Digestive System

Digestion is controlled disassembly. Food enters as complex polymers — proteins, carbohydrates, fats — and has to come out the other end as monomers and small molecules that can be absorbed. The tube that does this runs from mouth to anus with specialized chemistry at every segment, and it coordinates tightly with a second brain of its own.

At a glance

What it does

Breaks food down mechanically and chemically, absorbs useful molecules, and expels what is left. It also polices what gets in — the gut is your largest surface exposed to the outside world, and most of your immune cells live in or near it. Bile from the liver emulsifies fat. Enzymes from the pancreas cleave proteins and carbs. Stomach acid kills most incoming pathogens and primes pepsin for protein digestion.

How it works

Food enters the mouth, mixes with salivary amylase, and is swallowed. The stomach churns it with acid (pH 1-2) into chyme over a few hours. The small intestine is where almost all absorption happens — duodenum receives pancreatic enzymes and bile, jejunum absorbs most nutrients, ileum picks up B12 and bile salts. The large intestine recovers water and electrolytes, houses the microbiome, and forms stool.

Motility is governed by the enteric nervous system, which can run the tract autonomously, and modulated by the vagus nerve and gut hormones. Hunger and satiety are driven by a hormone network: ghrelin from the empty stomach, CCK and GLP-1 from the intestines after eating, leptin from fat tissue over longer timescales.

Accessory organs

• Liver — the largest internal organ and metabolic central processor. Handles drug metabolism, plasma protein synthesis, cholesterol handling, bile production, and roughly 500 other jobs.
• Gallbladder — stores and concentrates bile, contracts when fat arrives in the duodenum.
• Pancreas — exocrine acini make digestive enzymes; endocrine islets make insulin and glucagon.

When it goes wrong

Irritable bowel syndrome is common (10-15% lifetime prevalence) and poorly understood — the underlying pathology is mostly visceral hypersensitivity and altered motility, not obvious inflammation. Inflammatory bowel disease (Crohn's, ulcerative colitis) is a real autoimmune process and far more serious. GERD — stomach acid climbing into the esophagus — affects 20% of adults in wealthy countries and is largely a pressure and valve problem, not an acid-overproduction problem.

Celiac is autoimmune, triggered by gluten, affects roughly 1% of people, and is genuinely damaging to the small bowel. Non-celiac gluten sensitivity exists but is less defined and much rarer than self-diagnosis rates suggest. Colorectal cancer is the third most common cancer globally and is the reason you get a colonoscopy starting at 45.

Gut-brain axis

The gut and brain communicate constantly through the vagus nerve, hormonal signals (GLP-1, PYY, ghrelin), immune cytokines, and metabolites produced by gut bacteria. About 90% of serotonin lives in the gut, not the brain, controlling motility rather than mood. But the microbiome does influence CNS function via short-chain fatty acids, bile acid derivatives, and inflammation — the effect is real, the mechanism is complex, and probiotic claims are mostly ahead of evidence.

Honest take

Sources

• Sleisenger and Fordtran's Gastrointestinal and Liver Disease — the reference textbook.
• Cryan et al. (2019), Physiological Reviews — the landmark gut-microbiota-brain axis review.
• Lebwohl et al., NEJM — modern consensus on celiac and gluten-related disorders.