface epithelium
They migrate to the top of the villus and replace older cells that die by apoptosis (Cells which die are digested and reabsorbed)
What is the lifespan of enterocytes and goblet cells?
About 36 hours
They have a rapid turnover in comparison to other epithelial cells
This is because
-Enterocytes are the first line of defense against GI pathogens and may be directly affected by toxic substances in diet
-Effects of agents which interfere with cell function, metabolic rate etc will be diminished
-Any lesions are short lived
If there is an impairment of production of new cells severe intestinal dysfunction will occur
How is the duodenum different from the rest of the small bowel?
Has Brunner’s glands
These are submucosal coiled tubular mucus glands secrete alkaline fluids
They open into the base of the crypts
This alkaline secretion neutralises acidic chyme from stomach so protects proximal small bowel and optimises the pH for action of digestive enzymes
How does the jejunum compare to the ileum?
The jejunum has a thicker wall than the ileum due to numerous plicae circulares
The jejunal mesentery is above and left of the aorta whereas ileal mesentery is right and below the aorta
Jejunal mesenteric vessels forms only 1 or 2 archades whereas ileum mesentery has many short terminal vessels and 3-4 archades
The ileum has peyer’s patches whereas jejunum doesn’t have any
What are functions of the small intestine motility?
To mix ingested food with digestive secretions and enzymes
To facilitate contact between contents of intestine and intestinal mucosa
To propel intestinal content along alimentary tract
What are the 3 types of small bowel movement?
Segmentation
Peristalsis
Migrating motor contents
What is segmentation?
Mixes contents of lumen
Occurs by stationary contraction of circular muscles at intervals
There are more frequent contractions in the duodenum than ileum
Allows pancreatic enzymes and bile to mix with chyme
Although chyme moves in both directions the net effect is movement towards colon
What is peristalsis?
Involves sequential contraction of smooth muscle
Propels chyme towards colon
Most waves of peristalsis only travel about 10cm
Segmentation and peristalsis result in chyme being segmented, mixed and popelled towards the colon
What is the migrating motor complex?
Cycles of smooth muscle contractions sweeping through gut
Begin in stomach then small intestine, colon and next wave starts in duodenum
Prevents migration of colonic bacteria in ileum
What is pancreatic alpha amylase?
Secreted into duodenum in response to meal
Digests starch and glycogen (carbohydrates) in small bowel
Neds Cl- for optimum activity and neutral/slightly alkaline pH
Acts mainly in the lumen (some absorbs to brush border)
Digestion of amylase products and simple carbohydrates occurs at brush border
How are carbohydrate monomers absorbed?
Glucose and galactose is absorbed by second degree active transport
Carrier protein is SGLT1 on apical membrane
Fructose is absorbed by facilitated diffusion
Carrier protein is GLUT-5 on apical membrane
GLUT-2 facilitated exit at basolateral membrane
Human small intestine can absorb 10kg of simple sugars a day
How are proteins digested in the small intestine?
Protein digestion begins in lumen of stomach by pepsin
This pepsin is inactivated in alkaline duodenum
5x pancreatic proteases are secreted as precursors into the lumen of the small bowel
Trypsin is activated by enterokinase- located on duodenal brush border
Trypsin activates other proteases which hydrolyse proteins into amino acids and oligopeptides
What are the 5 pancreatic protease precursors secreted in the small bowel?
Trypsinogen Chymotrypsinogen Proelastase Procarboxypeptidase A Procarboxypeptidase B
What proteases are activated by trypsin?
Trypsin Chymotrypsin Elastase Carboxypeptidase A Carboxypeptidase B
What happens to oligopeptides?
Peptidases at brush border of enterocytes progressively hydrolyse them into amino acids
Enterocytes directly absorb some oligopeptides via action of H+/oligopeptide transporter PepT1 - they go on to be hydrolysed to amino acids by peptidases in enterocyte cytoplasm
How are lipids digested?
Lipids are poorly soluble in water
They have 4 stages of digestion in the small bowel:
1. Secretion of bile salts and pancreatic lipases
2.Emulsification (increases SA for digestion)
3.Enzymatic hydrolysis of ester linkages (colipase complexes and lipase enzymes- prevents bile salts displacing lipase from fat droplets)
4. Solubilisation of lipolytic products in bile salt micelles
How are lipids absorbed?
Fatty acids and monoglycerides leave micelles and enter enterocytes
FAs and MG resynthesized into triglycerides by 2 pathways: Monoglyceride acylation (major) and Phosphatidic acid pathway (minor)
Chylomicrons are lipoprotein particles (mostly triglyceride) synthesised as an emulsion in golgi apparatus- these are secreted across basement membrane by exocytosis
Chylomicrons enter a lacteal (lymph capillary)- 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 backflow of bacteria into ileum
