Digestive System Overview

Digestive System Overview

• The digestive system is responsible for breaking down large organic molecules into smaller molecules for absorption.
• It consists of the digestive tract (mouth to anus) and associated organs (salivary glands, liver, gallbladder, pancreas).
• Functions of the digestive system include ingestion, digestion, absorption, and elimination of waste.

Digestive Tract Layers

• Mucosa (innermost layer): secretes mucus, contains blood vessels, nerves, and small glands.
• Submucosa: contains blood vessels, nerves, and small glands.
• Muscularis: longitudinal, circular, and oblique muscles that help move food along.
• Serosa (outermost layer): connective tissue, peritoneum, and mesenteries.

Oral Cavity

• Stratified squamous epithelia, salivary glands, and teeth.
• Salivary glands produce saliva containing amylase (breaks down carbohydrates) and lysozyme (anti-bacterial).
• Teeth: incisors, canines, premolars, molars, and wisdom teeth.
• Tooth anatomy: enamel (hard covering), dentin, pulp cavity, and root.

Pharynx and Esophagus

• Pharynx: connects mouth to esophagus, has three parts (nasopharynx, oropharynx, laryngopharynx).
• Esophagus: muscular tube that transports food to stomach through peristalsis.

Swallowing Process

• Voluntary phase: forming a bolus in the mouth.
• Pharyngeal phase: bolus stimulates receptors, initiating a reflex to push food into the esophagus.
• Esophageal phase: peristalsis moves food down the esophagus.

Stomach

• Storage tank for food, produces mucus, hydrochloric acid, and protein-digesting enzymes.
• Three muscular layers: longitudinal, circular, and oblique.
• Rugae (folds) allow for expansion and contraction.
• Pyloric opening leads to the small intestine.

Regulation of Stomach Secretions

• Cephalic phase: sight, smell, and thought of food stimulate gastrin secretion.
• Gastric phase: distension of the stomach and presence of protein stimulate gastrin secretion.
• Intestinal phase: acidic chyme stimulates secretin and cholecystokinin to inhibit gastric secretion.

Small Intestine

• Major absorptive organ for nutrients.
• Six meters long, with three parts: duodenum, jejunum, and ileum.
• Microvilli and folds increase surface area for absorption.
• Bile and pancreatic ducts bring digestive enzymes.

Large Intestine

• Absorbs water and electrolytes, stores and eliminates waste.
• Cecum, colon, rectum, and anus make up the large intestine.
• Iliocecal sphincter separates the small and large intestines.

Liver

• Associated organ, weighs about 3 pounds, located in the right upper quadrant.
• Functions: detoxifies blood, filters harmful substances, synthesizes molecules, and secretes bile.
• Lobes, porta (gate), and hepatic ducts.

Gallbladder and Pancreas

• Gallbladder: stores bile, located under the liver.
• Pancreas: produces digestive enzymes, located posterior to the stomach, with endocrine and exocrine tissues.
• Pancreatic ducts carry enzymes to the small intestine.

Note: These study notes focus on key facts and context, and are written in a concise and engaging manner.### Large Intestine and Digestive Process

• The large intestine consists of the cecum, colon, rectum, and anal canal.
• The cecum is a blind pouch that joins the small intestine at the ileocecal junction and has an appendix attached.
• The appendix is a 9 cm long structure with no known function, but it can be removed if it gets inflamed or is about to burst.

Colon and Rectum

• The colon is about 1.5 meters long and consists of ascending, transverse, descending, and sigmoid regions.
• The rectum is a straight tube that begins at the sigmoid region and ends at the anal canal.
• The anal canal is about 2-3 cm long and is the final part of the digestive tract.

Digestive Process

• The digestive process consists of digestion, propulsion, absorption, and defecation.
• Digestion breaks down food into carbohydrates, lipids, and proteins.
• Propulsion moves food through the digestive tract.
• Absorption occurs mainly in the duodenum and jejunum.
• Defecation is the elimination of waste in the form of feces.

Carbohydrate Digestion

• Polysaccharides are broken down into disaccharides by salivary and pancreatic amylases.
• Disaccharides are broken down into monosaccharides on the surface of the intestines.
• Glucose is absorbed by co-transport with sodium into the intestinal epithelium and then carried to the liver.

Lipid Digestion

• Lipase breaks down triglycerides into fatty acids and monoglycerides.
• Bile salts surround fatty acids to form micelles, which attach to the plasma membrane of intestinal epithelial cells.
• Fatty acids and monoglycerides pass by simple diffusion into the intestinal epithelial cells.
• Lipid digestion occurs within the intestinal epithelial cell, where fatty acids and monoglycerides are converted to triglycerides.

Protein Digestion

• Pepsin is a protein-digesting enzyme secreted by the stomach.
• The pancreas secretes trypsin, chymotrypsin, and carboxypeptidase into the small intestine.
• Enzymes are activated in the small intestine and bound to the microvilli of the intestinal epithelium for further breakdown.
• Amino acids are absorbed through the intestinal epithelial cells by various co-transport mechanisms.

Water and Minerals

• Water can move across the intestinal wall in either direction, depending on osmotic pressures.
• 99% of water entering the intestine is absorbed back into the body.
• Minerals are actively transported across the wall of the small intestine.
• Fluid volumes in the digestive tract: 2 liters ingested, 1 liter secreted by salivary glands, 2 liters secreted by gastric secretions, 1 liter secreted by pancreatic secretions, 1 liter secreted by bile and small intestine secretions.