Digestive System: Structure, Function, and Processes

Digestive System Overview

Main Functions of the Digestive System

The digestive system is responsible for processing food to extract and absorb nutrients, and eliminate waste. Its main functions include:

• Ingestion: Taking in food.

• Mechanical and Chemical Breakdown: Breaking food into nutrient molecules.

• Absorption: Moving nutrient molecules into the bloodstream.

• Excretion: Removing indigestible remains from the body.

Digestive System Anatomy

Accessory Digestive Organs

Accessory organs aid digestion but are not part of the alimentary canal. They include:

• Teeth and Tongue

• Gallbladder

• Digestive glands: Salivary glands, liver, pancreas

Alimentary Canal

The alimentary canal is a continuous tube from mouth to anus, including the oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.

Digestive Processes

Six Essential Activities

• Ingestion: Eating food.

• Propulsion: Moving food through the canal (swallowing and peristalsis).

• Mechanical Breakdown: Chewing, mixing, churning, and segmentation.

• Digestion: Enzymatic breakdown of complex molecules.

• Absorption: Passage of digested fragments into blood or lymph.

• Defecation: Elimination of indigestible substances.

Functional Anatomy of the Digestive System

Mouth and Associated Organs

The mouth is the entry point for food, where mechanical and chemical digestion begins. Associated organs include the tongue, salivary glands, and teeth.

Teeth and Dentition

Teeth are essential for mastication (chewing), breaking food into smaller fragments. Dentition refers to the arrangement and types of teeth:

• Incisors: Cutting

• Canines: Tearing

• Premolars: Grinding

• Molars: Crushing

Dental formulas indicate the number and types of teeth:

Type	Formula	Total Teeth
Primary (deciduous)	2I, 1C, 2M (upper) / 2I, 1C, 2M (lower) × 2	20
Permanent	2I, 1C, 2PM, 3M (upper) / 2I, 1C, 2PM, 3M (lower) × 2	32

Microscopic Anatomy of the Digestive Tract

Esophagus Structure

The esophagus is a muscular tube lined with stratified squamous epithelium, transitioning to simple columnar epithelium at the stomach.

Stomach Structure

The stomach is a temporary storage tank that initiates protein digestion and converts food into chyme. It has specialized regions and a mucosa with gastric pits and glands.

Hormonal Regulation of Digestion

Hormones and Paracrines

Several hormones and paracrines regulate digestive processes. Key examples include:

Hormone	Site of Production	Stimulus	Target Organ	Activity
Cholecystokinin (CCK)	Duodenal mucosa	Fatty chyme	Stomach, Liver, Pancreas, Gallbladder	Inhibits stomach activity, stimulates enzyme-rich pancreatic juice, stimulates gallbladder contraction
Secretin	Duodenal mucosa	Acidic chyme	Stomach, Pancreas, Liver	Inhibits gastric secretion, stimulates bicarbonate-rich pancreatic juice, increases bile output
Gastrin	Stomach mucosa	Food in stomach	Stomach, Small intestine, Large intestine	Stimulates gastric glands and motility

Liver, Gallbladder, and Pancreas

Liver Anatomy and Function

The liver is the largest gland in the body, producing bile for fat emulsification. It consists of four lobes and is anchored by ligaments.

Microscopic Anatomy of the Liver

Liver lobules are hexagonal units composed of hepatocytes. Blood flows through sinusoids, and Kupffer cells remove debris. Hepatocytes produce bile, process nutrients, store vitamins, and detoxify blood.

Gallbladder and Pancreas

The gallbladder stores and concentrates bile. The pancreas produces digestive enzymes and bicarbonate, and has both exocrine and endocrine functions.

Small Intestine

Gross and Microscopic Anatomy

The small intestine is the major site of digestion and absorption, with three subdivisions: duodenum, jejunum, and ileum. Its surface area is increased by circular folds, villi, and microvilli.

Large Intestine

Gross Anatomy

The large intestine absorbs water and electrolytes, and forms and expels feces. It includes the cecum, appendix, colon, rectum, and anal canal.

Processing of Nutrients

Digestion and Absorption of Carbohydrates

Carbohydrate digestion begins in the mouth with salivary amylase, continues in the small intestine with pancreatic amylase and brush border enzymes, and results in monosaccharides absorbed into the blood.

Digestion and Absorption of Proteins

Protein digestion starts in the stomach with pepsin, continues in the small intestine with pancreatic proteases and brush border enzymes, and results in amino acids absorbed into the blood.

Digestion and Absorption of Lipids

Lipid digestion requires emulsification by bile salts and breakdown by pancreatic lipases. Monoglycerides and fatty acids are absorbed and transported via chylomicrons in the lymphatic system.

Absorption of Water

Most water absorption occurs in the small intestine by osmosis, with the remainder absorbed in the large intestine.