Digestive System Function and Anatomy

Digestive System Function

• Acquires nutrients from the environment
• Anabolism: Uses raw materials to build complex molecules, requiring energy and reducing power.
• Catabolism: Breaks down substances to provide energy for cellular functions.

Digestive (GI) Tract

• Oral Cavity, Teeth, Tongue: Mechanically processes food, moistens it, and mixes it with saliva.
• Mouth: Entrance to the digestive system.
• Liver: Secretes bile for lipid digestion, stores nutrients, and performs various vital functions.
• Gallbladder: Stores and concentrates bile.
• Pancreas: Secretes buffers and digestive enzymes. Also produces hormones.
• Large Intestine: Dehydrates and compacts indigestible materials for elimination.
• Salivary Glands: Secrete lubricating fluid containing enzymes that break down carbohydrates.
• Pharynx: Muscular propulsion of materials into the esophagus.
• Esophagus: Transports materials to the stomach.
• Stomach: Mechanically breaks down food; chemically breaks down food using acid and enzymes. Produces intrinsic factor needed for Vitamin B12 absorption.
• Small Intestine: Enzymatically digests and absorbs water, organic substrates, vitamins, and ions. Consists of three sections: duodenum, jejunum, and ileum.

Actions of the GI Tract

• Ingestion: Material enters the mouth.
• Secretion: Release of water, acids, buffers, enzymes, and salts by the GI tract and glandular organs.
• Mechanical Processing: Crushing and shearing of materials to make them easier to move through the tract.
• Absorption: Movement of organic substrates, electrolytes, vitamins, and water across the epithelium of the digestive tract.
• Digestion: Chemical breakdown of food into smaller components.
• Excretion: Removal of waste products from the body.

Movement of Digestive Materials

• Peristalsis: Rhythmic cycles of muscular contractions moving a bolus (a mass of food) along the digestive tract.

Functions of Oral Cavity

• Sensory Analysis: Of material before swallowing.
• Mechanical Processing: Through actions of teeth, tongue, and palatal surfaces.
• Lubrication: Mixing with mucus and salivary secretions
• Limited Digestion: Of carbohydrates and lipids.

Esophagus

• A hollow muscular tube, about 25 cm long and 2 cm wide.
• Conveys solid food and liquids to the stomach.

Stomach Function

• Major Functions: Storage of ingested food; Mechanical breakdown of ingested food; Chemical breakdown of chemical bonds in food materials by producing acid and enzymes; and Production of intrinsic factor for vitamin B12 absorption.

Digestion in the Stomach

• Performs preliminary digestion of proteins by pepsin
• Some digestion of carbohydrates (by salivary amylase)
• Lipids: digested by lingual lipase
• Stomach contents become more fluid; pH approaches 2.0; pepsin activity increases; protein disassembly begins.
• Nutrients are NOT generally absorbed in the stomach

Small Intestine

• 90% of absorption occurs in the small intestine
• The duodenum is the first part; receives chyme from the stomach and digestive secretions from the pancreas and liver. Neutralizes stomach acids.
• The jejunum is the middle segment; mostly responsible for chemical digestion and absorption of nutrients.
• The ileum is the final segment; absorbs remaining nutrients and ends at the ileocecal valve.

Small Intestine - Intestinal Secretions

• Watery intestinal juice is secreted in liters in the small intestine.
• The juice moistens chyme, assists in buffering acids, and keeps digestive processes active.
• Chyme arrives in the duodenum, and weak peristaltic contractions move it slowly to the jejunum.

Villi

• Tiny, finger-like projections in the small intestine that help absorb nutrients and water

Rugae

• Gastric rugae (folds) in the stomach expand to increase surface area.

Pancreas

• Located posterior to the stomach, extending toward the spleen
• Endocrine Function: Secretes insulin and glucagon into the bloodstream.
• Exocrine Function: Secretes pancreatic juice

Pancreatic Enzymes

• Pancreatic amylase breaks down starches, similar to salivary amylase
• Pancreatic lipase breaks down complex lipids into fatty acids for absorption

Liver

• Has a right and left lobe
• Hepatocytes: Are liver cells that adjust circulating nutrient levels through selective absorption and secretion. Many Kupffer cells are also located in the liver (resident macrophages that play a role in maintaining liver function).
• Functions: Metabolic regulator, Hematological regulator, Bile production

Gallbladder

• Pear-shaped, muscular sac
• Stores and concentrates bile, releases bile into the duodenum

Large Intestine

• Horseshoe-shaped
• Extends from the end of the ileum to the anus.
• Lies inferior to the stomach and liver, framing the small intestine; also called the large bowel).
• About 1.5 meters (4.9 ft) long, and 7.5 cm (3 in) wide.

Large Intestine Functions

• Reabsorption of water
• Compaction of intestinal contents into feces.
• Absorption of vitamins produced by bacteria
• Storage of fecal material prior to defecation

Parts of Large Intestine:

• Cecum: An expanded pouch that receives material from the ileum and begins compaction
• Appendix: A slender, hollow appendage, about 9 cm (3.6 in) long, dominated by lymphoid nodules.
• Colon: Has a larger diameter and thinner wall than the small intestine; forms a series of pouches called haustra to permit expansion. Parts of the colon include the ascending, transverse, descending and sigmoid colon.
• Rectum: Forms the last 15 cm (6 in) of the digestive tract, an expandable organ for temporary storage of feces; Movement of fecal material into the rectum triggers the urge to defecate.
• Anal Canal: Last portion of the rectum, with small longitudinal folds called anal columns
• Anus: Also called anal orifice, the exit of the anal canal, with keratinized epidermis like skin.

Physiology of the Large Intestine

• Absorption of water, bile salts, vitamins produced by bacteria, and organic wastes
• Bacteria generate ammonia and hydrogen sulfide from peptides in feces.

Three Vitamins Produced in the Large Intestine

• Vitamin K (fat soluble)
• Biotin (water soluble)
• Pantothenic acid (B5) (water soluble)

Organic Wastes:

• Bacteria break down peptides in feces.
• Generate ammonia and hydrogen sulfide
• Produce flatus (intestinal gas)

Digestion

• Digestive system handles each nutrient differently
• Large organic molecules must be digested before absorption.
• Water, electrolytes, and vitamins can be absorbed without processing.

Water Absorption

• Cells cannot actively absorb or secrete water.
• Water movement across the digestive tract lining involves passive water flow.