Digestive System Anatomy Quiz

Digestive System Overview

• The digestive system is a complex series of organs working together to process food, extract nutrients, and eliminate waste.
• The system includes the oral cavity, salivary glands, esophagus, stomach, pancreas, gallbladder, small intestine, and large intestine.

Accessory Digestive Organs of the Small Intestines

• The liver, gallbladder, and pancreas are accessory organs vital to small intestine function.
• Liver: produces bile, a crucial component for fat emulsification and absorption; produces 900 ml of bile per day. Stores glucose as glycogen, processes bloodborne nutrients, performs detoxification by converting ammonia to urea, and excretes bilirubin into bile. It also synthesizes clotting factors (most) and lipoproteins.
• Gallbladder: stores and concentrates bile, releasing it into the small intestine as needed. Its muscular contractions release bile into the common bile duct. The gallbladder has honeycomb folds supporting expansion during filling.
• Pancreas: produces and delivers enzymes for digesting proteins, carbohydrates, and fats; releases bicarbonate to neutralize stomach acid. It contains acinar cells producing pancreatic juice, containing digestive enzymes.

The Liver (Gross Anatomy)

• The liver is the largest gland in the body, weighing approximately 3 lbs.
• It has four primary lobes: right, left, caudate, and quadrate.
• The gallbladder resides in a recess on the inferior surface of the right lobe. The liver and gallbladder's location are related anatomically

Microscopic Anatomy of the Liver

• The liver is composed of hexagonal lobules, structural and functional units.
• Hepatocytes, the liver cells, filter and process nutrient-rich blood.
• Central veins run through the center of each lobule.
• Liver sinusoids: leaky capillaries housing stellate macrophages, crucial in removing debris and old red blood cells.

The Liver: Further Functions

• The liver produces approximately 900 ml of bile per day.
• It processes bloodborne nutrients, storing glucose as glycogen.
• The liver performs detoxification, converting ammonia to urea. It also excretes bilirubin into bile. It synthesizes clotting factors (primarily) and lipoproteins.

Bile: Composition and Enterohepatic Circulation

• Bile is a yellow-green alkaline solution.
• Bile contains bile salts, bilirubin, electrolytes, and other components playing roles in fat emulsification, absorption, and waste removal.
• Enterohepatic circulation involves the recycling of 95% of secreted bile salts.

The Gallbladder

• A thin-walled muscular sac on the inferior surface of the liver.
• It stores and concentrates bile, releasing it via the cystic duct into the common bile duct.
• Honeycomb folds enable expansion during filling.

Physiology of the Gallbladder

• The release of bile from the gallbladder is controlled by cholecystokinin (CCK).
• The release of CCK and the relaxation of the hepatopancreatic sphincter enables bile to enter the duodenum.

The Pancreas

• Primarily retroperitoneal, the pancreas has both endocrine and exocrine functions.
• Endocrine: pancreatic islet cells produce insulin and glucagon.
• Exocrine: acinar cells produce pancreatic juice, containing digestive enzymes, primarily zymogen granules containing inactive proenzymes that need activation.

Composition of Pancreatic Juice

• Pancreatic juice is a watery, alkaline solution, crucial for neutralizing acidic chyme from the stomach.
• It includes electrolytes, primarily bicarbonate.
• It contains digestive enzymes, including proteases (pepsin, trypsin, chymotrypsin, carboxypeptidase) amylase, lipases, and nucleases (ribonuclease, deoxyribonuclease).

Activation of Pancreatic Proteases

• Zymogen granules contain inactive enzymes.
• Enteropeptidase (enterokinase) is an enzyme residing in the duodenum, activating trypsinogen to trypsin.
• Trypsin further activates other pancreatic enzymes. This cascade of activation enables controlled enzyme activity in the duodenum.

Bile and Pancreatic Secretion into the Small Intestine

• Bile and pancreatic juices unite into the hepatopancreatic ampulla and sphincter, which delivers them into the duodenum through the major duodenal papilla.
• The sphincter regulates the entry of bile and pancreatic juice, remaining closed unless digestion is active.

Small Intestine

• The small intestine extends from the pyloric sphincter to the ileocecal valve, encompassing the duodenum, jejunum, and ileum.
• It is about 7-13 feet in length. The small intestine comprises three segments: Duodenum, Jejunum, and Ileum.

Blood Supply and Nerve Supply of the Small Intestine

• The superior mesenteric artery supplies oxygenated blood to the small intestine.
• Veins carry nutrient-rich blood into the superior mesenteric veins, and eventually into the liver via the hepatic portal vein.
• Innervation involves parasympathetic fibers from the vagus nerve and sympathetic fibers from thoracic splanchnic nerves.

Modifications of Small Intestine for Absorption

• Circular folds, villi, and microvilli significantly increase the surface area for efficient absorption of nutrients.
• Circular folds are deep folds of mucosa and submucosa.
• Villi are finger-like projections of mucosa.
• Microvilli are cytoplasmic extensions of absorptive cells creating a "brush border" important for final digestion and absorption.

Histology of the Small Intestine Wall

• The small intestine wall comprises four tunics (mucosa, submucosa, muscularis, and serosa).
• Mucosa: contains enterocytes, goblet cells, enteroendocrine cells, Paneth cells, and stem cells.
• Submucosa: contains areolar tissue and duodenal glands, which secrete alkaline mucus important for neutralizing acidic chyme.
• Muscularis: contains circular and longitudinal smooth muscle layers, assisting in propulsion.

Intestinal Juice

• Daily secretion of 1-2 liters of intestinal juice by intestinal glands.
• Its release is stimulated by distension or irritation of mucosa, hypertonic or acidic chyme.
• Constitutes a watery, alkaline, and isotonic solution crucial for transporting digested nutrients.

Digestive Processes in the Small Intestine

• It’s where the majority of digestion occurs with help from bile and pancreatic enzymes.
• Chyme contains nutrients, and largely undigested fats—with a usually hypertonic nature.
• Enterogastric reflex and enterogastrones regulate chyme movement into duodenum.
• Regulate duodenal filling.

Motility of the Small Intestine

• Two primary patterns—segmentation and peristalsis.
• Segmentation (post-meal): smooth muscle contraction mixes chyme with digestive enzymes.
• Peristalsis (inter-meal): sequentially contracting waves move chyme toward the ileocecal valve.
• Migrating motor complexes (MMCs) are also involved in removing residue
• Intestinal phase of digestion is supported by hormones (CCK, Secretin, Motilin).

Ileocecal Valve Control

• The ileocecal valve regulates the passage of chyme from the small intestine to the large intestine.
• Gastroileal reflex enhances intestinal motility and stimulates the valve’s relaxation for passing chyme.
• Gastrin secretion boosts ileum motility; the valve closes when chyme creates pressure to prevent regurgitation.

The Large Intestine

• Extending from the ileocecal valve to the anus, the large intestine (or colon) absorbs water, forms feces, and excretes waste materials.
• It has a shorter length but greater diameter compared to the small intestine.

Gross Anatomy of the Large Intestine

• The large intestine comprises four segments: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anal canal.
• Appendicitis is defined as an acute inflammation of the appendix.

Microscopic Anatomy of the Large Intestine

• The mucosa consists of simple columnar epithelium (except the anal canal, which has stratified squamous).
• Lacking circular folds and villi, the mucosa is thicker with abundant mucus-producing goblet cells in deep crypts.
• The anal canal possesses anal columns containing anal sinuses that secrete mucus to aid in the passage of feces.
• The mucosa of the anal canal has a unique stratified squamous epithelium, designed for abrasion resistance. The large intestine's mucosa lacks circular folds and villi, but contains a significant number of goblet cells.

Bacterial Microbiota in Large Intestine

• Bacteria residing in the large intestine (bacterial flora) outnumber our cells.
• These bacteria ferment indigestible carbohydrates and mucins, generating short-chain fatty acids and vitamins (e.g., B vitamins & K).
• They prevent pathogenic bacteria from growing excessively in the colon; also produce vitamins.
• The bacteria also generate gases (like H2, CO2), and certain acids, although some of these are irritating.

Gut Bacteria and Health

• Gut microbiota influences body weight, susceptibility to diseases (e.g. diabetes, atherosclerosis, fatty liver disease), and responses to various medications.
• Manipulating gut microbiota may in the future offer methods to improve health.

Digestive Processes in the Large Intestine

• Motility: involves haustral contractions (slow segmenting movement) which aid in the absorption of water and mass movements (powerful contractile movements, roughly 3-4x a day) moving feces towards the rectum.
• Defecation is initiated by distension of the rectum and/or by stimuli from the large intestine.
• The reflex involves stimulation, relaxing of the internal anal sphincter(smooth muscle), and stimulation of the external anal sphincter (skeletal muscle). This enables defecation.

Additional Notes

• Information regarding specific enzymes, chemicals, and locations found in the text are relevant but omitted here to be concise.