Digestive System Processes Overview

Digestive System Processes

• Ingestion: Intake of food and liquids into the oral cavity, the first step in digestion.
• Propulsion: Movement of food through the digestive tract via voluntary and involuntary muscle contractions, mixing and moving materials.
• Mechanical Digestion: Physical breakdown of food into smaller pieces, e.g., chewing and churning, without changing chemical composition.
• Chemical Digestion: Breakdown of food molecules into absorbable nutrients using enzymes and other chemicals.
• Absorption: Transport of digested molecules, electrolytes, vitamins, and water from the GI tract into the bloodstream or lymphatic system.
• Excretion: Removal of undigested food and waste products from the body.

Digestive Tract Organs and Functions

Upper Gastrointestinal Tract

• Oral Cavity and Salivary Glands: Mechanical digestion begins; saliva containing amylase (starch digestion) mixes with food to form a bolus.
• Pharynx: Bolus moves to the pharynx during swallowing; mucus facilitates swallowing.
• Esophagus: Transports the bolus from the pharynx to the stomach; lubricated by mucus.
• Stomach: Bolus mixes with gastric secretions and smooth muscle contractions to form chyme; secretions produced by stomach epithelial cells.
• Duodenum: Part of upper GI tract, where chyme from the stomach mixes with digestive juices from accessory organs.

Lower Gastrointestinal Tract and Accessory Organs

• Small Intestine: Major site of nutrient absorption, absorbing most nutrients, water, electrolytes, and vitamins within a few hours.
• Gallbladder: Stores, concentrates, and releases bile produced by the liver; aids in fat digestion.
• Pancreas: Produces enzymes (pancreatic juice) for digestion and hormones (insulin, glucagon) regulating blood sugar.
• Large Intestine: Absorbs water and electrolytes from remaining digested material; compacts chyme into feces and stores it until elimination.

Digestive Processes of Organic Molecules

Carbohydrates

• Digestion: Salivary amylase begins starch breakdown in the mouth, pancreatic amylase continues in the small intestine; maltase, sucrase, and lactase further break disaccharides into monosaccharides.
• Absorption: Monosaccharides (glucose, fructose, galactose) absorbed in the small intestine via active transport (glucose, galactose) or facilitated diffusion (fructose).

Proteins

• Digestion: Pepsin in the stomach breaks down proteins; pancreatic proteases (trypsin, chymotrypsin) and carboxypeptidase further break proteins into smaller peptides. Aminopeptidases break peptides down into amino acids.
• Absorption: Amino acids and small peptides absorbed in the small intestine via active transport.

Lipids

• Digestion: Lingual lipase (mouth), gastric lipase (stomach) begin triglyceride breakdown; pancreatic lipase (small intestine) breaks down triglycerides into monoglycerides and free fatty acids. Bile emulsifies fats, increasing surface area for lipase activity.
• Absorption: Monoglycerides and fatty acids form micelles (bile salt coated particles) for transport across intestinal membrane and enter enterocytes; Inside cells, they are re-esterified to triglycerides, packaged into chylomicrons, and released into lymphatic system and then into blood.

Nucleic Acids

• Digestion: Pancreatic nucleases (DNAse, RNAse) break down DNA and RNA into nucleotides; nucleotidases break down nucleotides.
• Absorption: Nucleotides absorbed into the small intestine and taken into bloodstream.

Summary of Enzymes and Hormones

• Specific enzymes are crucial for breaking down each type of organic molecule in the digestive process.
• Hormones like gastrin regulate digestive secretions.
• Absorption occurs primarily in the small intestine.