Human Digestive System Quiz

Gastrointestinal System 3: The Small Intestine

• The small intestine is divided into three main sections: duodenum, jejunum, and ileum.
• The duodenum and hepatopancreatic region: the pancreas, and gallbladder contribute.

The Duodenum & Hepatopancreatic Region

• The duodenum is the first 10 inches of the small intestine.
• It's the primary site for intensive digestion.
• It receives bile and pancreatic juice via the common bile and main pancreatic ducts.
• Chyme in the duodenum stimulates releases of hepatic and pancreatic secretions.
• Fatty chyme triggers the release of Cholecystokinin (CCK)
• Acid in chyme triggers the release of secretin which in turn causes release of pancreatic bicarbonate which neutralizes the acidity.
• Arrival of chyme stimulates the release of hepatic and pancreatic secretions.

Structure of the Pancreas

• The pancreas has endocrine (hormonal) and exocrine functions.
• Exocrine function involves secretory cells (acinar, duct cells)
• Acinar cells release proteases, amylase, lipase into ducts.
• Duct cells secrete bicarbonate solution.
• Endocrine function involves islets of Langerhans with specific cell types (alpha & beta cells)
• Some pancreatic enzymes are secreted as inactive precursors (zymogens).

Functional Anatomy of the Pancreas

• Exocrine pancreas
  • Duct cells secrete aqueous bicarbonate solutions
  • Acinar cells secrete pancreatic juice (proteases, amylase, lipase).
• Endocrine pancreas
• Islets of Langerhans
• Alpha (α) cells - secrete glucagon
• Beta (β) cells - secrete insulin
• Pancreatic enzymes are secreted as inactive precursors that require activation in the duodenum.

Secretions of the Pancreas

• The pancreas' exocrine portion releases enzymes and bicarbonate.
• The endocrine portion releases hormones insulin and glucagon
• Bicarbonate neutralizes stomach acid in the duodenum.

Hormonal Control of Pancreatic Exocrine Secretion

• Acid in the duodenum triggers the release of secretin, which stimulates the pancreas to release bicarbonate-rich secretions, neutralizing stomach acid.
• Fatty and protein products in the duodenum trigger the release of CCK, stimulating the release of pancreatic digestive enzymes.
• These regulatory hormones like secretin and CCK control pancreatic secretion.

Activation of Pancreatic Enzymes

• Inactive precursor proteases (zymogens) are activated by enteropeptidase.
• Trypsin is a crucial enzyme activating other enzymes, controlling enzymatic cascade.
• The pancreas produces trypsin inhibitors to prevent premature activation of the enzymes.
• Trypsin, a crucial enzyme, activates several other pancreatic enzymes.

The Liver & Hepatic Portal Circulation

• The liver receives blood from two sources:
  • Arterial blood (O2 supply and metabolites) via the hepatic artery
  • Venous blood (nutrients and absorbed substances) via the hepatic portal vein from the digestive tract.
• The blood leaves the liver via the hepatic vein.
• The liver is a crucial organ playing multiple roles in digestion, nutrient processing, and metabolic reactions.

The Liver

• The liver is the largest gland in the body.
• It is composed of "plates" of hepatocytes.
• A key function is bile production (and other metabolic processes).

Bile

• Bile is a yellow-green, alkaline solution.
• It contains bile salts, bile pigments, cholesterol, neutral fats, and electrolytes.
• Bile aids fat emulsification, increasing surface area for better digestion.
• There are also other important functions of bile.

Fat Emulsification by Bile Salts

• Bile salts emulsify fats by coating fat globules, preventing clumping and increasing surface area.
• This process aids the enzymes in digesting the triglycerides into monoglycerides and fatty acids.

Control of Bile Release

• Acidic or fatty chyme causes the release of cholecystokinin (CCK) and secretin to trigger bile release into the duodenum.
• Bile is regulated by cholecystokinin (CCK) and secretin.

Enterohepatic Circulation of Bile Salts

• Bile salts are largely reabsorbed into the enterohepatic circulation.
• About 95% of bile salts are reabsorbed, returning to the liver for reuse.
• A small portion is lost in the feces.

Digestion & Absorption in the Small Intestine

• Digestion and absorption are the core functions of the small intestine

Absorption in Small Intestine

• The duodenum and jejunum are the primary sites for nutrient absorption.
• Nutrients from carbohydrate, protein, and fat digestion are absorbed here.
• Most electrolytes and vitamins are also absorbed.
• Calcium and iron absorption quantities will vary based on body needs.
• Ileum absorbs bile salts, vitamin B12, electrolytes, and water.

Carbohydrate, Protein & Fat Digestion

• Specific enzymes break down these molecules into absorbable units
• This involves enzymatic action, and there are specific enzymes for each macronutrient.

Carbohydrate Digestion

• Starch and glycogen are broken down to glucose via amylase.
• Specific disaccharidases further breakdown to monosaccharides (glucose, galactose, fructose).
• Digestive enzymes like lactase, maltase, and sucrase break down disaccharides in the small intestine.

Carbohydrate Absorption

• Glucose and galactose enter enterocytes via SGLT1 and are subsequently transported to the bloodstream.
• Fructose enters enterocytes via GLUT-5 and is transported to the bloodstream via GLUT-2.
• Passive diffusion is another transport process.

Protein Digestion

• Proteins are broken down into amino acids by pepsin and various pancreatic proteolytic enzymes.

Protein Absorption

• Most amino acids use active transport aided by carrier proteins.
  • some peptides may also be absorbed
• Various transporter proteins facilitate the absorption of specific amino acids.

Fat Digestion and Absorption

• Bile salts emulsify fats, pancreatic lipase then breaks down triglycerides into monoglycerides & fatty acids.
• Monoglycerides, fatty acids enter cells embedded in water-soluble micelles.
• The micelles, then transport the monoglycerides and fatty acids to absorptive epithelial cells.
• These molecules diffuse across the intestinal cells and are reassembled into triglycerides.
• Then packaged with chylomicrons, these are released into the surrounding lymph vessels.
• Chylomicrons enter the lymphatic system.

Iron Absorption

• Iron, either heme or ferrous (Fe2+) is absorbed by the small intestine.
• Absorbed iron is used for red blood cell production.
• Unused iron is stored as ferritin.
• Excess iron is excreted in feces.
• Iron absorption is regulated and influenced by various factors and processes in our bodies.

The Large Intestine

• The large intestine is primarily for water absorption and waste storage
• Some vitamins are absorbed here, and beneficial bacteria reside within..
• Bacteria assist in the production of vitamin K and folic acid.
• Undigested materials are transformed into feces.
• The large intestine plays an essential role in water absorption and waste processing.

Recommended Reading (Not Study Notes).

• Provides supplementary reading materials to enhance learning about the human physiology systems.