Digestive System: Anatomy & Physiology Study Notes

Digestive Processes

Main Digestive Processes

The digestive system carries out several coordinated processes to break down food, absorb nutrients, and eliminate waste. These processes occur sequentially throughout the gastrointestinal tract.

• Ingestion: Intake of food into the digestive tract.

• Propulsion: Movement of food through the digestive tract, including swallowing and peristalsis.

• Mechanical Breakdown: Physical breakdown of food by chewing, mixing with saliva, and churning in the stomach.

• Digestion: Chemical breakdown of food by enzymes into smaller molecules.

• Absorption: Passage of digested nutrients from the lumen of the GI tract into blood or lymph.

• Defecation: Elimination of indigestible substances from the body as feces.

Digestion and Absorption of Nutrients

Digestion of Carbohydrates, Proteins, and Fats

Digestion involves enzymatic breakdown of macromolecules into absorbable units. Each nutrient class is digested by specific enzymes and absorbed by specialized mechanisms.

• Carbohydrates: Digestion begins in the mouth with salivary amylase, continues in the small intestine with pancreatic amylase. Disaccharides are broken down by brush border enzymes into monosaccharides, which are absorbed into epithelial cells and then into capillaries by facilitated diffusion and active transport.

• Proteins: Digestion starts in the stomach with pepsin and continues in the small intestine with pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase). Brush border enzymes further break down peptides into amino acids, which are absorbed by active and facilitated diffusion.

• Lipids: Digestion occurs mainly in the small intestine. Bile salts emulsify fats, increasing surface area for pancreatic lipases to break down triglycerides into fatty acids and monoglycerides. These are absorbed into epithelial cells, reassembled into triglycerides, and packaged as chylomicrons for transport via lymphatics.

Peritoneum and Control of Digestive Activity

Peritoneum Structure

The peritoneum is a serous membrane lining the abdominal cavity and covering abdominal organs, providing support and reducing friction.

• Visceral Peritoneum: Covers external surfaces of digestive organs.

• Parietal Peritoneum: Lines the body wall.

• Peritoneal Cavity: Space between layers, contains fluid for lubrication.

• Mesentery: Double layer of peritoneum; routes for blood vessels, nerves, and lymphatics.

Control of Digestive Activity

Digestive activity is regulated by neural and hormonal mechanisms.

• Intrinsic Nerve Supply: Enteric nervous system (submucosal and myenteric plexuses) controls local reflexes.

• Extrinsic Nerve Supply: Autonomic nervous system (sympathetic and parasympathetic fibers) modulates digestive activity.

• Hormonal Control: Hormones from stomach and small intestine stimulate or inhibit digestive processes.

Functions of Saliva, Chewing, and Swallowing

Saliva

Saliva is essential for moistening food, beginning digestion, and protecting oral tissues.

• Cleanses mouth

• Dissolves food chemicals for taste

• Moistens food and compacts into bolus

• Begins breakdown of starch with amylase

Composition of Saliva: 97-99.5% water; contains electrolytes, digestive enzymes (amylase), proteins (mucin, lysozyme, IgA), and metabolic wastes (urea, uric acid).

Chewing and Swallowing

Chewing (mastication) mechanically breaks down food. Swallowing (deglutition) moves food from mouth to stomach in three phases:

• Buccal Phase: Voluntary; tongue pushes bolus into oropharynx.

• Pharyngeal-Esophageal Phase: Involuntary; controlled by swallowing center in brain stem. Epiglottis closes off trachea, upper esophageal sphincter relaxes, bolus moves into esophagus.

• Esophageal Phase: Peristalsis moves bolus through esophagus to stomach; gastroesophageal sphincter relaxes to allow entry into stomach.

Gastric Secretion and Regulation

Gastric Barrier and Regulation

The stomach secretes gastric juice containing hydrochloric acid and enzymes for protein digestion. The mucosal barrier protects the stomach lining from self-digestion.

• Mucosal Barrier: Tight junctions between epithelial cells, bicarbonate-rich mucus, rapid cell turnover.

• Gastric Gland Cells: Parietal cells (secrete HCl), chief cells (secrete pepsinogen), enteroendocrine cells (secrete hormones).

• Intrinsic Factor: Required for absorption of vitamin B12 in the small intestine.

Regulation of Gastric Secretion

• Cephalic Phase: Occurs before food enters the stomach; triggered by sight, smell, taste, or thought of food.

• Gastric Phase: Begins when food enters the stomach; stimulates secretion via stretch and chemical receptors.

• Intestinal Phase: Begins when chyme enters the small intestine; initially stimulates, then inhibits gastric secretion.

Small Intestine: Absorption and Motility

Modifications for Absorption

The small intestine is specialized for nutrient absorption, with structural modifications to increase surface area.

• Plicae Circulares: Circular folds that slow movement and increase surface area.

• Villi: Finger-like projections with capillaries and lacteals for absorption.

• Microvilli: Brush border on absorptive cells, containing enzymes for final digestion.

Motility

Segmentation and peristalsis mix and move chyme, facilitating absorption.

Liver and Pancreas in Digestion

Liver Functions

The liver produces bile, which emulsifies fats and aids in their digestion and absorption.

• Liver Histology: Lobules with hepatocytes; blood from hepatic artery and portal vein flows through sinusoids.

• Bile: Contains bile salts, pigments, cholesterol, and phospholipids.

• Functions: Metabolism, detoxification, storage, synthesis of plasma proteins.

Pancreatic Functions

The pancreas secretes digestive enzymes and bicarbonate-rich fluid into the duodenum.

• Exocrine Function: Acinar cells secrete pancreatic juice containing enzymes (amylase, lipase, proteases, nucleases).

• Endocrine Function: Islets of Langerhans secrete insulin and glucagon.

Regulation of Pancreatic Secretion

• Secretin: Stimulates secretion of bicarbonate-rich pancreatic juice.

• Cholecystokinin (CCK): Stimulates secretion of enzyme-rich pancreatic juice and contraction of gallbladder.

Large Intestine: Functions

Major Functions

The large intestine absorbs water and electrolytes, forms and stores feces, and houses beneficial bacteria.

• Bacterial Flora: Ferment indigestible carbohydrates, synthesize B vitamins and vitamin K.

• Propulsion of Feces: Mass movements and defecation.

• Defecation Reflex: Initiated by stretching of rectal wall; involves relaxation of internal anal sphincter and voluntary relaxation of external anal sphincter.

Summary Table: Digestive Enzymes and Their Functions

Enzyme	Source	Substrate	Product
Amylase	Salivary glands, pancreas	Starch	Maltose, oligosaccharides
Pepsin	Stomach	Proteins	Peptides
Trypsin, Chymotrypsin	Pancreas	Proteins	Peptides
Lipase	Pancreas	Triglycerides	Fatty acids, monoglycerides
Nucleases	Pancreas	Nucleic acids	Nucleotides

Key Equations

• General Digestion Equation:

$\text{Macromolecule} + \text{Enzyme} \rightarrow \text{Smaller molecules}$

• Facilitated Diffusion (for absorption):

$J = P \times (C_1 - C_2)$

Where $J$ is the flux, $P$ is the permeability coefficient, and $C_1$ and $C_2$ are concentrations on either side of the membrane.

Additional info:

• Some content inferred for completeness, such as the summary table and equations.

• Scientific names and terms italicized where appropriate.