Small Intestine

CHAPTER 19
Small Intestine

James Lee and Dale A. Dangleben

Test Taking Tips

• Know which nutrients are absorbed in which parts of the small intestine.

• Know the various surgical options for strictures in Crohn disease. Try to lean more toward stricturoplasty in an attempt to preserve bowel. Know the various stricturoplasties.

EMBRYOLOGY

What does the cranial limb of the midgut loop develop into?

Distal duodenum, jejunum, and proximal ilium

What does the caudal limb of the midgut loop develop into?

Distal ilium and proximal two-thirds of the transverse colon

What structure joins the yolk sac to the juncture of the cranial and caudal limbs of the midgut loop?

The vitelline duct

What is a persistent vitelline duct otherwise known as?

Meckel diverticulum

How much does midgut loop rotate during normal physiologic herniation of bowel?

270° counterclockwise rotation

ANATOMY

How long is the entire small intestine, from the pylorus to the cecum?

~6 m (20 ft)

How long is the duodenum?

26 cm (9.8 in.)

How long is the jejunum?

2.5 m (8.2 ft)

How long is the ileum?

3.5 m (11.5 ft)

What peritoneal fold supports the duodenojejunal angle, which marks where the duodenum ends and the jejunum begin?

Ligament of Treitz

FIGURE 19-1. Blood supply and luminal surface of the small bowel. The arterial arcades of the small intestine increase in number from 1 or 2 in the proximal jejunum to 4 or 5 in the distal ileum, a finding that helps to distinguish proximal from distal bowel at operation. Plicae circulares are more prominent in the jejunum. (Reproduced with permission from Gerard M Doherty. CURRENT Diagnosis & Treatment: Surgery. 13th ed. http://www.accessmedicine.com. Copyright © The McGraw-Hill Companies, Inc. All rights reserved.)

What are plicae circulares?

Circular folds of mucosa in the small bowel lumen (also known as valvulae conniventes)

Excluding the proximal duodenum, what artery supplies the entire blood supply of the small bowel?

Except for the proximal duodenum that is supplied by branches of the celiac axis, the supply comes entirely from the superior mesenteric artery (SMA).

Where is the parasympathetic innervation of the small bowel derived from?

Parasympathetic fibers are derived from the vagus, which traverse the celiac ganglion to supply the small bowel.

Where is the sympathetic innervation of the small bowel derived from?

Sympathetic fibers originate from 3 sets of splanchnic nerves; ganglion cells found in a plexus around base of SMA.

Describe the lymphatic drainage of the small intestine:

From mucosa through wall of the bowel

Mesenteric lymph nodes adjacent to the bowel

Regional nodes adjacent to the mesenteric arterial arcades

Group of lymph nodes at the base of the superior mesentery vessels

Cisterna chyli

Thoracic ducts

Return to venous system

FIGURE 19-2. Layers of wall of the small intestine. The individual layers and their prominent features are represented schematically. (Reproduced from Brunicardi FC, Anderson DK, Billiar TR, et al. Schwartz’s Principles of Surgery. 9th ed. http://www.accessmedicine.com. Copyright © The McGraw-Hill Companies, Inc. All rights reserved.)

What are the 4 layers of the small bowel? From out to in:

Serosa

Muscularis propria

Submucosa

Mucosa

The muscularis propria comprises what 2 muscle layers?

Thin outer longitudinal layer; thicker inner circular layer (with myenteric [Auerbach] plexus between the muscle layers)

The strongest layer of the small bowel:

Submucosa

What extensive plexus of nerve fibers and ganglion cells are found in the submucosa of the small bowel?

Meissner plexus

What 3 layers comprise the mucosa of the small bowel?

Muscularis mucosae (thin layer of muscle that separates mucosa from submucosa) Lamina propria (connective tissue layer between epithelial cells and muscularis mucosae) Epithelial layer (continual sheet of epithelial cells covering villi and lining crypts)

Which layer of small bowel mucosa is responsible for combating microorganisms that penetrate the overlying small bowel epithelium?

Lamina propria (contains a rich supply of immune cells)

What are the 4 main cell types found in the small bowel mucosal layer?

Absorptive enterocytes

Goblet cells (secretes mucus)

Paneth cells (secrete lysozyme, tumor necrosis factor)

Enteroendocrine cells (produce gastrointestinal hormones)

Term for the fuzzy coat of glycoprotein that covers the microvilli to further increase absorption:

Glycocalyx

The portions of the duodenum that are retroperitoneal:

Second portion (descending), third portion (transverse)

The vascular supply of the duodenum is:

Anterosuperior pancreaticoduodenal and posterosuperior pancreaticoduodenal arteries (from gastroduodenal artery) and anteroinferior pancreaticoduodenal and posteroinferior pancreaticoduodenal arteries (from SMA)

What is the first branch off of the SMA?

Anteroinferior pancreaticoduodenal and posteroinferior pancreaticoduodenal arteries

PHYSIOLOGY

What is the primary responsibility of the small bowel?

The absorption of carbohydrates, proteins, fats, ions, vitamins, and water

What does the terminal ileum absorb?

Fatty acids; bile salts; vitamin B₁₂

What enzymes involved in carbohydrate digestion are contained in the brush border of the small intestine?

Lactase; maltase; sucrase-isomaltase; trehalase

Name the substrate and the product for lactase, located in the brush border of the small intestine:

Substrate: lactose

Product: glucose, galactose

What brush border enzyme in the duodenum activates pancreatic trypsinogen that is secreted into the intestine?

Enterokinase

What 3 principal pancreatic proteases comprise the endopeptidases?

Trypsin

Chymotrypsin

Elastase

What 2 principal pancreatic proteases comprise the exopeptidases?

Carboxypeptidase A

Carboxypeptidase B

What is the primary action of trypsin?

Forms products with basic amino acids at carboxyl-terminal end by attacking peptide bonds involving basic amino acids.

What is the primary action of chymotrypsin?

Forms peptide products with aromatic amino acids, leucine, glutamine, and methionine at carboxyl-terminal ends by attacking peptide bonds involving aromatic amino acids, leucine, glutamine, and methionine.

What is the primary action of elastase?

Forms products with neutral amino acids at carboxyl-terminal end by attacking peptide bonds involving neutral aliphatic amino acids.

What is the primary action of carboxypeptidase A?

Attacks peptides with aromatic and neutral aliphatic amino acids at carboxyl-terminal end

What is the primary action of carboxypeptidase B?

Attacks peptides with basic amino acids at carboxyl-terminal end

Normally, what percentage of digestion and absorption of protein is completed in the jejunum?

80% to 90%

Describe the process of the fat digestion in the small bowel:

Fat globules from the diet are emulsified with the help of bile

Pancreatic lipase splits triglycerides into free fatty acids and 2-monoglycerides

Bile salts form micelles with the monoglycerides and free fatty acids (dissolved in central hydrophobic portion of micelles) formed from lipolysis

Micelles carry the products of fat hydrolysis to brush border where absorption occurs

What is a micelle?

A small spherical globule with a lipophilic sterol nucleus and a polar hydrophilic group that projects outward and is composed of 20 to 40 molecules of bile salts

What kind of fatty acids are absorbed directly into the portal blood?

Short- and medium-chain fatty acids

What kind of fatty acid is absorbed via chylomicrons passing from the epithelial cells into lacteals, which pass through the lymphatics into the venous system?

Long-chain fatty acids

What is the composition of a chylomicron?

~90% triacylglycerol; ~10% phospholipid; cholesterol; protein

What percentage of fat absorption occurs by way of the thoracic lymphatics?

80% to 90%

What percentage of bile salts are reabsorbed?

95%

How are unconjugated bile acids absorbed into the jejunum?

Passive diffusion

Only location of the small bowel where conjugated bile is absorbed:

Terminal ileum

What is bile conjugated to?

Glycine and taurine

What are the primary bile acids?

Chenodeoxycholic; cholic

What are the secondary bile acids?

Dexoycholic; lithocholic

How much is the total bile salt pool in humans?

2 to 3 g

Approximately how much bile is lost in the stool in 1 day?

0.5 g

Number of times the bile recirculates in 24 hours (enterohepatic circulation):

~6 times

In the ileal enterocyte, what binds and transports free vitamin B₁₂ into the portal circulation?

Transcobalamin II

What are the 3 areas of the small bowel where gut-associated lymphoid tissue can be found?

Intraepithelial; lamina propria; Peyer patches

What cells contained within the follicle-associated epithelium of the small bowel serve as a site for the selective sampling of intraluminal antigens?

M cells (microfold cells)

What is the most important hormone in the migrating motor complex?

Motilin

During which phase of the migrating motor complex is motilin found at its peak plasma levels?

Phase III

What happens during Phase I of the migrating motor complex?

Rest

What happens during Phase II of the migrating motor complex?

Acceleration and gallbladder contraction

What happens during Phase III of the migrating motor complex?

Peristalsis

What happens during Phase IV of the migrating motor complex?

Deceleration

What is the primary effect of the hormone gastrin?

Stimulates gastric mucosal growth; stimulates acid and pepsinogen secretion

Location of the cells that release the hormone gastrin:

G cells in the duodenum; antrum

What are the major stimulants for the release of the hormone gastrin?

Amino acids; peptides; antral distention; gastrin-releasing peptide (bombesin); vagal and adrenergic stimulation

What is the primary effect of the hormone cholecystokinin?

Stimulates gallbladder contraction; relaxes sphincter of Oddi; stimulates pancreatic enzyme secretion; inhibits gastric emptying

Location of the cells that release the hormone cholecystokinin:

Duodenum; jejunum (I cells)

What are the major stimulants for the release of the hormone cholecystokinin?

Amino acids; peptides; fats

What is the primary effect of the hormone secretin?

Stimulates flow/alkalinity of bile and release of water and bicarbonate from pancreatic ductal cells; inhibits gastrin releases and gastric acid secretion and motility

Location of the cells that release the hormone secretin:

Duodenum; jejunum (S cells)

What are the major stimulants for the release of the hormone secretin?

Bile salts; fatty acids; luminal acidity

What is the primary effect of the hormone somatostatin?

Inhibits gastric acid secretion, release of gastrointestinal hormones, small bowel water and electrolyte secretion, and secretion of pancreatic hormones (universal “off” switch)

Location of the cells that release the hormone somatostatin:

Pancreatic islets (D cells); antrum; duodenum

What are the major stimulants for the release of the hormone somatostatin?

Pancreas: amino acids, cholecystokinin, and glucose;

Gut: acid, fat, protein, and other hormones (gastrin, cholecystokinin)

What is the primary effect of the hormone gastrin-releasing peptide?

Stimulates release of all gastrointestinal hormones (except secretin), gastric acid secretion and release of antral gastrin, gastrointestinal secretion and motility, and growth of intestinal mucosa and pancreas (universal “on” switch)

Location of the cells that release the hormone gastrin-releasing peptide:

Small bowel

What are the major stimulants for the release of the hormone gastrin-releasing peptide?

Vagal stimulation

What is the primary effect of the hormone gastric inhibitory peptide?

Stimulates pancreatic insulin release in response to hyperglycemia and inhibits gastric acid and pepsin secretion

Location of the cells that release the hormone gastric inhibitory peptide:

Duodenum; jejunum (K cells)

What are the major stimulants for the release of the hormone gastric inhibitory peptide?

Adrenergic stimulation

Glucose

Fat