The human body, what goes wrong, and how to make good decisions about your own health
Part A — The Human Body: A Working Map
What this module covers
Medicine is perhaps the most consequential body of knowledge an ordinary person can acquire. It decides how you interpret symptoms, evaluate treatments, navigate healthcare systems, and ultimately understand your own mortality. This module is organised in twelve parts: body architecture (A), disease mechanisms (B), diagnostics (C), treatments (D), the evidence base (E), public health (F), healthcare systems (G), nutrition (H), mental health (I), emergency first aid (J), rational decision-making (K), and the future of medicine (L). You do not need a science background. You need the willingness to think precisely about uncertain information.
Organ systems — interactive map
Click a system to explore it
Cardiovascular System
The cardiovascular system is a closed-loop pressure circuit: the heart (a 300-gram muscular pump) ejects roughly 70 mL of blood per beat, around 5 litres per minute at rest and up to 25 litres during intense exercise. Arteries carry oxygenated blood outward under high pressure; veins return deoxygenated blood under low pressure. The heart's two pumps work in parallel: the right side sends blood to the lungs (pulmonary circuit) and the left side sends it to the rest of the body (systemic circuit). The coronary arteries, which feed the heart muscle itself, are only 2–3 mm in diameter — which is why a small plaque can trigger a massive heart attack. Blood pressure is expressed as systolic/diastolic mmHg; 120/80 is textbook normal. Above 140/90 consistently constitutes hypertension, which silently damages artery walls for decades before causing strokes or kidney failure.
The cell — life's basic unit
The human body contains roughly 37 trillion cells, each a self-contained chemical factory. Every cell contains a nucleus (DNA blueprint), mitochondria (ATP generators, roughly 2,000 per cell), a cell membrane (selectively permeable lipid bilayer), and specialised organelles. A red blood cell lasts 120 days; a gut epithelial cell lasts 3–5 days; a neuron may last your entire lifetime. The key insight: disease almost always starts at the cellular level — a mutation, a pathogen entering, a receptor malfunctioning.
Homeostasis — the body's self-regulation
Core body temperature must stay within 36.1–37.2°C. Blood pH must stay within 7.35–7.45. Blood glucose must remain roughly 4–7 mmol/L. The body achieves this through negative feedback loops: a deviation triggers a corrective response that reverses the deviation. The thermostat analogy is imperfect — biological systems use hormone cascades, neural reflexes, and organ-organ communication simultaneously, which is why a single-point failure (e.g., insulin deficiency in type 1 diabetes) destabilises multiple systems at once.
Part B — How Disease Works
Disease taxonomy — click to explore
Infectious Disease
Infectious diseases are caused by pathogens: bacteria (living cells that replicate independently, killed by antibiotics), viruses (genetic material in a protein coat that hijack host cell machinery, requiring antivirals or vaccines), fungi (eukaryotes, treatable with antifungals), and parasites (complex organisms like Plasmodium falciparum, which kills 600,000 people yearly via malaria). The key concept is transmission: each pathogen has a characteristic R0 (basic reproduction number) — the number of new infections one case generates in a fully susceptible population. Measles has an R0 of 12–18. Influenza is around 2–3. SARS-CoV-2 (original strain) was 2–3; Omicron was 8–15. R0 above 1 means exponential growth; herd immunity requires a fraction (1 − 1/R0) of the population to be immune.
Global causes of death — top conditions (millions/year, WHO 2019 data)
Source: WHO Global Health Estimates 2020. Figures are pre-COVID 2019 baselines. Cardiovascular disease alone accounts for 32% of all global deaths.
The immune response — how your body fights infection
Innate and adaptive immunity — a schematic
The innate immune system responds within hours but cannot distinguish between specific pathogens. The adaptive system takes days but generates exquisitely specific antibodies and cytotoxic T cells, plus memory that makes future responses faster. Vaccines exploit this memory mechanism.
Part C — Diagnostics
Normal vital signs — what each measures
Heart Rate
60–100
beats per minute. Below 60 is bradycardia (normal in athletes); above 100 is tachycardia. Trained endurance athletes often rest at 40–50.
Blood Pressure
120/80
mmHg (systolic/diastolic). Above 140/90 on repeated measures is hypertension. Below 90/60 is hypotension.
Respiratory Rate
12–20
breaths per minute. Often the most ignored vital sign — a rate above 25 in an adult is a serious warning sign of respiratory or metabolic distress.
Temperature
36.1–37.2°C
Oral temperature. Above 38.0°C is fever. Average core temperature has been declining since the 19th century, from 37.0 to around 36.6°C.
O2 Saturation
95–100%
Pulse oximetry. Below 90% requires urgent attention. COPD patients may baseline at 88–92%. SpO2 is not the same as PaO2 (arterial oxygen).
Blood Glucose
4.0–7.8
mmol/L fasting. Above 7.0 fasting or 11.1 random = diabetes threshold. HbA1c over 6.5% confirms 3-month average hyperglycaemia.
Sensitivity, specificity & the base-rate problem — calculator
A test with 99% sensitivity and 99% specificity sounds almost perfect. But when the disease prevalence is low, most positive results are false positives. Enter numbers to see how misleading high-accuracy tests can be.
Medical imaging — when each modality is used
X-ray (Radiograph)
X-rays use ionising electromagnetic radiation that passes through soft tissue but is absorbed by dense structures like bone and calcium deposits. The image is a 2D shadow. Dose per chest X-ray is roughly 0.1 mSv — equivalent to about 10 days of natural background radiation. Ideal for: fractures, pneumonia (lung consolidation appears white), pneumothorax (collapsed lung), foreign bodies. Not ideal for: soft tissue, brain, abdomen organs (poor contrast). Speed: 5 minutes from request to image.
Part D — Treatment
Drug development — from discovery to prescription
Average time from molecule identification to approval: 10–15 years. Average cost: $1–2 billion (including failures). Only 1 in ~10,000 candidate molecules reaches clinical use.
Timeline is approximate. Phase III trials alone can enrol 10,000–30,000 participants. The FDA's expedited pathway (Breakthrough Therapy designation) can compress timelines by 30%.
Treatment strength vs. side-effect burden — a spectrum
Approximate relative positioning. Not a clinical guide. Individual drugs within each class vary widely.
Antibiotics — the resistance crisis
Antibiotics are bactericidal or bacteriostatic — they kill bacteria or stop their replication. They work by targeting bacterial-specific structures: the cell wall (penicillins, cephalosporins), protein synthesis (macrolides, tetracyclines), or DNA replication (fluoroquinolones). They have zero effect on viruses. Resistance emerges because bacteria evolve: random mutations that happen to inactivate an antibiotic are selected for when antibiotics are present. The WHO considers antimicrobial resistance one of the greatest threats to global health — 700,000 people die from resistant infections annually today; projections put this at 10 million by 2050.
Vaccines — how immunisation works
Vaccines present the immune system with an antigen (or instructions to make one) without causing disease. The immune system generates antibodies and memory B and T cells. On future exposure, the response is faster and stronger — before you develop symptoms. Types: live-attenuated (MMR, highest efficacy), inactivated (flu, polio Salk), subunit (hepatitis B, pertussis), and mRNA (Pfizer/Moderna COVID-19, the first mRNA vaccines approved for mass use). The mRNA in COVID-19 vaccines degrades within days; it does not enter the nucleus or alter DNA.
Part E — Evidence-Based Medicine
The hierarchy of evidence — from weakest to strongest
The hierarchy is not absolute: a well-designed observational study can be more informative than a poorly designed RCT. But it captures the general principle: the higher up, the more the study design controls for confounding variables and bias.
Absolute vs relative risk — the numbers that get manipulated
Drug manufacturers often report relative risk reduction (RRR) because it sounds more impressive. Absolute risk reduction (ARR) is what matters for individual decisions. Adjust the baseline risk to see how misleading relative framing can be.
Baseline annual risk of event (e.g., heart attack) in control group:
Your baseline risk:
Publication bias — what gets published vs what gets done
Estimates suggest only 50–60% of clinical trials ever publish results. Positive trials are 2.4x more likely to be published than negative ones (Turner et al., NEJM 2008). This systematically overstates treatment efficacy in the literature. Pre-registration of trials at ClinicalTrials.gov partially addresses this.
Part F — Public Health
What actually determines your health — proportional contribution (estimates)
These proportions (from US County Health Rankings model) are contested but directionally robust. The counterintuitive insight: clinical healthcare — what we typically mean by "medicine" — accounts for only about 20% of health outcomes. Your postcode predicts your lifespan better than your doctor.
Major pandemic timeline — death toll and origin
Bars show approximate years of peak mortality. Death tolls are historical estimates and vary by source. The Black Death (1347–1351) killed an estimated 30–60% of Europe's population — unmatched in proportional terms by any subsequent pandemic.
Part G — Healthcare Systems
The four basic healthcare financing models
Beveridge Model
Named after William Beveridge, architect of the 1948 NHS. Government collects taxes, owns or contracts hospitals, employs or contracts doctors, and provides care free at the point of use. The logic: healthcare is a public good, like roads or defence. Examples: UK (NHS), Spain, Scandinavia. Advantages: universal coverage, no administrative complexity for patients, lower per-capita cost. Disadvantages: waiting times, political vulnerability (funding decisions tied to electoral cycles), potential for rationing. The NHS costs the UK roughly £3,000 per person per year — less than half the US per-capita spend — with comparable or better outcomes on most population health metrics.
Healthcare spending vs outcomes — selected countries
Annual per-capita healthcare spending in USD (2022, OECD). Life expectancy at birth shown alongside.
The US spends more than $12,000 per person per year — more than twice the OECD average — yet ranks last among high-income countries on life expectancy, infant mortality, and chronic disease burden (Commonwealth Fund, 2023).
Part H — Nutrition & Lifestyle Medicine
Daily calorie needs estimator (Mifflin-St Jeor equation)
The Mifflin-St Jeor equation is currently the most validated formula for estimating resting metabolic rate (RMR). Note: this is an estimate; individual variation can be ±15%.
Ultraprocessed foods — the evidence
The NOVA classification defines ultraprocessed foods (UPF) as industrial formulations containing ingredients not used in domestic cooking: emulsifiers, flavour enhancers, stabilisers, hydrogenated oils. UPFs now account for 57% of UK caloric intake and 60% in the US. The NOVA system was controversial when proposed by Carlos Monteiro in 2010 — critics argued it ignored nutrient content. But prospective cohort studies have since linked high UPF intake to 32% higher cardiovascular mortality, 12% higher cancer risk, and 53% higher dementia risk (JAMA Internal Medicine, 2023). The mechanism is unclear: it may be the additives, the lack of fibre, the speed of digestion, or the food matrix disruption.
Sleep — the most underrated intervention
Chronic sleep restriction to 6 hours per night produces cognitive deficits equivalent to 24 hours of total sleep deprivation, yet subjects report feeling only "slightly sleepy" — they lose the ability to perceive their own impairment. During sleep, the glymphatic system (brain's waste clearance mechanism) pumps cerebrospinal fluid through brain tissue, flushing amyloid beta and tau — the proteins that aggregate in Alzheimer's disease. Adults need 7–9 hours. No amount of caffeine or adaptation compensates for the biological deficit. Matthew Walker's book "Why We Sleep" (2017) overstates some claims, but the core evidence base — from laboratory studies and population epidemiology — is solid.
What dietary patterns actually do — risk reduction estimates
Mediterranean diet data from PREDIMED trial (7,447 participants, Spain, 2013). Effect sizes are approximate. The PREDIMED trial was partially retracted due to randomisation issues but re-analysis confirmed the main findings. Note: dietary epidemiology is notoriously difficult — all estimates are from observational studies except Mediterranean diet, which has RCT evidence.
Part I — Mental Health
Major mental health conditions — click to explore
Depression (Major Depressive Disorder)
Depression affects 280 million people globally — 5% of adults — making it the leading cause of disability worldwide (WHO, 2023). The DSM-5 requires at least 5 of 9 symptoms for 2+ weeks, one of which must be depressed mood or anhedonia (inability to feel pleasure). The "chemical imbalance" explanation (low serotonin = depression) is a significant oversimplification. The serotonin theory was never well-supported, as a major 2022 umbrella review by Moncrieff et al. (Molecular Psychiatry) confirmed. Antidepressants do work — particularly SSRIs for moderate-severe depression — but likely through mechanisms beyond serotonin reuptake. Effect size vs placebo is around 0.3 (NNT roughly 7–8). Cognitive-behavioural therapy (CBT) has comparable efficacy for mild-moderate depression and better durability after treatment ends.
Global mental health burden — disability-adjusted life years (DALYs, millions)
DALYs combine years of life lost to premature death with years lived with disability. Mental disorders collectively account for around 13% of global DALYs, yet receive only 2% of health budgets in most low-income countries (WHO Mental Health Atlas, 2020).
Part J — First Aid & Emergency Medicine
The primary survey — ABCDE in any emergency
Before doing anything else in any medical emergency, assess in order: Airway, Breathing, Circulation, Disability, Exposure. Each step is a "treat as you find" approach — fix a problem before moving on.
CPR survival — how compression depth affects outcomes
Survival from out-of-hospital cardiac arrest without bystander CPR is around 5–10%. Optimal CPR (100–120 compressions/min, 5–6 cm depth) can double or triple survival rates. Use this slider to understand the relationship.
Emergency conditions — recognition and response
Stroke — FAST
Stroke occurs when blood supply to part of the brain is cut off (ischaemic, 85% of cases) or when a blood vessel bleeds into brain tissue (haemorrhagic, 15%). Brain cells die at the rate of 1.9 million per minute during a large ischaemic stroke. FAST: Face (drooping on one side?), Arms (one arm drifts down?), Speech (slurred, confused, unable to speak?), Time (call emergency services immediately). Time is brain: thrombolysis (clot-busting drug tPA) must be given within 4.5 hours, and mechanical thrombectomy within 24 hours for large vessel occlusion. Do not give aspirin until haemorrhagic stroke is excluded by CT scan — aspirin worsens a bleed. Do not let the person eat or drink; stroke impairs swallowing and aspiration is fatal.
Part K — Medical Decision-Making
Evaluating health claims — spotting pseudoscience
Red flags: testimonials instead of trials, mechanism too vague to be falsifiable ("boosts immunity"), cherry-picked studies, appeal to nature, appeal to conspiracy (doctors don't want you to know), before/after photos, products sold by the same person making the claims. The single most reliable question: what would change your mind? Pseudoscience producers have no answer. Also check: is the study on humans or petri dishes? Is the effect size clinically meaningful? Was it replicated? Who funded it?
Overdiagnosis — when more medicine harms
Overdiagnosis occurs when a condition is detected that would never have caused symptoms or death in a patient's lifetime. PSA screening for prostate cancer overdiagnoses roughly 20–50% of cases — men undergo surgery, radiation, and hormone therapy for cancers that would never have harmed them. CT colonoscopy finds polyps in 30–40% of adults; most will never progress to cancer. Overdiagnosis is not medical error — it's a structural consequence of improving test sensitivity without improving prognostic discrimination. The Choosing Wisely campaign (launched 2012) has identified over 600 tests and treatments that are overused.
Complementary & alternative medicine — evidence quality spectrum
Approximate positioning based on RCT evidence. "Some evidence" = effect size detectable but modest; "no reliable evidence" = trials fail to show effects beyond placebo or have been retracted.
Acupuncture has the strongest CAM evidence base, particularly for chronic pain and nausea — though effect sizes are small and sham acupuncture often performs as well as "real" acupuncture. Homeopathy is water: no plausible mechanism and no replicated trial evidence above placebo exists. Chiropody for musculoskeletal issues has better evidence than chiropractic for visceral disease claims.
Part L — The Future of Medicine
Genomics and personalised medicine — what it means in practice
Genomics and Personalised Medicine
Whole genome sequencing now costs under $200 (down from $3 billion for the first human genome in 2003). Pharmacogenomics is already clinical reality: variants in the CYP2C19 gene determine whether clopidogrel (an antiplatelet drug) is metabolically active; roughly 30% of people carry loss-of-function variants and receive ineffective therapy. The UK Biobank (500,000 participants with genotype + health records) has identified hundreds of genetic variants associated with disease risk. Polygenic risk scores can predict lifetime risk of type 2 diabetes, coronary artery disease, and breast cancer more accurately than family history alone. The key limitation: most variants explain tiny proportions of variance, and gene-environment interactions are poorly understood. Personalised medicine so far works best for cancer (selecting targeted therapies) and drug dosing — not yet for population-level prevention.
Antimicrobial resistance — projected annual deaths (millions)
Projections from the O'Neill Commission on AMR (2016). Current deaths (~700k/year) are already equivalent to one pandemic per year. Without action, resistant infections could overtake cancer as the leading cause of death by 2050. New antibiotic development has been commercially unattractive because drugs are used briefly and should be reserved — creating a market failure that requires public funding.
Part M · Q&A
If vaccines train the immune system, why does the flu vaccine have such low efficacy some years?
Why do randomised controlled trials occasionally produce results that turn out to be wrong?
If the US spends far more per capita on healthcare than any other country, why does it have worse outcomes?
Is the "serotonin hypothesis" of depression wrong, and does that mean antidepressants don't work?
Antibiotics have no effect on viruses — so why do doctors still prescribe them for viral infections?
What is a QALY and why is it controversial for rationing healthcare?
Can you prevent cancer through lifestyle changes, and by how much?