Digestive System: An Overview

Digestive System Overview

• Nutrients are essential for all living organisms to survive and carry out cellular functions.
• Plants obtain nutrients through photosynthesis, while animals get them by consuming other organisms.
• The animal body requires nutrients such as amino acids, lipid molecules, nucleotides, and simple sugars.
• Digestion is a multistep process that converts macromolecules into smaller components for absorption.
• Physical digestion involves actions like chewing to break down food.
• Chemical digestion utilizes enzyme-catalyzed reactions to further process food.
• Maintaining a balance between food intake, storage, and energy expenditure is a challenge to human nutrition.
• Excess food energy is stored as fat, contributing to obesity and related diseases like type 2 diabetes.
• Understanding diet and nutrition is important for maintaining overall health.

Human Digestive System

• Digestion starts in the mouth (oral cavity) and involves physical and chemical processes.
• Teeth aid in mastication, physically breaking down food into smaller particles and increasing surface area.
• Saliva initiates chemical digestion with enzymes like amylase (breaks down starch) and lipase (breaks down fats).
• Swallowed food moves through the esophagus, a tube connecting the mouth to the stomach.
• Peristalsis, wave-like muscle contractions, propels food through the esophagus.
• The stomach has a highly acidic environment (pH 1.5-2.5) to kill microorganisms, break down tissues, and activate enzymes.
• The small intestine continues food breakdown with bile from the liver and enzymes from the small intestine and pancreas.
• Smaller molecules are absorbed into the bloodstream through the epithelial cells lining the small intestine.
• The large intestine absorbs water from the remaining waste, which is then compacted into feces.
• Feces are stored in the rectum and excreted through the anus.
• The GI tract includes the oral cavity, esophagus, stomach, small intestine, large intestine, and rectum.
• Accessory organs such as salivary glands, liver, gall bladder, and pancreas, aid in digestion by secreting substances into the GI tract.

Oral Cavity: Physical and Chemical Digestion

• Both physical and chemical digestion begin in the oral cavity.
• Mastication (chewing) by the teeth breaks food into smaller particles.
• Salivary glands produce saliva containing mucus to moisten food and buffer pH.
• Lysozyme in saliva has antibacterial properties.
• Salivary amylase converts starches into maltose (a disaccharide).
• Lipase, produced by cells in the tongue, breaks down fats.
• Chewing and saliva create a bolus for swallowing.
• The tongue moves the bolus to the pharynx, which leads to the esophagus and trachea.
• The epiglottis covers the tracheal opening to prevent food from entering the lungs during swallowing.

Esophagus and Swallowing

• Bolus passes down to the stomach after being swallowed.
• Peristalsis within the smooth muscle of the esophagus propels food to the stomach.
• Peristaltic movement is unidirectional.
• Sphincters form valves in the digestive system.
• The gastro-esophageal sphincter (lower esophageal or cardiac sphincter) is at the stomach end of the esophagus.
• The gastro-esophageal sphincter opens when swallowing happens, allowing bolus into the stomach.
• When not swallowing, the gastro-esophageal sphincter remains closed to prevent stomach contents from rising into the esophagus.
• Acid reflux (heartburn) happens when acidic stomach juices escape.
• Prolonged exposure to acidity can cause physical damage to the esophagus.

Stomach Function in Digestion

• A large portion of protein digestion takes place in the stomach.
• The stomach is a sac-like organ that secretes gastric digestive juices.
• Pepsin, an enzyme, carries out protein digestion with in the stomach chamber.
• The stomach's acidic environment kills microorganisms and aids protein catabolism.
• Churning of smooth muscles facilitates chemical digestion.
• The mixture, called chyme, consists of partially digested food and gastric juice.
• Gastric emptying releases a small amount of chyme into the small intestine within 2-6 hours after a meal.
• Hormones, stomach distension, and muscular reflexes regulate chyme movement.
• The low pH of the stomach denatures amylase and lipase from the mouth.
• Over time, chemical digestion of starches and fats decreases in the stomach.
• Pepsin is released as pepsinogen and is activated by the low pH, protecting the stomach lining from acidity.
• A thick mucus lining also protects the stomach

Small Intestine: Completing Digestion

• Chyme moves from the stomach to the small intestine.
• The small intestine is the primary site for completing protein, fat, and carbohydrate digestion.
• Villi (finger-like projections) and microvilli (microscopic projections on villi) increase the surface area for absorption.
• Epithelial cells absorb nutrients and release them into the bloodstream.
• The small intestine facilitates nutrient absorption with active transport.
• The small intestine’s surface area enhances absorption efficiency.
• The human small intestine is over 6 m (19.6 ft) long and includes the duodenum, jejunum and ileum.
• The pyloric sphincter separates the duodenum from the stomach.
• Pancreatic juices, which contain bicarbonate to neutralize the acidic chyme from the stomach, mix with chyme.
• Appropriate enzymes in the neutralized environment raise the pH.
• Pancreatic juices contain digestive enzymes (amylase, trypsin, and lipase) to break down starches, proteins, and fats.
• Bile is produced in the liver and stored in the gallbladder, with bile salts, which make lipids accessible to water-soluble enzymes, entering the duodenum through the bile duct.
• Bile emulsifies fats by keeping fat droplets apart, increasing the surface area available to lipase.
• Disaccharidases, secreted by the small intestine, break down disaccharides (e.g. maltose, sucrose, lactose) into monosaccharides.
• Monosaccharides, amino acids, bile salts, vitamins, and other nutrients are absorbed by the intestinal lining.

Large Intestine: Water Reabsorption & Waste Processing

• Undigested food goes from the ileum to the colon via peristaltic movements.
• The ileocecal valve marks the beginning of the large intestine.
• The vermiform appendix is at the ileocecal valve and has a small role in immunity.
• The large intestine reabsorbs water from indigestible food material and processes waste.
• It is shorter but wider compared to the small intestine.
• The large intestine consists of the cecum, colon, and rectum.
• The cecum is a receiving pouch for waste.
• Colon contains intestinal flora (bacteria) that aid in digestion.
• The colon consists of the ascending colon, transverse colon, descending colon, and sigmoid colon.
• The main functions of the colon are to extract water and mineral salts and to store waste.
• The rectum stores feces until defecation.
• Peristaltic movements propel feces during elimination through the anus.
• Two sphincters regulate the exit of feces: an involuntary inner sphincter and a voluntary outer sphincter.

Accessory Organs & Digestion

• Accessory organs secrete enzymes that help break down food.
• Salivary glands, the liver, pancreas, and the gall bladder are accessory organs.
• Hormone are released according to food consumption and regulate the secretions of the liver, pancreas, and gallbladder .
• The liver (largest internal organ) produces bile for fat digestion and also detoxifies blood.
• The liver also processes absorbed vitamins and fatty acids and synthesizes plasma proteins.
• The gallbladder stores bile and concentrates bile salts.
• The pancreas secretes bicarbonate to neutralize acidic chyme and enzymes (trypsin, amylase, lipase) for digestion.

Nutrition & Balanced Diet

• A balanced human diet should provide the nutrients, minerals, and vitamins to maintain bodily functions and health.
• A balanced diet includes fruits, vegetables, grains, protein, and dairy.
• Digestion breaks down digestible carbohydrates into glucose for energy.
• Complex carbohydrates (polysaccharides) can be broken down into glucose biochemically.
• Humans lack the enzyme to digest cellulose (fiber), however, intestinal flora help extract/digest plant fibers.
• Excess sugars are converted into glycogen and stored for later use.
• Glycogen provides energy for prolonged exertions and during food shortage.
• Fats are stored under the skin of mammals for insulation and energy reserves.
• Digestion breaks down proteins, resulting in amino acids are absorbed.
• Body proteins comes from these amino acid constituents.
• Fats add flavor and promote satiety.
• Fatty acids are required for lipid membranes, vitamin absorption, and hormone production.
• Essential nutrients cannot be synthesized by the body.
• Essential nutrients include fatty acids, amino acids, vitamins, and minerals.
• Organs work together to digest food and absorb nutrients.
• The mouth ingests and starts mechanical and chemical breakdown of food.
• Saliva contains amylase (breaks down carbohydrates) and lipase (breaks down triglycerides).
• Peristalsis moves the food bolus through the esophagus to the stomach.
• Pepsin digests protein in the stomach's acidic environment.
• Absorption and digestion take place in the small intestine.
• The large intestine reabsorbs water and stores waste.
• Carbohydrates, proteins, and fats are the primary components of food.
• Essential nutrients are needed for cellular function but cannot be produced by the body.
• These include vitamins, minerals, some fatty acids, and some amino acids.
• Excess food intake is stored as glycogen and fat.
• Excess fat storage can lead to obesity and health problems.

Digestive System Glossary

• Amylase: Enzyme in saliva and secreted by the pancreas that converts carbohydrates to maltose.
• Anus: Exit point of the digestive system for waste material.
• Bile: Digestive juice produced by the liver; important for digestion of lipids.
• Bolus: A mass of food resulting from chewing action and wetting by saliva.
• Chyme: A mixture of partially digested food and stomach juices.
• Colon: The largest portion of the large intestine consisting of the ascending colon, transverse colon, and descending colon.
• Esophagus: A tubular organ that connects the mouth to the stomach.
• Essential Nutrient: A nutrient that cannot be synthesized by the body.
• Gallbladder: Organ that stores and concentrates bile.
• Large Intestine: Digestive system organ that reabsorbs water
• Liver: Organ that produces bile and processes vitamins and lipids.
• Mineral: An inorganic, elemental molecule that carries out bodily processes.
• Oral Cavity: The point of entry of food into the digestive system.
• Pancreas: A gland that secretes digestive juices.
• Pepsin: An enzyme found in the stomach with the main role of protein digestion.
• Peristalsis: Wave-like movements of muscle tissue.
• Rectum: The area of the body where feces is stored until elimination.
• Salivary Gland: One of three pairs of exocrine glands in the mammalian mouth that secretes saliva, a mix of watery mucus and enzymes.
• Small Intestine: The organ where digestion of protein, fats, and carbohydrates is completed.
• Stomach: A saclike organ containing acidic digestive juices.
• Vitamin: An organic substance necessary in small amounts to sustain life.