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


• 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


• Consists of two layers of smooth muscle

• The outer, thinner layer contains longitudinal fibers

• The inner, thicker layer contains circular fibers


• 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


• 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


• 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


• 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

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