Mechanical digestion is like the prep work in a kitchen — chopping, mixing, and moving ingredients around before the actual cooking (chemical digestion) begins. It doesn't change the chemical nature of food; it just makes it easier for enzymes to work.

Chewing (Mastication): Your teeth are the first mechanical processors. Incisors cut, canines tear, and molars grind. Chewing breaks food into smaller pieces, increasing surface area for enzymes. It also mixes food with saliva. This is partly voluntary — you control how thoroughly you chew.

Peristalsis — The Wave: This is the primary movement of the GI tract. Imagine squeezing a tube of toothpaste — you apply pressure behind the paste and it moves forward. Peristalsis works the same way: circular muscles contract behind the food bolus while longitudinal muscles contract in front, creating a wave that pushes food along. It's involuntary and controlled by the enteric nervous system.

Segmentation — The Mix: In the small intestine, another pattern predominates. Ring-like contractions appear and disappear at different points along the intestine, chopping and mixing the chyme. Think of it like kneading dough — it doesn't move the contents forward much, but it thoroughly mixes them and increases contact with the absorptive surface. Segmentation is most active after meals.

Mass Movements: In the large intestine, powerful peristaltic contractions occur 3-4 times daily, typically after meals (the gastrocolic reflex). These push feces toward the rectum and are what trigger the urge to defecate.

While mechanical digestion prepares food, chemical digestion actually breaks down complex molecules into absorbable units. Each macronutrient has specific enzymes that break it down through hydrolysis — adding water to break chemical bonds.

Did you know your gut has its own nervous system with about 100-500 million neurons — more than your spinal cord? This enteric nervous system (ENS) can operate independently, which is why it's sometimes called your "second brain."

Two Nerve Networks:

• Submucosal plexus (Meissner's): Located in the submucosa. Controls secretions, blood flow, and absorption. It's the "regulatory center" for gut functions.
• Myenteric plexus (Auerbach's): Located between muscle layers. Controls motility — peristalsis and segmentation. It coordinates the complex contractions that move and mix food.

Brain-Gut Connection: While the ENS can work independently, it communicates with your brain via the vagus nerve. This explains why stress affects your digestion ("butterflies in stomach"), why you feel emotions in your gut, and why 95% of your body's serotonin is actually in your GI tract!

Autonomic Control:

• Parasympathetic (vagus nerve): "Rest and digest" — increases motility, secretions, and blood flow. Digestion works best when you're relaxed.
• Sympathetic: "Fight or flight" — decreases motility and secretions, directing blood to muscles. This is why stress can cause constipation or why you shouldn't eat right before intense exercise.

Clinical note: Many GI medications work by affecting the ENS. Prokinetics enhance motility; anticholinergics reduce it. Understanding this system helps you understand both normal digestion and many GI disorders.

Once digestion breaks nutrients into absorbable units, they must cross the intestinal epithelium to enter your bloodstream or lymph. Different nutrients use different routes and mechanisms.

Transport Mechanisms:

• Simple diffusion: Small, lipid-soluble molecules (like fatty acids) diffuse down their concentration gradient. No energy required.
• Facilitated diffusion: Uses carrier proteins but no energy. Example: fructose uses GLUT5 transporter.
• Active transport: Uses carrier proteins AND energy (ATP) to move against concentration gradient. Example: glucose uses SGLT1 cotransporter with sodium.
• Paracellular transport: Between cells through tight junctions. Limited to small molecules and water.

Where Different Nutrients Go:

• Carbohydrates (as monosaccharides): Absorbed into blood capillaries in villi → portal vein → liver
• Amino acids: Same route as carbohydrates — into blood → liver
• Fats (as fatty acids): Absorbed into intestinal cells, reassembled into triglycerides, packaged into chylomicrons → lacteals (lymph) → eventually blood
• Water: Follows solutes by osmosis — about 9 liters of water are absorbed daily (7-8 liters from secretions + 1-2 liters from food/drink)
• Vitamins: Fat-soluble (A, D, E, K) absorbed with fats; water-soluble (B, C) absorbed into blood

Vitamin B12 Special Case: Requires intrinsic factor (made by stomach parietal cells) for absorption in the ileum. Pernicious anemia occurs when intrinsic factor is lacking.