Anatomy and Physiology of The Small Intestine

What is the basic function of the small intestine?

The site where 90% of digestion and absorption occurs.

What is the length of the small intestine in living people vs cadavers?

8.6–16.4 ft (living), 19.7–23 ft (cadavers) due to loss of muscle tone.

What are the three regions of the small intestine?

Duodenum, Jejunum, Ileum.

What is the function of the duodenum?

Receives secretions from the pancreas, liver, and gallbladder; begins digestion.

What does the hepatopancreatic sphincter (Sphincter of Oddi) do?

Controls the release of bile and pancreatic juice into the duodenum.

What does the pancreas and duodenal glands secrete?

Alkaline mucus to neutralize stomach acid.

What is bile and its function?

A secretion from the liver/gallbladder containing bile salts, cholesterol, and pigments; it emulsifies fats.

What are amphipathic bile salts?

Molecules with hydrophilic and hydrophobic regions that break and coat lipid droplets to prevent recombining.

What percentage of the small intestine is the jejunum?

40%

What percentage of the small intestine is the ileum?

55%

Where does the ileum end?

At the ileocecal valve.

What are villi?

Mucosal folds projecting into the lumen; increase surface area.

What cells line villi?

Simple columnar absorptive cells called enterocytes.

What are microvilli and their purpose?

Tiny folds on enterocytes’ apical membrane; create a brush border and increase surface area.

What else do villi contain besides enterocytes?

Goblet cells and a core of lamina propria with capillaries and a lacteal.

What is the function of the lacteal?

Absorbs lipids into the lymphatic system.

What are crypts of the small intestine?

Inward folds into the lamina propria; contain secretory cells.

What types of cells are in the crypts?

Goblet cells, enteroendocrine cells, Paneth cells, and stem cells.

What is the role of enteroendocrine cells?

Secrete digestive hormones.

What is the function of Paneth cells?

Regulate bacterial growth.

What are Peyer’s patches and where are they found?

MALT structures in the ileum's lamina propria; part of immune defense.

What are plica circulares?

Large folds in the submucosa that increase surface area; most prominent in the jejunum.

What are Brunner’s glands and their function?

Glands in the duodenal submucosa that secrete alkaline mucus to neutralize chyme.

What are the layers of the small intestine wall?

Mucosa, submucosa, muscularis externa, and serosa.

Where does most carbohydrate digestion occur?

Small intestine (90%) by pancreatic amylase and brush border enzymes.

What brush border enzymes digest disaccharides?

Sucrase, lactase, and maltase.

What are the digestion products of sucrose, lactose, and maltose?

Sucrose → glucose + fructose; Lactose → glucose + galactose; Maltose → glucose.

How are glucose and galactose absorbed?

By Na+/glucose cotransporter (SGLT) into enterocytes, then GLUT transporter out into blood.

How is fructose absorbed?

By facilitated diffusion via GLUT uniporters in and out of enterocytes.

Where are all monosaccharides absorbed?

Into cardiovascular capillaries in the lamina propria.

How much lipid digestion occurs before the small intestine?

20% by lingual lipase in the mouth and stomach.

Why is lipid digestion inefficient in the mouth and stomach?

Lipids form large droplets with low surface area for enzyme access.

How are lipids emulsified in the small intestine?

Bile salts break large lipid droplets into micelles and coat them.

What enzyme digests emulsified triglycerides in the small intestine?

Pancreatic lipase.

What are the final products of triglyceride digestion?

Monoglycerides and fatty acids.

How are lipids absorbed into enterocytes?

Micelles contact the membrane and monoglycerides/fatty acids diffuse in.

What are chylomicrons?

Lipid-protein vesicles formed inside enterocytes from absorbed fats.

Where do chylomicrons go after formation?

Exocytosed into interstitial fluid and absorbed into lacteals (lymph).

Where does protein digestion begin?

In the stomach, with HCl and pepsin.

What does pepsin do?

Breaks proteins into polypeptides (about 10% of protein digestion).

What enzymes continue protein digestion in the small intestine?

Trypsin and chymotrypsin (from pancreas), activated from trypsinogen/chymotrypsinogen.

What enzymes complete protein digestion into amino acids?

Peptidases (from pancreas and enterocytes).

How are amino acids absorbed?

By Na+/amino acid cotransporters and uniporters into enterocytes, then by uniporters into the blood.

How much water is absorbed by the small intestine daily?

About 9250 mL out of 9400 mL.

How is water absorbed?

By osmosis, due to high solute concentration in the interstitial fluid.

How are ions absorbed in the small intestine?

Through channels and active transport (primary and secondary).

Which vitamins are absorbed by diffusion?

Water-soluble vitamins (B, C), except B12.

How is vitamin B12 absorbed?

By active transport only when bound to intrinsic factor.

Which vitamins are fat-soluble?

Vitamins A, D, E, and K.

How are fat-soluble vitamins absorbed?

They dissolve in micelles and are absorbed along with lipids by diffusion.

Why can't fat-soluble vitamins be absorbed without dietary lipids?

They require lipids to dissolve into micelles for absorption.