Gastrointestinal Tract: Also known as the alimentary canal, it is the continuous passage through which food moves.
Accessory Organs: Assist in digestion but usually do not come into direct contact with food.
Functions of the Digestive System
Ingestion: Taking in food and fluids.
Secretion: Hormones, enzymes, acids, mucus, etc., from endocrine and exocrine glands aid digestion.
Propulsion: Rhythmic contractions (peristalsis) move food through the alimentary canal.
Digestion: Breakdown of food via:
Mechanical means (e.g. chewing)
Chemical means (e.g. enzymatic actions)
Absorption: Transfer of nutrients from the digestive tract into blood or lymphatic vessels.
Defecation: Elimination of indigestible substances and metabolic waste.
Stomach Anatomy and Functions
Muscular Layers: The stomach has a third layer of smooth muscle (muscularis externa) for churning food.
Chyme Formation: Mixture of food and gastric juices.
Cellular Composition
Goblet Cells: Produce mucus to protect the stomach lining.
Chief Cells: Secrete pepsinogen (inactive form of pepsin).
Parietal Cells: Produce HCl and intrinsic factor.
Gastric Juice Components
Hydrochloric Acid (HCl):
Functions:
Activates pepsinogen to pepsin for protein digestion
Provides an acidic environment for digestion
Defends against pathogens
Pepsin: Active enzyme that digests proteins.
Phases of Gastric Acid Production
Cephalic Phase: Initiated by sight, smell, or thought of food; involves acetylcholine release.
Gastric Phase: Triggered by food entering the stomach; releases gastrin and increases acid secretion.
Intestinal Phase: Regulates gastric secretion via partially digested food in the small intestine; secretin slows gastric activity.
Receptive Relaxation
Accommodates food as it enters the stomach; mediated by the parasympathetic nervous system via the vagus nerve.
Small Intestine Structure and Function
Regions: Duodenum, Jejunum, and Ileum.
Nutrient Absorption: Primarily occurs in the jejunum and ileum.
Surface Area Increase: Achieved through circular folds, villi, and microvilli.
Cell Types
Enterocytes: Special epithelial cells with microvilli that increase absorption surface area.
Lacteals: Lymphatic vessels in the villi that absorb lipids.
Digestion of Nutrients
Carbohydrates
Digestion Process:
Begins in the mouth with salivary amylase.
Continues in the small intestine with pancreatic enzymes.
Monosaccharide Absorption: Via facilitated diffusion across the basolateral membrane.
Proteins
Breakdown: Proteins are reduced to amino acids by:
Pepsin in the stomach
Pancreatic enzymes in the small intestine.
Transport: Amino acids absorbed through secondary active transport facilitated by Na+/K+ pumps.
Lipids
Emulsification: Bile salts break down fats into smaller droplets in the small intestine.
Micelle Formation: Lipids are surrounded by bile salts to facilitate absorption into enterocytes.
Chylomicron Formation: Lipids are repackaged into chylomicrons before entering the lymphatic system.
Pancreatic Function
Endocrine and Exocrine: Secretes pancreatic juices rich in digestive enzymes regulated by CCK and secretin.
Liver and Gallbladder Functions
Bile Production: Assists in lipid digestion and is stored in the gallbladder.
Other Functions:
Storage of carbohydrates (glycogen)
Protein production (e.g., albumin)
Detoxification of harmful substances.
Defecation
Stretch Receptors: Detect fecal matter in the distal colon, sending signals to the spinal cord.
Relaxation of Smooth Muscles: Results in the contraction of the sigmoid colon and rectum.
Voluntary Control: Adults control anal sphincter relaxation through the cerebral cortex.
Bristol Stool Chart
Types of Stools: Ranges from separate hard lumps (Type 1) to liquid consistency (Type 7) indicating digestive health issues such as constipation or diarrhea.
Summary
Understanding the components of gastric juices, mechanisms of digestion for nutrients, and the processes involved in absorption are crucial for grasping the broader functions of the digestive system.