Digestive System Quiz - Small Intestine Functions

Digestive System Overview

• The digestive system is responsible for processing food, extracting nutrients, and eliminating waste.
• It comprises organs like the oral cavity, esophagus, stomach, small intestine, large intestine, and accessory organs.

Digestive System Part II

• The digestive system's structures are the oral cavity, salivary glands, esophagus, stomach, pancreas, gallbladder, large intestine, and small intestine.
• Chapter 23, sections 23.1-23.11, cover the information.
• The specific page numbers are from 874 to 924.
• Accessory organs aid small intestine digestion: the liver, gallbladder, and pancreas.

The Liver

• The liver is the largest gland in the body (~3 lbs).
• It consists of four lobes: right, left, caudate, and quadrate.
• The gallbladder lies on the inferior surface of the right lobe.
• The falciform ligament separates the larger right lobe from the smaller left lobe and suspends the liver from the diaphragm and anterior abdominal wall.
• The lesser omentum anchors the liver to the stomach.
• The hepatic artery proper and hepatic portal vein enter the liver at the porta hepatis.
• Bile leaves via the left and right hepatic ducts, fusing into a common hepatic duct.
• The cystic duct connects with the gallbladder.
• The bile duct forms from the union of the common hepatic and cystic ducts.
• Liver lobules are structural and functional units with hexagonal shapes.
• Hepatocytes (liver cells) are in plates that filter and process nutrient-rich blood.
• The central vein is located along the longitudinal axis of the lobule.
• Liver sinusoids are leaky capillaries between hepatic plates, lined with stellate macrophages.
• Hepatocytes produce 900 ml of bile daily.
• The liver also processes bloodborne nutrients (storing glucose as glycogen, making plasma proteins), stores fat-soluble vitamins, performs detoxification (converting ammonia to urea), and excretes bilirubin and phagocytoses old red blood cells, leukocytes, and bacteria, and synthesizes most clotting factors.
• Bile is a yellow-green alkaline solution with bile salts, bilirubin (main bile pigment from hemoglobin), cholesterol, triglycerides, phospholipids, and electrolytes.
• The enterohepatic circulation recycles bile salts.
• Reabsorption in the ileum and return via the hepatic portal vein accounts for 95% of secreted bile salts.

The Gallbladder

• The gallbladder is a thin-walled muscular sac on the inferior surface of the liver.
• It stores and concentrates bile by absorbing water and ions.
• Honeycomb folds allow the gallbladder to expand as it fills
• Muscular contraction releases bile into the cystic duct and then into the common bile duct.
• The gallbladder stores and concentrates bile.

The Pancreas

• The pancreas is mostly retroperitoneal, with its head encircled by the duodenum and its tail abutting the spleen.
• It has endocrine and exocrine functions.
• Endocrine function is the secretion of insulin and glucagon by pancreatic islet cells.
• The pancreas secretes pancreatic juice
• Acini are clusters of secretory cells
• They produce zymogen granules containing proenzymes.
• Ducts secrete to the duodenum via the main pancreatic duct; smaller ducts produce water and bicarbonate.
• Pancreatic juice contains water, bicarbonate (neutralizes acidic chyme), and digestive enzymes (inactive forms requiring activation).
• Example enzymes are proteases (for proteins), amylase (for carbohydrates), lipases (for lipids), and nucleases (for nucleic acids).

The Small Intestine

• The small intestine is the major organ of digestion and absorption, extending from the pyloric sphincter to the ileocecal valve (~7-13 ft during life; ~20 ft in a cadaver).
• It has a small diameter of 2.5-4 cm (1.0-1.6 inches).
• The subdivisions are the duodenum (retroperitoneal), jejunum (~2.5 m or 8 ft), and ileum (~3.6 m or 12 ft).
• The jejunum has the most features.
• Modifications for absorption include: circular folds, villi, and microvilli, increasing the surface area significantly for absorption.
• The superior mesenteric artery supplies oxygenated blood, and veins drain nutrient-rich blood into the superior mesenteric veins, then the hepatic portal vein, and finally into the liver.
• The small intestine's nerve supply includes parasympathetic innervation (via the vagus nerve) and sympathetic innervation (from thoracic splanchnic nerves).
• The small intestine's mucosa contains enterocytes (absorptive cells), goblet cells, enteroendocrine cells, and Paneth cells.

The Large Intestine

• The large intestine frames the small intestine on three sides, extending from the ileocecal valve to the anus.
• It has a much shorter length than the small intestine (1.5 m vs. 6 m), but larger diameter (~7 cm).
• Its major functions include absorbing water from indigestible food residues, absorbing metabolites produced by bacteria, storing residues temporarily, and eliminating them as feces.
• The large intestine is divided into the cecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal. Different parts of the colon have specialized functions.
• The wall has the same basic architecture as other GI sections but includes modifications for function, like the lack of circular folds and villi, abundant deep crypts, and mucus-producing goblet cells.
• The anal canal has an internal anal sphincter (smooth muscle) and an external anal sphincter (skeletal muscle).
• The colon is retroperitoneal (except transverse and sigmoid parts), anchored to the posterior abdominal wall by mesocolons.
• Mucus is produced, and the mucus protects and assists in passage.
• The large intestine's nerve supply includes parasympathetic and sympathetic components.
• Haustral contractions, • Slow segmenting movements in the ascending and transverse colon. • Mass movements, powerful contraction, ~3-4 times a day.
• Gastrocolic reflex triggers mass movements during/after meals.
• Defecation reflex is triggered by distension in the rectum, involves relaxation of internal anal sphincter, and can be voluntarily controlled by the external anal sphincter.
• Bacterial microbiota in the large intestine has metabolic functions (fermentation of indigestible sugars / produce short-chain fatty acids; some vitamins), keep pathogenic bacteria in check (preventing harmful bacteria from crossing gut epithelium; immune system functions; dendritic cells).

Regulation of Hormone and Nerve Control in Digestion

• Hormonal controls involve cholecystokinin (CCK) release by duodenal enteroendocrine cells in response to proteins & fats, and secretin release in response to acidic chyme. • Stimulates pancreatic acinar cells for enzyme-rich pancreatic juice which causes contraction of the gallbladder and relaxation of the hepatopancreatic sphincter to release bile into duodenum. • Stimulates duct cells in pancreas for bicarbonate-rich pancreatic juice.
• Neural pathways include long reflexes (vagus nerve) and short reflexes that weakly stimulate gallbladder contraction and pancreatic secretion during cephalic and gastric phases.

Processing of Nutrients

• Different digestive enzymes break down different food molecules: proteins, carbohydrates, lipids, and nucleic acids.
• The breakdown products are absorbed into the bloodstream and transported to the liver and cells.
• Specific processes are detailed per nutrient (carbohydrates, proteins, lipids, and nucleic acids).

Absorption of Vitamins, Electrolytes and Water

• Water-soluble and fat-soluble vitamins are absorbed and transported in specific ways.
• Electrolytes (primarily Na⁺, Cl⁻, K⁺, HCO₃⁻, and Ca²⁺) are actively absorbed, often coupled with the absorption of other nutrients.
• Water absorption is primarily through osmosis based on solute concentration gradients generated by active transport of substances in the intestines.