Digestion, Absorption, and Transport

Key Concepts

• Digestion: Breaking down food so it is small enough to be absorbed.
• Absorption: When the body takes in nutrients to use them.
• Food needs to be small enough for absorption, which is the purpose of digestion.
• Terms for food in digestive pathway:
  • Food: Before entering the mouth.
  • Bolus: After entering the mouth until it leaves the stomach.
  • Chyme: After mixing with stomach acid and leaving the stomach.

Digestive Movements

• Peristalsis: Muscle contractions moving food along the digestive tract.
• Segmentation: Muscle contractions breaking food into smaller pieces.

Digestive System Organs

• Main Pathway: Mouth → Esophagus → Stomach → Small Intestine → Large Intestine.
• Accessory Organs: Liver, Gallbladder, Pancreas.

Sphincters

• Upper Esophageal Sphincter: Between mouth and esophagus, prevents food backflow.
• Lower Esophageal Sphincter: Between esophagus and stomach, prevents acid reflux.
• Pyloric Sphincter: Between stomach and small intestine.
• Ileocecal Valve: Between small and large intestine.

Digestive Processes and Enzymes

• Mouth:
  • Carbohydrate digestion begins with salivary amylase.
• Stomach:
  • Protein digestion begins with pepsin and HCl (denatures proteins).
  • Carbohydrate digestion pauses (salivary amylase is deactivated by stomach acid).
• Small Intestine:
  • Carbohydrates, Proteins, and Lipids finalize their digestion.
  • Bile: Emulsifies fats (hydrophobic & hydrophilic properties).
  • Pancreatic enzymes: Digest carbs, proteins, and lipids.
  • Bicarbonate: Neutralizes stomach acid in the small intestine.

Functions of Hormones in Digestion

• Gastrin: Tells stomach to release HCl.
• Secretin: Tells pancreas to release bicarbonate.
• Cholecystokinin (CCK):
  • Tells gallbladder to release bile.
  • Signals pancreas to release digestive enzymes.

Absorption

• Small Intestine:
  • First organ to absorb nutrients.
  • Majority of nutrient absorption occurs here due to its length and surface area (up to 20 feet long).
  • Villi and Microvilli: Increase surface area, aid in nutrient absorption.
• Nutrients absorbed into cells lining the walls of the small intestine.
  • Facilitated Diffusion: Uses carrier proteins.
  • Active Transport: Requires energy.

Nutrient Transport

• Two main systems: Vascular System and Lymphatic System.
• Vascular System:
  • Transports water-soluble nutrients and small fats.
  • Uses bloodstream (red blood cells).
  • Involves arteries and veins (e.g., pulmonary artery, hepatic vein).
• Lymphatic System:
  • Transports fat-soluble nutrients and large fats.
  • Uses lymph (white blood cells).
  • Nutrients enter through lymph, then access subclavian vein.
  • Bypasses liver on the first round.

Summary

• Vascular system: Nutrients detour via liver first for detox.
• Lymphatic system: Larger fats and fat-soluble nutrients enter bloodstream through a larger portal (subclavian vein) and bypass liver initially but join the vascular system eventually for detoxification.

Keywords

• Peristalsis: Movement of food along GI tract.
• Segmentation: Breaking food into smaller parts.
• Bolus: Chewed food mixed with saliva.
• Chyme: Semi-fluid mass of partly digested food.
• Sphincters: Muscular valves controlling food passage.
• Emulsification: Mixing of fats with digestive enzymes via bile.