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Digestive System

从口到肛门 · 9 米的化学工厂 · 70% 免疫驻地 · 与微生物共生 · 第二大脑就在这里

Story path

1Stomach acidStomach acid
2Pancreas + bilePancreas + bile
3Brush-border absorptionBrush-border absorption
4Microbiome · 38 trillionMicrobiome · 38 trillion
5Gut-brain axisGut-brain axis
6Barrier & tight junctionsBarrier & tight junctions

Chapter 1

Stomach acid

Stomach acid (HCl) is the strongest acid in the body — pre-meal pH 1.5-2.0, roughly equivalent to dilute hydrochloric acid in a battery.

It's secreted by parietal cells in the stomach wall, which pump H⁺ against the gradient into the stomach lumen via the H⁺/K⁺-ATPase proton pump. Each round of acid secretion consumes a lot of , which is why acid output drops under severe stress (energy diverted elsewhere).

Three jobs of stomach acid:

Chemical barrier: most bacteria die within seconds at pH < 2 — the first line of defense against foodborne infectionActivate pepsinogen → pepsin: pepsin only works at pH < 4 and breaks proteins into polypeptidesUnlock minerals: reduce Fe³⁺ → Fe²⁺, release B12 from protein, ionize calcium for absorption
The side effects of long-term PPIs (omeprazole and similar acid suppressants) are the inverse of these three: gut infections rise (*C. difficile*), iron deficiency, B12 deficiency, increased fracture risk. That's the mechanistic basis for why acid suppression is not free.

Stomach beyond grinding

Adult stomach capacity is about 1-1.5 L (shrinks to 50 mL when empty); 2-3 L of gastric juice are secreted daily. A meal sits in the stomach for 2-4 hours — protein plus fat stays longer, liquids leave fast.

Gastric emptying rate regulates the insulin curve: a high-sugar liquid hits the small intestine within 30 minutes and blood glucose spikes; protein + fiber together release slowly and glucose is smooth. This is why 'vegetables first' (vegetables before rice) and 'protein first' eating order lower post-meal glucose peaks — gastric emptying is slowed. One of the core mechanisms of weight-loss drugs (semaglutide) is directly slowing gastric emptying so people don't feel hungry for a long time.

The enemies of stomach acid:

*Helicobacter pylori*: the only bacterium that survives stomach acid; global infection rate ~50%; the main driver of gastric cancerChronic stress: downregulates acid via the vagus nerveCalcium carbonate / magnesium antacids: neutralize acid short-term, rebound long-termPPI-class drugs: irreversibly inhibit the proton pump; short half-life but effect lasts 24-48 hours

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Chapter 2

Pancreas + bile

Food leaves the stomach and enters the duodenum — the acidic chyme is immediately neutralized and processed jointly by the pancreas and the gallbladder.

The pancreas secretes:

Bicarbonate (HCO₃⁻): pulls pH from 2 up to 6-8Trypsin / chymotrypsin / elastase: break down proteinPancreatic lipase + colipase: break down fatPancreatic amylase: breaks down carbohydrateNucleases: break down DNA/RNA
The gallbladder stores concentrated bile made by the liver. The main components are bile acids (cholic acid / chenodeoxycholic acid) + phospholipids + cholesterol + bilirubin; the job is to emulsify fat — breaking large oil droplets into micro-droplets so pancreatic lipase has more surface area. About 500-800 mL is secreted daily, most of which is recycled — this is the entero-hepatic circulation, 5-10 times per day.

The hormonal triggers: chyme reaching the duodenum → intestinal I cells secrete CCK (cholecystokinin) → gallbladder contracts + pancreatic enzymes release; acid reaching the duodenum → intestinal S cells secrete secretin → pancreatic bicarbonate is released.

So this is a precise chemical concert, not a crude 'stomach grinds + intestine absorbs'.

Gallstones & malabsorption

Gallstones form when excess cholesterol crystallizes in bile — about 10-15% of Westerners have them, most asymptomatic. The core is an imbalance between cholesterol and the bile acids / phospholipids.

What happens after gallbladder removal? Bile is no longer concentrated and stored; instead it drips continuously into the intestine — fat absorption at large meals is inadequate, and long term this can lead to fat-soluble vitamin (A/D/E/K) deficiency. But most people compensate well and don't need special supplementation.

Pancreatic insufficiency (chronic pancreatitis / cystic fibrosis): insufficient enzyme secretion → steatorrhea, presenting as oily, floating, foul-smelling stool plus weight loss; requires oral pancreatic enzyme replacement plus high-dose ADEK.

Clinical significance of the entero-hepatic circulation:

95% of bile acids are actively reclaimed in the terminal ileum, returned via the portal vein, and re-secreted by the liverIleal resection or Crohn's involving the ileum → bile acid loss → steatorrhea + simultaneous B12 loss (the ileum is the only B12 absorption site)Bile acid-binding agents like cholestyramine exploit this loop to lower cholesterol — forcing the liver to consume cholesterol to make new bile acids
So the real exit route for cholesterol metabolism is this circuit — it also explains why dietary fiber lowers cholesterol: fiber adsorbs bile acids to block their reabsorption, forcing the liver to burn blood cholesterol to make new ones.

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Chapter 3

Brush-border absorption

The small intestine is where the body really absorbs nutrients — about 6-7 meters long, with the surface area amplified 600× by villi + microvilli, for a total absorption area of about 30-40 m² (roughly a tennis court).

The absorptive epithelium is called the brush border, dotted with specialized transporters:

SGLT1: Na⁺-coupled glucose co-transporter (the chemistry behind sports drinks)PEPT1: di-/tripeptide transporter (peptides are absorbed faster than individual amino acids)DMT1: divalent metal transporter (Fe²⁺ / Mn²⁺ / Cu²⁺ / Zn²⁺ all use it and compete with each other)NaPi-2b: phosphate absorptionCFTR: chloride / bicarbonate channel (cystic fibrosis mutation is here)
Enterocyte lifespan is only 3-5 days — one of the fastest-turnover tissues in the body, second only to olfactory neurons. The reason is that they are in direct contact with food, toxins, and microbes and take heavy wear.

This high turnover means: enterocytes need large amounts of amino acids (especially glutamine) plus energy — glutamine, not glucose, is the enterocyte's preferred fuel; during chemotherapy the entire mucosa from mouth to anus collapses first, which is why chemo patients commonly develop mucositis / diarrhea; under severe stress (sepsis, major burns) the gut barrier is the first to break down.

ORS — a lifesaving formula

Oral Rehydration Solution (ORS) is one of the most important low-cost medical discoveries of the 20th century. The WHO estimates it has saved ~50 million children's lives since 1978, mostly from diarrhea deaths.

The mechanism is SGLT1: cholera toxin makes intestinal cells frantically secrete water and electrolytes, causing severe dehydration; but SGLT1 still works — it requires both Na⁺ and glucose at the same time to transport; one Na⁺ + one glucose enter together, and water follows passively by osmosis back into the blood.

The formula (WHO 2002 revision):

Glucose 13.5 g/LSodium 75 mmol/LPotassium 20 mmol/LChloride 65 mmol/LCitrate 10 mmol/LTotal osmolality 245 mOsm/L (low osmolality is actually more effective)
This is why drinking plain water during diarrhea is worse: without Na⁺ + glucose, SGLT1 stays closed and water molecules can't cross the membrane to be reabsorbed. Sports drinks (Gatorade etc.) are sweetened ORS variants with different ratios but the same chemical principle.

Homemade ORS: 1 L water + 6 tsp sugar + 1/2 tsp salt + 1/2 cup orange juice (for K and citrate) — first-line for acute pediatric diarrhea.

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Chapter 4

Microbiome · 38 trillion

Human cells number about 30 trillion; gut bacteria about 38 trillion — Sender *PLoS Biol* 2016's corrected real ratio is close to 1:1, not the long-repeated 10:1. But by gene count, humans have about 22,000 while the gut microbiota collectively has 3 million to 10 million — the microbiome is the body's second genome, 100-500× larger.

Most gut bacteria live in the colon, with a total mass of about 1-2 kg. The dominant phyla are Bacteroidetes + Firmicutes, accounting for 90%.

What do they live on? Things we can't digest. Dietary fiber (resistant starch, inulin, pectin, β-glucan, etc.) is fermented into short-chain fatty acids ():

Acetate enters the blood and serves as a systemic metabolic substratePropionate enters the portal vein to the liver, inhibiting fat synthesis and appetite signalsButyrate is the colonocyte's preferred fuel (70% of energy), and is simultaneously an HDAC inhibitor and an anti-inflammatory signal
This is an important metabolic process the body has 'outsourced to microbes': eating 25-30 g of fiber, microbes produce about 200-400 mmol SCFA/day — a sizeable fraction of distal gut energy.

Without fiber there is no SCFA, and the colon goes hungry. A long-term low-fiber diet (typical Western) starves the microbes into nibbling on the mucus layer, directly weakening the gut barrier. This is the chemical root of 'fiber isn't just for regularity'.

Dysbiosis & diversity

'Good bacteria / bad bacteria' is an outdated simplification — the real markers of gut health are diversity + resilience, not a particular species.

Things that reduce diversity (and modern people do all of them):

Broad-spectrum antibiotics: one course drops microbial diversity by about 25%, and it doesn't fully recover within six monthsUltra-processed food: emulsifiers (polysorbate 80, carboxymethylcellulose) directly damage the mucus layerLow-fiber high-sugar diet: shifts toward pro-inflammatory speciesChronic stress + sleep deprivation: regulated via vagus nerve and cortisolExcessive hygiene (hand sanitizer / antibacterial soap): doesn't affect gut bacteria, but does affect skin / nasal microbiota
Things that increase diversity:

30+ different plants per week — American Gut Project shows this is the single strongest predictorFermented foods (yogurt, kimchi, kombucha) — Sonnenburg 2021 *Cell* shows these are more effective at lowering inflammation than fiber aloneAdequate fiber 25-40 g/dayAvoid unnecessary antibiotics
Fecal microbiota transplantation (FMT) is now the gold-standard treatment for recurrent *C. difficile* infection (80-90% cure rate). Other indications (IBD / IBS / metabolic syndrome / depression) are still under research, with B-C grade evidence — don't fall for the marketing.

Probiotic supplements: specific strains have evidence for specific indications (*S. boulardii* prevents diarrhea, *L. rhamnosus* GG for acute pediatric diarrhea); broad-spectrum 'gut health' probiotics have thin evidence — most strains die crossing stomach acid; live cultures in food (yogurt / kefir / natto / kimchi) have stronger evidence and are also cheaper.

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Chapter 5

Gut-brain axis

The gut has about 500 million neurons — more than the spinal cord, exceeding the entire peripheral nervous system combined. This network is called the enteric nervous system (ENS), the 'second brain', and can independently regulate motility, secretion, and local blood flow without input from the brain.

The three communication channels of the gut-brain axis:

1. Vagus nerve — the 10th cranial nerve; 80% of its fibers are gut → brain afferents (not brain → gut). The gut sends up signals of fullness, inflammation, and microbial state. Vagus nerve stimulation (VNS) is now a clinical treatment for refractory depression and epilepsy.

2. Neurotransmitter production:

Serotonin (5-HT) — 95% is made in the gut, mainly by enterochromaffin cells, regulating motility + local signals; only 5% in the brainDopamine — ~50% is in the gutGABA / short-chain fatty acids / tryptophan metabolites — directly produced by microbes
3. Immune + endocrine signaling:

Cytokines (, ) cross the blood-brain barrier and affect moodLPS (lipopolysaccharide) — gut bacterial cell-wall fragments; trace amounts entering blood is normal; large amounts entering blood = metabolic endotoxemia, linked to depression and metabolic disease
This is why gut health affects mental state: it's not abstract 'mind-body connection' — it's the physical circuit of 500 million neurons + 80% upgoing vagus + 95% serotonin + thousands of bacterial metabolites.

Mood, microbiome, depression

Clinical translation of the gut-brain axis is still early, but the direction is clear.

Depression and the microbiome:

Depressed patients have lower microbial diversity, Bacteroides ↑, *Faecalibacterium* / *Coprococcus* ↓Mouse fecal transplant — gut microbes from depressed humans transferred into mice → mice develop 'depression-like behavior' (longer immobility on the forced swim test)A few small RCTs (Akkasheh 2016 and others) show specific probiotic combinations reduce depression scores by ~25-30% — not strong but consistent
Anxiety and the vagus nerve:

*Lactobacillus rhamnosus* JB-1 reduces anxiety + regulates cortisol in mice through the vagus nerve — cutting the vagus abolishes the effect (a direct causal-chain piece of evidence)This sparked the 'psychobiotics' research field, but human evidence is below B-grade
IBS (irritable bowel syndrome) — a textbook example of gut-brain dysregulation:

Patients have heightened intestinal sensory sensitivity (the same gas distension feels much more painful)Stress → worsens symptomsThe low-FODMAP diet (reducing fermentable sugars) and cognitive behavioral therapy (CBT) / hypnotherapy both have RCT evidenceThis reflects a real three-way brain-gut-microbe dysregulation, not an 'imagined' illness
Practical advice (B-C grade evidence, but low risk and high upside):

Diverse plant-based diet + fermented foodsRegular exercise — directly increases microbial diversity (Allen 2018)Adequate sleep — sleep deprivation lowers microbial diversityDon't blindly take probiotics — unless there's a clear indication

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Chapter 6

Barrier & tight junctions

The gut barrier is the thinnest, longest, most-impacted border in the body — a single layer of enterocytes (about 25 μm thick, 1/4 the width of a hair) separating the outside world (gut lumen = food + trillions of microbes) from the inside world (blood + immune system).

This barrier has four layers of defense:

Mucus layer — goblet cells secrete mucin (primarily MUC2), a physical + chemical bufferAntimicrobial peptides — Paneth cells secrete defensins / cathelicidinEnterocytes + tight junctions — claudin / occludin / ZO-1 proteins 'stitch' adjacent cells tightSubmucosal GALT — 70% of the body's immune cells are stationed here
Tight junctions are not 'welded shut' — they're regulated:

Open — let water / ions / small molecules selectively passClose — block large molecules / bacteria / toxins
The regulating signal molecule is zonulin (zonula occludens toxin), discovered by Alessio Fasano's team in the 2000s. Gliadin (a gluten fragment) is one of the strongest known zonulin-releasing triggers — in celiac disease, this pathway is over-activated, tight junctions loosen, gluten fragments cross the barrier and trigger immune attack.

Normal people vs celiac patients: gluten activates zonulin in both, but in normal people the closing mechanism still works; in celiac it doesn't. That's the molecular distinction between 'sensitive vs not sensitive'.

Leaky gut — what's real

'Leaky gut syndrome' is overused in alternative medicine circles and dismissed in mainstream medicine — but the underlying science of intestinal permeability is real; the clinical application is just early.

Diseases confirmed to have elevated barrier permeability:

Celiac disease — most clearly, causal levelType 1 diabetes — most patients have elevated permeability before diagnosisInflammatory bowel disease (Crohn's / UC) — both consequence and triggerNAFLD (non-alcoholic fatty liver disease) — leaky gut → portal-vein LPS up → hepatic inflammation
Potentially related but B-C grade evidence: food allergy / intolerance, IBS, autoimmune diseases (thyroid, rheumatoid, SLE), depression / anxiety.

Clinical testing:

Lactulose / mannitol disaccharide ratio — drink it and measure the urinary excretion ratio; the classic research toolSerum zonulin — commercially available, but many false positivesLPS / LBP (LPS-binding protein) — indirect indicatorsNone has been accepted by mainstream consensus as a diagnostic standard for 'leaky gut syndrome'
Things that improve the barrier: adequate fiber + (butyrate directly fuels enterocytes and upregulates tight-junction genes); zinc, vitamin A, vitamin D, glutamine — basic raw materials for the barrier; less NSAIDs (long-term high-dose ibuprofen / aspirin is a classic leaky-gut trigger); less alcohol (acetaldehyde directly damages tight junctions); regular sleep (circadian rhythm regulates gut barrier genes); manage chronic stress (CRH directly opens tight junctions).

Claims without supporting evidence: 'leaky gut detox' supplements / teas / enemas; 'collagen / bone broth heals leaky gut' (the products themselves are fine, but effects are exaggerated); '*H. pylori* / candida overgrowth' framed as the root of all disease.

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