Overview
This lecture covers the structure and function of the human gastrointestinal system, focusing on the processes of ingestion, digestion, absorption, and elimination, along with key organs, enzymes, and hormones involved.
Functions of the Gastrointestinal System
- The digestive system performs ingestion, digestion, absorption, and elimination.
- Ingestion is the intake of food.
- Digestion breaks down large biomolecules into building blocks using mechanical and chemical processes.
- Absorption allows nutrients to enter the body for cell use.
- Elimination removes undigested waste from the body.
Pathway of Digestion
- Digestion begins in the mouth with saliva containing enzymes like salivary amylase (chemical digestion) and teeth (mechanical digestion).
- Saliva also lubricates food, forming it into a bolus for swallowing.
- The epiglottis prevents food from entering the trachea during swallowing.
- Peristalsis moves food down the esophagus to the stomach.
- The stomach uses acid (HCl) and pepsin for protein breakdown and churns food into chyme.
- The lower esophageal sphincter prevents acid reflux; the pyloric sphincter controls chyme entry into the duodenum.
Small Intestine Segments and Roles
- The small intestine consists of the duodenum, jejunum, and ileum.
- Duodenum digests chyme with enzymes from the pancreas and bile from the liver; absorbs iron and minerals.
- Jejunum absorbs most carbohydrates and proteins with villi increasing absorption surface area.
- Ileum absorbs vitamin B12, bile salts, and remaining nutrients, especially fat-soluble vitamins (A, D, E, K).
Large Intestine Structure and Function
- The large intestine includes the ascending, transverse, and descending colons.
- Ascending colon absorbs water and salts, forming stool.
- Transverse colon stores and continues absorbing water and salts.
- Descending colon stores and consolidates feces before elimination.
- The large intestine is the main site for water absorption.
Accessory Organs & Secretions
- Liver produces bile (emulsifies fats) and metabolizes carbohydrates and proteins.
- Gallbladder stores and releases bile as needed.
- Pancreas secretes digestive enzymes and neutralizes stomach acid in the small intestine.
Key Hormones and Enzymes
- Gastrin: stimulates stomach glands to secrete pepsinogen and hydrochloric acid for protein digestion.
- Cholecystokinin (CCK): stimulates gallbladder to release bile and pancreas to release digestive enzymes.
- Secretin: stimulates pancreas to release bicarbonate and inhibits gastric acid, maintaining pH balance in the duodenum.
- Insulin: lowers blood glucose by increasing cellular uptake and storing glucose as glycogen.
- Glucagon: raises blood glucose by converting glycogen to glucose in the liver.
- Bile: emulsifies fats for easier digestion by enzymes.
Key Terms & Definitions
- Ingestion — taking food into the body.
- Digestion — breaking food into smaller molecules mechanically and chemically.
- Absorption — uptake of nutrients into blood or lymph.
- Elimination — expulsion of undigested waste.
- Peristalsis — wave-like muscle contractions moving food through the digestive tract.
- Chyme — semi-liquid mixture produced in the stomach.
- Villi — small finger-like projections increasing absorption surface in the small intestine.
- Sphincter — circular muscle controlling passage between digestive tract sections.
Action Items / Next Steps
- Review the order and function of each segment in the digestive tract.
- Memorize the main digestive enzymes and hormones and their functions.
- Study the roles of accessory organs in digestion.