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

4

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)