Powerpoint presentation focusing on the highlights of the anatomy, physiology and common absite tested material regarding the small intestines.

1. Small Intestines
Dr. Joseph Di Como
Conemaugh Memorial Medical
Center

2. Movement of Digestive Materials
• By muscular layers of digestive tract
– Consist of visceral smooth muscle
– Along digestive tract:
• Has rhythmic cycles of activities (PERISTALSIS)
– Consists of waves of muscular contractions
– Move a bolus along the length of the tract
• Controlled by pacesetter cells
– Surrounding the lumen of the tract
• Cells undergo spontaneous depolarization
– Triggering wave of contraction through entire muscular sheet

3. Small Intestine
• Intestinal Secretions
– Watery intestinal juice
– 1.8 liters per day enter
intestinal lumen
– Moisten chyme
– Assist in buffering acids
– Keep digestive enzymes and
products of digestion in
solution
• Intestinal Movements
– Chyme arrives in duodenum
– Weak peristaltic contractions
move it slowly toward
jejunum
• Myenteric reflexes
• Not under CNS control
• Parasympathetic stimulation
accelerates local peristalsis
and segmentation

6. Histological Structure of the Digestive
(GI) Tract

7. Small Intestine
• 90% of absorption occurs in the small intestine

8. Anatomy
• 270 to 290 cm
• – Duodenum 20 cm
• – Jejunum 100 to 110 cm
• – Ileum 150 to 160 cm
• Mucosa has transverse folds ( plicae
• circulares circulares)
• Jejunum starts at the ligament of Treitz
• No obvious jej jej-ileal demarcation
• Jejunum has larger circumference, is thicker and has different mesenteric
vessels

9. Innervation
• Autonomic only
– Parasympathetic- Vagus Vagus, celiac ganglion
– Sympathetic - 3 pairs of nerves, superior
mesenteric plexuses, pain

10. Lymphatic Drainage
• Major deposits of lymphatic tissue- Peyer
patches in distal small bowel
• Mucosa -> nodes adjacent to bowel > -> >
nodes at the mesenteric arterial arcades ->
group of nodes at the base of superior
mesenteric vessels -> > cisterna chyli
• Fat absorption

11. • 4 layers
• Mucosa
• Muscularis mucosa: thin, separate from
submucosa
• Lamina propria: connective tissue, immune
function
• Epithelial layer: covers vili and crypts–
 Goblet cells: secrete mucus
 Panethcells: mucosal defense system; secrete lysozyme, TNF, cryptidins
 Enterocytes: absorption with microvilli, covered by the , glycocalyx
 Enteroendocrinecellscells

15. Villi and Microvilli
• The surface area inside the small intestine is greatly increased
by circular folds, villi, and microvilli.
• Intestinal Villi - A series of fingerlike projections: in mucosa of
small intestine
• Covered by simple columnar epithelium:covered with
microvilli Intestinal glands have goblet cells between
columnar epithelial cells Eject mucins onto intestinal surfaces
Brush Border Enzymes are integral membrane proteins ,on
surfaces of intestinal microvilli
• Break down materials in contact with brush border by
trypsinogen

16. Small Intestine
• The Duodenum
– The segment of small intestine closest to stomach
– 25 cm (10 in.) long
– “Mixing bowl” that receives chyme from stomach and
digestive secretions from pancreas and liver
– Functions of the duodenum
• To receive chyme from stomach
• To neutralize acids before they can damage the absorptive
surfaces of the small intestine

17. Small Intestine
• The Jejunum
– Is the middle segment of small intestine
– 2.5 meters (8.2 ft) long
– Is the location of most
• Chemical digestion
• Nutrient absorption
– Has few plicae circulares
– Small villi

18. Small Intestine
• The Ileum
– The final segment of small intestine
– 3.5 meters (11.48 ft) long
– Ends at the ileocecal valve, a sphincter that
controls flow of material from the ileum into the
large intestine

19. Secretions of Small Intestine
• peptidase – breaks down peptides into amino acids
• sucrase, maltase, lactase – break down disaccharides into
monosaccharides
• lipase – breaks down fats into fatty acids and glycerol
• enterokinase – converts trypsinogen to trypsin somatostatin –
hormone that inhibits acid secretion by stomach
• cholecystokinin – hormone that inhibits gastric glands,
stimulates pancreas to release enzymes in pancreatic juice,
stimulates gallbladder to release bile
• secretin – stimulates pancreas to release bicarbonate ions in
pancreatic juice

20. Absorption Sites
• Duodenum and jejunum: Carbohydrates,
amino acids, lipids, iron, and Ca 2+ .
• Ileum: Bile salts, vitamin B 12 , electrolytes,
and H20.

21. Intestinal Enzymes
• Microvilli contain brush border enzymes that
are not secreted into the lumen. Brush border
enzymes remain attached to the cell
membrane with their active sites exposed to
the chyme. Absorption requires both brush
border enzymes and pancreatic enzymes.
• Brush border enzymes activates zymogens
complete digestion of carbohydrates and
proteins

22. Small Intestine

23. Carbohydrates––Broken down by intra luminal amylase and
amylopectin
Brush border: maltase, lactase, sucrasesucrase, , trehalase-->
break disaccharides
Monosaccharidesare absorbed by Na cotransport and
facilitated diffusion
Carbohydrate Digestion

24. Protein Digestion
• 80-90% absorbed in jejunum
• Pancreatic trypsinogen (enterokinase)
• Endopeptidases: trypsin, chymotrypsin,
elastase

25. Digestion and Absorption of
Protein
• Digestion begins in the stomach when pepsin
digests proteins to form polypeptides.
• In the duodenum and jejunum:
- Endopeptidases cleave peptide bonds in the interior
of the polypeptide: Trypsin. Chymotrypsin. Elastase.
- Exopeptidases cleave peptide bonds from the ends
of the polypeptide: Carboxypeptidase.
Aminopeptidase.

26. Protein
• Free amino acids absorbed by cotransport
with Na + . Dipeptides and tripeptides
transported by secondary active transport
using a H + gradient to transport them into
the cytoplasm. Hydrolyzed into free amino
acids and then secreted into the blood.

27. Fat Absorption
• Emulsification
- Breakdown of fat globules into smaller sizes
- Facilitated by bile (Bile salts, lecithin)
- Allows action of pancreatic lipase
• Micelle formation
- Bile Salts are amphipathic
- Core of free fatty acids and monoglycerides
- They simply diffuse into the interior of the cell, without the
bile salts

28. Fat
• Intracellular processing
- Reformation of triglycerides
- Combination with lipoproteins: short and medium chain fatty
acids may be diluted in blood (portal system). Chylomicrons to
lacteals and then lymphatics.
• Enterohepatic circulation
- Conjugated bile acids are absorbed in the distal ileum ->
portal system -> back to the liver
- Pool of 2-3g
- Recirculates 6 times every day
- 5% lost: resynthesis from cholesterol

29. Lipids
• Arrival of lipids in the duodenum serves as a
stimulus for secretion of bile. Emulsification:
Bile salts are secreted into duodenum to break
up fat droplets. Pancreatic lipase and colipase
hydrolyze triglycerides to free fatty acids and
monglycerides. Colipase coats the
emulsification droplets and anchors the lipase
enzyme to them. Form micelles and move to
brush border.

30. Lipids
• Free fatty acids, monoglycerides, and
lysolecithin leave micelles and enter into
epithelial cells. Resynthesize triglycerides and
phospholipids within cell. Combine with a
protein to form chylomicrons. Secreted into
central lacteals.

31. Lipid Transport
• In blood, lipoprotein lipase hydrolyzes
triglycerides to free fatty acids and glycerol for
use in cells. Remnants containing cholesterol
are taken to the liver. Form VLDLs which take
triglycerides to cells. Once triglycerides are
removed, VLDLs are converted to LDLs. LDLs
transport cholesterol to organs and blood
vessels

32. Water, Electrolytes and Vitamins
• Daily: 10 liters water in, 500cc out
• Water absorbed by simples diffusion
• Na: active transport
• Cl: passive diffusion
• HCO3: indirect active transport (NA)
• Ca: active transport in duodenum and jejunum
• Iron: active transport in Duodenum
• Vitamins: Fat soluble (DEAK): distal ileum. Water
soluble: variable

33. Intestinal Contractions and Motility
• 2 major types of contractions occur in the
small intestine:
• Peristalsis: Slow movement. Pressure at the
pyloric end of small intestine is greater than at
the distal end.
• Segmentation: Major contractile activity of
the small intestine. Contraction of circular
smooth muscle. Mix chyme

34. Contractions of Intestinal Smooth
Muscles
• Occur automatically in response to
endogenous pacemaker activity. Rhythm of
contractions is paced by graded
depolarizations called slow waves. Slow waves
produced by interstitial cells of Cajal. Slow
waves spread from 1 smooth muscle cell to
another through nexuses.

35. Contractions of Intestinal Smooth
Muscles
• When slow waves above threshold, it triggers
APs by opening of VG Ca 2+ channels. Inward
flow of Ca 2+ : Produces the upward
depolarization phase. Stimulates contraction
of smooth muscle. Repolarization: VG K +
channels open. Slow waves decrease in
amplitude as they are conducted. May
stimulate contraction in proportion to the
magnitude of depolarization

36. Peristaltic Contractions Myenteric
reflexes
• Not under CNS Parasympathetic stimulation:
accelerates local peristalsis and segmentation
• Gastroenteric Reflex Stimulates motility and
secretion: along entire small intestine
• Gastroileal Reflex triggers relaxation of
ileocecal valve: Allows materials to pass: from
small intestine into large intestine

37. Mechanical Processes
• Motility of the Small Intestine
- Segmentation is the most common motion of the
small intestine, which is the contracting and relaxing
of smooth muscle.
- Pacemaker cells in the smooth muscle initiate
segmentation, although the duodenum depolarizes
more frequently (12 –14 contractions per minute)
then the ileum (8 – 9 contractions per minute).

38. Mechanical Processes
• This allows ample time for complete digestion
and absorption as contents move towards the
ileocecal valve.
• Long and short reflexes and hormones alter
the intensity of segmentation.
• Parasympathetic activity enhances and
sympathetic activity decreases segmentation.

39. Peristalsis
• 1-2 cm/s
• Movement of intestinal chyme
• Duodenum sees to be pace setter in fed state
• Migrating myoelectric complex during fasting
periods: motilin
• Parasympathetic: cholinergic vagus stimulates
• Sympathetic: adrenergic inhibits

40. Peristalsis
• Peristalsis occurs after most nutrients have been
absorbed and is regulated on the basis of which
neurons are stimulated.
• Peristaltic waves sweep slowly along the duodenum to sweep
out debris, bacteria, and meal remnants.
• This is called the migrating mobility complex and it’s function
is to keep bacteria from settling into the small intestine.
• The enteric neurons of the GI tract coordinate the mobility
patterns.
• Impulses sent proximally by the cholinergic neurons cause
contraction and shortening of the muscle layer.

41. Peristalsis
• Impulses sent distally to certain interneurons cause
shortening of the longitudinal muscle layer and
distension of the intestine, in response to Ach
releasing neurons.
• Other impulses sent distally by activated VIP or NO
releasing enteric neurons that cause relaxation of the
circular muscle. As a result, as the proximal area
constricts and forces chyme along the tract, the
lumen of the intestine enlarges to receive it, where it
moves toward the ileocecal sphincter.

42. Peristalsis
• Impulses sent distally to certain interneurons cause
shortening of the longitudinal muscle layer and
distension of the intestine, in response to Ach
releasing neurons.
• Other impulses sent distally by activated VIP or NO
releasing enteric neurons that cause relaxation of the
circular muscle.
• As a result, as the proximal area constricts and forces
chyme along the tract, the lumen of the intestine
enlarges to receive it, where it moves toward the
ileocecal sphincter.

43. Peristalsis
• Most of the time, the ileosphincter valve is
constricted and closed. Two mechanisms; one
neural, one hormonal cause it relax and allow chyme
to enter the cecum.
• Enhanced activity in the stomach initiates the
gastroileal reflex that enhances force of
segmentation in the ileum.
• Gastrin release by stomach increases motility of the
ileum and relaxes the ileocecal sphincter.
• Once the chyme has passed through, it exerts
backpressure that closes the valve’s flaps and
prevents regurgitation.

44. Peristalsis Segmentation
• A ring of muscle contraction appears on the oral side
moves toward the anus, propelling the contents of
the lumen in that direction; as the ring moves, the
muscle on the other side of the distended area
relaxes, facilitating smooth passage of the bolus.
Segmentation contractions are a common type of
mixing motility seen especially in the small intestine -
segmental rings of contraction chop and mix the
ingesta. Alternating contraction and relaxation of the
longitudinal muscle in the wall of the gut also
provides effective mixing of its contents.

45. Peristalsis Rush
• Powerful and rapid peristalsis Travel long
distances to sweep the contents of the
intestine into the colon and thereby relieve
the small intestine irritative chyme and
excessive distention Is produced by intense
irritation of the intestinal mucosa, infectious
diarrhea

46. Ileocecal valve and ileocecal
sphincter
• The lips of the ileocecal valve protrude into
the lumen of the cecum to prevent the
backflow of fecal contents from the colon into
the small intestine
• Sphincter remains constricted and slows the
emptying of ileal contents into the cecum

47. Coordination of Secretion &
Absorption

48. Coordination of Secretion &
Absorption
• Intestinal Absorption
– It takes about 5 hours for materials
to pass from duodenum to end of ileum
– Movements of the mucosa increases absorptive
effectiveness
• Stir and mix intestinal contents
• Constantly change environment around epithelial cells

49. Digestion
• Digestive Enzymes
– Break molecular bonds in large organic molecules
• Carbohydrates, proteins, lipids, and nucleic acids
• In a process called hydrolysis
– Are divided into classes by targets
• Carbohydrases break bonds between simple sugars
• Proteases break bonds between amino acids
• Lipases separate fatty acids from glycerides

50. Digestion
• Water Absorption
– Cells cannot actively absorb or
secrete water
– All movement of water across
lining of digestive tract
• Involves passive water flow down
osmotic gradients

51. Thank you
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