The Small Intestine

The Small Intestine

The Small
Intestine

• Most digestion and absorption of nutrients occur

• Length provides a large surface area

• Area is further increased by circular folds, villi, and
microvilli

• Begins at the pyloric sphincter of the stomach

• Eventually opens into the large intestine

• Length – 3 m in a living person and about 6.5m in a
cadaver

Parts of
Small Intestine

Anatomy of
Small Intestine

• Duodenum – 25 cm

– Shortest region 25 cm

– Retroperitoneal

– Merges with the jejunum

• Jejunum – 1 m

– 1 m long and extends to the ileum

– Jejunum means “empty” – found at death

• Ileum – 2 m

– The final and longest region of the small intestine

– Joins the large intestine at ileocecal sphincter

Functions
of Small Intestine

• Segmentations mix chyme with digestive juices and bring
food into contact with the mucosa for absorption

• Peristalsis propels chyme through the small intestine

• Completes the digestion of carbohydrates, proteins, and
lipids

• Begins and completes the digestion of nucleic acids

• Absorbs about 90% of nutrients and water

Layers of
Small Intestine

Histology
of Small Intestine

• Composed of a layer of epithelium, lamina propria &
muscularis mucosae

• Epithelial layer:
consists of simple columnar epithelium

– Absorptive cells, Goblet cells & Paneth cells

– Enteroendocrine cells: S cells, CCK cells and K cells

• Intestinal glands (crypts of Lieberkühn) and secrete
intestinal juice

Mucosa

• The lamina propria

– Contains areolar connective tissue

– Abundance of mucosa-associated lymphoid tissue (MALT)

– Ileum: Groups of lymphatic nodules – aggregated lymphatic
follicles (Peyer’s patches)

– The muscularis mucosae

– Consists of smooth muscle

Enlarged Villus

Histology
of Small Intestine – Submucosa

• Duodenum contains duodenal (Brunner’s) glands – secrete an
alkaline mucus

• Helps neutralize gastric acid in the chyme

Muscularis

• Consists of two layers of smooth muscle

• The outer, thinner layer contains longitudinal fibers

• The inner, thicker layer contains circular fibers

Serosa

• Except for a major portion of the duodenum, the serosa (or
visceral peritoneum) completely surrounds the small intestine

Special
Features of Small Intestine

• Facilitate the process of digestion and absorption

• The structural features include:

– Circular folds, Villi & Microvilli

Circular
folds or Plicae circulares

• Folds of the mucosa and submucosa

• These permanent ridges – 10 mm long

• Begin near the proximal portion of the duodenum

• End at about the midportion of the ileum

• Enhance absorption by increasing surface area

• Causing the chyme to spiral, rather than move in a
straight line

Villi (Tufts
of Hair)

• Fingerlike projections of the mucosa – 0. 5–1 mm long

• Gives the intestinal mucosa a velvety appearance

• Each villus – covered by epithelium and has a core of
lamina propria

Microvilli

• Projections of the apical (free) membrane of the
absorptive cells

• Each microvillus is a 1 m-long cylindrical, membrane
-covered projection

• Contains a bundle of 20–30 actin filaments

• Greatly increase the surface area

• Viewed through a light microscope as fuzzy line – brush
border

• Brush border contains several brush-border enzymes – have
digestive functions

• 200 million microvilli per square millimeter of small
intestine

Role of
Intestinal Juice and Brush Border Enzyme

Intestinal
juice

• Clear yellow fluid

• Contains water and mucus and is slightly alkaline (ph 7.6)

• Together, pancreatic and intestinal juices provide a
liquid medium

• Aids the absorption of substances from chyme

Brush-border
Enzymes

• The absorptive cells of the small intestine synthesize
several digestive enzymes brush-border enzymes

• Insert them in the plasma membrane of the microvilli

Carbohydrate
digesting enzyme

• Dextrinase – dextrins into glucose

• Maltase – Maltose to glucose

• Sucrase – Sucrose to glucose and fructose

• Lactase – Lactose to glucose and galactose

Protein-digesting
enzymes – Peptidases

• Aminopeptidases – Break off amino acids at the amino ends
of peptides

• Dipeptidases – Split dipeptides into aminoacids

Nucleotide-digesting
enzymes

• Nucleosidases and Phosphatases – Nucleotides to pentoses
and nitrogenous bases

Mechanical
Digestion in Small Intestines 

• The two types of movements of the small intestine:

– Migrating Motility Complexes – type of peristalsis called
governed by the myenteric plexus

– Segmentations

• Segmentations

– Localized, mixing contractions

– Occur in portions of intestine distended by a large volume
of chyme

– Segmentations mix chyme with the digestive juices

– Bring the particles of food into contact with the mucosa
for absorption

– They do not push the intestinal contents along the tract

– Duodenum – 12 times per minute

– Progressively slow to – 8 times per minute in the ileum

MMC

• Less distension of the small intestine, segmentation stops
and peristalsis begins

• Begins in the lower portion of the stomach

• Pushes chyme forward along a short stretch of small
intestine

• Slowly migrates down the small intestine, reaching the end
of the ileum in 90–120 minutes

• Altogether, chyme remains in the small intestine for 3–5
hours

Chemical
Digestion in Small Intestines 

• In the mouth
salivary amylase converts starch (a polysaccharide) to:

• Maltose (a disaccharide)

• Maltotriose (a trisaccharide)

• Dextrins (short-chain, branched fragments of starch with
5– 10 glucose units)

• Lingual and gastric
lipases convert some triglycerides into:

– Fatty acids

– Diglycerides

– monoglycerides

• In the stomach,
pepsin converts proteins to:

– Peptides (small fragments of proteins)

• Thus, chyme entering the small intestine contains
partially digested carbohydrates, proteins and lipids

• The completion of the digestion of carbohydrates,
proteins, and lipids is a collective effort of pancreatic juice, bile, and
intestinal juice in the small intestine

Digestion
of Carbohydrates

• Pancreatic amylase

– Acts on both glycogen and starches

– No effect on another polysaccharide called cellulose, an
indigestible plant fiber that is commonly referred to as “roughage”

• After amylase, a brush-border enzyme called -dextrinase –
acts on the resulting -dextrins, clipping off one glucose unit at a time

• Ingested molecules of sucrose, lactose, and maltose—three
disaccharides—are not acted on until they reach the small intestine

• Three brush-border enzymes digest the disaccharides into
monosaccharides

• Sucrase breaks
sucrose into a molecule of glucose and a molecule of fructose

• Lactase digests
lactose into a molecule of glucose and a molecule of galactose

• Maltase splits
maltose and maltotriose into two or three molecules of glucose

• Ends with the production of monosaccharides

Digestion
of Proteins

• Protein digestion starts in the stomach

• Pepsin: Fragments
protein into peptides

• Enzymes in
pancreatic juice: continue to break down proteins into peptides

• Protein digestion is completed by two peptidases in the
brush border

• Aminopeptidase
cleaves off the amino acid at the amino end of a peptide

• Dipeptidase
splits dipeptides (two amino acids joined by a peptide bond) into single amino
acids

Digestion
of Lipids

• The most abundant lipids in the diet are triglycerides

• Lipases: Split triglycerides and phospholipids

• Most digestion occurs in the small intestine

• Pancreatic lipase – Breaks down by pancreatic lipase into
fatty acids and monoglycerides

• The liberated fatty acids can be either short (with fewer
than 10–12 carbons) or long-chain fatty acids

Emulsification

• Process in which the large lipid globule is broken down
into several small lipid globules

• Bile contains bile salts, the sodium salts and potassium
salts of bile acids (mainly chenodeoxycholic acid and cholic acid)

• The amphipathic nature of bile salts allows them to
emulsify

• The hydrophobic regions interact with the large lipid
globule

• The hydrophilic regions of bile salts interact with the
watery intestinal chyme

Digestion of Nucleic Acids

• Pancreatic juice contains two nucleases:

– Ribonuclease – digests RNA

– Deoxyribonuclease – digests DNA

• The nucleotides that result from the action of the two
nucleases are further digested by brush-border enzymes Nucleosidases,
Phosphatases

• Into pentoses, phosphates, and nitrogenous bases

• These products are absorbed via active transport

Absorption
of Carbohydrates

Absorption
of Proteins

Absorption
of Lipids

Movement of
absorbed nutrients into the Blood and Lymph

Absorption
of Electrolytes & Vitamins

• Sodium ions are actively transported out of absorptive
cells by basolateral sodium–potassium pumps after they have moved into
absorptive cells via diffusion and secondary active transport

• Negatively charged bicarbonate, chloride, iodide, and
nitrate ions can passively follow Na or be actively transported

• Calcium ions also are absorbed actively in a process
stimulated by calcitriol

• Other electrolytes such as iron, potassium, magnesium, and
phosphate ions – active transport mechanisms

• Fat-soluble vitamins A, D, E, and K are included with
ingested dietary lipids in micelles – Absorbed via simple diffusion

• Most water-soluble vitamins – simple diffusion

• Vitamin B combines with intrinsic factor – absorbed in the
ileum via an active transport mechanism

Absorption
of Water

• Osmosis from the lumen of the intestines through
absorptive cells and into blood capillaries

• Depends on the absorption of electrolytes and nutrients to
maintain an osmotic balance with the blood

• The absorbed electrolytes, monosaccharides, and amino
acids establish a concentration gradient for water

Daily volumes of fluid
ingested, secreted, absorbed, and excreted from the GI tract