small intestine 1
DESCRIPTION
UG lectureTRANSCRIPT
Small Intestine
Dr Raghuveer Choudhary
Overview
- Nearly all chemical
digestion and nutrient
absorption occur in the
small intestines.
Gross anatomy
• Three regions:• Duodenum - first 25 cm• Jejunum – next 2.5 m• Ileum – last 3.6 m ; ends at ileocecal
junction – joins cecum of large intestine
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Three Parts of Small Intestine
The Small Intestine plays key role in digestion and absorption of nutrient,
90% of nutrient absorption occurs in the small intestine
• The Duodenum• The segment of small intestine closest to
stomach• 25 cm (10 in.) long• “Mixing bowl” that receives:
– chyme from stomach– digestive secretions from pancreas and liver
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
The Ileum• The final segment of small intestine• 3.5 meters (11.48 ft) long
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Wall of Small Intestine
Small Intestine
• Nearly all chemical digestion & nutrient absorption occurs here
• Longest part of digestive tract• Circular folds of mucosa , villi, and
microvilli – enhance surface area for absorption of nutrients
Digestive System
Or folds Kerckring – well developed in duodenum and jejunum; inc absorptive area 3x
Less in distal small int; inc Absorptive area another 10x
10
Structure of the Villi in the Small Intestine
On each epithelial cell on each villus; inc absorptive capacity for another 20x
Microvilli (brush border)
brush border enzymes
- The surface area inside the small intestine is greatly increased by circular folds, villi, and microvilli.
villi
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Intestinal Villus
Epithelial cells cover the mucosa
Small Intestine
• Each villus is a fold in the mucosa.
• Covered with columnar epithelial cells interspersed with goblet cells.
• Epithelial cells at the tips of villi are exfoliated and replaced by mitosis in crypt of Lieberkuhn.
• Lamina propria contain lymphocytes, capillaries, and central lacteal.
Insert fig. 18.12
Digestive Anatomy: Histological Overview
Figure 21-2e: ANATOMY SUMMARY: The Digestive System
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
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Small Intestine
• Absorbs– 80% ingested water– Electrolytes– Vitamins– Minerals– Carbonates
• Active/facilitated transport• Monosaccharides
– Proteins• Di-/tripeptides• Amino acids
– Lipids• Monoglycerides• Fatty acids• Micelles• Chylomicrons
Intestinal Secretions
• Watery intestinal juice• 1.8 liters per day enter intestinal lumen• Moistens chyme• Assists in buffering acids• Keeps digestive enzymes and products of
digestion in solution
Intestinal Movements• Chyme arrives in duodenum• Weak peristaltic contractions move it slowly
toward jejunum
Digestive Secretions:
(7 L / Day From Tissues into Lumen)
Figure 21-5: Daily mass balance in the digestive system
• Salivary glands• Pancreas• Water• Enzymes• Mucus• Ions: H+, K+, Na+
• HCO3-, Cl-
• Mass Balance (H2O)
• 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.
Intestinal Enzymes
• Duodenum and jejunum:– Carbohydrates, amino acids, lipids, iron, and Ca2+.
• Ileum:– Bile salts, vitamin B12, electrolytes, and H20.
Absorption in Small Intestine
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Small Intestine
• Secretes digestive enzymes– Peptidases
• Amino-• Di-• Tri-
– Sucrases– Maltase– Lactase– Saccharidases
• Di-• Tri-
– Lipase– Nucleases
brush border enzymes
- activates zymogens- complete digestion of carbohydrates and proteins
Chemical Digestion and Absorption of Nutrients
Chemical Digestion and Absorption of Nutrients
Carbohydrates
Proteins
Lipids
Nucleic Acids
Vitamins
Minerals
Water
Digestion and Absorption of Carbohydrate
- Most digestible dietary carbohydrate is starch.
- The starch digestion begins in the mouth by salivary amylase.
- But fully digestion of starch occurs in the small intestines.
CHEMICAL DIGESTION OF CARBOHYDRATES
STARCH SUCROSE LACTOSE
mouth
stomach
small intestine
salivary amylase
(absorbed into blood of villus)
MALTOSE
pancreaticamylase
brush border maltase
glucose + glucose
SUCROSE
glucose + fructose(absorbed into blood of villus)
brush border sucrase
LACTOSE
glucose + galactose(absorbed into blood of villus)
brush border lactase
Digestion and Absorption of Carbohydrates
• Salivary amylase:– Begins starch digestion.
• Pancreatic amylase:– Digests starch to
oligosaccharides.– Oligosaccharides
hydrolyzed by brush border enzymes.
• Glucose is transported by secondary active transport with Na+ into the capillaries.
Insert fig. 18.32
- Starch is digested to oligosaccharides (3-8 glucose residues), disaccharide maltose, and glucose.
starcholigosaccharides glucose
pancreatic amylaseBrush borderenzymes
Intestinal lumen Intestinalepithelialcells
blood
glucose glucose
Glucose is absorbed by: - sodium-dependent glucose transporter (SGLT). - solvent drag
Digestion and Absorption of Proteins
CHEMICAL DIGESTION OF PROTEINS
PROTEINS
mouth
stomach
small intestine
(absorbed into blood of villus)
SMALLER PROTEINS, POLYPEPTIDES
trypsin trypsinogen
brush border peptidases
peptides
pepsin pepsinogen
HCl
enterokinase
chymotrypsin chymotrypsinogentrypsin
carboxypeptidase procarboxypeptidasetrypsin
amino acids
from pancreas
from intestinal glands
• 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.
Digestion and Absorption of Protein
- Proteins are digested by proteases and peptidases.
- Protein digestion starts in the stomach.
Protein digestion continues in the small intestine by pancreatic enzymes trypsin and chymotrypsin.
Protein digestion is completed in the small intestine by brush border enzymes carboxypeptidase, aminopeptidase, and dipeptidase.
Amino acid absorption is similar to that of monosaccharides, via several sodium-dependent amino acid cotransporters.
Digestion and Absorption of Protein (continued)
• 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.
Insert fig. 18.33
Digestion and Absorption of Lipids
- Lipids are digested by enzymes called lipases.
- Most fat digestion occurs in the small intestine via several steps.
CHEMICAL DIGESTION OF FATS
FATS
mouth
stomach
small intestine
lingual lipase
(absorbed into lymph of villus)
EMULSIFIED FATS
pancreatic lipase + cholesterol esterase
CHYLOMICRONS
gastriclipase
from pancreas
reassembled into
bile
FATTY ACIDS, GLYCEROLS, GLYCERIDES
from gallbladder/liver
minimal effects
minimal effects
• Arrival of lipids in the duodenum serves as a stimulus for secretion of bile.
• Emulsification:– Bile salt micelles 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.
Digestion and Absorption of 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.
Digestion and Absorption of Lipids (continued)