Small Intestine Anatomy and Physiology

Physiology

1. Digestion and Absorption
1. Carbohydrate
• in a typical adult diet, 60% of carbohydrates are ingested as starch, 30% as sucrose, and 10% as lactose


1. Starch
• consists of 2 glucose polymers: amylose (20%) and amylopectin (80%)
• pancreatic and salivary amylase break down amylose and amylopectin into maltose (glucose-glucose), maltotriose (glucose-glucose-glucose), and dextrins (the residual branched saccharides)
• these breakdown products are further broken down into glucose by enzymes located on the microvilli brush border
• glucose, along with sodium, is then actively transported across the intestinal membrane by carrier-mediated transport


2. Sucrose
• disaccharide made up of glucose and fructose molecules
• broken down into individual glucose and fructose molecules by sucrase, an enzyme located on the microvilli brush border


3. Lactose
• disaccharide made up of glucose and galactose molecules
• broken down into glucose and galactose by lactase, which is located on the microvilli brush border


2. Protein
• initial digestion begins in the stomach, where pepsin hydrolyzes proteins into polypeptides
• in the duodenum, pancreatic trypsinogen is activated to trypsin by the duodenal mucosal enzyme, enterokinase
• trypsin then activates the remaining pancreatic proteases
• the final products of protein digestion are amino acids and oligopeptides, which enter the intestinal cell by sodium dependent, carrier-mediated active transport
• the oligopeptides are broken down into free amino acids by cytosolic peptidases
• digestion and absorption of proteins is 80% to 90% completed in the jejunum


3. Fat
• fat in the duodenum stimulates the secretion of cholecystokinin and secretin, resulting in gallbladder emptying and the secretion of pancreatic enzymes
• pancreatic lipase hydrolyzes triglycerides into 2 fatty acids and beta monoglyceride
• after this initial digestion, bile salts and lecithin combine with lipids to form micelles
• when the micelles encounter the brush border of the microvilli, the enclosed lipids diffuse into the intestinal cell, a process which requires no energy
• once in the cell, the triglyceride is reformed in the smooth endoplasmic reticulum
• triglycerides then combine with cholesterol, phospholipids, and apoproteins to form chylomicrons
• chylomicrons preferentially enter the central lacteal of the villus, where they eventually enter the thoracic duct and then the bloodstream
• normally, all ingested fat is absorbed
• jejunum is the site of most fat absorption


1. Medium-Chain Fatty Acids
• C₈ to C₁₀ chain lengths
• may be absorbed without hydrolysis by lipase
• pass directly into portal blood without being formed into chylomicrons
• clinically, may be useful in the management of patients with deficiencies in fat digestion and absorption


2. Bile Salts
• the conjugated bile acids that form micelles are reabsorbed by active transport in the distal ileum
• reabsorbed bile acids then pass via portal venous blood to the liver, where they are resecreted in bile (enterohepatic circulation)
• normal bile acid pool is about 5 gm and it recirculates six to eight times a day
• only 500 mg of bile acids are lost in the stool per day
• lost bile acids are replaced by hepatic synthesis from cholesterol


4. Water, Electrolyte, and Mineral Absorption
1. Water Absorption
• the small intestine secretes and absorbs huge amounts of water
• 1 to 2 liters of water per day are ingested
• in addition, salivary, gastric, biliary, pancreatic, and intestinal secretions contribute 5 to 10 liters of water per day
• all but 500 cc of this fluid is reabsorbed in the small intestine
• osmotic pressure and diffusion are the forces governing movement of water across the intestinal cell membrane
• osmotic pressure results from the active transport of solutes (sodium, glucose, amino acids) into the intestinal cell
• diffusion occurs through pores in cell membranes


2. Electrolyte Absorption
• in the jejunum, sodium is usually cotransported along with glucose and amino acids
• in the ileum, sodium is absorbed against a steep electrochemical gradient
• potassium is passively absorbed along its electrochemical gradient
• chloride is absorbed by active transport and by coupling to organic solutes
• bicarbonate is absorbed by active transport


3. Mineral Absorption
• calcium is absorbed in the duodenum and proximal jejunum by active transport and is facilitated by an acid environment, vitamin D, and parathyroid hormone
• iron is absorbed in the duodenum
• magnesium absorption occurs in the ileum, predominantly by passive diffusion


4. Vitamin Absorption
• fat-soluble vitamins first become incorporated into micelles, and then pass freely into intestinal cells where they packaged into chylomicrons before entering the lymphatic system
• ileum is the major site of vitamin C absorption
• vitamin B₁₂ absorption occurs in the distal ileum and requires binding to gastric intrinsic factor


2. Motility
1. Migrating Myoelectric Complex (MMC)
• between meals, there are cyclically occurring contractions that move down the intestine every 90 minutes
• most of these contractions originate in the stomach or duodenum
• the purpose may be to sweep the intestine clean between meals
• motilin appears to regulate the MMC


2. Regulation of Motility
• pacesetter potentials, probably originating in the duodenum, initiate contractions that propel food through the small intestine
• 2 types of contractions: segmentation and peristalsis
• sympathetic activity inhibits intestinal motility; parasympathetic activity increases it
• denervation of the small intestine does not produce long-term disturbance of function
• many hormones influence intestinal motility: gastrin, cholecystokinin, and motilin stimulate muscle contraction; secretin, glucagon, and somatostatin inhibit muscle contraction


3. Endocrine Function
1. Secretin
• primarily found in duodenal S cells
• released in response to intraluminal acid
• stimulates the release of water and bicarbonate from the pancreas, neutralizing gastric acid
• also inhibits gastrin release, gastric acid secretion, and gastrointestinal motility


2. Cholecystokinin
• release is stimulated by amino acids and fatty acids
• causes gallbladder emptying and relaxation of the sphincter of Oddi
• stimulates the secretion of pancreatic enzymes


3. Other Gut Hormones
1. Enteroglucagon
• found primarily in the distal small intestine
• major action is to inhibit intestinal motility


2. Vasoactive Intestinal Peptide
• functions chiefly as a neuropeptide
• potent vasodilator
• stimulates pancreatic and intestinal secretions
• inhibits gastric acid secretion
• is the primary agent in the watery diarrhea syndrome caused by pancreatic endocrine tumors (VIPomas)


3. Motilin
• regulates the MMC


4. Somatostatin
• paracrine agent
• functions as an ‘off’ switch to inhibit GI hormone release


4. Immune Function
• the large surface area of the small bowel mucosa represents a huge potential portal of entry for pathogens
• the small bowel is a major source of immunoglobulin A, which is produced by plasma cells located in the lamina propria
• IgA antibody binds to a carrier protein and is secreted into the intestinal lumen
• the antibodies collect in the glycocalyx, where they are best able to interact with antigen
• antigens from the intestinal lumen are transported through specialized epithelial cells called M cells that are in contact with underlying lymphoid cells
• antigens then come into contact with macrophages, leading to the secretion of interleukin-1 and activation of helper T cells