Digestive System Anatomy and Physiology

• Digestion is the breakdown of food into smaller molecules through physical and chemical processes.
• Absorption involves bringing these molecules into the body for cellular use.

Digestive System Division

• The digestive system is divided into the alimentary canal and accessory organs.
• The alimentary canal is a continuous tube from mouth to anus where food passes through.
• Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Abdominal Cavity

• The abdominal cavity, or peritoneal cavity, can be intraperitoneal or retroperitoneal.
• Intraperitoneal is within the peritoneal cavity.
• Retroperitoneal is behind the peritoneal cavity.
• The mesentery contains blood and lymph vessels, nerves, and anchors organs.
• The lesser omentum connects the stomach to the liver.
• The greater omentum is the "beer belly."
• Mesocolons connect the large intestine to the abdominal wall.

Digestive System Processes

• Ingestion: intake of food.
• Propulsion: the ability to move substances.
• Mechanical digestion: physically breaking down food.
• Chemical digestion: chemically breaking down food into smaller molecules.
• Absorption: bringing small molecules into blood/lymph vessels.
• Defecation (excretion): eliminating indigestible material.

Histology

• Blood for digestive tissues comes from splanchnic circulation.
• Digestive organs share four basic layers: mucosa, submucosa, muscularis externa, and serosa.
• The mucosa, the deepest layer, houses MALT and facilitates secretion, absorption, and protection.
• The submucosa contains blood/lymph vessels and nerve fibers.
• The muscularis externa consists of muscle layers for movement, forming sphincters at organ junctions.
• The serosa is the most superficial layer. However, in the esophagus, the outermost layer is adventitia.
• The enteric nervous system regulates gland activity and GI tract motility.

Anatomy of Mouth and Organs

• The mouth, responsible for ingestion, is a mucosa-lined cavity bordered by lips, cheeks, palate, and tongue.
• The soft palate closes off the nasopharynx during swallowing.
• The tongue forms bolus and is important for swallowing, speech, and taste.
• Salivary glands secrete saliva for food breakdown, containing water (>97%), digestive enzymes, mucus, and antimicrobials.
• Salivary amylase is an important digestive enzyme for carbohydrates.
• Major salivary glands are parotid, submandibular, and sublingual.
• Humans have 20 deciduous teeth, replaced by 32 permanent teeth.
• Incisors are for biting, canines for tearing, while premolars and molars are for grinding and crushing.

Tooth Structure

• The crown is the visible portion outside the gum.
• The root is embedded in the bone.
• The pulp cavity contains blood vessels and nerves inside the tooth. Enamel covers the crown, made of hydroxyapatite crystals, the body's hardest substance.
• Dentin is calcified, protein-rich tissue composing the bulk of the tooth.

Pharynx and Esophagus

• The pharynx (throat) passes food through the oropharynx and laryngopharynx to the esophagus.
• The esophagus, a muscular tube, connects the pharynx and stomach, forming the gastroesophageal sphincter at the stomach junction.

Digestive Processes from Mouth to Stomach

• Mastication is the act of chewing.
• Deglutition is the act of swallowing.

Anatomy of the Stomach

• The stomach stores food and mixes it with water/gastric juice to form chyme.
• The major regions include the cardiac, fundus, body, and pyloric regions.
• The stomach can expand up to 4L due to folds called rugae.
• The pyloric sphincter regulates chyme movement into the small intestine.
• The stomach contains an additional oblique muscularis layer to increase churning.

Stomach Lining

• The mucosa contains gastric pits filled with cells that secrete gastric juice.
• Mucous neck cells secrete an acidic mucus.
• Parietal cells secrete hydrochloric acid (HCl) and intrinsic factor, needed for vitamin B₁₂ absorption.
• Enteroendocrine cells secrete hormones.
• Chief cells secrete pepsinogen, an inactive form of pepsin.
• Regenerative cells constantly undergo mitosis.

Regulation of Gastric Secretion

• Gastric secretion is hormonally and neutrally regulated.
• The cephalic phase is preparing for digestion.
• The gastric phase involves swallowing, which initiates gastric secretion of hormones to stimulate digestion and release.
• During the intestinal phase, chyme entering the duodenum stimulates the release of secretin and cholecystokinin, causing gastrin secretions to decline

Anatomy of Small Intestine

• The function of the small intestine is absorption.
• The small intestine extends from the pyloric sphincter to the ileocecal valve.
• The duodenum is the first section, receiving chyme, pancreatic juice, and bile.
• The jejunum is the middle section.
• The ileum is the last section, ending at the ileocecal valve.
• Plicae circulares are folds that slow chyme movement and increase surface area.
• Villi are fingerlike projections containing capillaries and lacteals and increase surface area.
• Microvilli make the brush border and increase surface area.
• Intestinal crypts contain cells that secrete intestinal juice.
• Absorptive cells secrete brush border enzymes for carbohydrate and protein digestion.
• Goblet cells secrete mucus.
• Enteroendocrine cells secrete hormones.
• Paneth cells secrete antimicrobials.
• Peyer's patches are MALT tissue found within the ileum.
• Brunner's glands secrete alkaline mucous to neutralize gastric acid in the duodenum.

Anatomy of Liver

• The liver produces bile to break down fats.
• It regulates blood glucose by glycogenesis which converts glucose to glycogen, and glycogenolysis which breaks down glycogen and releases it into blood.
• The liver stores fat-soluble vitamins, synthesizes proteins, breaks down ammonia and other toxic substances.
• It also breaks down old RBCs to be recycled.
• The liver has four major lobes: right, left, caudate, and quadrate.
• Porta hepatis is the entrance for blood and lymph vessels and the common hepatic duct.
• Functional units are lobules containing hepatocytes to filter blood and produce bile.
• Kupffer cells are the main immune cells here.
• Bile breaks down fats in the intestines.

Anatomy of Gallbladder

• The gallbladder stores and concentrates excess bile from the liver.

Anatomy of Pancreas

• Pancreatic acini cells produce pancreatic juice, containing amylase for starch digestion, proteases for protein/peptide digestion, lipase for lipid digestion, nucleases for nucleic acid digestion, and sodium bicarbonate to neutralize stomach acid.

Digestive Processes

• It takes up to 6 hours for chyme to get through the small intestine, where most nutrients are absorbed.
• Segmentation facilitates absorption.
• Peristalsis propels substances.

Anatomy of Large Intestine

• The large intestine absorbs water from indigestible food and stores it until defecation.
• The cecum is the first pouch where the appendix attaches.
• Parts of the colon are ascending, transverse, descending, and sigmoid.
• Haustra are pouches and the taenia coli is a smooth muscle that form haustra.
• The anal canal contains the internal (involuntary) and external (voluntary) anal sphincters.

Large Intestine lining

• Absorptive cells absorb water.
• Goblet cells secrete mucus.
• Bacteria metabolize indigestible carbohydrates and produce gases, vitamin K, and some B vitamins through fermentation.

Digestive Processes

• Digestion is minimal and absorption is limited to water, vitamins B and K, and some electrolytes.
• Haustral contractions produce segmentation for motility.
• Defecation releases feces, aided by Valsalva's maneuver.

Chemical Digestion

• Chemical digestion breaks down polymers into monomers via hydrolysis for absorption.
• Carbohydrates break down into monosaccharides with amylase.
• Proteins break down into amino acids aided by proteases like pepsin, rennin, and trypsin/chymotrypsin.
• Lipids break down into fatty acids by lipases.
• Nucleic acids break down into nucleotides via pancreatic nucleases

Absorption

• Absorption involves moving monomers from the GI tract into the body.
• Tight junctions ensure molecules pass through cells.
• Carbohydrates are absorbed through secondary active transport and facilitated diffusion.
• Proteins and nucleic acids are absorbed via active transport.
• Lipids, being insoluble in water, are transported in micelles via chylomicrons.

Explore the digestive system's anatomy and key functions, from nutrient absorption and the enteric nervous system to the roles of saliva and teeth. Understand the layers of the alimentary canal and the differences between intraperitoneal and retroperitoneal organs.