The Digestive System

• The digestive system explains how nutrients are extracted from food.
• Nutrients from food provide the body with fuel and building materials.
• Digestive organs become active when triggered by the sight or smell of food.
• The digestive system converts complex foods in to basic nutrients.
• This process needs a group of digestive organs, each having specific jobs.
• Digestion is a two-part process.

Mechanical Digestion

• Includes chewing, crushing, and churning of food.
• It occurs in the mouth and stomach.
• Teeth break food into smaller pieces for easier swallowing.
• Churning in the stomach further breaks down food into smaller particles.

Chemical Digestion

• It involves enzymatic reactions that occur throughout the digestive tract.
• Enzymes, hydrochloric acid (from stomach glands), and bile (from the liver) dissolve food, enabling nutrients to be released.

Gastrointestinal Tract

• Key sites for digestion and nutrient absorption include:
  • Mouth
  • Esophagus
  • Stomach
  • Small intestine
  • Large intestine
  • Rectum
  • Anus

Accessory Organs

• Organs include:
  • Liver
  • Pancreas
  • Gallbladder

Eyes and Nose

• Appetite-triggering food stimulates the brain, which prepares the digestive organs.
• Aromatic food molecules travel from the nose to the brain.
• Receptor cells communicate with the brain, which sends messages to the mouth and digestive tract.
• This results in saliva production and stomach activity.

Mouth

• Involved in mastication (chewing).
• Contains 9,000 taste buds.
• Produces saliva which contains salivary amylase that aids in starch breakdown.
• Saliva also moistens, softens, and dissolves food.
• No protein digestion occurs here.
• Fat digestion via lingual lipase is minimal.

Esophagus

• It is a long tube that connects the throat with the stomach.
• Food is moved to the stomach via peristalsis (muscle wave action).
• The sphincter muscle at the end, prevents backflow from the stomach.

Stomach

• Acts as a food processor to further break down food.
• It receives food from the esophagus.

Gastric Juices

• Gastric juices contain hydrochloric acid (HCL) to kill microorganisms.
• It has mucus to protect the stomach lining from its own strong acids.
• It has digestive enzymes to begin fat and protein digestion.
• The stomach’s acidic environment deactivates amylases, stopping carbohydrates digestion.
• Food remains in the stomach for 2-3 hours, depending on the type of food.
• In the stomach liquids empty faster than solids.
• Carbohydrates and proteins digest faster than fats.
• The stomach absorbs alcohol, but no other nutrients.

Small Intestine

• Approximately 6 meters long with a narrow diameter of 2.5cm.
• The main site for digestion and absorption.
• Villi (fingerlike protrusions) on its surface supports absorption.
• Food takes 5-14 hours to pass through the small intestine.
• It is less acidic than the stomach.
• Chyme that passes from the stomach to the small intestine, contains partially digested food and gastric juices.
• Digestive enzymes released in the small intestine:
  • Pancreatic and intestinal enzymes complete protein digestion into amino acids.
  • Bile (from the liver, stored in the gallbladder) enables fats to mix with water.
• Alkaline pancreatic juices in the small intestine reduce acidity, allowing amylases to continue breaking down carbohydrates.

Small Intestine Enzymes

• Intestinal alcohol dehydrogenase within the small intestine digests alcohol that has not been previously absorbed.
• Nutrients are carried through the bloodstream to the liver after digestion and absorption in the small intestine for processing and distribution to body cells.

Large Intestine

• Also called the colon.
• The role is to absorb water from the undigested mixture it receives from the small intestine.
• It squeezes the remaining matter into feces.
• Bacteria in the colon produce small amounts of vitamins like B12 and K.
• The rectum is the end of the colon where feces collect before exiting through the anus.

Liver

• Accessory organ that produces bile, which is essential for fat absorption.
• It detoxifies dangerous substances.

Gallbladder

• Accessory organ that stores bile and secretes it into the small intestine when needed.

Pancreas

• Accessory organ that secretes pancreatic juice containing digestive enzymes into the small intestine.

Absorption Sites

• Stomach absorbs alcohol.
• Small intestine absorbs vitamins, minerals, and all other nutrients.
• Large intestine absorbs water and forms feces.