Digestive System: Structure, Function, and Nutrient Absorption

Digestive System Overview

Pathway of Food Through the Digestive Tract

The digestive system is responsible for breaking down food, absorbing nutrients, and eliminating waste. Food follows a specific pathway through the digestive tract:

• Mouth

• Pharynx

• Esophagus

• Stomach

• Small Intestine

• Large Intestine

• Rectum

• Anus

Mechanical and Chemical Digestion

Digestion involves two main processes:

• Mechanical Digestion: Physical breakdown of food into smaller pieces, primarily by teeth and smooth muscle contractions.

• Chemical Digestion: Enzymatic breakdown of food into its monomers for absorption.

Motility in the Digestive Tract

Peristalsis and Segmentation

The movement of food through the digestive tract is accomplished by two types of muscle contractions:

• Peristalsis: Coordinated contraction of longitudinal muscles propels food forward.

• Segmentation: Circular muscles mix and churn food, pushing it against the epithelial lining for chemical breakdown and absorption.

Stomach Structure and Function

Anatomy of the Stomach

The stomach is a J-shaped organ located in the left upper quadrant of the abdomen. It serves as a reservoir for food and initiates protein digestion.

Gastric Secretions: Phases and Regulation

Gastric secretions occur in three phases:

• Cephalic Phase: Triggered by the sight, smell, taste, or thought of food. The vagus nerve stimulates the release of acetylcholine (AcH), activating gastric glands.

• Gastric Phase: Initiated when food enters the stomach. Stretch receptors and chemoreceptors activate neural and hormonal responses, including the release of gastrin and histamine.

• Intestinal Phase: Begins as chyme enters the duodenum, stimulating further secretions and motility.

Stimulation and Inhibition of Gastric Secretions

• Stimulation: Physical distension and chemical signals (peptides, amino acids) activate stretch and chemoreceptors, leading to the release of AcH, gastrin, and histamine.

• Inhibition: High acidity (pH < 2), decreased vagal stimulation, increased sympathetic input, and duodenal distension reduce gastric gland activity.

Gastric Emptying and Retropulsion

Peristalsis moves food toward the pylorus, where the pyloric sphincter opens to allow small amounts of chyme into the duodenum. Retropulsion occurs when the sphincter closes, forcing contents backward.

Hormonal Regulation of Digestion

Hormones and Paracrines in Digestion

Several hormones and paracrines regulate digestive processes, including gastrin, secretin, cholecystokinin (CCK), and histamine.

Hormone/Paracrine	Site of Production	Stimulus	Target Organ	Activity
Gastrin	Stomach	Food, AcH	Stomach	Stimulates gastric glands
Secretin	Duodenum	Acidic chyme	Pancreas, liver	Stimulates bicarbonate secretion
Cholecystokinin (CCK)	Duodenum	Fatty chyme	Pancreas, gallbladder	Stimulates enzyme and bile release
Histamine	Stomach	Food	Stomach	Stimulates acid secretion
Somatostatin	Stomach, duodenum	Food	Stomach, pancreas	Inhibits gastric secretion
Additional info: Table is a summary of key hormones and their actions in digestion.

Absorption of Nutrients by the Small Intestine

Carbohydrate Absorption

Carbohydrates are broken down into monosaccharides by brush border enzymes. Glucose is absorbed via sodium-glucose transport proteins (SGLT) and facilitated diffusion. Increased osmolarity draws water and dissolved nutrients into the extracellular fluid (ECF), which are then absorbed by blood capillaries and transported to the liver.

Protein Absorption

• Proteins are denatured and hydrolyzed by pepsin in the stomach.

• Pepsin is inactivated in the duodenum; trypsin and chymotrypsin continue hydrolysis.

• Brush border enzymes (dipeptidase) further break down peptides.

• Free amino acids are absorbed via sodium-dependent transport and facilitated diffusion into the bloodstream.

Lipid Absorption

Lipid digestion is complex due to their hydrophobic nature.

• Begins in the mouth with lingual lipase, continues in the stomach with gastric lipase.

• In the small intestine, pancreatic lipase and bile emulsify fats.

• Micelles, aggregates of bile molecules, transport lipids to absorptive cells.

• Inside cells, lipids are reassembled into triglycerides, packaged as chylomicrons, and absorbed into lymphatic lacteals.

Large Intestine and Defecation

Absorption and Elimination

Most digestion and absorption occur before the large intestine. The large intestine absorbs water and final nutrients.

Defecation Reflex

• Stretching of rectal walls activates the parasympathetic spinal reflex.

• Sigmoid colon and rectum contract; internal anal sphincter relaxes.

• Voluntary relaxation of the external anal sphincter allows feces to exit the body.

Common Aliments of the GI System

Overview

Disorders of the gastrointestinal system can affect any part of the digestive tract, impacting digestion, absorption, and elimination. Additional info: Examples include gastritis, peptic ulcers, and irritable bowel syndrome.