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Digestive System: Structure, Function, and Processes

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Digestive System Overview

General Structure and Function

The digestive system is responsible for the breakdown and absorption of nutrients, as well as the elimination of waste. It consists of the alimentary canal and accessory organs.

  • Alimentary canal: Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus

  • Accessory organs: Liver, gallbladder, pancreas

Small Intestine

Anatomy and Regions

The small intestine is the primary site for digestion and absorption. It is divided into three regions:

  • Duodenum: ~10 inches, C-shaped, loops around the pancreas head, distal to pyloric sphincter

  • Jejunum: Middle section, major site of absorption

  • Ileum: Distal section, connects to large intestine at ileocecal valve

Duodenum Features

  • Duodenal (Brunner's) glands: Secrete alkaline mucus to neutralize acidic chyme

  • Intestinal crypts: Contain enteroendocrine cells (G-cells, CCK, Secretin)

Small Intestine Mucosa

The mucosa is specialized for absorption and protection.

  • Simple columnar cells: With microvilli for increased surface area

  • Goblet cells: Produce mucin for lubrication

  • Intestinal crypts: Release hormones

  • MALT/GALT: Mucosa-associated lymphoid tissue for immune defense

  • Lacteals: Lymphatic vessels for fat absorption

Three Orders of Folds

These structures increase surface area for absorption:

  • Folds (plicae circulares): Submucosa

  • Villi: Mucosa

  • Microvilli: Apical surface of epithelial cells

Digestive Processes in the Small Intestine

Types of Digestion

  • Mechanical digestion: Segmentation mixes chyme

  • Chemical digestion: Enzymes from accessory organs break down food

  • Propulsion: Peristalsis moves contents forward

  • Absorption: Majority occurs in the jejunum

Intestinal Reflexes

Duodenal Regulation of Gastric Activity

  • Duodenum inhibits gastric secretions when pH is low:

    • Acidic chyme stimulates inhibition

    • Vagal (X) nerve inhibition

    • High pH excites gastric secretions

Accessory Digestive Organs

Liver

  • Lobules: Hexagonal units

  • Hepatocytes: Filter nutrient-rich blood, produce bile

  • Portal triad: Hepatic artery, hepatic portal vein, bile duct

Gallbladder

  • Bile: Yellow-green fluid, contains salts and bilirubin

  • Storage: Bile made in liver, stored in gallbladder

  • Function: Emulsifies fats (mechanical digestion)

  • Bilirubin breakdown: Gives feces brown color

Pancreas

  • Exocrine function: Acinar cells produce enzyme-rich pancreatic juice

  • Pancreatic ducts: Secrete bicarbonate () to neutralize acid (pH 8)

Enteroendocrine Regulation

CCK & Secretin

  • Cholecystokinin (CCK): Decreases stomach contraction force, stimulates bile release

  • Secretin: Inhibits gastric secretions, stimulates pancreatic bicarbonate secretion

  • Both regulate bile and pancreatic juice secretion

Absorption and Digestion Examples

Brush Border Enzymes

  • Chemical digestion: Hydrolysis by enzymes ending in "-ase"

  • Absorption: Transmembrane transport into blood (except fat, which is absorbed into lacteals)

  • Trypsin: Activates other proteases; analogous to pepsin in the stomach

Carbohydrate Digestion

Foodstuff

Enzyme(s) & Source

Site of Action

Starch & Disaccharides

Salivary amylase, Pancreatic amylase

Mouth, Small intestine

Oligosaccharides & Disaccharides

Brush border enzymes (dextrinase, glucoamylase, lactase, maltase, sucrase)

Small intestine

Lactose, Maltose, Sucrose

Brush border enzymes

Small intestine

Galactose, Glucose, Fructose

Absorbed

Small intestine

Protein Digestion

Foodstuff

Enzyme(s) & Source

Site of Action

Protein

Pepsin (stomach glands, HCl)

Stomach

Large polypeptides

Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase)

Small intestine

Small polypeptides, small peptides

Brush border enzymes (aminopeptidase, carboxypeptidase, dipeptidase)

Small intestine

Amino acids

Absorbed

Small intestine

Nucleic Acid Digestion

Foodstuff

Enzyme(s) & Source

Site of Action

Nucleic acids

Pancreatic ribonuclease, deoxyribonuclease

Small intestine

Pentose sugars, N-containing bases, phosphate ions

Brush border enzymes (nucleosidases, phosphatases)

Small intestine

Fat Digestion and Absorption

Foodstuff

Enzyme(s) & Source

Site of Action

Unemulsified fats

Bile (emulsification), Pancreatic lipases

Small intestine

Monoglycerides, fatty acids, glycerol

Absorbed

Small intestine

Fat Absorption Process

  • Fat droplets are emulsified by bile to form micelles

  • Pancreatic lipase breaks down fats into free fatty acids (FFA) and glycerol

  • Triglycerides are reformed in epithelial cells and packaged into chylomicrons

  • Chylomicrons are released by exocytosis into lacteals (lymphatic vessels)

Large Intestine

Processes and Functions

  • Absorbs water and electrolytes: No food breakdown occurs

  • Gut flora: Outnumber body cells 10:1, produce B and K vitamins

  • Primary role: Mass bowel movements

Reflexes and Movements

  • Gastro-colic reflex: Presence of food in stomach triggers distention and haustral contractions (slow, powerful peristaltic waves), producing mass movements 3-4 times/day

  • Spinal defecation reflex: Parasympathetic efferents inhibit sympathetic "resting" tone of internal anal sphincter, allowing defecation

Key Terms and Concepts

  • Segmentation: Mixing contractions in the small intestine

  • Peristalsis: Propulsive contractions moving contents along the tract

  • Chyme: Semi-fluid mass of partially digested food

  • Chylomicron: Lipoprotein particle for fat transport

  • Haustral contractions: Segmental movements in the colon

  • GALT: Gut-associated lymphoid tissue

Summary Table: Digestive Enzymes and Sites of Action

Macromolecule

Enzyme(s)

Source

Site of Action

Carbohydrates

Amylases, brush border enzymes

Salivary glands, pancreas, intestinal mucosa

Mouth, small intestine

Proteins

Pepsin, trypsin, chymotrypsin, carboxypeptidase, brush border enzymes

Stomach, pancreas, intestinal mucosa

Stomach, small intestine

Nucleic acids

Ribonuclease, deoxyribonuclease, nucleosidases, phosphatases

Pancreas, intestinal mucosa

Small intestine

Fats

Bile (emulsification), pancreatic lipase

Liver, pancreas

Small intestine

Equations and Chemical Reactions

  • General hydrolysis reaction:

  • Bicarbonate secretion:

Additional info:

  • Gut flora (microbiota) is an active area of research, with implications for health and disease.

  • Segmentation and peristalsis are regulated by the enteric nervous system and autonomic input.

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